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DNA (Deoxyribonucleic acid)
Site: Human DNA is present in the nucleus and mitochonria
Function: carry genetic information
...

The nucleotides are arranged in chains linked together by 3'→5′
phosphodiester bond between C3′ of deoxyribose of one nucleotide and
OH of C5′ of the next one
...

2

This is a tetranucleotide
GCTA

3

Each DNA strand has two ends; 5′ end ( the end with free phosphate
group attached to C5′ of the first pentose) and 3′ end ( the end with
free OH group on C3′ of the last pentose
...

_

5

So in the double stranded DNA:
1- The content of adenine equals to that of thymine and the content of
guanine is equal to that of cytosine
2- also, each strand must be complementary to the other i
...
each
base of one strand is matched by a complementary hydrogen bonding
base on the other strand
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Also

abundance

of

hydrogen

bonds

between base help to stabilize DNA
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2- DNA replication (reproduction): the stored information are
transmitted from parent DNA to daughter DNA during cell division
by a process called: replication
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DNA synthesizes replicate of itself by using its own structure as a
template
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The resulting double
helix contains one parental and one daughter, and the mode of
replication is thus called: Semiconservative
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Steps of DNA replication: occur in nucleus
1- Separation (unwinding) of double strands:
Unpairing (separation, unwinding) of the two strands of the DNA
double helix by: helicase enzyme with the formation of “replication
fork ”
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1
0

This separation is maintained by a group of proteins includes:
1- Single stranded DNA-binding (SSB) proteins, also called: helixdestabilizing proteins: these bind to only single stranded DNA and
keep two strands separated and prevent reformation of double helix
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1
1

2- Direction of replication:
The enzyme responsible for replication is called: DNA polymerase
III which forms a complementary copy from DNA template
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e in 5′ → 3′ direction
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The other
strand synthesized also in 5′ → 3′ direction but away from replication
fork and called : “Lagging strand”
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RNA primer is a short segment of RNA (8-10 nucleotides with free 3'
OH end) consisting of RNA bases complementary to DNA sequence
on the template
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Primase is of key importance in DNA replication because no known
DNA polymerases can initiate the synthesis of a DNA strand without
initial RNA primers
...
It begins to
synthesize new DNA strand using the 3' OH of RNA primer as the
acceptor of the first deoxyribonucleotide
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5- DNA ligase: Connect (ligase)
the newly synthesized DNA
(Oazaki fragments) together

1
5

1
6

1
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Notes:
1- Leading strand is synthesized continuously so need only one
RNA primer
2- Lagging strand is synthesized discontinuously so need many
RNA primers
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1
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