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Terms in this set (34)
8 components requiremed
-DNA POL III
Properties of Replication
since strand of double helix are antiparralel and complementary, replication must produce a strand that is antiparallel and complementary to the template strand
is copied and directs the synthesis of the complementary DNA sequence.
hat is the initial sequence of DNA at the 5' end of the new DNA. The primer provides a 3' hydroxyl that attacks the incoming nucleotide. (RNA synthesis does not require a primer.)
enzyme catalyzes energy dependent strand separation.
unwinds the dna double helix into individual strandstwo
-Single strand binding proteins (SSB's
help keep the strands apart after they are separated.
-Four deoxynucleotide triphosphates
that provide the nucleoside monophosphate unit that is added to the growing chain of DNA.
coat single stranded dna prevents reannealing to double helix dna
enzyme, which is a complex of enzymes that catalyze the process of DNA elongation.
DNA polymerase is a hand-shaped enzyme that strings nucleotides together to form a DNA strand. The sliding clamp is an accessory protein that helps hold the DNA polymerase onto the DNA strand during replication
collectively relax the supercoiled DNA that is created by the action of the helicase.
creates an RNA primer region in lagging strand fragments. This provides the starting material to which DNA polymerase III can add segments of DNA.
Primase is an RNA polymerase that synthesizes the short RNA primers needed to start the strand replication process.
nucleotide triphosphates (NTP's)
provide energy for the action of helicase and topoisomerase.
DNA Polymerase I
removes the RNA primer nucleotides from the lagging strand segments and replaces them with the appropriate deoxynucleotides.
closes the gaps between segments of lagging strand DNA.
DNA ligase links short stretches of DNA together to create one long continuous DNA strand.
How its copied
each of two template strands is copied and becomes half a new double helix
-To begin the process of DNA replication, the two double helix strands are unwound and separated from each other by the helicase enzyme.
-The point where the DNA is separated into single strands, and where new DNA will be synthesized, is known as the replication fork.
-Single-strand binding proteins, or SSBs, quickly coat the newly exposed single strands.
-SSBs maintain the separated strands during DNA replication.
-Without the SSBs, the complementary DNA strands could easily snap back together.
-SSBs bind loosely to the DNA, and are displaced when the polymerase enzymes begin synthesizing the new DNA strands.
New Strand Synthesis
To begin the process of DNA replication, the two double helix strands are unwound and separated from each other by the helicase enzyme. The point where the DNA is separated into single strands, and where new DNA will be synthesized, is known as the replication fork. Single-strand binding proteins, or SSBs, quickly coat the newly exposed single strands. SSBs maintain the separated strands during DNA replication. Without the SSBs, the complementary DNA strands could easily snap back together. SSBs bind loosely to the DNA, and are displaced when the polymerase enzymes begin synthesizing the new DNA strands.
Even when the strands are separated, however, DNA polymerase cannot simply begin copying the DNA. DNA polymerase can only extend a nucleic acid chain but cannot start one from scratch. To give the DNA polymerase a place to start, an RNA polymerase called primase first copies a short stretch of the DNA strand. This creates a complementary RNA segment, up to 60 nucleotides long that is called a primer.
Now DNA polymerase can copy the DNA strand. The DNA polymerase starts at the 3' end of the RNA primer, and, using the original DNA strand as a guide, begins to synthesize a new complementary DNA strand. Two polymerase enzymes are required, one for each parental DNA strand. Due to the antiparallel nature of the DNA strands, however, the polymerase enzymes on the two strands start to move in opposite directions.
One polymerase can remain on its DNA template and copy the DNA in one continuous strand. However, the other polymerase can only copy a short stretch of DNA before it runs into the primer of the previously sequenced fragment. It is therefore forced to repeatedly release the DNA strand and slide further upstream to begin extension from another RNA primer. The sliding clamp helps hold this DNA polymerase onto the DNA as the DNA moves through the replication machinery. The sliding clamp makes the polymerase processive.
The continuously synthesized strand is known as the leading strand, while the strand that is synthesized in short pieces is known as the lagging strand. The short stretches of DNA that make up the lagging strand are known as Okazaki fragments.
THE LAGGING STRAND
Before the lagging-strand DNA exits the replication factory, its RNA primers must be removed and the Okazaki fragments must be joined together to create a continuous DNA strand. The first step is the removal of the RNA primer. RNAse H, which recognizes RNA-DNA hybrid helices, degrades the RNA by hydrolyzing its phosphodiester bonds. Next, the sequence gap created by RNAse H is then filled in by DNA polymerase which extends the 3' end of the neighboring Okazaki fragment. Finally, the Okazaki fragments are joined together by DNA ligase that hooks together the 3' end of one fragment to the 5' phosphate group of the neighboring fragment in an ATP- or NAD+-dependent reaction.
The process begins when the helicase enzyme unwinds the double helix to expose two single DNA strands and create two replication forks. DNA replication takes place simultaneously at each fork. The mechanism of replication is identical at each fork. Remember that the proteins involved in replication are clustered together and anchored in the cell. Thus, the replication proteins do not travel down the length of the DNA. Instead, the DNA helix is fed through a stationary replication factory like film is fed through a projector.
Single-strand binding proteins, or SSBs, coat the single DNA strands to prevent them from snapping back together. SSBs are easily displaced by DNA polymerase.
The primase enzyme uses the original DNA sequence as a template to synthesize a short RNA primer. Primers are necessary because DNA polymerase can only extend a nucleotide chain, not start one.
DNA polymerase begins to synthesize a new DNA strand by extending an RNA primer in the 5' to 3' direction. Each parental DNA strand is copied by one DNA polymerase. Remember, both template strands move through the replication factory in the same direction, and DNA polymerase can only synthesize DNA from the 5' end to the 3' end. Due to these two factors, one of the DNA strands must be made discontinuously in short pieces which are later joined together.
As replication proceeds, RNAse H recognizes RNA primers bound to the DNA template and removes the primers by hydrolyzing the RNA.
DNA polymerase can then fill in the gap left by RNase H.
The DNA replication process is completed when the ligase enzyme joins the short DNA pieces together into one continuous strand.
replication occurs from the 5 oh end toward the 3 oh end of the strand being synthesized
this directionality of the synthesis is the consequence of the mechanism of synthesis, names attack of the 3 oh of the primer on the alpha phosphate of the incoming dntp
new subunits are added
at the 3 end
dna is synthesized
in the 5 3 direction only
Basics of DNA replication 1
-each strand must be copied once
-replication begins in a special seuqnee called origin
-prok = 1 origin
from the origin replication proceeds bidirectionally this yield two site where dna is synthesized called replication forks
Basics of Dna replication2
-two strand are replicated differently because dna polymerase can only add on a to a 3 end
-for one strand 5-->3 is in the same direction as the fork
- the other strand 5--. 3 is in the opposite direction
-all dna polymerases require a primer of about 20 nucleotides
-many proteins are involved in replication:
some are required for initiation
cells contain multiple dna polymers
-the entire process has a very low error rate. this is part because DNA polymerase has proofreading body
nucleotides can not be added to the 5 end
This means that to strand can't be replicated in the identical matter
dna polymerase can only synthesize dna in the 5 3 direction
leading strand: synthesis goes in the same direction as the replication fork is moving:no problem
lagging strand: synthesis proceed in the opposite direction from fork movement. this strand must be made as a series of fragments called Okazaki fragments
more nte in pork then elk
because dna polymerase can only act in the 5 3 direction replication of one strand has to be discontinuous
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