DNA synthesis and Cell cycle Flashcards
What recognizes the replication origin?
By an initiation complex
In what phase of the cell cycle does DNA synthesis occur?
S phase of the cycle and DNA is completely unwounded
What does SSB do?
Keeps the strands seperate
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 three enzymes are required to produce a continuous strand of DNA? Include there function
○ 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 is the issue of accessing nucleotides at the center of the helix addressed?
○ DNA helicases uses energy from the hydrolysis of ATP to propel itself forward breaking the double helix apart
What does localised unwinding of DNA cause and how is it resolved?
causes the other side of the fork to get wound more tightly creating tension in the DNA
DNA topoisomerase is used to relieve the tension by producing nicks to temporarily release the tension and then reseal the nick before falling of the DNA.
How are enzymes involved in replication held together?
large multienzyme complex and moves as a unit parallel to the parental DNA
Bacterial cell cycle
• S Phase is longer than M phase
Divide once every 20-30 minutes
Mammalian cell cycle
• G1 phase ○ Pairing DNA ○ Replicate protein • S Phase ○ DNA replication ○ Semi conservative • G2 Phase ○ Prep for mitosis • M phase ○ Mitosis
How many replication origins does bacteria have?
One
How many does eukaryotic DNA have?
Multiple
What happens to the replication bubble on the parent strand as replication advances?
Becomes larger and larger
Types of DNA polymerase in bacteria
○ I repair
○ II repair
○ III replication
DNA polymerase in eukaryotic cells
Refer to table
