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Discovery of a “ transforming principle”

Frederick Griffith, 1928Pneumonia ( Diplococcus pneumoniae )infects mice.Mice develop pneumonia and die.

Two types of bacteria:R bacteria rough coat - no pneumonia

S bacteria smooth coat- pneumonia

Coat type is associated with virulence.

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Avery, MacLeod and McCarty Avery, MacLeod and McCarty

Isolated DNA from heat killed type SIsolated DNA from heat killed type S

bacteria and injected it along with type Rbacteria and injected it along with type Rbacteria into micebacteria into mice

The mice died and their bodies containedThe mice died and their bodies containedactive type S bacteriaactive type S bacteriaConcluded that DNA passed from type SConcluded that DNA passed from type S

bacteria to type R, making it lethalbacteria to type R, making it lethal

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DNA is the genetic material

Alfred Hershey and Martha Chase, 1953

Viruses can infect the E. coli bacteria. A virus has protein “ head” and DNA core.

Infection occurs when virus injects DNA

into a bacterial cell.

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Fig. 9.4

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Chemical Nature of Nucleic Acids

Levene’s work lead to the idea that thestructure of DNA was a simple repeatingunit of GATCGATCGATC

This is why no one thought it could be thegenetic material with a structure this simple

• Purines - Large organic bases – Adenine and Guanine

• Pyrimidines - Small organic bases – Cytosine and Thymine, Uracil (RNA)

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CHARGAFF’S RULES

• In all DNA molecules: – The proportion of adenine is equal to

thymine• A = T

– The proportion of guanine is equal tocytosine

• G = C

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DNA bases pair via hydrogen bonds

•Erwin Chargaff observed:

• % adenine = % thymine•% guanine = % cytosine•Complementary bases pair:

– A and T pair – C and G pair

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Replication as a process

•Double-stranded DNAunwinds.

The junction of the unwoundmolecules is a replication fork.

A new strand is formed by pairingcomplementary bases with theold strand.

Two molecules are made.Each has one new and one oldDNA strand.

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E i DNA li i

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Enzymes in DNA replication

Helicase unwindsparental double helix

Binding proteinsstabilize separate

strands

DNA polymerasebinds nucleotides

to form new strands

Ligase joins Okazakifragments and seals

other nicks in sugar-phosphate backbone

Primase addsshort primer

to template strand

Exonuclease removesRNA primer and insertsthe correct bases

li i

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Binding proteins prevent single strands from rewinding.

Replication

Helicase protein binds to DNA sequencescalled origins and unwinds DNA strands.

5’

3’

5’

3’

Primase protein makes a short segment of RNAcomplementary to the DNA, a primer.

3’5’

5’3’

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Replication

DNA polymerase enzyme adds DNA nucleotides

to the RNA primer.

5’

5’

Overall directionof replication

5’

3’

5’3’

3’

3’

DNA polymerase proofreads bases added andreplaces incorrect nucleotides .

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Replication

3’5’ 5’

5’3’

5’3’

3’

5’

3’Overall directionof replication

Okazaki fragment

Leading strand synthesis continues in a5’ to 3’ direction.

Discontinuous synthesis produces 5’ to 3’ DNA

segments called Okazaki fragments.

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Replication

5’

5’

3’ 3’

5’

3’

5’3’

5’

3’

3’

5’

Discontinuous synthesis produces 5’ to 3’ DNAsegments called Okazaki fragments

Exonuclease enzymes remove RNA primers.

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Replication

Exonuclease enzymes remove RNA primers.

Ligase forms bonds between sugar-phosphate

backbone.

3’

5’

3’

5’3’

5’

3’

3’

5’

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PROTEIN PRODUCTION

First step in protein production isFirst step in protein production istranscriptiontranscription

Transcription makes a copy of the DNATranscription makes a copy of the DNAcalled messenger RNAcalled messenger RNAmRNAmRNA

Called messenger RNA because it carriesCalled messenger RNA because it carriesthe genetic message from the DNA to thethe genetic message from the DNA to the

protein factory, theprotein factory, the ribosomesribosomes in thein thecytoplasmcytoplasmTranscription is directed by the enzymeTranscription is directed by the enzymeRNA polymeraseRNA polymerase

• RNA is also a nucleic acid

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• RNA is also a nucleic acid

RNA has a slightly different sugarRNA has U instead of T

Figure 10.2C, D

Phosphategroup

Uracil (U)

Sugar (ribose)

Nitrogenous base(A, G, C, or U)

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RNA

Messenger RNA is synthesized from DNA by aMessenger RNA is synthesized from DNA by aprocess called transcriptionprocess called transcriptionThis process is similar to DNA replication inThis process is similar to DNA replication inthat i t depends on complementary base pairingthat it depends on complementary base pairing

DNADNA RNARNAGuanineGuanine CytosineCytosine

CytosineCytosine

GuanineGuanine

ThymineThymine Adenine Adenine Adenine Adenine UracilUracil

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