<PAGE>

                                                                     EXHIBIT 2.7


                              DNA SCIENCES, INC.

                 ____________________________________________

                  SERIES C PREFERRED STOCK PURCHASE AGREEMENT
                               December 17, 2000

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                               Table of Contents

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SECTION 1 Authorization of Series C Preferred Stock...............   1

      1.1  Authorization of Series C Preferred Stock..............   1

      1.2  Sale of Series C Preferred.............................   1

SECTION 2 Closing Date; Delivery..................................   1

      2.1  Closing Date...........................................   1

      2.2  Delivery...............................................   1

SECTION 3 Representations and Warranties of the Company...........   2

      3.1  Organization and Standing; Articles and Bylaws.........   2

      3.2  Corporate Power........................................   2

      3.3  Subsidiaries...........................................   2

      3.4  Capitalization.........................................   2

      3.5  Authorization..........................................   3

      3.6  Validity of Shares.....................................   3

      3.7  Title to Properties and Assets; Liens, etc.............   3

      3.8  Material Contracts and Commitments.....................   4

      3.9  Patents, Trademarks, etc...............................   4

     3.10  Compliance with Other Instruments......................   4

     3.11  Litigation, etc........................................   5

     3.12  Employees..............................................   5

     3.13  Insurance..............................................   5

     3.14  Registration Rights....................................   5

     3.15  Governmental Consents, etc.............................   5

     3.16  Offering...............................................   6

     3.17  Disclosure.............................................   6

     3.18  Brokers or Finders.....................................   6

     3.19  Permits................................................   6

     3.20  No Conflict of Interest................................   6

     3.21  Actions................................................   7

     3.22  Voting Rights..........................................   7

     3.23  Proprietary Information and Inventions Agreements......   7

     3.24  Employee Benefit Plans.................................   7

     3.25  Tax Returns and Payments...............................   7
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                               Table of Contents
                                  (CONTINUED)

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     3.26  Corporate Documents....................................   7

     3.27  Environmental Matters..................................   7

     3.28  Investment Company Act.................................   8

     3.29  Tax Elections..........................................   8

     3.30  Financial Statements...................................   8

     3.31  Financial Condition....................................   8

     3.32  Qualified Small Business Stock.........................   9

     3.33  U.S. Real Property Holding Corporation.................   9

SECTION 4 Representations, Warranties and Covenants of Purchaser..   9

      4.1  Accredited Investor....................................   9

      4.2  Investment.............................................   9

      4.3  Restricted Securities; Rule 144........................   9

      4.4  No Public Market.......................................  10

      4.5  Access to Data.........................................  10

      4.6  Authorization..........................................  10

      4.7  Government Consents....................................  10

      4.8  Further Limitations on Disposition.....................  10

      4.9  Legends................................................  10

SECTION 5 Conditions to Closing of Purchaser......................  11

      5.1  Representations and Warranties Correct.................  11

      5.2  Covenants..............................................  11

      5.3  Compliance Certificate.................................  11

      5.4  Blue Sky...............................................  11

      5.5  Related Agreements.....................................  11

      5.6  Restated Certificate of Incorporation..................  11

      5.8  Opinion of Company Counsel.............................  12

SECTION 6 Conditions to Closing of the Company....................  12

      6.1  Representations........................................  12

      6.2  Blue Sky...............................................  12

      6.3  Related Agreements.....................................  12

      6.4  Restated Certificate of Incorporation..................  12

      6.5  The Merger.............................................  12

SECTION 7 Affirmative Covenants of the Company....................  12
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                               Table of Contents
                                  (CONTINUED)

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      7.1  The Board of Directors.................................  12

      7.2  Proprietary Information Agreements.....................  12

      7.3  Option Vesting.........................................  12

      7.4  Insurance..............................................  13

      7.5  Termination of Covenants...............................  13

      7.6  U.S. Real Property Holding Corporation.................  13

SECTION 8 Miscellaneous...........................................  13

      8.1  Governing Law..........................................  13

      8.2  Survival...............................................  13

      8.3  Successors and Assigns.................................  13

      8.4  Entire Agreement; Amendment............................  13

      8.5  Waiver of Conflicts....................................  14

      8.6  Notices, etc...........................................  14

      8.7  Delays or Omissions....................................  14

      8.8  California Corporate Securities Law....................  14

      8.9  Counterparts...........................................  15

     8.10  Severability...........................................  15
</TABLE>

                                      iii
<PAGE>

                              DNA SCIENCES, INC.

                  SERIES C PREFERRED STOCK PURCHASE AGREEMENT

     This Agreement is made as of December 17, 2000, between DNA Sciences, Inc.,
a Delaware corporation (the "Company") and the entity listed on Exhibit A
("Purchaser").

                                   SECTION 1

              Authorization and Sale of Series C Preferred Stock

     1.1  Authorization of Series C Preferred Stock. The Company has authorized
the sale and issuance hereunder of up to Five Million Five Hundred Thousand
(5,500,000) shares of its Series C Preferred Stock (the "Series C Preferred"),
having the rights, preferences, privileges and restrictions set forth in the
Company's Amended and Restated Certificate of Incorporation in the form attached
hereto as Exhibit B (the "Restated Certificate"). The shares of Series C
Preferred to be sold hereunder are collectively referred to as the "Shares."

     1.2  Sale of Series C Preferred. Subject to the terms and conditions
hereof, the Company will issue and sell to Purchaser, and Purchaser will
purchase, at a purchase price of ten dollars and fifteen cents ($10.15) per
share from the Company, the number of shares of Series C Preferred specified
opposite the name of Purchaser listed on Exhibit A at the Closing (as
hereinafter defined).

