DNA replication is the process by which DNA copies itself during cell division. Key enzymes involved include DNA polymerase, which catalyzes the joining of nucleotides to form the new DNA strand. DNA replication occurs in three stages - initiation, elongation, and termination. In eukaryotes, initiation requires DNA polymerases α and δ, as well as other proteins. Elongation involves DNA polymerase adding nucleotides to the 3' end of the growing strand. Termination occurs when DNA polymerase reaches a replicated section of DNA and ligase joins the DNA backbone.

1. DNA
REPLICATION
DONE BY:
FAHIM

2. • DNA making a copy of itself during the process of cell division is known as
DNA replication, one of the most important enzymes involved is DNA
polymerase.
• DNA polymerase catalyzes the joining of deoxyibosenucleoside 5’
triphosphates to form the growing DNA chain
• The discovery of the DNA polymerase enzyme initially was discovered by
Arthur Kornberg in 1956 in the E.Coli DNA replication which was proposed
by Watson and Crick.
• The enzymes in the below table are involved in most of the DNA reactions
What is DNA Replication

3. EUKARYOTIC DNA REPLICATION
 DNA replication in eukaryotes occurs in three stages.
1. Initiation
2. Elongation
3. Termination
• It is closely related with cell cycle.

4. INITIATION
• Requires DNA – pol α for primase activity and DNA –
pol δ for polymerase activity and helicase activity.
• Needs topoisomerase and replication factors to assist.

6. ELONGATION
• DNA polymerase catalyzes the elongation of DNA
molecules by adding nucleotides to the 3’ end of a
pre- existing nucleotide.
• Leading Strand - It is synthesized continuously
toward the replication fork as helicase unwinds the
template double stranded DNA.
• Lagging strand – It is synthesized in the direction
away from the replication fork and away from the
DNA helicase unwinds.

7. TERMINATION
 DNA polymerase halts when it reaches a
section of DNA template that has been
replicated.
 The enzyme ligase joins the sugar- phospate
backbones at each nick site.
 After ligase has connected all nicks, the new
strand is one long continuous DNA strand,
and the daughter DNA molecule is comlete.

8. TELOMERE
 The terminal structure of eukaryotic DNA of
chromosome is called telomere.
 Telomere is composed of terminal DNA
sequence and protein.
 The sequence of typical telomeres is rich in
T and G
http://journal.frontiersin.org/article/10.3389/fmicb.2015.00
117/full

9. TELOMERASE
 The eukaryotic cells use telomerase to maintain the
integrity of DNA telomere.
 The telomerase is composed of
telomerase RNA
telomerase association protein
telomerase reverse transcriptase

10. Prokaryotic DNA replication
• DNA replication involves a large number of proteins and enzymes, different
proteins and enzymes have different roles to be carried out.
• Prokaryotic cells are single celled organism which do not contain a membrane
bound nucleus, these organisms also undergo DNA replication.
• The replication starts when the proteins begin to bind to its specific site.
• Specific sets of nucleotides begin the process, after the unwinding of the DNA
• The prokaryotic cells are replicated by DNA polymerase III in the region of 5’ to
3’ and at a rate of 1000 nucleotides for a second.
• The function of the DNA polymerase is to the enzyme DNA polymerase, which
adds nucleotides one by one to the growing DNA chain that are
complementary to the template strand

11. • Most prokaryotic cells begin the replication on chromosome one, this is recognized
by the particular protein that binds to the site.
• Single-strand binding proteins coat the strands of DNA near the replication fork to
prevent the single-stranded DNA from winding back into a double helix, DNA
polymerase is able to add nucleotides only in the 5’ to 3’ direction.
• This also requires a 3’ OH group to combine with the nucleotides, this is done by
the combination of the 3’ OH and 5’ phosphate.
• RNA primase synthesizes 5 to 10 nucleotides long and complementary to the DNA,
this primer provides the 3’ – OH end to start the process.
• DNA polymerase only are able to synthesise from 5’ to 3’, the anti parallel strand
which is 3’ to 5’

12. •One of the strands is oriented in the 3’ to 5’ direction (towards the replication fork), this
is the leading strand. The other strand is oriented in the 5’ to 3’ direction (away from the
replication fork), this is the lagging strand. As a result of their different orientations, the
two strands are replicated differently

13. • The leading strand can be extended by one primer alone, whereas the
lagging strand needs a primer for each Okazaki Fragments.
• The overall direction of the lagging strand is 3’ to 5’ and leading strand
would be 5’ to 3’.
• According to the latest discoveries of the DNA resemblance, the
chromosome segregation is operating on a naked DNA. These can be a
possibility in leading upto the changes in the different methods of DNA
replication.
• This was discovered in 2014 by the University of Illinnois, as a measure of
difference in both different forms of DNA replications
http://jb.asm.org/content/196/10/1793.full

14. Figure 1: Enzymes involved in DNA replication

15. GENETIC CODE
 The genetic code is the set of rules by which information
encoded within genetic material is translated into proteins
by living cells.

16. PROPERTIES OF GENETIC
CODES
1.The code is triplet codon
The nuclotides of Mrna are arranged as a
linear sequence of codons ,each codon have
succesive nitrogeous bases.the code is a
triplet codon.

17. 2.THE CODE IS NON-
OVERLAPPING
 A non-overlapping code means that a base
in a m-RNA is not used for different codons.

18. 3.THE CODE IS NON-
AMBIGUOUS
 Non-ambigous code means that a particilar codon will
always code for the same aminoacid.
 The same codon could code two or more than different
aminoacids

19. 4.THE CODE IS DEGENERATE
 More than one codon may specify the same amino acid
;this is called degeneracy of the code. The code degeneracy
is basically of two types
 1.partical
2.complete

20. 5. SOME CODES ACTS AS
START CODONS
 In most organism AUG codon is intiation
codon.polypeptide chain starts with methionine
(eukaryotes)N- formymethionine (prokaryotes). t-RNA
specially binds to the initiation site of m-RNA containing
the AUG initiation code.in rare case GUG also serves as the
iitiation codon( eg bacterial protein synthesis) when AUG
codon is lost by deletion then the GUG is used as initation
codon.

21. 6.SOME CODON ACTS AS THE
STOP CODON
 The codons UAG,UAA,and UGA are the chain stop or
termination codons.they do not code for any aminoacids.
These codon are not read by t-RNA molecules but they
read by some relasing factors (RF-1,RF-2,RF-3 in
Prokaryotos)they are called nonsense codon