the function of dna is information transfer and storage

The function of DNA is information transfer and storage

1. DNA is copied to more DNA in DNA replication

2. Gene expression i.e.

Transcription- synthesis of RNA from only one strand of a double stranded DNA helix

Translation- ribosome mediated synthesis of a polypeptide from a messenger RNA molecule

DNA replication

A polymerisation reaction with the typical 3 phases

• initiation,

• elongation,

• termination

Initiation occurs at replication origins

Initiation

• occurs at Ori C (origin of chrm replication)

• a 245 bp region with 2 conserved sequences

• 3 tandem 13bp repeats

• 4 copies of a 9 bp sequence

• Dna A protein molecules with bound ATP bind to 9 bp repeats

• Facilitated by HU protein• The 3 x 13 bp repeats are sequencially

denatured to give the open complex• A complex formed by Dna B and Dna C bind to

the melted region, Dna C is released• In the presence of SSB protein and Dna gyrase

the Dna B helicase unwinds the DNA in preparation for priming and DNA synthesis

Replication occurs at origins

Elongation

• Leading and lagging strand synthesis

• Parental DNA is unwound (helicases) and topological stress removed (gyrases/topoisomerases)

• Separated strand stabilised by SSB

• Leading strand

• Primase (DnaG protein) synthesises a short RNA primer at origin

• DNA pol III builds complimentary strand from ds primer

• continuous process

• Lagging

• Primase synthesizes many RNA primers along lagging strand

• Each primer is extended by DNA pol III

• Synthesis proceeds in 5' to 3' direction i.e. the direction opposite to the fork movement

• Synthesis is discontinuous in the form of multiple Okazaki fragments

lagging continued..........• Synthesis continues until fragment

extends as far as the primer of the previously added Okazaki sequence

• Note both strands are synthesised by a single asymetric dimer of DNA pol III that moves in the direction of the replication fork

• This is achieved in the case of the lagging strand by the DNA looping around the part of the dimer DNA pol III

• DnaB (helicase) and Dna G (primase) together form the primosome

• 1000 nucleotides of new DNA added per second to each strand

• RNA primers are removed (exonuclease activity) and replaced with DNA (polymerase) by DNA pol I and the remaining nick is sealled by a ligase using NAD as a cofactor

Termination• Eventually the 2 relication forks of E.coli meet

at a terminus region containing many copies of a 20 bp sequence called Ter

• These sequences are binding sites for a protein called Terminal utilization substance (Tus)

• Tus-Ter complex arrests the replication fork in one direction

• The other replication fork halts when they meet.

• the few hundred base pairs in between the protein complexes are replicated by an as yet unknown mechanism resulting in 2 interlinked circular chromosomes

• separation requires Topoisomerase IV• separate chrm are then segregated at

cell division

Proteins at the E coli replication Fork

SSB binds to ssDNA and

stabilizes it

DnaB protein (helicase) DNA unwinding; primosome

constituent

Dna G protein (Primase) RNA primer synthesis;

primosome constituent

DNA pol III New strand elongation

DNA pol I Excision of primers and filling

of gaps

DNA ligase Ligation

DNA gyrase supercoiling

(DNA topoisomerase II)

DNA polymerasesDNA pol I DNA pol II DNA pol III

Mol.wt.

(Daltons) 103,000 88,000 900,000

polymerizatn

rate 16-20 7 250-1000

(nucleotides/second)

3' to 5'

exonuclease

activity yes yes yes

5' to 3‘

exonuclease

activity yes no no

Functions proof reading and repair replicatn