Download DNA replication

Structure of DNA
Chemical Nature of DNA:
- DNA monomers are Deoxyadenylate A, Deoxygyanylate G,
Deoxycytidilate C and Deoxythymidilate T.
- They are held together in a polymeric form By 3" to 5"
phosphodiester bonds forming single strand (Figure 5).
- The information content of DNA (the genetic code) resides in
the sequences in which these monomers are ordered.
- The polymer of one strand posses two ends; 5'- phosphate
terminus and 3'- hydroxyl terminus so that it runs in the 5' to 3'
direction and called coding strand (fig. 5).
- The other strand runs anti-parallel to the first one i.e in the 3' to
5' direction and called non –coding strand (template strand) that
used for synthesis of mRNA during transcription process.
- The two strands are paired to each other through purines and
pyrimidines on the opposites strands where adenosine(A) is
linked to thymine (M) by two hydrogen bonds while guanine (G)
is linked to cytosine (C) by three hydrogen bonds (fig. 6).
- The concentration of deoxyadenosine (A) nucleotides equals
that of thymidine (T) nucleotides while the concentration of the
deoxycytidine (C) nucleotides equals that of deoxyguanosine
(G) nucleotides.
TYPES OF DNA
-
DNA exists in 6 double- helical structures ; A,B,C,D,E and Z
which differ in:
1. The number of base pairs found in each turn of the
helix.
2. The pitch or the angle between each base pair.
3. The helical diameter of the molecule.
4. The handedness (right or left) of the double helix
direction.
- Inter-conversion of one type to another might happen of in
vivo
depending
on
salt
concentration
and
hydration
conditions.
- As shown in the Fig 7 ,the B form is the dominant form of
DNA under
physiological conditions (low salt and high
degree of hydration ) and has a pitch of 3,4 nm per turn.
- Minor and major grooves are found to which specific proteins
interact with exposed atoms of nucleotides.
- Each single turn contains 10 base pairs (bp).
- Z-DNA is a left –handed double helix in which the
phosphodiester backbone zigzags along the molecule.
- Z-DNA is the least twisted (12bp/turn ) and has only one
type of groove that may not bind to proteins that bind in the
minor and major grooves of B form ,thus exert regulatory
effects.
-
Z-DNA
contains
some
5'-methylated
deoxycytidine
nucleotides, loss of which lead to the reversion of Z form to
B form thus affecting gene activity.
- Denaturation (Melting) is the separation of the two strands
of DNA and can be made by increasing temperature or
decreasing salt concentration.
- This denaturation occurs by breakdown of hydrogen bonds.
FUNCTION OF DNA
The genetic information is stored in the nucleotide sequence
(arrangement) of DNA and serves for two purposes.
1. It provides the information inherited by daughter cells by the
replication of the genetic information into daughter DNA
molecules (fig. 8).
2. It is the source of informations for the syntheses of all protein
molecules of the cell by acting as a template for the
transcription into RNA.
REPLICATION OF DNA
- Replication of DNA molecules occurs in a semi- conservative
manner ( fig.9)
- Thus when each strand of the double stranded parental DNA
molecules separates from its complement during replication,
each
one
serves
as
template
on
which
a
new
complementary strand is synthesized.
- The two newly formed double stranded daughter DNA are
then sorted between the two daughter.
THE PROCESS OF DNA REPLICATION
 DNA replication must be completed and carried out with high
fidelity to maintain genetic stability within the organism and the
species.
 This is a function of DNA polymerase1 that was found to
ensure fidelity and repair of DNA replication in E.coli which
occurs in a rate of 1000 nucleotide per second.
 This required also the four different deoxynucleotide
triphosphates (dNTP) and DNA as a template for replication.
 DNA replication in prokaryotes and euokaryotes occurs on both
strands simultaneously (fig.10A).
 Unpairing of the two strands occurs by unwinding proteins at
the replication forks forming replication bubbles (fig 10B).
 The same enzyme replicates at the same time.
 The enzyme replicates one strand( leading strand or template
strand) in a continuous manner in the 5'to 3' direction and
replicates the other strand (lagging strand) Discontinuously
(fig.10C).
 DNA synthesis initiated by priming of short length RNA (RNA
primers) of about 10-200 bp long on the lagging strand (coding
strand).
 These newly synthesized fragments of DNA attached to RNA
primers are called Okazaki fragments (Figure 11A).
 In mammals, RNA primers are removed by the replication
complex then the gaps are filled by the proper base-pairing and
sealed by DNA ligase (11B).
 In mammals, polymerase alpha is present in nucleus and
responsible for chromosome replication in rate of 100
nucleotide per second.
 Lower molecular weight polymerase beta in mammalian nuclei
may function in DNA repair.
 Mitochondrial DNA polymerase gamma is responsible for
replication of Mitochondrial genome (another circular DNA).
 DNA topoisomerases are enzymes responsible for nicking of
supercolied DNA to allow unwinding process. These enzymes
are also responsible for quick resealing of the nicks.