                                   SECTION 2

                            Closing Date; Delivery

     2.1  Closing Date. The closing of the purchase and sale of the Series C
Preferred hereunder (the "Closing") shall be held at the offices of Cooley
Godward, 5 Palo Alto Square, 3000 El Camino Real, Palo Alto, California 94304 on
the date of the closing (the "Closing Date") of the merger of PIPO Acquisition
Corp., a Delaware corporation and a wholly owned subsidiary of the Company
("Merger Sub"), with and into PPGx, Inc., a Delaware corporation ("PPGx"),
pursuant to that certain Agreement and Plan of Merger and Reorganization, by and
among the Company, Merger Sub and PPGx, dated as of December [17], 2000 (the
"Reorganization Agreement"), pursuant to which PPGx shall become a wholly owned
subsidiary of the Company (the "Merger"). On the Closing Date and immediately
prior to the closing of the Merger, pursuant to the terms of this Agreement, the
Company shall issue and sell to Purchaser, and Purchaser shall purchase, at a
purchase price of ten dollars and fifteen cents ($10.15) per share from the
Company, the number of shares of Series C Preferred specified in the column
labeled "Total Number of Series C Shares" opposite the name of Purchaser listed
on Exhibit A.

     2.2  Delivery. At the Closing, the Company will deliver to Purchaser a
certificate or certificates representing the number of Shares set forth opposite
Purchaser's name on Exhibit A of this Agreement, against payment of the purchase
price therefor, by check or wire transfer payable to the Company.

                                       1
<PAGE>

                                   SECTION 3

                 Representations and Warranties of the Company

     Except as set forth on Exhibit C and other exhibits attached hereto, or
otherwise contemplated hereunder, the Company hereby represents and warrants to
Purchaser as of the date hereof as follows:

     3.1  Organization and Standing; Articles and Bylaws. The Company is a
corporation duly organized and validly existing under, and by virtue of, the
laws of the State of Delaware and is in good standing under such laws. The
Company has all requisite corporate power to own and operate its properties and
assets, and to carry on its business as presently conducted and as proposed to
be conducted. The Company is qualified to do business as a foreign corporation
in any jurisdiction in which failure to qualify would have a material adverse
effect on the Company's business. The Company has furnished Purchaser true and
complete copies of its Restated Certificate and Bylaws in effect as of the date
hereof.

     3.2  Corporate Power. The Company will have prior to the Closing Date all
requisite legal and corporate power to execute and deliver this Agreement and
the Amended and Restated Investor Rights Agreement in the form attached hereto
as Exhibit D (the "Investor Rights Agreement"), the Amended and Restated Right
of First Refusal and Co-Sale Agreement in the form attached hereto as Exhibit E
(the "Refusal Agreement") and the Voting Agreement in the form attached hereto
as Exhibit F (the "Voting Agreement") (the Investor Rights Agreement, the
Refusal Agreement and the Voting Agreement are collectively referred to as the
"Related Agreements"), to sell and issue the Shares hereunder, to issue the
Common Stock issuable upon conversion of the Shares and to carry out and perform
its obligations under the terms of this Agreement and the Related Agreements and
the transactions contemplated hereby and thereby.

     3.3  Subsidiaries. The Company has no subsidiaries or affiliated companies
and does not otherwise own or control any other corporation, association or
business entity. The Company has no other equity interest in any other
corporation, association or business entity.

     3.4  Capitalization. Immediately prior to the Closing, the authorized
capital stock of the Company consists of 40,000,000 shares of Common Stock,
4,721,364 shares of which are issued and outstanding, and 22,000,000 shares of
Preferred Stock, 8,283,000 shares of which are designated as Series A Preferred
Stock, all of which are issued and outstanding, 7,099,006 shares of which are
designated Series B Preferred Stock, all of which are issued and outstanding and
5,500,000 of which are designated as Series C Preferred Stock, 1,644,076 of
which are issued and outstanding.  All such issued and outstanding shares have
been duly authorized and validly issued, and are fully paid and non-assessable
and have been issued in compliance all federal and applicable state securities
laws.  The Company has reserved 8,283,000 shares of Common Stock for issuance
upon conversion of the Series A Preferred, 7,099,006 shares of Common Stock for
issuance upon conversion of the Series B Preferred,  5,500,000 shares of Common
Stock for issuance upon conversion of the Series C Preferred, 650,000 shares of
Common Stock for issuance to officers, directors, employees, sales
representatives and consultants of the Company pursuant to stock option grants
outside of the Company's 2000 Equity Incentive Plan (the "Stock Plan"), and
5,257,136 shares of its Common Stock for issuance to officers, directors,
employees,

                                       2
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sales representatives and consultants of the Company pursuant to the Stock Plan,
of which 3,687,395 shares are currently subject to outstanding options under the
Stock Plan, and 1,569,741 shares are available for future issuance. The Series C
Preferred shall have the rights, preferences, privileges and restrictions set
forth in the Restated Certificate. Except as referenced on the Schedule of
Exceptions on Exhibit C hereto, there are no options, warrants, conversion
privileges or other rights presently outstanding to purchase or otherwise
acquire any authorized but unissued shares of the capital stock or other
securities of the Company, nor any agreements or understandings with respect
thereto. The Company is not a party or subject to any agreement or
understanding, and, to the Company's knowledge, there is no agreement or
understanding between any persons and/or entities, which affects or relates to
the voting or giving of consents with respect to any security or by a director
of the Company.

