DNADNA

RNARNA

ProteinProtein

Scientists call this the:

Central

Dogma of

Molecular

Biology!

How do we know that all of our genetic information

comes from DNA?

• What type of experiment would you design to determine that DNA is the source of all genetic information?

Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation

• Frederick Griffiths was a bacteriologist studying pneumonia

• He discovered two types of bacteria:– Smooth colonies– Rough colonies

CONCLUSION:

The smooth colonies must carry

the disease!

Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation

• When heat was applied to the deadly smooth type…

• And injected into a mouse…

• The mouse lived!

• Griffith injected the heat-killed type and the non-deadly rough type of bacteria.

• The bacteria “transformed” itself from the heated non-deadly type to the deadly type.

Griffith’s Experiment with Pneumonia and the accidental discovery of Transformation

Griffith’s Experiment did not prove that DNA was responsible for

transformationHow would you design an

experiment to prove that DNA was responsible for

transformation?

Avery, McCarty, and MacLeodRepeated Griffith’s Experiment

Oswald Avery Maclyn McCarty Colin MacLeod

Avery, McCarty, and MacLeodAdded the non-deadly Rough Type of Bacteria to the Heat-Killed Smooth

Type

Carbohydrates Lipids

Proteins

RNA DNA

To the Heat-Killed Smooth Type, added enzymes that destroyed…

S-Type Carbohydrates

Destroyed

S-Type Lipids

Destroyed

S-Type Proteins Destroye

d

S-Type RNA

Destroyed

S-Type DNA

Destroyed

Conclusion:

DNA was the transforming factor!

The Hershey-Chase Experiment

Alfred Hershey & Martha

Chase worked with a

bacteriophage:

A virus that invades

bacteria. It consists of a

DNA core and a protein

coat

DNA

Protein coat

The Hershey-Chase results reinforced the Avery,

McCarty, and MacLeod conclusion:

DNA carries the genetic code!

However, there were still important details to

uncover…

How did DNA:1. Store information?

2. Duplicate itself easily?

These questions would be answered by

discovering DNA’s structure

The Race to Discover DNA’s Structure

The Race to Discover DNA’s Structure

Linus Pauling

1940s

Discovered the alpha-helical structure of proteins.

The Race to Discover DNA’s Structure

1950

Chargaff’s Rule: Equal amounts of Adenine and Thymine, and equal amounts of Guanine and Cytosine

Erwin Chargaff

Why do you think the bases match up

this way?

Purine + Purine = Too wide

Pyrimidine + Pyrimidine = Too Narrow

Purine + Pyrimidine = Perfect Fit from X-ray data

The Race to Discover DNA’s Structure

Maurice Wilkins

Rosalind Franklin

X-Ray diffraction image of DNA taken by Franklin in

1951

The Race to Discover DNA’s Structure

James Watson Francis Crick

1953

Compiled data from previous scientists to build a double-helical model of DNA

DNA Structure

Deoxyribonucleic acid Double helix (twisted ladder or strands)

of nucleotides (Sugar (deoxyribose), phosphate and a nitrogen base)

Each strand has a sugar and phosphate backbone covalently bonded to a nitrogen base

One single strand of DNA…

DNA Structure

Double helix is made of covalently bonded strands that are hydrogen bonded to complementary covalently bonded strands

One strand bonds to the second strand via hydrogen bonds (weak enough to break in order to separate the 2 strands)

Each strand measures 3.4 nm/twist or 10 base pairs

DNA Double Helix

DNA Structure Strands of DNA

are different – they are oriented in opposite directions to each other – they are ANTIPARALLEL

Each end has a number (5’ or 3’ – you say 5 prime or 3 prime)

Four Nitrogen Bases

Adenine (A), Guanine (G), Cytosine (C), Thymine (T)

Purines (double ring structures) – Adenine and Guanine

Pyrimidines (single ring structures) – Cytosine and Thymine

Chargaff rules: A –T and T – A G – C and C - G

Adenine GuanineCytosine

Thymine

PhosphateSugar (deoxyribose)

Purines Pyrimidines

Nitrogenous Bases

Chromosome Structure DNA packs tightly around histones to

form chromatin.

DNA and histones form bead-like structures called nucleosomes.

Nucleosomes pack together to form supercoils.

Supercoils condense to form chromosomes.

Chromosome

Nucleosome

Coils

Supercoils

Histones

DNA

Chromosome Structure

DNA Replication

• The double helix did explain how DNA copies itself

• We will study this process, DNA replication, in more detail

How does DNA replicate?

Conservative Semi-Conservative

Dispersive

Hypotheses:

DNA Replication

DNA copies itself in the “S” phase of interphase.

1 parent DNA molecule produces 2 daughter DNA molecules, each daughter being made up of “parent” DNA and a strand of “new” DNA (semiconservative process)

Steps of DNA Replication

1. DNA unzips – Helicase enzyme breaks hydrogen bonds, unzipping the double helix at the origin of replication (about 100 on a human chromosome). A replication bubble is formed when DNA

unzips DNA polymerization is bi-directional

because of the antiparallel orientation of the DNA strand.

DNA ReplicationSection 12-2

Growth

Growth

Replication fork

DNA polymerase

New strand

Original strand DNA

polymerase

Nitrogenous bases

Replication fork

Original strand

New strand

DNA Replication

Steps of DNA Replication

2. Bases pair up – DNA Polymerase bonds free nucleotides to complementary bases DNA POL reads DNA in 3’ to 5’ direction,

thus a new strand elongates only in the 5’ to 3’ direction

Nucleotides are added at a rate of about 50 per second in mammals and 500 per second in bacteria.

Steps of DNA Replication

Leading strand has continuous elongation starting at RNA primer since it is read in 3’ to 5’ direction (towards replication fork) by DNA polymerase Lagging strand has discontinuous

elongation DNA strand is read 3’ to 5’ away from the

replication fork in a series of segments called Okazaki fragments

Once fragments are finished, they are joined to previous fragment with enzyme Ligase.

DNA Replication

Steps of DNA Replication

3. Proofreading and repair DNA polymerase “proof-reads” newly

created DNA strand and identifies incorrect base pairs.

Nuclease (exonuclease) enzyme cuts out the identified incorrect nucleotides.

DNA polymerase places correct nucleotides into DNA strand.

Ligase fuses these corrected nucleotides into the DNA strand