Presented by …summaya asghar

Dna replication

DNA DNA is an essential molecule for life. It

acts like a recipe holding the instructions telling our bodies how to develop and function.

What is DNA made of? DNA is a long thin molecule made up of

something called nucleotides. There are four different types of nucleotides

and . They are usually represented by their first

letter: A- adenine T- thymine C - cytosine G - guanine

Definition ofdna replication

DNA replication is the process by which DNA makes a copy of itself during cell division. 

DNA polymerase DNA polymerases are a

family of enzymes thatcarry out all forms of DNA

replication. 

Steps of dna replication The first step in DNA replication is to

‘unzip’ the double helix structure of the DNA molecule.

This is carried out by an enzyme called helicase which breaks the hydrogen bonds holding the complementary

bases of DNA together (A with T, C with G).

Cont… The separation of the two single

strands of DNA creates a ‘Y’ shape called a replication ‘fork’.

Cont… The two separated strands will act as

templates for making the new strands of DNA.

One of the strands is oriented in the 3’ to 5’ direction (towards the replication fork), this is the leading strand.

Cont…. 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:

Cont… Once all of the bases are matched up

(A with T, C with G), an enzyme called exonuclease strips away the primer(s).

The gaps where the primer(s) were are then filled by yet more complementary nucleotides.

Cont…. The new strand is proofread to make

sure there are no mistakes in the new DNA sequence.

Finally, an enzyme called DNA ligase seals up the sequence of DNA into two continuous double strands.

Cont…. The result of DNA replication is two

DNA molecules consisting of one new and one old chain of nucleotides.

This is why DNA replication is described as semi-conservative, half of the chain is part of the original DNA molecule, half is brand new.Following replication the new DNA automatically winds up into a double helix. 

Replication process DNA Replication, like all biological

polymerization processes, proceeds in three enzymatically catalyzed and coordinated steps:

initiation elongation and termination

References  Imperfect DNA replication results in mutations. Berg JM, Tymoczko JL, Stryer L, Clarke ND (2002). "

Chapter 27: DNA Replication, Recombination, and Repair".Biochemistry. W.H. Freeman and Company. ISBN 0-7167-3051-0.

Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2002). "Chapter 5: DNA Replication, Repair, and Recombination". Molecular Biology of the Cell. Garland Science. ISBN 0-8153-3218-1.

 Berg JM, Tymoczko JL, Stryer L, Clarke ND (2002). "Chapter 27, Section 4: DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites".Biochemistry. W.H. Freeman and Company. ISBN 0-7167-3051-0.

 Alberts, B., et al., Molecular Biology of the Cell, Garland Science, 4th ed., 2002, pp. 238–240 ISBN 0-8153-3218-1

 Allison, Lizabeth A. Fundamental Molecular Biology. Blackwell Publishing. 2007. p.112 ISBN 978-1-4051-0379-4

 Berg JM, Tymoczko JL, Stryer L, Clarke ND (2002). Biochemistry. W.H. Freeman and Company. ISBN 0-7167-3051-0. Chapter 27, Section 2: DNA Polymerases Require a Template and a Primer

 McCulloch SD, Kunkel TA (January 2008). "The fidelity of DNA synthesis by eukaryotic replicative and translesion synthesis polymerases". Cell Research 18(1): 148–61. doi:10.1038/cr.2008.4. PMC 3639319. PMID 18166979.

 McCarthy D, Minner C, Bernstein H, Bernstein C (1976). "DNA elongation rates and growing point distributions of wild-type phage T4 and a DNA-delay amber mutant". J Mol Biol 106 (4): 963–81. doi:10.1016/0022-2836(76)90346-6.PMID 789903.

 Drake JW (1970) The Molecular Basis of Mutation. Holden-Day, San FranciscoISBN 0816224501 ISBN 978-0816224500