     3.5  Authorization. All corporate action on the part of the Company, its
officers, directors and shareholders necessary for the authorization, execution,
delivery and performance of this Agreement and each of the Related Agreements by
the Company, the authorization, sale, issuance and delivery of the Shares (and
the Common Stock issuable upon conversion of the Shares) and the performance of
the Company's obligations hereunder and thereunder has been taken or will be
taken prior to the Closing. This Agreement and each of the Related Agreements,
when executed and delivered by the Company, shall constitute the valid and
binding obligations of the Company, enforceable against the Company in
accordance with their respective terms, subject to laws of general application
relating to bankruptcy, insolvency and the relief of debtors and rules of law
governing specific performance, injunctive relief or other equitable remedies.

     3.6  Validity of Shares. The Shares, when issued, sold and delivered in
compliance with the provisions of this Agreement, will be duly and validly
issued and will be fully paid and nonassessable and free and clear of all liens
and encumbrances, and the Common Stock issuable upon conversion of the Shares
has been duly and validly reserved and, when issued and delivered in compliance
with the provisions of the Restated Certificate, will be duly and validly issued
and will be fully paid and nonassessable and free and clear of all liens and
encumbrances and restrictions on transfer other than as set forth in this
Agreement and the Related Agreements, provided, however, that the Shares (and
the Common Stock issuable upon conversion of the Shares) may be subject to
restrictions on transfer under state and/or federal securities laws. There are
no outstanding rights of first refusal or preemptive rights applicable to the
Shares.

     3.7  Title to Properties and Assets; Liens, etc. The Company has good and
marketable title to all its properties and assets, and is in compliance with the
lease of all material properties leased by it, in each case subject to no
mortgage, pledge, lien, lease, encumbrance or charge, other than the lien of
current taxes not yet due and payable. The Company is not in default under or in
breach of any provision of its leases, and the Company holds valid lease-hold
interests in the properties which it leases. The Company's material properties
and assets are in good condition and repair, ordinary wear and tear excepted, in
all material respects.

     3.8  Material Contracts and Commitments. Exhibit C sets forth a list of all
agreements, contracts, indebtedness, liabilities and other obligations to which
the Company is a party or by which it or its assets are bound that (a) involve
in excess of $50,000 aggregating similar agreements or obligations to the same
party; (b) involve to the Company's knowledge any of the officers, consultants,
directors, employees or shareholders of the Company or any

                                       3
<PAGE>

members of the immediate family of the foregoing; or (c) obligate the Company to
share, license or develop any product. Copies of such agreements and contracts
and documentation evidencing such liabilities and other obligations have been
made available by the Company to Purchaser. True and complete copies of all
items set forth on Exhibit C have been made available to Purchaser.

     3.9   Patents, Trademarks, etc. To the knowledge of the Company, the
Company has sufficient title and ownership of all patents, trademarks, service
marks, trade names, copyrights, trade secrets, information, proprietary rights
and processes necessary for its business as now conducted, and as proposed to be
conducted, and, to the knowledge of the Company, without conflict with or
infringement of the rights of others. The Company has not received any
communications alleging that the Company has violated or, by conducting its
business as proposed, would violate, the patents, trademarks, service marks,
trade names, copyrights, trade secrets, information or proprietary or
intellectual property rights of any other person or entity. The Company is not
aware of any violation or infringement by a third party of any of the Company's
patents, licenses, trademarks, service marks, trade names, copyrights, trade
secrets or other proprietary rights. The Company has disclosed trade secrets to
other persons solely as required for the conduct of its business and solely
under nondisclosure agreements that are enforceable by the Company. The Company
has at all times maintained reasonable procedures to protect and have enforced
all of its trade secrets.

     3.10  Compliance with Other Instruments. The Company is not in violation of
any term of its Restated Certificate or Bylaws, or in any material respect of
any term or provision of any mortgage, indenture, contract, agreement,
instrument, judgment or decree, and to its knowledge is not in violation of any
order, statute, rule or regulation applicable to the Company. The execution,
delivery and performance of and compliance with this Agreement and each of the
Related Agreements and the issuance of the Shares (and the Common Stock issuable
upon conversion of the Shares), have not resulted and will not result in any
violation of, or conflict with, or constitute a default under any of the
foregoing, or result in the creation of any mortgage, pledge, lien, encumbrance
or charge upon any of the properties or assets of the Company; and there is no
such violation or default which materially and adversely affects the business,
financial condition, results of operations or prospects of the Company or any of
its properties or assets. All of the contracts and agreements set forth on
Exhibit C are in full force and effect and constitute legal, valid and binding
obligations of the Company. The Company and, to the best knowledge of the
Company, each other party thereto, has performed in all material respects all
obligations required to be performed by it under the contracts and agreements
set forth on Exhibit C, and no material violation or default exists in respect
thereof, nor any event that with notice or lapse of time, or both, would
constitute a default thereof; on the part of the Company or, to the best
knowledge of the Company, any other party thereto; none of the contracts and
agreements set forth on Exhibit C is currently being renegotiated; and the
validity, effectiveness and continuation of all contracts and agreements set
forth on Exhibit C will not be materially adversely affected by the transactions
contemplated by this Agreement or any of the Related Agreements.

     3.11  Litigation, etc. There are no actions, suits, proceedings or
investigations pending or, to the Company's knowledge, threatened against the
Company or its properties before any court or governmental agency, which, either
in any case or in the aggregate, might result in any

                                       4
<PAGE>

material adverse change in the business, prospects, financial condition,
affairs, operations or equity ownership of the Company or any of its properties
or assets, or in any material impairment of the right or ability of the Company
to carry on its business as now conducted or as proposed to be conducted, or in
any material liability on the part of the Company, and none which questions the
validity of this Agreement or any of the Related Agreements or any action taken
or to be taken in connection herewith or therewith. The Company is not a party
or subject to the order, writ, judgement, injunction, decree or other provisions
of any court or governmental agency or instrumentality. There is no action,
suit, proceeding or investigation by the Company currently pending or that the
Company intends to initiate.

