DNA replication

Replication Requirements

-a template of the DNA (leading/lagging strands) to be copied. (from double helix. each strand will serve as a template. -an enzyme (DNA polymerase) to make a copy of the template strands (makes new DNA from template strand) - an RNA primer (made from DNA primase) to initiate DNA replication -building blocks (nucleotides) to assemble the polymers (nucleic acids

In what direction is DNA synthesized?

5' to 3'

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If damage of DNA occurs during replication, Rate of synthesis slows down so that DNA can be repaired. If one strand slows down to repair DNA damage so will the other. When RNA primer is encountered lagging strand slows down so leading strand slows down too until primer is removed by DNA pol 1 complex.

Prokaryotic DNA replication

starts in a single spot and goes around in 2 directions until the entire chromosome is copied - bacteria have a single, double-stranded, circular chromosome - DNA replication is initiated at the origin or replication (oriC) - DNA elongation (of both strands of parental DNA) ensues bi-directionally - DNA replication ends at a termination site

DNA primase

synthesis of RNA primer on the 5' end which then initiates dna synthesis -the rna primer is complementary to the dna template strand

DNA polymerase III

synthesizes new DNA only in the 5' to 3' direction -goes towards the replication fork. - 2 one on each strand - beta clamps hold pol on strand

parent strands

the DNA strands which serve as a template for DNA replication each individual template strand serve as a template strand for the synthesis of a daughter strand *also antiparallel

Telomere

repetitive DNA at the end of a eukaryotic chromosome (G/T)

dispersive replication

replication results in both original and new DNA dispersed among the two daughter strands

Semidiscontinuous Process (lagging strand)

- Pol III can only add nucleotides to the 3' end of strands - DNA strands are antiparallel to each other - leading strand is synthesized continuously - lagging strand is synthesized discontinuously

EUKARYOTIC DNA REPLICATION

- eukaryotic chromosomes have multiple origins of replication - more complex DNA replication enzymes than prokaryotes - synthesizing the ends of the chromosomes is difficult because of the lack of a primer; linear chromosomes shorten with each round of replication removing primer causes overhang

DNA pol 2

- requires a primer - adds nucleotides in the 5' - 3' direction - has 3' - 5' exonuclease activity (proofreading)

DNA pol 1

- requires a primer - adds nucleotides in the 5' - 3' direction - has 3' - 5' exonuclease activity (proofreading) - has 5' - 3' exonuclease activity (primer removal*)

DNA pol 3

- requires a primer - adds nucleotides in the 5' - 3' direction (major polymerase) - has 3' - 5' exonuclease activity (proofreading)

lagging strand

A discontinuously synthesized DNA strand that elongates by means of Okazaki fragments, each synthesized in a 5' to 3' direction away from the replication fork. -5' side is facing fork

DNA ligase

A linking enzyme essential for DNA replication; catalyzes the covalent bonding of the 3' end of a new DNA fragment to the 5' end of a growing chain. -joins ends of dna segments -joins after dna pol 1 removes primer -forms last phosphodiester bond

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After replacement fragments are joined by enzyme called DNA ligase

Telomerase

An enzyme that catalyzes the lengthening of telomeres in eukaryotic germ cells. (fixes linear chromosomes shortening) -linear molecules have ends. after each round of replication, the ends shorten because of the removal of the rna primer on the 5' end of the lagging strand. this is fixed by a repeating sequence called the telomere (G/T) which is made by telomerase (A/C) -telomerase is a rna template complementary to telomere sequence adds NT's to 3' end

DNA helicase

An enzyme that unwinds the DNA double helix during DNA replication by breaking H bonds

DNA structure

DNA consists of two long chains of nucleotides twisted into a double helix and joined by hydrogen bonds between the complementary bases adenine and thymine or cytosine and guanine

Proofreading

DNA pols can correct errors. it detects mispaired nucleotides and removes it and inserts the correct one -reduces mutations

meselson and stahl

Determined that DNA replication is semiconservative. 1) both strand were labeled with N15 (heavy) 2) the heavy medium was removed. the parental strands still contain N15 but the daughter strand contains the lighter N14 3) after two rounds, half duplex will have one strand with N15 and one with N14. the other half will have strands (both) N14 4) two bands are formed. one intermediate and one light.

1

Each new DNA strand must begin with a short stretch of of RNA that serves as a primer for DNA synthesis. The primer is needed because DNA polymerase complex (enzyme that makes copy of template strand) cannot begin a new strand on its own it can only elongate. this primer is made by DNA primase

semiconservative replication

Method of DNA replication in which parental strands separate, act as templates, and produce molecules of DNA with one parental DNA strand and one new DNA strand (daughter) - each new DNA duplex consists of one strand that was originally part of the parental duplex and one newly synthesized strand.

2

Once the primer is made, DNA polymerase takes over and elongates the primer, adding DNA nucleotides to the three prime end of growing strand. All new DNA strands have a short stretch of RNA at the five prime end. There are many primers on the lagging strand.

DNA pol I

Removes RNA nucleotides of primer from 5' end and replaces them with DNA nucleotides (fills gap)

the leading strand

The new continuous complementary DNA strand synthesized along the template strand in the mandatory 5' to 3' direction. -daughter strand in which the 3' is facing the replication fork. continuous because nucleotides are added to the 3' side

3

To ensure that both daughters strands occur at the same time, as each new primary for an okazaki fragment is synthesized, the lagging strand forms a loop so that both DNA polymerases 3 (on each strand) can move in the same direction. So with the loop on the lagging strand the three prime end of the lagging strand and the three prime end of the leading strands are elongated together. The loop persists until new lagging strand encounters primer of previous fragment

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When the fragment comes in contact with the primer on the fragment in front of it, a different DNA polymerase complex (1) removes the RNA primer and extends the growing fragment with DNA nucleotides to fill the space left by the removal of the RNA primer

the replisome creates

a loop in the lagging strand so both pols 3 can run in the same direction. - the loop in the lagging strand template allows replication to occur 5' to 3' on both strands

DNA strands are

antiparallel

conservative replication

the parental molecule serves as a template for the synthesis of an entirely new molecule. -original DNA duplex remains intact and the daughter DNA is completely new.

single stranded binding proteins

bind to and stabilize single-stranded DNA - hold the strands apart so they dont reanneal.

DNA strands can only grow

by the addition of nucleotides to the 3' end

results of this experiment

half dna molecule will have one heavy old strand and one light new strand and an intermediate density. half the dna molecule will have one light old and one light new and a light density.

what does DNA pol 3 add to the 3' end

nucleotide triphosphate

How are DNA connected?

phosphodiester bond links the 3' OH of one DNA to the 5' carbon of another through the 5' phosphate group.

replication fork

region where parental strands are separated and new strands are being synthesized

DNA gyrase (topoisomerase)

relax supercoiling -prevents opposite side of DNA from supercoiling -on opposite side of replication fork