Page 172 - MNU microbiology theoritical 2025

General Microbiology & Immunology (PM 401) Second level Semester 4 2024/2025

2- Elongation

• DNA replication (addition of nucleotides to form a new strand) occurs

through DNA polymerase III

• DNA polymerase III can only add nucleotides to the 3’ end of the DNA.

• This causes the NEW strand to be built in a 5’ to 3’ direction.

• Therefore, in one strand (the template 3‘→5‘leading strand) it is

continuous, hence called continuous replication

• while on the other strand (the template 5‘→3‘lagging strand) it is

discontinuous replication. They occur as fragments called Okazaki

fragments. The enzyme called DNA ligase joins them later.

● The Leading Strand: is synthesized as a single strand in the direction 5’-
3’ of DNA template by DNA polymerase III

• The lagging strand

⮚ Primase: synthesizes a short RNA primer of 10-12 nucleotides, to which
DNA polymerase III adds nucleotides in the direction of 5’-3’ to form
Okazaki fragments.

General Microbiology & Immunology (PM 401) theoretical handbook Page 161