Why and when is DNA replication necessary?
It is necessary in: 1) Duplication of the genome during S phase (prior to mitosis or meiosis) 2)To repair damage to the genome.
What is the mechanism of DNA replication?
DNA replication occurs through semiconservative mechanism.
Describe semiconservative mehcanism of replication:
Each parent strand serves as a template for a new strand and the new strand is made of an old strand and a new strand.
What are the two steps involved in DNA replication?
1) DNA helix is unwound to separate the two template strands. 2)New nucleotides are added and form complementary base paring with the template DNAs.
What are the four things that DNA replication needs in order to occur?
1)A template 2)Nucleotides 3)Enzymes that catalyze the addition of nucleotides 4)A primer to initiate the polymerization reaction.
Where does the enrgy used to drive the condensation reaction comes from?
It comes from the hydrolisis of dNTPs which contains three phospates group and two are removed during the formation of phosphodiester linkages.
What's the first step in the polymerization of DNA during replication?
The replication begins with the binding of the pre-replication complex to the origin of replication.
What happens after the pre-replication complex binds to the origin of replication?
DNA is unwound by helicase and replication proceeds in both direction forming two replication forks.
What are replication forks?
The sites ehere DNA unwinds to exposes the bases so they can act as templates.
What does helicase do?
Helicase uses energy from ATP to unwind the two strands of DNA held together by hydrogen bonds.
What do single strand binding proteins (SSBP) do?
They bind to the single strands in order to avoid reassociation.
What does DNA polymerase III do?
DNA polymerase III is an enzyme that catalyses the addition of nucleotides by covalently bonding them and it requires a primer.
What's a primer?
A short strand of RNA that is complementary to DNA template and it's synthetized by primase.
Primase is an RNA polymerase that requires a DNA template and generates a primer for DNA polymerase III.
What's DNA polymerase I?
An enzyme that hydrolise the RNA primer and fill the gap adding DNA.
What are the two types of strands formed after replication forks?
-Leading strand -Lagging strand
Describe leading strand:
Leading strand is orientated so that it can continuously grows at its 3' end as the fork open up. I t requires just one primer.
Describe the lagging strand:
Lagging strand is oriented so that as the fork opens up its exposed 3' end gets further and further away from the replication fork.
Describe the synthesis of the lagging strand:
It requires the synthesis of small, discontinous strands of DNA called Ozazaki fragments and each of those requires its own primer.
What's DNA ligase?
Is an enzyme that attache the Ozazaki fragments together.
Why is DNA polymerase III processive?
Because they catalyse the fromation of many phosphodiester linkages at the same time.
How is processivity of DNA polymerase III mantained
It's maintained with the help of a sliding DNA clamp.
What does the sliding DNA clamp do?
It keeps the enzyme and the DNA associated so that they remain attached longer.
What are the three different mechanism to correct DNA errors?
-Proofreading -Mismatch Repair -Escision repair.
It corrects errors in replication ad the DNA polymerase III makes them by exciding the wrong nucletides and replacing them with correct nucleotides.
Describe mismatch repair:
It occurs after proofreading and scans DNA immediately after it has been replicated correcting any base pairing mismatching.
Describe Excision repair:
It removes abnormal bases that have been formed and replace them with functional bases.
Describe telomere and its function:
Telomere is a repetitive sequence at the end of the chromosome that prevents DNA repair system to recognize them as broken.
What happens during transcription?
The information in the DNA sequence (a gene) is copied into a complementary RNA sequence
State the three components required for transcription to occurs:
1) A DNA template for complementary base pairing. 2)The four ribonucelotides trisphosphate. 3)RNA polymerase.