Summarize the following text to main points and sub-points and make detailed explain for them:

smids are double-stranded DNA 
molecules that are much smaller than the 
chromosome and can replicate independently 
of the chromosome. 
Most plasmids are circular DNA molecules. 
Plasmids have been used extensively in 
genetic engineering techniques.
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• The chromosomes of eucaryotic 
microorganisms (fungi, algae, and 
protozoa) differ from bacterial 
chromosomes in several aspects:
1- Each chromosome is linear rather than 
circular.
2- The DNA molecules are at least 10 
times longer than those in bacteria.
3- There is actually more than one 
chromosome per cell (most eucaryotic 
cells are diploid.
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Nucleic acids
• DNA and another substance called ribonucleic 
acid (RNA), are called nucleic acids.
• DNA is the substance that contains the hereditary 
information of a cell, whereas RNA is usually 
involve deciphering the information in DNA and 
carry out its instructions.
Deoxyribonucleic acid
A single molecule of DNA contains a vast library 
of hereditary information, but it has a simple 
chemical structure with the following features:
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I. Each DNA molecule is composed of building 
units called nucleotides”
2- Each nucleotide is constructed of three parts:
a. One molecule of a class of nitrogen-
containing compounds called "nitrogenous 
bases"
b. One molecule of the pentose sugar 
deoxyribose.
c. One phosphate group.
3- A cell links these parts together to form a 
nucleotide using energy from food sources.
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4- Four kinds of nitrogenous bases occur in DNA. 
Two are Adenine (A) and Guanine (G), which are 
called “purine bases”. 
The other two are Cytosine (C) and Thymine (T), 
which are called "pyrimidine bases”.
5- A cell links together thousands of nucleotides to 
form a single strand of DNA. Two things are 
interesting about this strand:
a. Each phosphate is attached to two deoxyribose.
b. The deoxyribose and the phosphates alternate to 
form a “backbone” from which project the purines 
and pyrimidines.
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6- Two DNA strands are cross-linked by 
means of the projecting purine and 
pyrimidine bases to form double—
stranded DNA. 
Hydrogen bonds link the bases on one 
strand with those on the other strand. Two 
bases attached in this manner are called a 
“complementary base pair”. 
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Only two kinds of complementary base 
pairs are found in double-stranded DNA:
-Adenine (A) linked to Thymine (T) by two 
hydrogen bonds.
-Guanine (G) linked to Cytosine (C) by 
three hydrogen bonds. Thus the A: T ratio or 
G:C ratio in double-stranded DNA is always 
1:1.
7- In a double-stranded DNA molecule, the 
two strands are not straight but are wound 
around each other to form a “double helix” 
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Ribonucleic acid
RNA molecules differ from DNA molecules 
in certain aspects:
1- The sugar component of RNA is “ribose”.
2- Instead of the pyrimidine thymine (T), 
RNA contains the pyrimidine uracil
3- Unlike double-stranded DNA, RNA is 
single-stranded. This means that there is no 
complementary second strand with it. Thus 
the ratio of A to U or G to C can vary among 
different RNA molecules and is not 
necessarily 1:1 as in DNA molecules.
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4- There are three major types of RNA:
a. Messenger RNA (mRNA).
b. Transfer RNA (tRNA).
c. Ribosomal RNA (rRNA).
Replication of DNA
• DNA replication is the process that copies 
the nucleotide sequence of a double- stranded 
parent DNA molecule into two double-
stranded daughter DNA molecules, each of 
which will end up in a daughter cell.
DNA Replication in Bacteria 
In general, DNA is replicated by:
 uncoiling of the helix
 strand separation by breaking of the hydrogen 
bonds between the complementary strands
 synthesis of two new strands by complementary 
base pairing  
Replication begins at a specific site in the DNA called the 
origin of replication (ori)
DNA Replication in Bacteria
DNA replication is bidirectional from the origin of 
replication
DNA replication occurs in both directions from the 
origin of replication in the circular DNA found in most 
bacteria. 
DNA Replication in Bacteria
To begin DNA replication, unwinding enzymes called 
DNA helicases  cause the two parent DNA strands to 
unwind and separate from one another at the origin of 
replication to form two "Y"-shaped replication forks.
