DNA replication Flashcards
What does DNA replication do and when does it happen?
Produces two identical copies of the
chromosome during S phase of
interphase.
DNA replication is catalyzed by many..
enzymes
The structure of DNA allows it to be easily…….
replicated
_________ between base pairs can be broken (unzipped) and each strand acts as a ________ for adding new nucleotides.
H-bonds, template
What are the three steps of replicating DNA?
Separating the DNA strands
Building complementary strands
Linking of Nitrogenous Bases
What happens in the first step of DNA replication, separating the DNA strand?
Enzymes and other proteins work
together to unwind and stabilize the
double helix
What are the players/enzymes in step one (separating the DNA strand) in DNA replication?
DNA helicase
DNA gyrase
SSBs
What does DNA helicase do?
Recognizes specific nucleotide
sequence (origin of replication)
Unwinds double helix by breaking H
bonds between complementary base
pairs
Opens up replication bubble
What does DNA Gyrase do?
Relieves the tension produced by
the unwinding of DNA
What does SSBs (Single-stranded Binding Proteins) do?
Keep separated DNA strands apart
by blocking hydrogen bonding
Keep the templates (single DNA
strands) straight
What is the replication fork?
The junction where the DNA strands
are still joined
What happens during the second step of DNA replication (Building complementary strands)?
Replication begins in two directions
from the origin(s) as a region of DNA is unwound
Replication proceeds towards the
replication fork on one strand, and away from the fork on the other strand.
Because there may be
more than one origin of
replication in eukaryotes,
more than one ___________ _____ may exist.
replication fork
What are the players in the second step of DNA replication (Building complementary strands)?
DNA polymerase
primase
DNA ligase
What does DNA polymerase III do?
Takes free nucleotides found
within the cell and adds them in the
5’ to 3’ direction
The parent strand is used as a
template
What is the leading strand what does it do?
The daughter strand that grows
continuously towards the replication
fork as the double helix unwinds
Occurs quickly
Requires a single RNA primer
What is the lagging strand in DNA replication?
Built in short segments (in the 5’ to 3’ direction) away from the replication fork
Requires many RNA primers
Why is the 3’ to 5’ strand a problem for DNA polymerase?
(More details for lagging strand.)
The 3’ to 5’ parent strand is a problem for DNA polymerase since it must synthesize in the 5’ to 3’ direction.
What does Primase do in the second step of DNA replication (Building complementary strands)?
Builds RNA primers (used to initiate DNA replication)
What are Okazaki Fragments?
Short fragments of DNA built by
DNA polymerase off of the RNA
primers
What does DNA Polymerase I do in step two of DNA replication?
Removes the RNA primers once they
have been used and replaces them with the appropriate DNA sequence
What does DNA ligase do in step two of DNA replication?
Joins the Okazaki fragments into one
strand by the creation of phosphodiester bonds
What happens in step 3 of DNA replication (Linking Nitrogenous Bases) ?
Nitrogen bases of nucleotides of
opposite strands (parent and daughter)
form new H bonds
Once bonds are formed, DNA twists
automatically into a double helix
Why is DNA polymerase consider the quality controller enzyme thingy
DNA polymerase acts as a
proof-reader by checking the newly
synthesized strand for any incorrectly inserted bases
A human cell can copy all of its DNA in a couple of hours
with an error rate of __ ____ _________ nucleotide pairs.
1 per billion
The most common type of error is mispairing, such that a __
is paired with __.
T is paired with G
Another error is caused by _______ _________ during
replication.
strand slippage
What does strand slippage result in?
This can result in addition or deletion of nucleotides in the newly synthesized strand.
Most polymerases have the ability to ___________.
proofread.
When an incorrect nucleotide is placed on the newly
synthesized strand, replication is stalled as the __’ end is in
the wrong position for the next to attach.
3’
The two strands of DNA are..
Antiparallel
What holds the nucleotides together?
H-bonds
How many bonds between A and T? C and G?
2 h-bonds for A and T. 3 h-bonds for C and G
What is Exonuclease enzyme?
It is the enzyme that removes nucleotides from a strand, starting at one end of the strand. If a mistake is found, it immediately cuts out the mistake
What are Okazaki Fragments?
Short fragments of DNA built by
DNA polymerase off of the RNA
primers
DNA polymerase acts as a
proof-reader by checking the newly
synthesized strand for any incorrectly inserted bases
If a mistake is found, act as an
____________ – & cut out the mistake
exonuclease
What is another mechanism that corrects errors in both eukaryotes and prokaryotes that were not fixed
during proofreading?
Mismatch repair
What are Telomeres?
Telomeres are long, non-coding, highly-repetitive sequences of DNA at the ends of eukaryotic chromosomes.
Video for DNA replication
https://www.youtube.com/watch?v=3jslVQDGkLU
Why are telomeres necessary?
They are necessary as DNA synthesis can only occur in the
5’-3’ direction.
The length of the telomeres
is closely linked to the
lifespan of an __________.
organism
Telomerase is enzyme that
can synthesize _________ _________ that have been lost.
telomeric regions
Telomerase activity high
during __________ when
tissues are growing rapidly.
Telomerase activity slows in
_______ _______ as
people age.
childhood, somatic cells (body)
If we could maintain high telomerase activity, we would
___ _____ ________.
age much slower.
High telomerase activity is what allows for ________ _______ ____ _______. High telomerase activity allows for the tumor cells to continue to ________ as important genes would not be lost.
cancerous tumors to grow, replicate
What do telomeres prevent?
They prevent the loss of important DNA during replication, as the copying process can’t fully replicate the very ends of chromosomes.
Over time, telomeres shorten, which is linked to …..
aging and cell death.
How does DNA replicate?
Back to interphase.
In mitosis the ______ DNA molecule is replicated, but for RNA synthesis, only ________ of the DNA are replicated.
entire, portions
In DNA replication, it is controlled by the enzyme called
DNA polymerase
