DNA Synthesis Flashcards
what recognises the replication origins?
By an intiation complex
in what phase of the cell cycle does DNA synthesis occur?
S phase of the cycle and DNA is completely unwounded
What does single stranded binding protein (SSB) do?
keeps the strands separate
DNA synthesis
1)Initiator proteins bind to the replication origins separating the two DNA strands. • Breaking individual hydrogen bonds doesn’t require as much energy. • There are many replication origins on the human genome therefore DNA replication can occur at many different places of the genome and shortens the time it takes to copy the cells entire genome. 2)At each replication origin, two replication forks are formed. These forks move in opposite directions away from the origin, replicating DNA as they move. • There maybe a slower rate of replication due to difficulties replicating through complex histones. 3)DNA polymerase catalyses the addition of nucleotides to the 3 end of a growing DNA strand, using one of the paternal strands as a template. • Polymerisation reaction involves the formation of a phosphodiester bond between the 3 end of growing DNA and the 5 phosphate group of the nucleotide. • Energy for this polymerisation is provided from the hydrolysis of deoxyribonucleotide triphosphate 4)The replication fork is asymmetrical because one new DNA strand is made in the 5 to 3 direction and another in the opposite direction in the 3 to 5 direction. • DNA polymerase only adds subunits to the 3 end of a DNA strand so only the 5 to 3 direction. • The 3 to 5 direction is discontinuously synthesised. 5)Primase synthesizes a primer(a short RNA sequence). • For leading strand, RNA primer is only needed to start replication • For lagging strand new primers are needed for polymerisation to keep going. DNA polymerase adds deoxyribonucleotide to the 3 end of each primer to start new Okazaki fragments and this will continue to elongate until the next primer is reached.
what is needed in order to to produce a continuous strand of DNA?
Nuclease - to degrade the RNA primer
repair polymerase - to replace RNA with DNA
DNA ligase - joins the 5 phosphate end of one DNA fragment to the adjacent 3 hydroxyl end of the next
how are the nucleotides lying at the centre of the helix accessed?
Using the energy obtained from the hydrolysis of ATP, the DNA helicases propel forward breaking the double helix apart
to prevent the single stranded DNA from reforming its base pairs and to keep it in it s elongated form, single stranded DNA binding proteins cling to the single stranded DNA
what does localised unwinding of DNA cause?
it causes the other side of the fork to get wound more tightly creating tension in the DNA
how is this resolved?
DNA topiosomerase is used to relieve the tension by producing nicks to tmeporarily release the tension and then reseal the nick before falling off the DNA
a protein known as the sliding clamp keeps the DNA polymerase firmly attached to the template strand so that it does’t fall off
how are enzymes located in the replication of DNA held together?
they are held in a large multienzyme complex and moves as a unit parallel to tehe parental DNA
describe the bacterial cell cycle
S phase is longer than M phase
Every 20-30 mins divides once
describe the mammalian cell cycle
G1 phase - pairing DNA, replicate protein
S phase - DNA replication, semi conservative
G2 phase - Prep for mitosis
M phase - mitosis
describe bacterial DNA replication
replication fork
one origin
SSB protein keeps the strand separate
describe Eukaryotic DNA replication
multiple replication origins
replication bubbles on the parent strand become larger and larger as replication advances
what are the types of DNA polymerase in bacteria?
I repair
II repair
III repair
in what direction does DNA polymerase act?
5’ to 3’ direction
