DNA REPLICATIONSBI4U Ms. Manning

DNA Replication Produces two identical copies of

the chromosome during S phase of interphase

Catalyzed by many enzymes including: DNA polymerase, helicase, ligase and primase

Replication takes place at a point where the DNA double helix separates (called the replication fork)

STEPS OF DNA REPLICATION1. UNWINDING AND SEPARATION OF THE

PARENT STRAND – “Initiation”

-separation is initiated at one or more sites called ORIGINS OF REPLICATION

-the structure of 2 unwound DNA strands is referred to as a replication fork

Three enzymes work together to unwind and stabilize the double helix:

A. DNA HELICASE – breaks the hydrogen bonds that connect the nitrogenous bases which allows the double helix to unwind and separate.

B. DNA GYRASE – an enzyme that relieves the tension produced by unwinding of DNA – is a type II topoisomerase.

C. SINGLE STRANDED BINDING PROTEINS (SSBs) – work to keep separated strands of DNA apart

gyrase

2. DNA SYNTHESIS (“Elongation”

Replication begins in 2 directions from the origins as a region of DNA is unwound. Replication proceeds towards the direction of the replication fork on one strand, and away from the fork on the other.

In eukaryotes, more than one replication fork may exist on a DNA molecule.

A replication bubble forms when 2 replication forks are in close proximity to each other

Fig 5.17 p.223

DNA Synthesis cont’d… DNA POLYMERASE III takes free

nucleotides found within the cell and adds them in the 5’ 3’ direction to form the new strand

The parent strand is used as a template i.e. If A is in the parent, then T is inserted into the daughter

LEADING STRAND The daughter strand that grows

continuously towards the replication fork as the double helix unwinds.

This occurs quickly The new strand grows from 5’

3’

LAGGING STRAND The other daughter strand that

cannot grow towards the replication fork, therefore it grows in the opposite direction

It is built in short segments (in the 5’ 3’ direction) away from the replication fork.

This is much slower than the leading strand!

Details about the lagging strand: The 3’ to 5’ parent strand is a problem

for DNA polymerase since it must synthesize in the 5’ 3’ direction

Short RNA primer sequences of 10-60 RNA bases are bonded to regions of the lagging strand with the purpose of initiating DNA replication

PRIMASE – an enzyme that binds the RNA primers to the DNA

Lagging strand cont’d… DNA polymerase builds short sequences

of DNA off of the RNA primers called OKAZAKI fragments.

This lagging strand…is…slow!

The lagging strand takes longer to replicate than the leading strand because it involves more steps and is not continuous replication.

2b) LINKING OF NITROGENOUS BASES

Nitrogen bases of nucleotides of opposite strands (parent and daughter) form new H bonds

Once hydrogen bonds are formed, DNA automatically twists into a double helix

3) Termination DNA polymerase I excises the RNA

primers and replaces them with the appropriate deoxyribonucleotides.

DNA ligase joins the gaps in the Okazaki fragments by the creation of a phosphodiester bond.

4) QUALITY CONTROL – “proofreading”

DNA POLYMERASE III & I – act as a proof-reader by checking the newly synthesized strand for any incorrectly inserted bases.

If a mistake is found, polymerase acts as an EXONUCLEASE – cutting out the mis-paired base and replacing it with the correct nucleotide.

Errors missed by proofreading can be corrected by one of several repair mechanisms after replication is complete.

What happens at the ends of chromosomes?

Telomeres

Shrinking telomeres and aging