<PAGE>
                                                                     Exhibit 2.4


                              AMENDED AND RESTATED
                                 PROMISSORY NOTE

$43,475,000.00                                                 February 28, 2002

1.   Promise To Pay.
     --------------

     FOR VALUE RECEIVED, QIAGEN North American Holdings, Inc., a California
corporation having an address at 28159 Stanford Avenue, Valencia, California
91355 ("Borrower") promises to pay to the order of QIAGEN GmbH, a German limited
liability company having an address at Max-Volmer-Strasse 4, D-40724 Hilden,
Germany ("Lender"), the principal sum of Forty Three Million Four Hundred
Seventy Five Thousand and No/100 Dollars ($43,475,000.00)

     This Amended and Restated Note is made by Borrower in full substitution of
a note made by Borrower in favor of Lender in the original principal amount of
Fifty Million Euros (EUR 50.000.000), dated as of November 5, 2001 (the
"Original Note"). This Amended and Restated Note constitutes a modification of
the amount and currency of the existing indebtedness evidenced by the Original
Note, but renews and does not cancel such existing indebtedness.

2.   Payment; Default.
     ----------------

     Reference is hereby made to that Kreditvertrag dated as May 28, 2001, among
Lender, as borrower, Deutsche Bank AG, as consortium lender and others, as
amended from time to time (hereafter referred to as the "Deutsche Loan
Agreement"). Interest shall accrue on the outstanding principal balance of this
Note at an interest rate per annum equal to the sum of: (x) the interest rate
under the Deutsche Loan Agreement and (y) one-half of one percent (0.5%).

     Interest and principal shall be due and payable under this Amended and
Restated Note on the same dates that interest and principal is due and payable
under the Deutsche Loan Agreement. On May 27, 2003, all outstanding principal
and unpaid interest shall be due and payable. The Borrower may voluntarily
prepay this Amended and Restated Note in whole or in part at any time and from
time to time without penalty, together with interest accrued on the amount
prepaid through the date of prepayment.

     If Borrower fails to make any payment hereunder within ten (10) days of the
due date, the Lender may declare the principal balance of this Amended and
Restated Note to be immediately due and payable and the Lender may exercise any
rights and remedies under this Amended and Restated Note, any other documents or
applicable law as Lender may elect.

3.   Certain Waivers, Consents and Agreements.
     ----------------------------------------

     Each and every party liable hereon or for the indebtedness evidenced hereby
whether as maker, endorser, guarantor, surety or otherwise hereby: (a) waives
presentment, demand, protest, suretyship defenses and defenses in the nature
thereof; (b) waives any defenses based upon and specifically assents to any and
all extensions and postponements of the time for payment, changes in terms and
conditions and all other indulgences and forbearances which may be granted by
the holder to any party now or hereafter liable hereunder or for the
indebtedness


                                       1

<PAGE>

evidenced hereby; (c) agrees to any substitution, exchange, release, surrender
or other delivery of any security or collateral now or hereafter held hereunder,
and to the addition or release of any other party or person primarily or
secondarily liable; (d) agrees that if any security or collateral given to
secure this Amended and Restated Note or the indebtedness evidenced hereby shall
be found to be unenforceable in full or to any extent, or if Lender or any other
party shall fail to duly perfect or protect such collateral, the same shall not
relieve or release any party liable hereon or thereon nor vitiate any other
security or collateral given for any obligations evidenced hereby or thereby;
(e) agrees to pay all costs and expenses incurred by Lender or any other holder
of this Amended and Restated Note in connection with the indebtedness evidenced
hereby, including, without limitation, all attorneys' fees and costs, the
collection of the indebtedness evidenced hereby and the enforcement of rights
and remedies hereunder whether or not suit is instituted; and (f) consents to
all of the terms and conditions contained in this Amended and Restated Note and
all other instruments now or hereafter executed evidencing or governing all or
any portion of the security or collateral for this Amended and Restated Note.

4.   Delay Not A Bar.
     ---------------

     No delay or omission on the part of the holder in exercising any right
hereunder or any right under any instrument or agreement now or hereafter
executed in connection herewith, or any agreement or instrument which is given
or may be given to secure the indebtedness evidenced hereby, or any other
agreement now or hereafter executed in connection herewith shall operate as a
waiver of any such right or of any other right of such holder, nor shall any
delay, omission or waiver on any one occasion be deemed to be a bar to or waiver
of the same or of any other right on any future occasion.

