DNA synthesis Flashcards
cell cycle stages
G1, S, G2, M
what happens during the S phase of the cell cycle?
DNA is replicated, histone and other protein synthesis is greatly increased
DNA replication
DNA unzips into two parts and splits with the cell; is semi-conservative
what is the purpose of methyl tags in DNA replication?
as methyl copying enzymes lay down methyl tags during replication, they silence the expression of genes in cells where they are not necessary (e.g. albumin in skin cells)
how does bacterial replication work?
since bacteria have circular DNA with an origin, replication begins there in both directions
how does eukaryotic replication work?
have multiple origins in DNA; replication bubbles arise at points along DNA and synthesis proceeds along a fork in each direction from these points
how is helix unwinding accomplished?
the DNA is unwound and opened up by a series of enzymes and single-strand binding proteins to keep DNA from “re-annealing”
what enzyme unwinds DNA?
helicase
what does topoisomerase do?
it is an enzyme responsible for allowing breaks and rejoining of the DNA to relieve tension from super-coiling
what does gyrase do?
it is a prokaryotic enzyme (type of topoisomerase) used to twist the single circular chromosome of prokaryotes upon itself to form supercoils; relieves the stress caused by unwinding DNA
what direction do DNA polymerases synthesize?
they synthesize in a 5’ to 3’ direction, but read along the template strand from 3’ to 5’)
exonuclease activity
the ability of DNA polymerases to move backwards to cut out incorrect bases from the end of a DNA strand in replication
processivity in replication refers to:
how well DNA polymerase stays associated with DNA template during replication
RNA primers
a short strand of RNA, made by DNA primase, that is used to elongate a strand of DNA during DNA replication
why are RNA primers needed in DNA replication?
DNA polymerase cannot add bases w/o a free 3’ OH group, so RNA primers are synthesized to start replication
what is the role of primase?
to add RNA bases in a 5’ to 3’ direction, copying from the template strand (this is followed by DNA polymerase using the 3’ OH on the last base to continue synthesis)
leading strand
the new complementary DNA strand synthesized continuously along the template strand toward the replication fork in the mandatory 5’ to 3’ direction
lagging strand
discontinuously synthesized DNA strand (stepwise fashion) that elongates by means of Okazaki fragments, each synthesized in a 5’ to 3’ direction away from the replication fork
what occurs after Okazaki fragments are synthesized?
RNA primers used for synthesis are removed and are replaced with DNA
what is the purpose of ligase?
final binding of nicks in DNA during synthesis and repair
telomeres
repeated DNA sequences at the ends of eukaryotic chromosomes; exist so that when chromosomes are replicated and the lagging strand is shorter, replication does not result in shortening of the coding regions
(cells age as telomeres shorten)
telomerase
an RNA-dependant DNA polymerase enzyme that catalyzes the lengthening of telomeres; includes a complex of proteins, RNA and enzyme that lengthens ends of chromosomes
what does telomerase contain?
reverse transcriptase (similar to polymerase used by HIV)
how does reverse transcriptase work?
a single strand of RNA is reverse transcribed to single stranded DNA, (complementary DNA/cDNA)
what is the purpose of DNA repair?
prevent the accumulation of mutations (e.g. large side groups causing errors)
what are the steps of DNA repair?
- damaged DNA is recognized
- enzymes that break down the phosphodiester bond remove the damaged area
- a DNA polymerase designed to repair rebuilds the damaged area
- DNA ligase seals the nicks
DNA repair mechanisms
any of several processes by which enzymes repair DNA damage
nucleotide excision repair (NER)
a DNA repair system in which several nucleotides in the damaged strand are removed from the DNA and the undamaged strand is used as a template to resynthesize a normal strand (this is defective in Xeroderma pigmentosum)