     3.12  Employees. To the Company's knowledge, no employee of the Company is
or will be in violation of any judgment, decree or order of any court or
administrative agency or any term of any employment contract or any other
contract (including without limitation any covenant not to compete) or agreement
relating to the relationship of any such employee with the Company or any other
party because of the nature of the business conducted or to be conducted by the
Company or to the utilization by the employee of such employee's reasonable
efforts with respect to such business. The Company does not have any collective
bargaining agreements covering any of its employees. To the Company's knowledge,
the Company is not using any inventions of any of its employees, consultants or
officers made before their employment by the Company. To the Company's
knowledge, no employee, consultant or officer has taken, removed or made use of
any proprietary documentation, manuals, products, materials or any other
tangible items from the employee's previous employers relating to the Company's
business. To the best knowledge of the Company, no labor union has sought to
represent any of the employees of the Company. There is no strike or other labor
dispute involving the Company pending, or to the best knowledge of the Company,
threatened. To the best knowledge of the Company, no officer or key employee
intends to terminate his or her employment with it.

     3.13  Insurance. The Company has in full force and effect fire, casualty
and liability insurance policies with recognized insurers with such coverages as
are sufficient in amount to allow replacement of the tangible properties of the
Company that might be damaged or destroyed.

     3.14  Registration Rights. Except as contemplated by the Investor Rights
Agreement, the Company is not under any obligation to register any of its
presently outstanding securities or any of its securities that may hereafter be
issued.

     3.15  Governmental Consents, etc. No consent, approval, order or
authorization of or designation, declaration or filing with any state, federal
or local governmental authority on the part of the Company is required in
connection with the valid execution and delivery of this Agreement, or the
offer, sale or issuance of the Shares (and the Common Stock issuable upon
conversion of the Shares), or the consummation of any other transaction
contemplated hereby, except (a) filing of the Restated Certificate in the office
of the Secretary of State of the State of Delaware, (b) qualification (or taking
such action as may be necessary to secure an exemption from qualification, if
available) under the California Corporate Securities Law and other applicable
Blue Sky laws, of the offer and sale of the Shares (and the Common Stock
issuable upon conversion of the Shares), which filings and qualification, if
required, will be accomplished in a timely manner prior to or promptly upon
completion of the Closing, and (c) such filings as

                                       5
<PAGE>

may be determined by counsel to the Company to be necessary to secure an
exemption from registration under the Securities Act of 1933, as amended (the
"Securities Act"), which filing, if required, will be accomplished in a timely
manner prior to or promptly upon completion of the Closing.

     3.16  Offering. Based in part upon the accuracy of Purchaser's
representations in Section 4 hereof, the offer, sale and issuance of the Shares
to be issued in conformity with the terms of this Agreement (and the issuance of
the Common Stock to be issued upon conversion of the Shares) constitute
transactions exempt from the registration requirements of Section 5 of the
Securities Act.

     3.17  Disclosure. The Company has fully provided Purchaser with all the
information which Purchaser have requested for deciding whether to acquire the
Shares. No statement by the Company contained in this Agreement and the exhibits
attached hereto, or in any certificate furnished or to be furnished to Purchaser
pursuant hereto, when taken as a whole, contains or will contain any untrue
statement of a material fact or omits or will omit to state a material fact
necessary in order to make the statements contained herein or therein not
misleading in light of the circumstances under which they were made.

     3.18  Brokers or Finders. The Company has not incurred, and will not incur,
directly or indirectly, any liability for brokerage or finders' fees or agents'
commissions or any similar charges in connection with this Agreement or any
transaction contemplated hereby.

     3.19  Permits. The Company has all franchises, permits, licenses and any
similar authority necessary for the conduct of its business as now being
conducted, the lack of which could materially and adversely affect the business,
properties, prospects, financial condition or results of operations of the
Company, and the Company believes it can obtain, without undue burden or
expense, any similar authority for the conduct of its business as planned to be
conducted. The Company is not in default in any material respect under any of
such franchises, permits, licenses or other similar authority.

     3.20  No Conflict of Interest. The Company is not indebted, directly or
indirectly, to any of its officers or directors or to their respective spouses
or children, in any amount whatsoever other than in connection with expenses or
advances of expenses incurred in the ordinary course of business or relocation
expenses of employees. To the best of the Company's knowledge, none of the
Company's officers or directors, or any members of their immediate families,
are, directly or indirectly, indebted to the Company (other than in connection
with purchases of the Company's stock) or have any direct or indirect ownership
interest in any firm or corporation with which the Company is affiliated or with
which the Company has a business relationship, or any firm or corporation which
competes with the Company except that officers, directors and/or shareholders of
the Company may own stock in (but not exceeding two percent of the outstanding
capital stock of) any publicly traded company that may compete with the Company.
To the best of the Company's knowledge, none of the Company's officers or
directors or any members of the immediate families are, directly or indirectly,
interested in any material contract with the Company. The Company is not a
guarantor or indemnitor of any indebtedness of any other person, firm or
corporation.

                                       6
<PAGE>

     3.21  Actions. Neither the Company nor any of its subsidiaries has (i)
declared or paid any dividends, or authorized or made any distribution upon or
with respect to any class or series of its capital stock, (ii) incurred any
indebtedness for money borrowed or incurred any other liabilities individually
in excess of $25,000 or in excess of $50,000 in the aggregate, (iii) made any
loans or advances to any person, other than ordinary advances for travel
expenses, or (iv) sold, exchanged or otherwise disposed of any of its assets or
rights, other than the sale of its inventory in the ordinary course of business.

