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DNA Replication

DNA replication is the process by which DNA copies itself. It occurs during cell division to produce two identical copies of the DNA for each new cell. DNA has two complementary strands that both undergo replication. Replication begins with initiation at the origin of replication site. RNA primers are used to begin DNA synthesis which proceeds bidirectionally from the replication fork in the 5' to 3' direction. Replication of the lagging strand is discontinuous and produces Okazaki fragments which are later joined. Termination occurs once replication is complete.

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20 views26 pages

DNA Replication

DNA replication is the process by which DNA copies itself. It occurs during cell division to produce two identical copies of the DNA for each new cell. DNA has two complementary strands that both undergo replication. Replication begins with initiation at the origin of replication site. RNA primers are used to begin DNA synthesis which proceeds bidirectionally from the replication fork in the 5' to 3' direction. Replication of the lagging strand is discontinuous and produces Okazaki fragments which are later joined. Termination occurs once replication is complete.

Uploaded by

Zainab Ashroff
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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DNA replication

Prof. U.P.K. Hettiaratchi


Dept. of Biochemistry

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Cell cycle

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DNA replication
• Process in which DNA copies it self to produce
identical daughter molecules of DNA
• Happens during cell division
• Parent DNA has two complementary strands
• Both the strands undergo replication to produce two
daughter strands

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DNA replication is
semi conservative
(half of the original
DNA is conserved)

New strand is
complementary to
the parent strand.

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DNA replication
• Initiation
• Elongation
• Termination

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Initiation

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Initiation of replication
• Occurs at a site called origin of replication.

• dnaA protein binds to the site of origin and


causes the strands to separate.

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Replication bubbles

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RNA primer
• Synthesis of new DNA requires a RNA primer

• Primer synthesized by a RNA polymerase


enzyme called primase.

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Elongation

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DNA synthesis
• Synthesis occurs on both strands in 5’to 3’
direction
• DNA synthesis is semi discontinuous. WHY??
– Leading strand – Synthesis is continuous
– Lagging strand – Synthesis is discontinous
• Small fragments of DNA are produced on the
lagging strand. These are called Okazaki
pieces.
• DNA synthesis is bi directional

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Replication fork
• Synthesis of
– leading strand – towards the replication fork
– lagging strand – away from the replication fork

• Okazaki pieces are later joined by DNA ligase/


DNA polymerase III.
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Proteins/enzymes involved in replication

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DNA polymerase III
• DNA polymerase III has two functions
1. DNA synthesis - synthesis of leading and
lagging strands in 5’ to 3’ direction.
2. Proof reading – only allows the
complementary base to be inserted. Checks
and removes the wrongly placed bases.

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Termination

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Replacement of RNA primer by DNA

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Supercoils
• When the two strands are separated, positive
supercoils (also called supertwists) appear in
the region of DNA ahead of the replication
fork

• DNA topoisomerases remove supercoils in the


helix.

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Positive supercoiling

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Eukaryotic DNA replication
• Closely follows that of prokaryotic translation
• Multiple origins of replication

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Eukaryotic DNA polymerases

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