AP Biology Chapter 9

Griffith 1928

• Proved transformation of bacteria into a mouse

• Had two strains of bacteria– An avirulent or nonlethal strain (R)– A virulent of lethal strain (S)

Griffith’s Experiment

• When he put the virulent strain in the mouse, it died

• When he put the avirulent strain in the mouse, the mouse lived!

• Then he heated the virulent strain and then put it into the mouse and the mouse lived!

• When he put the heated virulent strain + the nonvirulent strain into the mouse, the mouse died

• Why?

Explanation

• Transformation had occurred

• The nonvirulent bacteria took up something in the dead heated virulent strain: a “transforming principle” and changed the nonvirulent strain into a virulent strain!!

Other Scientists of interest

• Before 1940, biologists thought that proteins were the information molecules because they were so complex and had a lot of variety

• Avery, Macleod and McCarty in 1944 proved that the transformation principle in Griffith’s experiment was DNA!

Other Scientists of interest

• Hershey and Chase in 1952 proved that bacteriophages (viruses that attack bacteria) inject DNA into bacterial cells

• Franklin and Wilkins used x ray diffraction on DNA to determine the distances between molecules

• Watson and Crick in 1953 came up with the model of DNA

Chargaff’s rules

• He found a simple relationship in DNA called the base pairing rules– Adenine = Thymine– Guanine = Cytosine

Watson and Crick• Crick: English phage geneticist at the Cavendish

labs at Cambridge University, London England• Watson: American postdoc in Crick’s lab• Both visited Wilkins & Franklin routinely 1951-53• Derived the overall concept of the chemical

relationship• Considered how Chargaff’s rules represented the

structure of DNA• Franklin’s X ray data• Built little tin models of the nucleotides and put the

DNA model together like a TinkerToy set• Correctly deduced the structure of DNA (double

helix)

The Double Helix• This is the Watson

and Crick model worked out in 1953 and published in a single-page article in Nature of that year.

• Was convincing structurally: gave evidence for how DNA replicated

• Most famous biology paper ever written!

DNA Structure

• Called deoxyribonucleic acid

• Made up of nucleotides which have 3 parts

1. sugar – deoxyribose

2. Phosphate

3. Nitrogen base

Deoxyribose

• Pentose sugar = 5 carbons

• Carbons on the sugar are numbered 1 through 5 and the first carbon (1’) is linked to one of the four nitrogen bases (ACTG)

phosphate

• Is attached to the 5’ and 3’ carbon, making a phosphodiester linkage

• Forms the sugar phospate backbone of DNA (or the ladder)

Nitrogen base

• Remember these are connected to the 1’ carbon of the sugar

• 2 groups– Purines

• Have two ring structures• Adenine and guanine

– Pyrimidines• Have one ring structure• Thymine and cytosine

• The number of purines = number of pyrimidines

DNA molecule

• Consists of 2 polynucleotide chains arranged in a coiled double helix

• Helix is like a ladder

• Two strands run in opposite directions and are said to be antiparallel to each other– The nitrogen bases are bonded by hydrogen

bonds– A = T and C=G according to Chargaff’s rules

Hydrogen bonds

What is the complement of 3’AGCTAC5’?

How Does DNA Replicate?• Several research groups worked on this.

We’ll discuss one• 1957: Matthew Meselsohn and Fred Stahl• They had 3 hypotheses

1. DNA replication is “semiconservative”– One old strand kept with each of the new molecules;

one old paired with one new strand

2. DNA replication is “conservative”– Double strand maintained intact; new strands are

together in the new molecule

3. DNA replication is “dispersive”– Strands cut up and the old and new DNA interspersed

in both new strands

Semiconservativeis correct!• Each strand

acts as a template for the other, and so the mutation will propagate through successive generations.

How Does Replication Start?• The replication complex binds at the origin of

replication, which is identified by a particular base sequence. This is initiated by RNA primer

• Helicase unwinds the DNA, which is held open with helix-destabilizing proteins. Replication starts in the Y-shaped replication fork.

Replication Proceeds on Two Strands• Nucleotides are always added to the 3’ end by DNA

polymerase, thus moving in the 3’ to 5’ direction

• but the new strands elongate in opposite directions

• The leading strand elongates into the fork

• The lagging strand elongates away from the fork

• Elongation proceeds smoothly on the leading strand

Leading and Lagging Strands• As the fork grows, both new strands elongate further• Subunit addition to the lagging strand is by 100-2000 base Okazaki fragments.• The lagging strand grows in a discontinuous manner because of the size of the Okazaki

fragments• That’s why it lags

The Lagging Strand• Notice that the lagging strand is always

growing away from the replication fork• The gaps between the Okazaki

fragments are joined together by DNA ligase

In Action!

Review of Enzymes

• Unwinds the DNA

• Puts down RNA primer

• Adds bases to strands

• Seals Okazaki fragments up

• Winds the DNA molecule back together

DNA Repair

• DNA polymerase proofreads what bases had been laid down

• If there is a mistake, it will go back and remove the wrong base and fix it

How does DNA fit in the cell?

• By histones– Positively charged proteins (due to the high

number of amino acids)– Are able to associate with DNA which is

negatively charged (due to the phosphate groups)

• Histones and DNA form structure called nucleosome

Nucleosome

• Are 8 histones with DNA wrapped around it

• Are part of the chromatin

• Prevent DNA strands from becoming tangled

Cellular Ageing and DNA• The replication process never entirely

completes at the ends of the chromosomes

• However, DNA is protected at its ends with long strands that do not carry any genetic information, called telomeres

• as we age, they become shorter• They are repaired and lengthened with

an enzyme called telomerase• Loss of telomerase activity may be an

important cause of cellular aging