DNA Replication

DNA Replication


       At the end of this lecture, student
will be able to

      Explain steps of DNA Replication

Replication of DNA

       Replication is a process in which
DNA copies itself to produce identical daughter molecules of DNA

       Process by which new identical DNA
is formed from existing DNA as template

       Replication of DNA occurs in 5′ to
3′ direction
simultaneously on both the strands of DNA

       On the leading strand-the DNA
synthesis is continuous

       On the lagging strand-the
synthesis of DNA is discontinuous

       Short pieces of DNA (15-250
nucleotides) are produced on the lagging strand

Copying DNA

of DNA

pairing allows each strand to serve as a template for a new strand

strand is 1/2 parent template & 1/2 new DNA

     Semiconservative model:

       Parent DNA has two strands
complementary to each other

        Both the strands undergo simultaneous
replication to produce two daughter molecules

       Each one of the newly synthesized
DNA has one-half of the parental DNA (one strand from original) and one-half of
new DNA

       This type of replication is known as
semiconservative since half of the original DNA is conserved in the daughter

       Synthesis of a new DNA molecule is a
complex process involving a series of steps

1.       Replication fork formation

2.       Primer binding

3.       Elongation

4.       Termination

Step 1-Replication fork

       Double stranded DNA is separated
into 2 single strands

       To unwind DNA, interactions between
base pairs must be broken & this is performed by DNA helicase

       DNA helicase disrupts H bonding to
separate strands into a Y shape – replication fork at replication origin

       Single-stranded binding proteins
(SSB) work with helicase to keep the parental DNA helix unwound

DNA Replication

team of enzymes coordinates replication

Replication: 1st step



part of DNA helix

by single-stranded binding proteins

Starting DNA Synthesis: RNA Primers

RNA primer built by primase serves as starter
sequence for DNA polymerase III

Step 2-Primer binding

       Leading strand is the simplest to

       As the DNA strands have been
separated, a short piece of RNA called a primer binds to 5’end of the

       Primer always binds at the starting
point for replication

       Primers are generated by the enzyme DNA

Step 3-Elongation

       DNA polymerases are responsible for
creating the new strand by a process called elongation

       5 different types of DNA polymerases
are known in bacteria and human cells

       DNA polymerase III is the main
replication enzyme involved in leading stand synthesis

       DNA polymerase I is involved in
lagging strand synthesis

       Other enzymes are involved in error
checking and repair

       Lagging strand begins replication by
binding with multiple primers and each primer is only several bases apart

       DNA polymerase then adds pieces of
DNA called Okazaki fragments to the strand between primers

       This process of replication is
discontinuous as the newly created fragments are disjointed

Leading & Lagging strands

Lagging strand

Okazaki fragments

Joined by ligase “spot welder” enzyme

Leading strand

Continuous synthesis

Step 4-Termination

       After the formation of continuous
and discontinuous strands, exonuclease enzyme removes all RNA primers from the
original strands

       Primers are then replaced with
appropriate bases

       DNA ligase enzyme joins Okazaki
fragments together forming a single continuous strand

       Ends of parent strands consists of
repeated DNA sequences called telomeres

       Telomeres act as protective cap to
prevent nearby chromosome from fusing

       Telomerase enzyme catalyses
synthesis of telomere sequences

       Parent strand and complementary
strand then coil into helical shape

Replacing RNA Primers with DNA

DNA polymerase I removes sections of RNA primer and
replaces with DNA nucleotides

But DNA polymerase I still can only build onto 3 end of an
existing DNA strand


Protective cap at the end of chromosome to prevent nearby
chromosome from fusing


Enzyme extends telomeres

Replication fork



2 main steps




        leading strands


only in 3I-5I direction

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