Unit 6 - topic 2 Flashcards
DNA replicates during
the S phase of the cell cycle
3 alternative models for DNA replication
conservative
semi-conservative
dispersive
conservative model
the parental strands direct synthesis of an entirely new double stranded molecule
the parental strands are fully conserved
semi-conservative model
the two parental strands each make a copy of itself
after one round of replication, the two daughter cells each have one parental and one new strand
dispersive model
material in the two parental strands in randomly dispersed between the two daughter molecules
after one round of replication, daughter molecules contain a random mix of parental and new DNA
in 1954 Meselson and Stahl
performed an experiment using bacteria
process of Meselson and Stahl’s experiment
- bacteria was cultured with a heavy isotope, 15N
- bacteria was transferred to a medium with a light isotope, 14N
- DNA was centrifuged and analyzed after each replication
by analyzing samples of DNA after each generation
it was found that the parental strands were following the semi-conservative model
DNA replication begins at sites called
origins of replication
various proteins will attach to the origin of replication and open the DNA to form a
replication fork
replication fork
prime spot for action in DNA replication
helicase
unwinds the DNA strands at each replication fork
to keep the DNA from re-bonding with itself
proteins called single stranded binding proteins (SSBPs) bind to the DNA to keep it open
topoisomerase
will help prevent strain ahead of the replication fork by relaxing supercoiling
primase
an enzyme that initiates replication by adding short segments of RNA called primers to the parental DNA strand
the enzymes that synthesize DNA can only attach new DNA nucleotides to
an existing strand of nucleotides
primers serve as the
foundation for DNA synthesis
DNA Polymerase III
attaches to each primer on the parental strand and moves in the 3’ to 5’ direction
as DNAP III moves, it adds
nucleotides to the new strand in the 5’ to 3’ direction
the DNAP III that follows helicase is known as the
leading strand and it only requires one primer
the DNAP III on the parental strand that moves away from helicase is known as the
lagging strand and requires many primers
leading strand
synthesized in one continuous segment
since the lagging strand moves away from the replication fork
it is synthesized in chunks
okazaki fragments
segments of the lagging strand
after DNAP III forms an okazaki fragment
DNAP I replaces RNA nucleotides with DNA nucleotides
DNA ligase
joins the okazaki fragments forming a continuous DNA strand
since DNAP III can only add nucleotides to a 3’ end
there is no way to finish replication on the 5’ end of a lagging strand
over many replications of DNA
the DNA will become shorter and shorter
telomeres
repeating units of short nucleotide sequences that do not code for genes
form a cap at the end of DNA to help postpone erosion
telomerase
enzyme that adds telomeres to DNA
as DNAP adds nucleotides to the new DNA strand
it proofreads the bases added
if errors still occur
mismatch repair will take place
enzymes remove and replace the incorrectly paired nucleotide
if segments of DNA are damaged
nuclease can remove segments of nucleotides and DNAP and ligase can replace the segments
