DNA Replication and Repair Flashcards
Helicase
Unwinds parental double helix at replication forks. First step in DNA replication
Single-Strand Binding protein
Binds to and stabilizes single-stranded DNA until it is used as a template. Prevents rebinding.
Topoisomerase
Relieves overwinding strain ahead of replication forks by breaking, swiveling, and rejoining DNA strands
Primase
Synthesizes an RNA primer at 5’ end of leading strand and at 5’ end of each Okazaki fragment of lagging strand
DNA Polymerase III
Using parental DNA as a template, synthesizes new DNA strand by adding nucleotides to an RNA primer or a pre-existing DNA strand
DNA Polymerase I
Removes RNA nucleotides of primer from 5’ end and replaces them with DNA nucleotides added to 3’ end of adjacent fragment
DNA Ligase
Joins Okazaki fragments of lagging strand; on leading strand, joins 3’ end of DNA that replaces primer to rest of leading strand DNA
Phosphodiester bond
Where does the energy come from for these bonds?
The linkage bonding the phosphate group of the new nucleotide and the hydroxyl group of the previous one
Energy comes from the other two phosphates
Where are attached nucleotides found?
dATP, dCTP, dGTP, dTTP
are attached by DNA polymerase and are found free floating in the nucleus. The energy to form this Phosphodiester Bond is stored in the two phosphates that get released as pyro-phosphate.
Leading Strand
The leading strand is built continuously by DNA Polymerase III towards the replication fork because DNA must be built from 5’ to 3’.
Lagging Strand
What are the fragments called? How are they joined?
The Lagging Strand is synthesized as a series of Okazaki fragments each with their own primer, which are joined later by DNA ligase. DNA must be built from 5’ to 3’.
Primers are about 200 bases apart.
Nucleotide Excision Repair
How does it work?
- A nuclease enzyme cuts out damaged stretches of DNA
- DNA Polymerase I comes and replaces the damaged Nucleotides
- DNA Ligase joins the repaired DNA to the rest of the strand
Telomeres
Dna Sequence? Consequence of shortening?
Telomeres postpone the erosion of genes near the ends of DNA molecules. The consequence of the shortening of telomeres can be connected to aging, including premature aging diseases.
DNA Sequence: TTAGGG
What is the main cause of Xeroderma Pigmentosum?
Xeroderma Pigmentosum is a disease in which individuals have increased sensitivity to UV rays. Normally, a DNA repair mechanism can go and fix the damaged DNA nucleotides but people with XP have mutations that affect this mechanism, mainly nucleotide excision repair.