     3.22  Voting Rights. To the Company's knowledge, no shareholders of the
Company have entered into any agreements with respect to the voting of capital
shares of the Company.

     3.23  Proprietary Information and Inventions Agreements. Each employee,
consultant and officer of the Company has executed an agreement with the Company
regarding confidentiality and proprietary information substantially in the form
or forms delivered to the counsel for Purchaser. The Company, after reasonable
investigation, is not aware that any of its employees or consultants is in
violation thereof, and the Company will use its best efforts to prevent any such
violation. All consultants to or vendors of the Company with access to
confidential information of the Company are parties to a written agreement
substantially in the form or forms provided to counsel for Purchaser under
which, among other things, each such consultant or vendor is obligated to
maintain the confidentiality of confidential information of the Company. The
Company, after reasonable investigation, is not aware that any of its
consultants or vendors are in violation thereof, and the Company will use its
best efforts to prevent any such violation.

     3.24  Employee Benefit Plans. The Company does not have any Employee
Benefit Plan as defined in the Employee Retirement Income Security Act of 1974.

     3.25  Tax Returns and Payments. The Company has filed all tax returns and
reports as required by law. These returns and reports are true and correct in
all material respects. The Company has paid all taxes and other assessments due.

     3.26  Corporate Documents. The Restated Certificate and Bylaws of the
Company are in the form provided to counsel for Purchaser. The copy of the
minute books of the Company provided to Purchaser's counsel contains minutes of
all meetings for directors and shareholders and all actions by written consent
without a meeting by the directors and shareholders since the date of
incorporation and reflects all actions by the directors (and any committee of
directors) and shareholders with respect to all transactions referred to in such
minutes accurately in all material respects.

     3.27  Environmental Matters. The business, assets and properties of the
Company are and have been operated and maintained in material compliance with
all applicable federal, state, city, county and local environmental protection
laws and regulations (collectively, the "Environmental Laws"). To the best
knowledge of the Company, no event has occurred which, with or without the
passage of time or the giving of notice, or both, would constitute noncompliance
by the Company with, or a violation by the Company of, the Environmental Laws.
To the best knowledge of the Company, the Company has not caused or permitted to
exist, as a result of an intentional or unintentional act or omission, a
disposal, discharge or

                                       7
<PAGE>

release of solid wastes, pollutants or hazardous substances, on or from any site
which currently is or formerly was owned, leased, occupied or used by it, except
where such disposal, discharge or release was in material compliance with the
Environmental Laws.

     3.28  Investment Company Act.  The Company is not an "investment company"
nor is the Company directly or indirectly controlled by or acting on behalf of
any person which is an "Investment Company" within the meaning of the Investment
Company Act of 1940, as amended.

     3.29  Tax Elections.  The Company has not elected pursuant to the Internal
Revenue Code of 1986, as amended (the "Code"), to be treated as an "S"
corporation or a collapsible corporation pursuant to Section 341(f) or Section
1362(a) of the Code, nor has it made any other elections pursuant to the Code
(other than elections which relate solely to matters of accounting, depreciation
or amortization) which would have a material affect on the Company, its
financial condition, its business as presently conducted or presently proposed
to be conducted or any of its properties or material assets.

     3.30  Financial Statements. The Company has made available to Purchaser its
(i) unaudited financial statements (including balance sheet and profit and loss
statement) as of August 31, 2000, and (ii) audited financial statements as of
December 31, 1999 (collectively, the "Financial Statements"). The Financial
Statements have been prepared in accordance with generally accepted accounting
principles applied on a consistent basis throughout the periods indicated,
except that such unaudited Financial Statements may not contain all footnotes
required by generally accepted accounting principles. The Financial Statements
fairly present the financial condition and operating results of the Company as
of the dates, and for the periods, indicated therein, subject to normal yearend
audit adjustments. Except as set forth in the Financial Statements, the Company
has no material liabilities, contingent or otherwise, other than (i) liabilities
incurred in the ordinary course of business subsequent to August 31, 2000, and
(ii) obligations under contracts and commitments incurred in the ordinary course
of business and not required under generally accepted accounting principles to
be reflected in the Financial Statements, which, in both cases, individually or
in the aggregate are not material to the financial condition or operating
results of the Company.

     3.31  Financial Condition. The Company was incorporated on May 11, 1998.
Since that time, the Company has only engaged in startup and development
activities and has generated no revenue. Since inception, the Company has been
operated only in the ordinary course of business, and there has not been any
event or condition of any character that has adversely affected the Company's
business in a material manner, or any adverse change in the condition, assets or
liabilities of the Company except current liabilities incurred in the ordinary
course of business, none of which materially and adversely affect the business,
properties or prospects of the Company.

     3.32  Qualified Small Business Stock. The Shares will be issued directly
from the Company to Purchaser. The Company has not, during the period beginning
on the date one year before the first issuance of Series C Preferred to certain
purchasers on December 15, 2000 (the "Issuance"), made one or more purchases of
its capital stock with an aggregate value (at the time of the respective
purchases) exceeding five percent (5%) of the aggregate value of all its capital

                                       8
<PAGE>

stock at the beginning of such one-year period. The "aggregate gross assets" as
defined in Section 1202(d)(2) of the Internal Revenue Code of 1986, as amended
(the "IRC"), of the Company at all times before the Issuance did not exceed
Fifty Million Dollars ($50,000,000), and the "aggregate gross assets"
immediately after the Issuance will not exceed Fifty Million Dollars
($50,000,000). At the time of the Closing, the Company intends to engage in the
active conduct of a "qualified trade or business" as defined in Section
1202(e)(3) of the IRC. At the time of the Closing, the Company is an "eligible
corporation" as defined in Section 1202(e)(4) of the IRC. Not more than ten
percent (10%) of the Company's "aggregate gross assets" by value immediately
after the Issuance will consist of real estate.

