DNA Replication | Molecular biology | Biochemistry | First Aid lectures | Lec7 #Drjamillectures
Channel: Dr. Jamil Lectures
Date: 17/05/2022

In this lecture we will discuss about DNA replication,Origin of 
replication,Replication fork,Helicase,Single-stranded 
binding proteins,DNA 
topoisomerases ,Primase,DNA polymerase III ,DNA polymerase I,DNA ligase and Telomerase.



https://youtu.be/_dg4HoY8GIs
https://youtu.be/48gsdeWRdwM
https://youtu.be/aP06bUvyqCA


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DNA replication
 Eukaryotic DNA replication is more complex than in prokaryotes but uses many enzymes analogous to those listed below. In both prokaryotes and eukaryotes, DNA replication is semiconservative, involves continuous and discontinuous (Okazaki fragment) synthesis, and occurs in the 5′  3′ direction. 
Origin of replication
 A Particular consensus sequence in genome where DNA replication begins. May be single (prokaryotes) or multiple (eukaryotes).
AT-rich sequences (such as TATA box regions) are found in promoters and origins of replication.
Replication fork
  Y-shaped region along DNA template where leading and lagging strands are synthesized.
Helicase 
 Unwinds DNA template at replication fork. Helicase Halves DNA.Deficient in Bloom syndrome (BLM gene mutation).
Single-stranded binding proteins D
Prevent strands from reannealing.
DNA topoisomerases 
Create a single- or double-stranded break in the helix to add or remove supercoils.
In eukaryotes: irinotecan/topotecan inhibit 
topoisomerase (TOP) I, etoposide/teniposide
inhibit TOP II. 
In prokaryotes: fluoroquinolones inhibit TOP II 
(DNA gyrase) and TOP IV.
Primase
 Makes an RNA primer on which DNA polymerase III can initiate replication.
DNA polymerase III 
 Prokaryotes only. Elongates leading strand by adding deoxynucleotides to the 3′ end. Elongates lagging strand until it reaches 
primer of preceding fragment.DNA polymerase III has 5′  3′ synthesis and 
proofreads with 3′  5′ exonuclease.
Drugs blocking DNA replication often have a 
modified 3′ OH, thereby preventing addition of 
the next nucleotide (“chain termination”).
DNA polymerase I 
 Prokaryotes only. Degrades RNA primer; replaces it with DNA.Same functions as DNA polymerase III, also excises RNA primer with 5′  3′ exonuclease.
DNA ligase  
 Catalyzes the formation of a phosphodiester bond within a strand of double-stranded DNA.Joins Okazaki fragments. 
Ligase
 Links DNA.
Telomerase
 Eukaryotes only. A reverse transcriptase (RNAdependent DNA polymerase) that adds DNA (TTAGGG) to 3′ ends of chromosomes to avoid loss of genetic material with every duplication. Often dysregulated in cancer cells, allowing unlimited replication.



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