DNA Replication

DNA Replication

Objective

       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

       Replication of DNA

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

      new 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 DNA

       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 formation

       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

       Large team of enzymes coordinates replication

Replication: 1st step

       Unwind DNA

      helicase enzyme

       unwinds part of DNA helix

       stabilized 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 replicate

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

       Primer always binds at the starting point for replication

       Primers are generated by the enzyme DNA primase

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

Telomeres

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

Telomerase

Enzyme extends telomeres

Replication fork

Summary

       DNA replication

      Involves 2 main steps

       Unwinding

       Replication

      Two strands

        leading strands

       lagging strands

      Replication only in 3I-5I direction

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