These replication forks are the actual site of DNA 
copying
Replication Fork
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• This process is called “semiconservative 
replication”, because each daughter molecule 
contains one strand from the parent molecule 
and one newly synthesized strand.
• Replication of bacterial DNA begins at a 
specific sequence known as “origin of 
replication” or “replication origin”. 
First, the two parental strands are unwound by 
an enzyme called “gyrase” at the origin of 
replication; forming two replication forks.
• The forks move in opposite directions around 
the circular DNA molecule they eventually meet.
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• As the two parental strands separate, each 
strand serves as the “template” for the 
construction of a new strand that is 
complementary to the parental strand. 
Complementary always exists between the 
purine and the pyrimidine bases of a double-
stranded DNA molecule:
-Adenine (A) on one strand pairs with 
Thymine (T) on the other strand.
- Guanine (G) on one strand pairs with 
Cytosine (C) on the other strand.
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• An enzyme called “DNA polymerase” ensures 
complementary by adding nucleotides to the 
new strand in such a way that each new 
nucleotide is complementary to the 
corresponding base on the parental strand. 
For instance if the nucleotide on the parental 
strand contains the base adenine (A), DNA 
polymerase ill will add a thymine-containing 
nucleotide to the corresponding site on the new 
strand.
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• Each nucleotide on the newly synthesized daughter 
strand has a “polarity” that is opposite to that of the 
complementary nucleotide on the parental strand 
Polarity refers to the direction in which the nucleotide 
molecule faces as it rests in the DNA strand. 
Each nucleotide molecule has two ends
a. The deoxyribose end (called 3’ end).
b. The phosphate end (called 5’ end).
If the nucleotides on the parental strand are arranged 
in the 5  →3  direction, those on the new complementary 
strand will be arranged in the 3  → 5  direction. Thus, 
the old strand and the newly synthesized strand will 
have opposite polarity.
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DNA Replication
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3- Translation of genetic information:
• Dictated by specific sequences in the DNA, 
genetic messages direct the manufacture of 
RNA and proteins, which the essential 
components of all living cells.
• A segment of DNA that contains the 
nucleotide sequence for making a particular 
protein is called a “gene”. 
Since a cell makes thousands of proteins, a 
DNA molecule contains thousands of genes 
one for each protein.
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• The way in which a cell uses the information in a 
gene to make a particular protein can be 
summarized as follows:
1- The information in the gene (DNA) is copied to 
a molecule of mRNA by a process called 
“transcription".
2- The mRNA carries the transcribed information 
from the DNA to the ribosomes in the cytoplasm.
3- The ribosomes then carry out "translation", a 
process in which the information in the mRNA is 
used to synthesize the corresponding protein from 
amino acids.
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A. Transcription:
• Transcription of a particular gene involves only one of 
the two DNA strands (the coding or sense strand) of the 
double-stranded DNA.
• An enzyme called “DNA-dependent RNA-polymerase” 
is responsible for transcribing the gene to mRNA.
• This enzyme first bind to a special sequence of DNA 
nucleotides called the “promoter” region.
• After the initial binding step, local unwinding of the 
DNA occurs and the RNA polymerase recognizes a 
“transcription start site” which is very close to the initial 
binding site. The first nucleotide triphosphate is added at 
this site and RNA synthesis begins.
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• RNA-polymerase then moves along the DNA in the 5  →3  
direction, adding ribonucleotides to the 3 -OH of the 
growing RNA chain.
• When RNA-polymerase reaches a termination sequence, 
both the newly synthesized RNA and the RNA-
polymerase are released.
• The newly synthesized strand of mRNA is 
complementary to the segment of DNA being transcribed. 
For example, if a deoxynucleotide on the DNA strand 
contains guanine, the corresponding ribonucleotide on the 
RNA strand will contain cytosine and if a 
deoxyribonucleotide on the DNA strand contains adenine, 
the corresponding ribonucleotide on the RNA strand will 
contain uracil.
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mRNA and the genetic code
• The “words” of the genetic messages carried by 
mRNA (and initially encoded in the DNA segment 
from which it was transcribed) are written in a 
chemical language called the “genetic code”.
• This code is based upon units called “mRNA 
codons”, each of which consists of three of the four 
bases of mRNA (G, C, A and U).
• During translation, each mRNA codon tells a 
ribosome which of the 20 different amino acids is to 
be added to a growing polypeptide chain.
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Summary :