Ch 4,11 DNA Replication

1928 - Griffith

• Transforming principle - a change in the genotype and phenotype due to assimilation of external cellular material by a cell

1952 - Hershey Chase

• radioactive labeled DNA from T2 phages to show virus injects DNA into the cell rather than the protein

1952 - Chargaff

Base Ratios

A = T

G - C

A + G = T + C

1952 - Franklin & Wilkins

suggested DNA is made of 2 strands with sugar-phosphate backdone

1953 - Waston & Crick

DNA is a double helix and established base pairing rules

DNA structure

• double stranded alpha helix

• antiparallel

• base faces interior and hydrogen bonds with one another

A:T = 2 hydrogen bonds

C:G = 3 hydrogen bonds

• purine binds with pyrimidine

• sugar phosphate groups make up the backbone

DNA Replication is Semiconservative

two strands of DNA - complementary, serves as a template to make a complementary strand

• Waston & Crick

DNA Replication - Step 1

The double strand must be separated from one another

DNA Replication - Step 2

The two strands must be prevented from reforming H bonds

DNA Replication - Step 3

the two strands must be duplicated (each strand serving as a template due to the base pairing rule)

DNA Replication - Step 4

Duplicated DNA must remain together

DNA Replication - Step 5

mistakes/damage must be repaired to prevent introduction of mutations

replication bubble

a region of DNA where the double helix has unwound and separated to allow for DNA replication

replication fork

a Y shaped structure that forms during DNA replication where the double-stranded DNA molecule is unwound and separated allowing for the synthesis of new DNA strands

helicases

are enzymes that untwist the double helix at the replication forks

single-strand binding proteins

bind to and stabilizes single-stranded DNA

Topoisomerase

relieves the strain of twisting of the double helix by breaking, swiveling, and rejoining DNA strands

primase

synthesizes a RNA primers and DNA strands starts from 3’end of RNA primer

DNA polymerases

catalyze the synthesis of new DNA at a replication fork

• nucleotides can only be added to the free 3’ end of a growing DNA strands

leading strand

moving towards the replication fork

lagging strand

DNA polymerase must work in the direction away from the replication fork

Okazaki fragments

the lagging strand is synthesized as a series of segments called

DNA ligase

an enzyme that joins together two DNA fragments by forming a phosphodiester bond, effectively sealing a gap in the DNA backbone

• forms bonds between DNA fragments

DNA pol 1

replaced the RNA primers with DNA

telomerase

lengthens telomeres in germ cells

mismatch repair

repair enzymes corrects errors in base pairing

nucleotide excision repair

nuclease cuts out and replaces damaged stretches of DNA