     3.33  U.S. Real Property Holding Corporation. The Company is not now, nor
will it be immediately after the Closing, a "United States Real Property Holding
Corporation" as defined in Section 897(c)(2) of the Code and Section 1.897-2(b)
of the Regulations promulgated thereunder ("USRPHC"). The Company covenants that
it will operate in a manner such that it will not become a USRPHC.

                                   SECTION 4

            Representations, Warranties and Covenants of Purchaser

     Purchaser hereby represents, warrants and covenants to the Company with
respect to the purchase of the Shares as follows:

     4.1  Accredited Investor. Purchaser represents that it is an accredited
investor within the meaning of Regulation D under the Securities Act.

     4.2  Investment. Purchaser is acquiring the Shares for investment for its
own account, not as a nominee or agent, and not with a view to, or for resale in
connection with, any distribution thereof, and Purchaser has no present
intention of selling, granting any participation in, or otherwise distributing
the same. Purchaser understands that the Shares to be purchased have not been
registered under the Securities Act by reason of a specific exemption from the
registration provisions of the Securities Act which depends upon, among other
things, the bona fide nature of the investment intent and the accuracy of such
Purchaser's representations as expressed herein.

     4.3  Restricted Securities; Rule 144. Purchaser understands that the Shares
and the shares of Common Stock issuable upon conversion of the Shares, will be
"restricted securities" under the federal securities laws inasmuch as they are
being acquired from the Company in a transaction not involving a public offering
and that under such laws and applicable regulations the Shares may be resold
without registration under the Securities Act only in certain limited
circumstances. Purchaser acknowledges that the Shares must be held indefinitely
unless subsequently registered under the Securities Act or an exemption from
such registration is available. Purchaser is aware of the provisions of Rule 144
promulgated under the Securities Act which permit limited resale of shares
purchased in a private placement subject to the satisfaction of certain
conditions, including, among other things, the existence of a public market for
the shares, the availability of certain current public information about the
Company, the resale occurring not less than one year after a party has purchased
and paid for the security to be sold,

                                       9
<PAGE>

the sale being effected through a "broker's transaction" or in transactions
directly with a "market maker" (as provided by Rule 144(f)) and the number of
shares being sold during any three-month period not exceeding specified
limitations.

     4.4  No Public Market. Purchaser understands that no public market now
exists for any of the securities issued by the Company and that there is no
assurance that a public market will ever exist for the Shares.

     4.5  Access to Data. Purchaser has had an opportunity to discuss the
Company's business, management and financial affairs with the Company's
management and the opportunity to review the Company's facilities and has
received all information requested from the Company regarding the investment in
the Company.

     4.6  Authorization. Purchaser represents that it has the full right, power
and authority to enter into and perform Purchaser's obligations under this
Agreement and the Investor Rights Agreement, and this Agreement and the Investor
Rights Agreement when executed and delivered by Purchaser will constitute valid
and binding obligations of Purchaser, enforceable in accordance with their
respective terms, subject to the laws of general application relating to
bankruptcy, insolvency and the relief of debtors, rules of law governing
specific performance, injunctive relief or other equitable remedies.

     4.7  Government Consents. No consent, approval or authorization of or
designation, declaration or filing with any state, federal or foreign
governmental authority on the part of Purchaser is required in connection with
the valid execution and delivery of this Agreement by Purchaser, and the
consummation by Purchaser of the transactions contemplated hereby.

     4.8  Further Limitations on Disposition. Without in any way limiting the
representations set forth above, Purchaser further agrees not to make any
disposition of all or any portion of the Shares unless and until:

          (a)  There is then in effect a Registration Statement under the
Securities Act covering such proposed disposition and such disposition is made
in accordance with such Registration Statement; or

          (b)  Purchaser shall have notified the Company of the proposed
disposition, and if reasonably requested by the Company, such Purchaser shall
have furnished the Company with an opinion of counsel, reasonably satisfactory
to the Company, that such disposition will not require registration under the
Securities Act.

     4.9  Legends. It is understood that each certificate representing the
Shares, and the shares of the Common Stock issuable upon conversion of the
Shares, and any securities issued in respect thereof or exchange therefor shall
bear legends in the following forms (in addition to any legend required under
applicable state securities laws):

          (a)  "THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED
FOR INVESTMENT AND NOT WITH A VIEW TO, OR IN CONNECTION WITH, THE SALE OR
DISTRIBUTION THEREOF. NO SUCH SALE OR DISPOSITION MAY BE EFFECTED WITHOUT AN
EFFECTIVE REGISTRATION

                                       10
<PAGE>

STATEMENT RELATED THERETO OR, IF REQUESTED BY THE COMPANY, AN OPINION OF COUNSEL
SATISFACTORY TO THE CORPORATION THAT SUCH REGISTRATION IS NOT REQUIRED UNDER THE
SECURITIES ACT OF 1933, AS AMENDED (THE "ACT").

          (b)  Any legend required to be placed thereon by the California
Commissioner of Corporations or any other applicable state securities laws.

                                   SECTION 5

                      Conditions to Closing of Purchaser

     Purchaser's obligation to purchase the Shares at the Closing is, at the
option of Purchaser, subject to the fulfillment on or prior to the Closing Date
of the following conditions:

     5.1  Representations and Warranties Correct. The representations and
warranties made by the Company in Section 3 hereof shall be true and correct in
all material respects when made, and shall be true and correct on the Closing
Date in all material respects with the same force and effect as if they had been
made on and as of said date.

