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DNA Structure

& Function

Chapter 13

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DNA Structure& Function 2

Outline

Genetic Material

Transformation

DNA Structure

Watson and Crick

DNA Replication

Prokaryotic versus Eukaryotic

Replication Errors

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DNA Structure& Function 3

Genetic Material

Frederick Griffith investigated virulence ofStreptococcus pneumoniae

Concluded that virulence passed from the

dead strain to the living strainTransformation

Further research by Avery et al

Discovered that DNA is the transformingsubstance

DNA from dead cell was being incorporated

into genome of living cells

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4Griffiths Transformation Experiment

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DNA Structure& Function 5

Reproduction of Viruses

Viruses consist of a protein coat (capsid)surrounding a nucleic acid core

Bacteriophages are viruses that infectbacteria

Hershey and Chase:

Radioactively labeled the DNA core andprotein capsid of a phage

Results indicated that DNA, not the protein,enters the host

The DNA of the phage contains genetic

information for producing new phages

B i d B i h

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6Bacteria and Bacteriophages

H h d Ch E i

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7Hershey and Chase Experiments

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DNA Structure& Function 8

Structure of DNA

DNA contains:

Two Nucleotides with purine bases

-Adenine (A)

-Guanine (G)

Two Nucleotides with pyrimidine bases-Thymine (T)

-Cytosine (C)

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DNA Structure& Function 9

Chargaffs Rules

The amounts of A, T, G, and C in DNA:Identical in identical twinsVaries between individuals of a speciesVaries more from species to species

In each species, there are equal amounts of:A & TG & C

All this suggests DNA uses complementary

base pairing to store genetic infoHuman chromosome estimated to contain, onaverage, 140 million base pairs

Number of possible nucleotide sequences4,140,000,000

N l tid C iti f DNA

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10Nucleotide Composition of DNA

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DNA Structure& Function 11

Watson and Crick Model

Watson and Crick, 1953

Constructed a model of DNA

Double-helix model is similar to a twistedladder

-Sugar-phosphate backbones make up the sides

-Hydrogen-bonded bases make up the rungs

Received a Nobel Prize in 1962

X R Diff ti f DNA

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12X-Ray Diffraction of DNA

W t /C i k M d l f DNA

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13Watson/Crick Model of DNA

S

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DNA Structure& Function 14Replication:

Prokaryotic vs. Eukaryotic

Prokaryotic Replication

Bacteria have a single circular loop

Replication moves around the circular DNAmolecule in both directions

Produces two identical circles

Cell divides between circles, as fast as every20 minutes

DNA S

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DNA Structure& Function 15Replication:

Prokaryotic vs. Eukaryotic

Eukaryotic ReplicationDNA replication begins at numerous points alonglinear chromosome

DNA Unwinds and unzips into two strands

Each old strand of DNA serves as a template for anew strand

Complementary base-pairing forms new strand oneach old strand

Replication bubbles spread bi-directionally until theymeet

Semiconservative:

One original strand is conserved in each daughter

molecule

Semiconservative Replication

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16Semiconservative Replicationof DNA

Meselson and Stahls

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17Meselson and StahlsDNA replication experiment

Replication:

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18Replication:Prokaryotic vs. Eukaryotic

DNA St t 19

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DNA Structure& Function 19

Replication Errors

Genetic variations are the raw material forevolutionary change

Mutation:

A permanent (but unplanned) change in base-pair sequence

-Some due to errors in DNA replication

-Others are due to to DNA damage

DNA repair enzymes are usually available to

reverse most errors

DNA Structure 20

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DNA Structure& Function 20

Review

Genetic Material

Transformation

DNA Structure

Watson and Crick

DNA Replication

Prokaryotic versus Eukaryotic

Replication Errors

Ending Slide Chapter 13

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DNA Structure

& Function

Ending Slide Chapter 13