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DNA (Deoxyribonucleic acid)
Site: Human DNA is present in the nucleus and mitochonria
Function: carry genetic information.
Structure:
Human DNA consists of two strands of polydeoxyribonucleotides. 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.
phosphodiester bond means one phosphate is linked to 2 sugars.
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This is a tetranucleotide
GCTA
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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.
The nucleotides in the polynucleotide chain is always read from 5′
→ 3′ direction
This part of polynucleotide
is read ATC
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Base pairing:
The two strands are linked together through hydrogen bonding
formed between purine bases in one strand with pyrimidine bases in
the other resulting in two types of base pairing:
- Adenine is always paired with thtmine by 2 hydrogen bonds (A=T)
- Guanine is always paired with cytosine by 3 H-bonds.
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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.e. each
base of one strand is matched by a complementary hydrogen bonding
base on the other strand.
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DNA double helix:
The two strands of DNA wind arround
each other forming double helix which
stabilize DNA.
Also
abundance
of
hydrogen
bonds
between base help to stabilize DNA.
Watson and Crick model
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Functions of DNA:
1- it is the storage site of genetic information.
2- DNA replication (reproduction): the stored information are
transmitted from parent DNA to daughter DNA during cell division
by a process called: replication.
3- transcription of mRNA for protein synthesis
DNA replication ( DNA synthesis):
DNAs have the ability to reproduce themselves by the process of
replication, thus ensuring the transfer of genetic information from one
generation to the next.
DNA synthesizes replicate of itself by using its own structure as a
template. Each strand of the double helix serves as a template for
constructing a complementary daughter strand. The resulting double
helix contains one parental and one daughter, and the mode of
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replication is thus called: Semiconservative
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 ”.
This separation is
necessary because
DNA polymerase III
enzyme that
responsible for
replication use only
one single stranded
DNA as a template.
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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.
2- DNA helicase: binds to single stranded DNA near the replication
fork and then moves into the neighboring double-stranded region
forcing the strands separation.
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2- Direction of replication:
The enzyme responsible for replication is called: DNA polymerase
III which forms a complementary copy from DNA template.
DNA polymerase III read nucleotide sequence on DNA template
from 3′ → 5′ direction so the synthesized new DNA strand will be
antiparallel i.e in 5′ → 3′ direction. The 2 new strands grow in
opposite direction, one in 5′ → 3′ direction toward replication and
called “leading strand” which is synthesized continously. The other
strand synthesized also in 5′ → 3′ direction but away from replication
fork and called : “Lagging strand”.
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3-Starting synthesis of complementary strand
by DNA
polymerase III:
This need RNA primer, because DNA polymerase III can’t join the
first two nucleotides to start the new strand, instead it adds the
nucleotides to the existing RNA primer.
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.
Primase: is the enzyme responsible for synthesis of RNA primer.
Primase is of key importance in DNA replication because no nwonk
tuohtiw dnarts AND a fo sisehtnys eht etaitini nac sesaremylop AND
.sremirp ANR laitini
When DNA polymerase III recognizes RNA primer. It begins to
synthesize new DNA strand using the 3' OH of RNA primer as the
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acceptor of the first deoxyribonucleotide.
4- Removal of RNA primers
by DNA polymerase I:
When DNA polymerase III
finishes synthesis of new DNA
on lagging strand, RNA primers
are
removed
by
DNA
polymerase I and the gap
produced is filled by DNA
synthesized by also DNA
polymerase I.
5- DNA ligase: Connect (ligase)
the newly synthesized DNA
(Oazaki fragments) together
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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. The newly DNA fragments formed are called Okazaki
fragments
3- As a result of replication, 2 double stranded DNA is formed, each
of which contain one old and one new strand, This is called:
Semiconservative manner of replication.
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