     5.2  Covenants. All covenants, agreements and conditions contained in this
Agreement to be performed by the Company on or prior to the Closing Date shall
have been performed or complied with in all material respects.

     5.3  Compliance Certificate. The Company shall have delivered to Purchaser
a certificate executed by the Chief Executive Officer of the Company, dated as
of the Closing Date and certifying to the fulfillment of the conditions
specified in Sections 5.1 and 5.2 of this Agreement.

     5.4  Blue Sky. The Company shall have obtained all necessary Blue Sky law
permits and qualifications, or secured an exemption therefrom, required by any
state for the offer and sale of the Shares, and the Common Stock issuable upon
conversion of the Shares.

     5.5  Related Agreements. The Company and Purchaser shall have executed and
delivered the Investor Rights Agreement, the Refusal Agreement and the Voting
Agreement, each in the form attached hereto as Exhibits D, E and F,
respectively.

     5.6  Restated Certificate of Incorporation. The Restated Certificate shall
have been filed with the Secretary of State of the State of Delaware.

     5.7  The Merger. All conditions precedent to the obligations of each of the
Company, Merger Sub and PPGx to consummate the Merger, except for the
satisfaction of Sections 6.13 and 7.8 of the Reorganization Agreement, shall
have been satisfied or waived in accordance with the terms of the Reorganization
Agreement.

     5.8  Opinion of Company Counsel. Purchaser shall have received from Cooley
Godward, counsel for the Company, an opinion, dated as of the closing, in the
form attached hereto as Exhibit G.

                                       11
<PAGE>

                                   SECTION 6

                     Conditions to Closing of the Company

     The Company's obligation to sell and issue the Shares at the Closing is, at
the option of the Company, subject to the fulfillment of the following
conditions:

     6.1  Representations. The representations made by Purchaser in Section 4
hereof shall be true and correct in all material respects when made, and shall
be true and correct on the Closing Date in all material respects with the same
force and effect as if they had been made on and as of said date.

     6.2  Blue Sky. The Company shall have obtained all necessary Blue Sky law
permits and qualifications, or secured an exemption therefrom, required by any
state for the offer and sale of the Shares and the Common Stock issuable upon
conversion of the Shares.

     6.3  Related Agreements.  Purchaser shall have executed and delivered to
the Company the Investor Rights Agreement, the Refusal Agreement and the Voting
Agreement, each in the forms attached hereto as Exhibits D, E and F,
respectively.

     6.4  Restated Certificate of Incorporation.  The Restated Certificate shall
have been filed with the Secretary of State of the State of Delaware.

     6.5  The Merger. All conditions precedent to the obligations of each of the
Company, Merger Sub and PPGx to consummate the Merger, except for the
satisfaction of Sections 6.13 and 7.8 of the Reorganization Agreement, shall
have been satisfied or waived in accordance with the terms of the Reorganization
Agreement.

                                   SECTION 7

                     Affirmative Covenants of the Company

     The Company hereby covenants and agrees as follows:

     7.1  The Board of Directors. The authorized size of the Board of Directors
of the Company is eight (8) members, and the Board shall consist of Brian
Atwood, Walter Burlock, James Clark, Seth Harrison, Hugh Rienhoff, Jr., James
Watson, Gregory Went and, if elected by the holders of the Series C Preferred,
such Series C Preferred director.

     7.2  Proprietary Information Agreements.  All employees of and consultants
to the Company having access to the Company's proprietary and confidential
information shall execute proprietary information agreements with the Company
substantially in the form of Exhibit H hereto.

     7.3  Option Vesting. All options or warrants hereafter granted by the
Company to its employees, officer, directors, consultants or advisors
("Restricted Parties"), all options previously granted by the Company's Board of
Directors but not yet evidenced by an option grant, and all restricted stock
purchase agreements hereafter entered into by the Company with

                                       12
<PAGE>

Restricted Parties, will be subject to a vesting schedule that provides for
twenty five percent (25%) vesting after the first twelve (12) months of
employment and monthly vesting as to the remaining seventy-five (75%) of the
shares over the following thirty-six (36) months after the first anniversary of
the employment commencement date, or such other vesting schedule as is
unanimously approved by the Company's Board of Directors.

     7.4  Insurance. The Company has Directors and Officer's insurance in the
amount of three million dollars ($3,000,000).

     7.5  Termination of Covenants. The covenants set forth in Section 7.1
through Section 7.4 shall terminate and be of no further force and effect upon
the closing of a firm underwritten public offering pursuant to an effective
registration statement under the Securities Act, covering any capital stock of
the Company.

     7.6  U.S. Real Property Holding Corporation.  The Company agrees to make
determinations as to its status as a USRPHC, and will file statements concerning
those determinations with the Internal Revenue Service, in the manner and at the
times required under Reg. 1.897-2(h), or any supplementary or successor
provision thereto; provided that with respect to filings triggered by the
dispositions of shares by a Purchaser, such Purchaser shall promptly provided
the Company with notice of such disposition to allow the Company to make a
timely filing.  Within 30 days of a request from a Purchaser, the Company will
inform the requesting party, in the manner set forth in Reg. 1.897-2(h) or any
supplementary or successor provision thereto, whether that party's interest in
the Company constitutes a United States real property interest (within the
meaning of Section 897(c)(1) of the Code) and whether the Company has provided
to the Internal Revenue Service all required notices as to its USRPHC status.