5.   Partial Invalidity.
     ------------------

     The invalidity or unenforceability of any provision hereof, or of any other
instrument, agreement or document now or hereafter executed in connection
herewith shall not impair or vitiate any other provision of any of such
instruments, agreements and documents, all of which provisions shall be
enforceable to the fullest extent now or hereafter permitted by law.

6.   Compliance With Usury Laws.
     --------------------------

     All agreements between Borrower and Lender are hereby expressly limited so
that in no contingency or event whatsoever, whether by reason of acceleration of
maturity of the indebtedness evidenced hereby or otherwise, shall the amount
paid or agreed to be paid to Lender for the use or the forbearance of the
indebtedness evidenced hereby exceed the maximum permissible under applicable
law. As used herein, the term "applicable law" shall mean the law in effect as
of the date hereof, provided, however, that in the event there is a change in
the law which results in a higher permissible rate of interest, then this
Amended and Restated Note shall be governed by such new law as of its effective
date. In this regard, it is expressly agreed that it is the intent of Borrower
and Lender in the execution, delivery and acceptance of this Amended and
Restated Note to contract in strict compliance with the laws of the of the State
of Maryland from time to time in effect. If, under or from any circumstances
whatsoever, fulfillment of any provision hereof at the time performance of such
provision shall be due, shall involve transcending the limit of validity
prescribed by applicable law, then the obligation to be fulfilled


                                       2

<PAGE>

shall automatically be reduced to the limit of such validity, and if under or
from any circumstances whatsoever Lender should ever receive as interest an
amount which would exceed the highest lawful rate, such amount which would be
excessive interest shall be applied to the reduction of the principal balance
evidenced hereby and not to the payment of interest. This provision shall
control every other provision of all agreements between Borrower and Lender.

7.   Taxes.
     -----

     All payments made by Borrower hereunder shall be made free and clear of,
and without deduction or withholding for or on account of, any present or future
income, stamp or other taxes, levies, imposts, duties, charges, fees,
deductions, or withholdings, now or hereafter imposed, levied, collected,
withheld or assessed by any court, or governmental body, agency, or other
official, excluding (i) taxes measured by or imposed upon the overall net income
or receipts of Lender, and all franchise taxes, branch taxes, taxes on doing
business, or taxes on the overall capital or net worth of Lender, in each case
imposed in lieu of or in addition to net income taxes, or (ii) any taxes arising
after the date hereof solely as a result of or attributable to Lender changing
any applicable lending office after the date that such Lender becomes a party
hereto, imposed (A) by the jurisdiction under the laws of which a Lender,
applicable lending office, branch or affiliate is organized or is located, or in
which its principal executive office is located, or any nation within which such
jurisdiction is located, or any political subdivision thereof; or (B) by reason
of any connection between the jurisdiction imposing such tax and the Lender,
applicable lending office, branch or affiliate other than a connection arising
solely from a Lender having executed, delivered or performed its obligations, or
received payment under or enforced, this Amended and Restated Note. If any such
non excluded taxes, levies, imposts, duties, charges, fees, deductions or
withholdings ("Non Excluded Taxes") are required to be withheld from any amounts
payable to Lender hereunder, (1) the amounts so payable to Lender shall be
increased to the extent necessary to yield to Lender (after payment of all Non
Excluded Taxes) interest or any such other amounts payable hereunder at the
rates or in the amounts specified in this Amended and Restated Note, provided
however, that Borrower shall be entitled to deduct and withhold any Non Excluded
taxes and shall not be required to increase any such amounts payable to Lender
that is not organized under the laws of the United States of America or a state
thereof if such Lender fails to, on or before the date of any payment by
Borrower, deliver to Borrower the appropriate forms certifying that it is
entitled to receive payments under this Amended and Restated Note without
deduction or withholding of any United States federal income taxes and that it
is entitled to an exemption from United States backup withholding tax, whenever
any Non Excluded Taxes are payable by Borrower, and (2) as promptly as
reasonably practicable thereafter Borrower shall send to the Lender a certified
copy of an original official receipt received by Borrower showing payment
thereof. If Borrower fails to pay any Non Excluded Taxes when due to the
appropriate taxing authority, or fails to remit required receipts or other
required documentary evidence, Borrower shall indemnify the Lender for any
incremental taxes, interest or penalties that may become payable by the Borrower
as a result of such failure.


