Gentics Chap 12

Genetics Chap 12

replication

-must be extremely accurate

-takes place at high speed

proposed DNA replication models

-conservative

-semiconservative

-dispersive

Meselson’s & Stahl’s ecperiment

-2 isotopes of nitrogen

-14N - normal form

-15 N -rare, heavy form

semiconservative

All DNA replications takes place in a _____ manner

theta replication

circular DNA, single origin of replication forming a replication fork, & it is usually a bidirectional replication; 2 circular molecules

rolling circle replication

virus; single origin of replication; circular; unidirectional; 1 circular molecule and 1 linear molecule that may circularize

-requires a break in the nucleotide strand to get started

LInear Eukaryotic Replication

linear; many replicons, bidirectional; 2 linear molecules

-requirements of replication

• template strand

• raw material: nucleotides (dNTPs)

• enzymes and other proteins

-direction of replication: DNA polymerase adds nucleotides only to the 3’ end

-replication can only go from 5’ → 3

-continous and discontinuous replication

eukaryotes- many origins of replication; prokaryotes- single origin of replication

DNA réplication in eukaryotes and prokaryotes differs in replication..

leading strand

undergoes continuous replication

lagging strand

undergoes discontinuous replication

ozaki fragments

discontinuously synthesized short DNA fragments forming the lagging strand

DNA helicase

unwinds DNA by binding to the lagging- strand template at each replication fork and moving in the 5’→ 3” direction

order they go

1. initiator protein

2. helicase

3. single-strand binding protein

4. DNA gyrase

short stretch of RNA nucleotides

All DNA polymerases require a primer with a 3’ OH group to begin DNA synthesis. The primer is a …

elongation

carried out by DNA polymerase III

each active replication fork requires 5 basic components

1. helicase

2. single strand binding proteins

3. DNA gyrase

4. primase

5. DNA polymerase

helicase

to unwind the DNA

single strand binding proteins

to protect the single nucleotide strands and prevent secondary structures.

DNA gyrase

to remove strain ahead of the replication fork

Primase 

to synthesize primers with a 3’ OH group at the beginning of each DNA fragment

DNA polymerase

to synthesize the leading and lagging nucleotide strands

Removing RNA primer

DNA polymerase I

DNA ligase

connecting nicks after RNA primers are removed

-seals the nick left by the DNA polymerase I in the sugar-phosphate backbone

-joins ozaki fragments

termination

when replication fork meets termination protien.

DNA polymerase I

removes and replaces primers

DNA polymerase III

elongates DNA

all dna polymerases

1. synthesized any sequence specified by the template strand

2. synthesize in the 5’→ 3’ direction by adding nucleotides to the 3’OH group

3. use dNTPs to synthesize new DNA

4. Require a 3’OH group to initiate syntheisize

5. Catalyze the formation of a phosphodiester bond by joining the 5’-phosphate group of the incoming nucleotide to the 3’-OH group of the preceding nucleotide on the growing strand, cleaving off 2 phosphates in the process

6. Produce newly synthesized strands that are complementary and antiparallel to the template strands

7. Are associated w/ a # of other proteins

Ligase

Seals the nick left by DNA polymerase I in the sugar-phosphate backbone

Initiator protein

Binds to origin and separates strands of DNA to initiate replication

Helicase

Unwinds DNA at replication fork

Single strand binding proteins

Attach to single stranded DNA and prevent secondary structures from forming

DNA primase

Synthesizes a short RNA primer to provide a 3’OH group for the attachment of DNA nucleotides

Proofreading

DNA polymerase exonuclease actively removes the incorrectly paired nucleotide

-provided by dna polymerase I and III

Mismatch repair (post replication)

• enzymes detect structural distortions in dna

• Removed mismatched bases from the new (unmethylated) strand. Old strand is methylated.

• Further reduces overall error rate

Eukaryotes

Do not need initiator protein

Creation of nucleosomes requires

• Disruption of original nucleosomes on the parental DNA

• Redistribution of preexisting histones on the parental dna

• The addition of newly synthesized histones to complete the formation of new nucleosomes

Chromosomes would shorten each generation

What would be the result if an organisms telomerase were mutated and not fuctional

Homologous recombination

Exchange is between homologous dna molecules during crossing over; increases genetic variation

Models of recombination

• Holiday junction and single stranded DNA break

• The double strand break module

Importance of recombination

Genetic variation and DNA repair