Topic 9: DNA replication Flashcards
how is each dna strand used
as a template for a new strand
why is dna replication needed
during process of cell division so daughter cells have same amount of DNA
what part of the cell cycle does dna replication take place
S phase
what are some key features of dna replication
semi-conservative - each old dna strand acts as a template for a new strand. so after dna replication, one old strand and one new strand
how does new strand form
bases from new and template strand attach by H bonds
creating a new phopshodiester bond
PPi is released (as dATP -> PPi and dGTP -> PPi)
template strand = read from 3 to 5 prime
new strand read from 5 to 3 prime
what is required for dna replication
catalysed by DNA polymerase
hydrolysis of PPi by pyrophosphatase drives the reaction
dNTP’s needed (for building blocks)
the double stranded dna template need to be separated first so both strands used as template
how does dna replication work in prokaryotes
initiation -
elongation
termination
of circular naked chromosome
what happens in initiation of dna replication in prokaryotes
dna polymerase required which is kick started by DNA primase to make an RNA primer to make new dna. circular double stranded DNA molecule, the strand seperates, two replication forks forms moving in either direction to form an origin of replication (one origin = 2 replication forks)
what happens in elongation of dna replication in prokaryotes
dna helicase unwinds the 2 strands of dna so the two replication forks move as replication continues. everytime it is unzipped, needed to be kickstarted and polymerase again from 5 end so forms little fragments (okazaki) - this is the lagging strand and the template strand is the leading strand.
what happens in termination of dna replication in prokaryotes
replication forks meet each other and two circular double stranded dna molecules that are identical copies of each other form. dna ligase fuses gaps in lagging strand. 2 leading strands and 2 lagging strands which are two identical dna molecules
what does dna polymerase do
synthesises dna from 5 prime, reads template in 3 to 5 direction
what happens in dna replication in eukaryotes
many origins of replication (each consist of 2 replication forks) - until whole chromosome replicated
one dna molecule (one chromosome -1 chromatids) to two dna molecules (one chromosome - 2 chromatids held together by centromere to form X shaped)