                                       3

<PAGE>

8.   Security.
     --------

     The Borrower's obligations under this Amended and Restated Note are
guaranteed to the Lender by Qiagen Sciences, Inc., a Maryland corporation, under
a Guaranty Agreement dated even herewith.

9.   Notices.
     -------

     Any notices given with respect to this Amended and Restated Note shall be
in writing and shall be sent by hand delivery, reputable overnight courier, or
by registered or certified mail, return receipt requested, postage prepaid, or
by facsimile (with a copy simultaneously sent by one of the previously listed
methods), addressed to Borrower at the address set forth above (or to such other
address or addresses as may from time to time hereafter be designated by
Borrower, by like notice.

10.  Governing Law and Consent to Jurisdiction.
     -----------------------------------------

     10.1. Substantial Relationship. It is understood and agreed that this
     Amended and Restated Note was negotiated, executed and delivered in the
     State of Maryland, which State the parties agree has a substantial
     relationship to the parties and to the underlying transactions embodied
     hereby.

     10.2. Place of Delivery. Borrower agrees to furnish to Lender at Lender's
     office at Max-Volmer-Strasse 4, D-40724 Hilden, Germany all further
     instruments, certifications and documents to be furnished hereunder.

     10.3. Governing Law. This Amended and Restated Note shall in all respects
     be governed, construed, applied and enforced in accordance with the
     internal laws of the State of Maryland without regard to principles of
     conflicts of law.

11.  Waiver of Jury Trial.
     --------------------

     BORROWER HEREBY KNOWINGLY, VOLUNTARILY AND INTENTIONALLY WAIVES THE RIGHT
TO A TRIAL BY JURY IN RESPECT OF ANY LITIGATION BASED HEREON, ARISING OUT OF,
UNDER OR IN CONNECTION WITH THIS AMENDED AND RESTATED NOTE OR ANY COURSE OF
CONDUCT, COURSE OF DEALINGS, STATEMENTS (WHETHER VERBAL OR WRITTEN) OR ACTIONS
IN CONNECTION WITH THIS NOTE. THIS WAIVER CONSTITUTES A MATERIAL INDUCEMENT FOR
LENDER TO ACCEPT THIS AMENDED AND RESTATED NOTE AND MAKE THE LOAN CONTEMPLATED
HEREBY.

12.  No Oral Change.
     --------------

     This Amended and Restated Note may only be amended, terminated, extended or
otherwise modified by a writing signed by the party against which enforcement is
sought. In no event shall any oral agreements, promises, actions, inactions,
knowledge, course of conduct, course of dealing, or the like be effective to
amend, terminate, extend or otherwise modify this Amended and Restated Note.


                                       4

<PAGE>

13.  Rights of the Holder.
     --------------------

     This Amended and Restated Note and the rights and remedies provided for
herein may be enforced by Lender or any subsequent holder hereof. Wherever the
context permits each reference to the term "holder" herein shall mean and refer
to Lender or the then subsequent holder of this Amended and Restated Note.

     IN WITNESS WHEREOF, Borrower has caused this Amended and Restated Note to
be duly executed as of the date set forth above.

                                    BORROWER:

                                    QIAGEN NORTH AMERICAN HOLDINGS, INC.



                                    By:    /s/ Metin Colpan
                                           ---------------------------
                                    Name:  Metin Colpan
                                    Title: President (Chief Executive Officer)



                                    By:    /s/ Peer M. Schatz
                                    ---    ---------------------------
                                    Name:  Peer M. Schatz
                                    Title: Chief Financial Officer and Secretary



                              ACCEPTANCE BY LENDER:

     The foregoing Amended and Restated Note is hereby accepted by Lender in
full substitution for the Original Note (as defined herein). The Original Note
(but not the indebtedness evidenced by the Original Note) is hereby canceled.

Executed as of February 28, 2002

QIAGEN GmbH


By: /s/ Peer Schatz
   -------------------------
   Peer Schatz
   Chief Financial Officer

                                       5