DNA: The Chemical Basis of Heredity

Chapter Goals

1.History of how DNA’s structure and function were discovered

2.DNA replication

3.Role of DNA in directing RNA synthesis (transcription)

4.Role of DNA in directing protein synthesis (translation)

The “Transforming Principle”Frederick Griffith (1920s)

(Serendipity is important in science!)

By 1920s…Chromosomes made up of proteins and DNABut…most scientists did not think DNA was the hereditary material.

From Griffith’s work…Some substance causes a heritable change between the

S and R cells.What is it?

Avery & MacLeod, 1944:Sequentially destroyed in S bacteria: Proteins Lipids Carbohydrates DNAThen asked: Which component of S bacteria, when NOT destroyed, was able to “transform” the R bacteria to become virulent?

Very important experiment…but not well received and generally disregarded by the scientific community.

“DNA is not complex enough to be the material of heredity.”

“Bacteria are so simple, it is unlikely that they have DNA.”

1952: Hershey and Chase

Used T2 bacteriophages to show that DNA

was the genetic material in viruses

Making use of viruses: The T2 bacteriophage

Viruses, in general: Are parasites! Not considered to be living organismsInsert their genetic material into host cell (Ex. bacteria, mucosa, T helper cells)Hijack host cell nutrients to make more virus particlesMay or not lyse (blow up) the host cellFairly simple…contain a shell of protein and a strand of

DNA inside

“Lytic”

Hershey & Chase (1952): Which part of the virus enters the bacteria? The DNA or the PROTEIN?

Phosphorus is found in DNA…NOT in proteins

Use 32P to label DNA

Sulfur is found in proteins…not in DNA

Use 35S to label protein

DNA is indeed the hereditary material in things

as simple as viruses!

(Just 60 years ago!)

The Structure of DNA

X-Ray Crystallography(Rosalind Franklin)

DNA is helical

Chargoff’s Rule

Amount of A = Amount of T

Amount of C = Amount of G

Quantity of Pyrimidines = Quantity of Purines

Previous Modeling Experiments

DNA is likely made up of 2 chains

These 2 chains run antiparallel

This is the information that Watson & Crick started with…

Watson & Crick’s Model

PentoseSugar

C

C

C C

C

Nitrogenous Base

(A, C, T, G)

P P P-O-5

4

3 2

1

Anatomy of a Nucleotide

3’

S

S

S

S

P

P

P

P

S

S

S

S

P

P

P

P

5’

5’

3’

Nucleotide is made of:

Nitrogenous base (A,C,G,T)

Deoxyribose Sugar

Phosphate

How Does DNA Replicate?3 possibilities

YES!

NO!

NO!

Semiconservative Replication:

Each new molecule has 1 old strand and 1 new strand.

Meselson-Stahl Experiment(Proof that DNA Replication is Semiconservative)

Rules of DNA Replication

1. All chromosomes have at least one “origin of replication” (ori)

• Prokaryotes have only 1.• Eukaryotic chromosomes have many.

2. A huge protein complex, call the REPLICATION COMPLEX, binds to the “ori”

3. Each new DNA strand grows ONLY IN THE 5’ to 3’ DIRECTION

What does 5’ to 3’ mean?C #5

C #3

Steps Involved in DNA Replication1.Helicase unwinds the double helix (Helicase hacks!)2.Primase lays down an RNA primer (Primase primes!)3.DNA polymerase pastes in nucleotides (Polymerase pastes!)4.Ligase joins the smaller Okazaki fragments and rips up the RNA primer (Ligase links!)

What Happens at the End of the Line?

Part of the end of each chromosome is “lost” following each round of replication (anywhere from 50-200 base pairs).

Telomeres: The end portions of a chromosome made up of a repeating sequence of DNA—does NOT contain a gene.

Telomerase: An Enzyme to the Rescue

But...Is telomerase always a good thing?

What happens when there is a mistake?

• DNA polymerase: 1,000 base pairs per second

• 1:1,000,000 error rate (1,000 errors every time DNA is replicated

Only 1 error in 1010 bases remains