DNA Replication

Strand Seperation

-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.

Basics of DNA replication 1

-each strand must be copied once -replication begins in a special seuqnee called origin -prok = 1 origin -euk=multiple from the origin replication proceeds bidirectionally this yield two site where dna is synthesized called replication forks

8 components requiremed

-template-old strands -helicase -ssbs -DNA POL III - DNTPs -Primase (NTPS) -DNA polymerase -Ligase

Review 5

As replication proceeds, RNAse H recognizes RNA primers bound to the DNA template and removes the primers by hydrolyzing the RNA.

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.

Review 6

DNA polymerase can then fill in the gap left by RNase H.

Next

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.

Review 7

The DNA replication process is completed when the ligase enzyme joins the short DNA pieces together into one continuous strand.

new subunits are added

at the 3 end

Ligase

closes the gaps between segments of lagging strand DNA. DNA ligase links short stretches of DNA together to create one long continuous DNA strand.

e Primase

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.

primer

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.)

-Single strand binding proteins (SSB's

help keep the strands apart after they are separated.

dna is synthesized

in the 5 3 direction only

Next

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.

Next

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.

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

Review 4

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.

Next

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.

Review 1

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.

Review 2

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.

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.

Topoisomerases

collectively relax the supercoiled DNA that is created by the action of the helicase.

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

Helicase

enzyme catalyzes energy dependent strand separation. unwinds the dna double helix into individual strandstwo

DNA polymerase

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

Dna Template

is copied and directs the synthesis of the complementary DNA sequence.

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.

Directionality

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

How its copied

semi conservative each of two template strands is copied and becomes half a new double helix

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

Review 3

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.

-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