DNA Replication

Presentation by: Md. Shabab Mehebub

DNA Replication

• Copying genetic information for transmission to the next generation is called

DNA replication.

Three model of DNA Replication

• We will follow the Semiconsevative Model because it is accepted by everyone.

DNA Replication Steps:• Initiation

– involves assembly of replication fork (bubble) at origin of replication • sequence of DNA found at a specific site

• Elongation– Parental strands unwind and daughter strands are synthesized. – the addition of bases by proteins

• Termination: – the duplicated chromosomes separate from each other. Now, there are

two IDENTICAL copies of DNA.

Segments of single-stranded DNA are called template strands.Copied strand is called the complement strand

• LEADING STRAND: synthesized 5’ to 3’ in the direction of the replication fork movement.-continuous requires a single RNA primer• LAGGING STRAND: synthesized 5’ to 3’ in the opposite direction.– semidiscontinuous requires many RNA primers , DNA is synthesized in short fragments.

Initiation Step

3

Polymerase III

5’ 3’

Leading strand

base pairs

5’

5’

3’

3’

Supercoiled DNA relaxed by gyrase & unwound by helicase + proteins:

Helicase +

Initiator Proteins

ATP

SSB Proteins

RNA Primer

primase

2Polymerase III

Lagging strand

Okazaki Fragments1

RNA primer replaced by polymerase I& gap is sealed by ligase

Gyrase

Elongation

DNA polymerase only adds nucleotides at the free 3’ end of NEW STRAND forming new DNA strands in the

5’ 3’ direction only…

DNA ligase seals the gaps between Okazaki fragments with a phosphodiester bond

Mistakes during Replication

Polymerase uses its proofreading ability to cleave the phosphodiester bond of improper nucleotide

Thank you Everyone