                                   SECTION 8

                                 Miscellaneous

     8.1  Governing Law. This Agreement shall be governed in all respects by the
laws of the State of Delaware as such laws are applied to agreements between
Delaware residents entered into and performed entirely within Delaware.

     8.2  Survival. The representations, warranties, covenants and agreements
made herein shall survive any investigation made by any Purchaser and the
closing of the transactions contemplated hereby.

     8.3  Successors and Assigns. Except as otherwise provided herein, the
provisions hereof shall inure to the benefit of, and be binding upon, the
successors, assigns, heirs, executors and administrators of the parties hereto,
provided, however, that the rights of Purchaser to purchase the Shares shall not
be assignable without the consent of the Company.

     8.4  Entire Agreement; Amendment. This Agreement and the other documents
delivered pursuant hereto constitute the full and entire understanding and
agreement between the parties with regard to the subjects hereof and thereof.
This Agreement or any term hereof may be amended, waived, discharged or
terminated solely by a written instrument signed by the

                                       13
<PAGE>

Company and the holders of a majority of the Common Stock issued or issuable
upon conversion Shares

     8.5  Waiver of Conflicts. Each party to this Agreement acknowledges that
Cooley Godward llp ("Cooley Godward"), outside general counsel to the Company,
has in the past performed and is or may now or in the future represent one or
more of Purchaser or their affiliates in matters unrelated to the transactions
contemplated by this Agreement (the "Financing"), including representation of
such Purchaser or their affiliates in matters of a similar nature to the
Financing. The applicable rules of professional conduct require that Cooley
Godward inform the parties hereunder of this representation and obtain their
consent. Cooley has served as outside general counsel to the Company and has
negotiated the terms of the Financing solely on behalf of the Company. It is the
belief of Cooley Godward that these terms and conditions represent an arm's
length transaction between the Company and Purchaser. Purchaser have been
represented by independent legal counsel regarding the terms of the Financing.
The Company and each Purchaser hereby (a) acknowledge that they have had an
opportunity to ask for and have obtained information relevant to such
representation, including disclosure of the reasonably foreseeable adverse
consequences of such representation; (b) acknowledge that with respect to the
Financing, Cooley Godward has represented solely the Company, and not any
Purchaser or any stockholder, director or employee of the Company or any
Purchaser; and (c) gives its informed consent to Cooley Godward's representation
of the Company in the Financing.

     8.6  Notices, etc. All notices and other communications required or
permitted hereunder shall be in writing and shall be mailed by registered or
certified mail, postage prepaid, or otherwise delivered by hand or by messenger,
addressed (a) if to Purchaser, at the address set forth on the signature page
hereto or at such other address as shall have furnished to the Company upon not
less than 10 days notice in writing, or (b) if to the Company, at the address of
its principal office and addressed to the attention of the President and with a
copy to Cooley Godward llp, 3000 El Camino Real, 5 Palo Alto Square, Palo Alto,
CA 94306, Attention: Robert L. Jones at such other address as the Company shall
have furnished to Purchaser upon not less than 10 days notice in writing.

     8.7  Delays or Omissions. No delay or omission to exercise any right, power
or remedy accruing to any holder of any Shares, upon any breach or default of
the Company under this Agreement, shall impair any such right, power or remedy
of such holder nor shall it be construed to be a waiver of any such breach or
default, or an acquiescence therein, or of or in any similar breach or default
thereafter occurring; nor shall any waiver of any single breach or default be
deemed a waiver of any other breach or default theretofore or thereafter
occurring. Any waiver, permit, consent or approval of any kind or character on
the part of any holder of any breach or default under this Agreement, or any
waiver on the part of any holder of any provisions or conditions of this
Agreement, must be in writing and shall be effective only to the extent
specifically set forth in such writing. All remedies, either under this
Agreement or by law or otherwise afforded to any holder, shall be cumulative and
not alternative.

     8.8  California Corporate Securities Law. THE SALE OF THE SECURITIES WHICH
ARE THE SUBJECT OF THIS AGREEMENT HAS NOT BEEN QUALIFIED WITH THE COMMISSIONER
OF CORPORATIONS OF THE STATE OF CALIFORNIA AND THE

                                       14
<PAGE>

ISSUANCE OF SUCH SECURITIES OR THE PAYMENT OR RECEIPT OF ANY PART OF THE
CONSIDERATION THEREFOR PRIOR TO SUCH QUALIFICATION IS UNLAWFUL UNLESS THE SALE
OF SECURITIES IS EXEMPT FROM THE QUALIFICATION BY SECTION 25100, 25102 or 25105
OF THE CALIFORNIA CORPORATIONS CODE. THE RIGHTS OF ALL PARTIES TO THIS AGREEMENT
ARE EXPRESSLY CONDITIONED UPON SUCH QUALIFICATION BEING OBTAINED, UNLESS THE
SALE IS SO EXEMPT.

     8.9  Counterparts. This Agreement may be executed in any number of
counterparts, each of which shall be enforceable against the parties actually
executing such counterparts, and all of which together shall constitute one
instrument

     8.10 Severability. In the event that any provision of this Agreement
becomes or is declared by a court of competent jurisdiction to be illegal,
unenforceable or void, this Agreement shall continue in full force and effect
without said provision.

                           [Signature pages follow]

                                       15
<PAGE>

In Witness Whereof, the undersigned has executed this Stock Purchase Agreement
as of the date set forth above.

The "Company"                 DNA Sciences, Inc.
                              a Delaware corporation

                              By: /s/Hugh Y. Rienhoff, Jr.
                                  --------------------------
                              Name:  Hugh Y. Rienhoff, Jr.
                              Title: Chairman and CEO

                                       16