DNADNA

DNA or Protein the Genetic DNA or Protein the Genetic material??material??

Hershey-Chase ExperimentHershey-Chase Experiment

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Structure:Structure:

Built of nucleotides:Built of nucleotides: Pentose sugar Pentose sugar PhosphatePhosphate Nitrogen baseNitrogen base

Purines – adenine, guaninePurines – adenine, guaninePyrmidines – thymine, cytosinePyrmidines – thymine, cytosine

N base attached to 1’ C of sugarN base attached to 1’ C of sugar3’ C of 1 sugar bonds to 5’ phosphate to 3’ C of 1 sugar bonds to 5’ phosphate to form phosphodiester bondform phosphodiester bondhttp://207.207.4.198/pub/flash/24/http://207.207.4.198/pub/flash/24/menu.swfmenu.swf

Chargaff’s RulesChargaff’s Rules

# of purines = # pyrimidines# of purines = # pyrimidines

A = TA = T

C = GC = G

From here, conclusion that adenine H-From here, conclusion that adenine H-bonds with thymine and cytosine H-bonds bonds with thymine and cytosine H-bonds with guaninewith guanine

Complementary StrandsComplementary Strands

DNA arranged in double helix (Rosalind DNA arranged in double helix (Rosalind Franklin’s work)Franklin’s work)

Antiparallel – run 5’Antiparallel – run 5’3’ on 1 strand and 3’ on 1 strand and 3’3’5’ on other5’ on other

2 strands are complementary; ie2 strands are complementary; ie 3’—AGTAC—5’3’—AGTAC—5’ 5’—TCATG—3’5’—TCATG—3’

DNA Replication is DNA Replication is semiconservativesemiconservative

Confirmed by Confirmed by Messelson-Stahl:Messelson-Stahl:

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Steps in DNA ReplicationSteps in DNA Replication

Begins at specific sites called origins of Begins at specific sites called origins of replicationreplicationDNA helicase unwinds double helix by DNA helicase unwinds double helix by breaking H-bonds – forms replication forksbreaking H-bonds – forms replication forks

Single-stranded binding proteins hold Single-stranded binding proteins hold strands openstrands open

DNA polymerases add nucleotides to 3’-DNA polymerases add nucleotides to 3’-end of growing DNA strandend of growing DNA strand Synthesis is always in 5’Synthesis is always in 5’3’ direction3’ direction Requires a RNA primer to build off ofRequires a RNA primer to build off of DNA primase synthesizes a short DNA primase synthesizes a short

complementary RNA strand for DNA complementary RNA strand for DNA polymerase to build on topolymerase to build on to

DNA Replication occurs on both DNA Replication occurs on both strands at the same timestrands at the same time

Strands run in opposite directionsStrands run in opposite directions

DNA only replicates in 5’DNA only replicates in 5’3’ direction3’ direction

Therefore, only 1 strand can replicate Therefore, only 1 strand can replicate toward the replication fork: toward the replication fork: leading leading strandstrand

Strand replicating away from the fork is Strand replicating away from the fork is called the called the lagging strandlagging strand Can only synthesize short pieces at a timeCan only synthesize short pieces at a time Okazaki fragmentsOkazaki fragments

Synthesis on leading strand is continuousSynthesis on leading strand is continuous

Synthesis on lagging strand requires multiple Synthesis on lagging strand requires multiple primersprimers

When RNA primer of previous Okazaki fragment When RNA primer of previous Okazaki fragment is reached, DNA polymerase breaks it downis reached, DNA polymerase breaks it down

DNA ligase seals Okazaki fragments togetherDNA ligase seals Okazaki fragments together

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ProofreadingProofreading

DNA polymerase proofreads new DNA polymerase proofreads new nucleotides against templatenucleotides against template

If mistake made, DNA polymerase repairs If mistake made, DNA polymerase repairs mistakemistake

If mistake not corrected, mutation has If mistake not corrected, mutation has occurredoccurred

Pro vs. EukaryotesPro vs. Eukaryotes

Prokaryotes have 1 origin of replicationProkaryotes have 1 origin of replication DNA synthesis proceeds in both directions DNA synthesis proceeds in both directions

around the circular chromosome until 2 around the circular chromosome until 2 replication forks meetreplication forks meet

Eukaryotic chromosomes have multiple Eukaryotic chromosomes have multiple origins of replicationorigins of replication Replication bubbles eventually meet and Replication bubbles eventually meet and

mergemerge Speeds up processSpeeds up process

TelomeresTelomeres

End of each chromosome is left short, End of each chromosome is left short, unreplicated strands of DNAunreplicated strands of DNA

These ends are repeating, non-coding These ends are repeating, non-coding sections called telomeressections called telomeres Help to regulate the # of times of cell can Help to regulate the # of times of cell can

dividedivide Cancer cells possess telomerase, which adds Cancer cells possess telomerase, which adds

repeating sequences to ends of repeating sequences to ends of chromosomes chromosomes