DNA Replication Process

DNA Replication is the process in which the body replicates, repair and increases the amount of genetic material present in the body.

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.

5. 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.

6. 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 .

7. 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.2015.00117/full

8. 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

9. 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

10. 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’

11. 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

12. 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

13. Figure 1: Enzymes involved in DNA replication

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

15. 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 .

16. 2.The code is non-overlapping A non-overlapping code means that a base in a m-RNA is not used for different codons .

17. 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

18. 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

19. 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 .

20. 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