chapter 6 Flashcards
The process of DNA synthesis is begun by _ that bind to specific DNA sequences called replication origins.
initiator proteins
DNA polymerase is so accurate that it makes only about one error in every _ nucleotide pairs it copies.
10^7
when DNA polymerase does make a rare mistake and adds the wrong nucleotide, it can correct the error through an activity called _.
proofreading
proofreading mechanism is possible only for DNA polymerases that synthesize DNA exclusively in the _ direction, otherwise, it would be _
5’-to-3’
unable to proofread
when a nucleotide is removed from the 5’ end, the remaining strand lacks the necessary chemical group (-_) on the 5’ end for the next nucleotide to be added. Without this group, the polymerase _, so the process is blocked, making it a _ for further synthesis.
OH
can’t continue building the DNA
dead end
Thus, for a DNA polymerase to function as a self-correcting enzyme that removes its own polymerization errors as it moves along the DNA, it must proceed only in the _ direction.
5’-to-3’
An _ fragment thus serves as a _ for DNA synthesis, and the enzyme that synthesizes the RNA primer is known as _.
RNA
primer
primase
Enzymes called DNA _ relieve this ten- sion. A DNA _ produces a transient, single-strand _ in the DNA backbone, which temporarily releases the built-up tension; the enzyme then reseals the _ before falling off the DNA
topoisomerases
topoisomerase
nick
nick
purine bases (_)
A and G
The rep- lication machine makes approximately one mistake per 10^7 nucleotides synthesized; DNA mismatch repair corrects 99% of these replication errors, increasing the overall accuracy to one mistake in _ nucleotides synthesized.
10^9
a _ is especially difficult to repair.
double-strand break
double-strand break can be repaired by:
nonhomologous end joining
homologous recombination
Homologous recombination most often occurs _ before _
shortly after a cell’s DNA has been replicated
cell division
Before a cell divides, it must _.
accurately replicate the vast quantity of genetic information carried in its DNA
Because the two strands of a DNA double helix are _, each strand can act as a _ for the synthesis of the other. Thus DNA replication produces _, double-helical DNA molecules, enabling genetic information to be copied and passed on from a cell to its _ and from a parent to its _.
complementary
template
two identical
daughter cells
offspring
During replication, the two strands of a DNA double helix are _ at a _ to form two _. DNA _ at each fork produce a new, _ DNA strand on each _.
pulled apart
replication origin
Y-shaped replication forks
polymerases
complementary
parental strand
DNA polymerase replicates a DNA template with remarkable _, making only about one error in every _ nucleotides copied. This accuracy is made possible, in part, by a _ process in which the enzyme _.
fidelity
_
10^7
_
proofreading
_
corrects its own mistakes as it moves along the DNA
Because DNA polymerase synthesizes new DNA in the _, only the _ at the _ can be synthesized in a _ fashion. On the _, DNA is synthesized in a _ process, producing _ that are later joined together by _.
5’-to-3’ direction
leading strand
replication fork
continuous
lagging strand
discontinuous backstitching
short fragments of DNA (Okazaki fragments)
DNA ligase
DNA polymerase is _ of starting a new DNA strand from scratch. Instead, DNA synthesis is _ called _, which makes short lengths of _ that are then elongated by _. These _ are subsequently _ and _.
incapable
primed by an RNA polymerase
primase
RNA primers
DNA polymerase
primers
removed
replaced with DNA
DNA replication requires the _ that form a _ that _ the double helix and _ the information contained in both DNA strands.
cooperation of many proteins
multienzyme replication machine
pries open
copies
In eukaryotes, a special enzyme called _ replicates the DNA at the ends of the chromosomes, particularly in rapidly dividing cells.
telomerase
The rare copying mistakes that escape _ are dealt with by _, which increase the accuracy of DNA replication to one mistake per _ nucleotides copied.
proofreading
mismatch repair proteins
10^9
Damage to one of the two DNA strands, caused by _, is repaired by a variety of _ that _ damaged DNA and _. The _ DNA is then resynthesized by a _, using the _ as a template.
unavoidable chemical reactions
DNA repair enzymes
recognize
excise a short stretch of the damaged strand
missing
repair DNA polymerase
undamaged strand
If both DNA strands are broken, the double-strand break can be rapidly repaired by nonhomologous end joining. Nucleotides are _, _.
often lost in the process
altering the DNA sequence at the repair site
Homologous recombination can _ repair double-strand breaks (and many other types of DNA damage) using an _ as a template.
flawlessly
undamaged homologous double helix
Highly accurate DNA replication and DNA repair processes play a key role in protecting us from the _.
uncontrolled growth of somatic cells known as cancer
each strand of a DNA double helix—labeled here as the _ strand and its complementary _ strand—can serve as a template to specify the sequence of nucleotides in its _.
S
S’
complementary strand
A DNA double helix is opened at _.
replication origins
DNA sequences
at replication origins are recognized by _, which _. The exposed single strands can then serve as _ for copying the DNA.
initiator proteins
_
locally pull apart the two strands of the double helix
_
templates
The _ formed at a replication origin move _.
two replication forks
away in opposite directions
The particles visible along the DNA are _, structures made of _.
nucleosomes
_
DNA and the histone protein complexes around which the DNA is wrapped
At each replication fork, the lagging DNA strand is synthesized in _.
pieces
Because both of the new strands at a replication fork are synthesized in the _ direction, the lagging strand of DNA must be made initially as _, which are later _
5’-to-3’
a series of short DNA strands
joined together.
To replicate the lagging strand, DNA polymerase uses a _ mechanism: it synthesizes _ (called _ fragments) in the 5-to-3 direction and then _.
backstitching
_
short pieces of DNA
_
Okazaki
_
moves back along the template strand (toward the fork) before synthesizing the next fragment
DNA polymerase contains separate sites for DNA _
and _.
synthesis
proofreading
When the polymerase adds an incorrect nucleotide, the newly synthesized DNA strand (red ) transiently _ from the template strand (orange), and its _ to allow the incorrect nucleotide to be removed.
unpairs
3’ end moves into the editing site (E)
DNA polymerases are held on the leading- and lagging-strand templates by _ that allow the polymerases to slide.
circular protein clamps
On the lagging-strand template, the clamp _.
detaches each time the polymerase completes an Okazaki fragment
A _ is required to attach a sliding clamp each time a new Okazaki fragment
is synthesized.
clamp loader
At the head of the fork, a_ unwinds the strands of the parental DNA double helix.
DNA helicase
_ keep the DNA strands apart to provide access for the primase and polymerase.
Single-strand DNA-binding proteins
the lagging strand shown in (A) has been _ to bring its DNA polymerase in _ with the leading-strand DNA polymerase. This folding process also brings the _ close to the start site for the _.
folded
contact
3’ end of each completed Okazaki fragment
next Okazaki fragment
_ relieve the tension that builds up in front of a replication fork.
DNA topoisomerases
DNA topoisomerases _.
relieve the tension that builds up in front of a replication fork
As a DNA helicase moves forward, unwinding the DNA double helix, it generates a section of _ DNA ahead of it.
overwound
Some of this torsional stress
is relieved by additional coiling of the
DNA double helix to form _.
supercoils
