BABS1201 - DNA Replication PDF

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LO1 Explain the semi-conservative model of DNA replication. ●

Semiconservative model  = The two strands of the parental molecule separate, and each functions as a template for synthesis of a new, complementary strand.

LO2 Describe the basic steps involved in the process of DNA replication. ●

Initiation of DNA replication is achieved via several enzymes: helicases, SSB, topoisomerase, primase & DNA polymerase.

●

DNA Polymerase III  adds nucleotides to the 3’ end of the primer, complementary to the template strand (i.e. it is synthesised in the 5’ to 3’ direction). ○

The nucleotides have a pentose and triphosphate attached to the base.

○

A dehydration reaction occurs and pyrophosphate is produced.

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Newly synthesised strand is called the l eading strand .

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Lagging strand synthesis: 1. Primase creates an RNA primer. 2. DNA polymerase III synthesises a short complementary strand of DNA called an Okazaki fragment . 3. DNA polymerase I replaces the RNA primer with DNA. 4. DNA ligase acts like glue and joins the Okazaki fragments together, forming a continuous strand.

LO3 Describe the function of the major enzymes involved in DNA replication.

Enzyme

Function

Helicase

Unwides

parental

double

helix

at

replication forks Single-strand binding protein (SSB)

Binds to and stabilises single-stranded DNA until it is used as a template (keeps the separated strands from re-pairing).

Primase

Initiates the synthesises of the new strand by synthesising an RNA primer at the 5'

end of leading strand and at 5’ end of each Okazaki fragment of lagging strand. DNA pol I

Removes RNA nucleotides of primer form 5’ end and replaces them with DNA nucleotides added to 3’ end of adjacent fragments.

DNA pol III

Using

parental

DNA

as

a template,

synthesises new DNA strand by adding nucleotides

to

an

RNA

primer

or

pre-existing DNA strand. DNA ligase

Joins Okazagi fragments of lagging strand; on the leading strand, joins 3’ end of DNA that replaces primer to rest of leading strand DNA.

Topoisomerase

Relieves

overwinding

strain ahead of

replication forks by breaking and rejoining the DNA strands. Telomerase

Responsible for maintenance of the length of telomeres by addition of guanine-rich repetitive sequences....