The Discovery, Replication,DNA vs. RNA

DNA

• In 1952, American biologists Alfred Hershey and Martha Chase set out to determine what composed the genetic material of a bacteriophage.

• They knew that a bacterial virus was an extremely simple organism, composed only of protein and DNA. The protein makes up the exterior of the virus, and the DNA is contained within it.

HERSHEY & CHASE

• When a bacterium is infected by a bacteriophage, the bacterium's internal machinery falls under the control of the virus, which uses the bacterium to produce more viruses.

• What Hershey and Chase wanted to know was: Which substance directed this takeover - DNA or protein?

HERSHEY & CHASE

• The Experiment:oProtein contains sulfur. DNA contains

phosphorus.oThey added bacteriophage (a bacterial

virus) to cultures containing either radioactive sulfur (S35) or radioactive phosphorus (P32).

oHershey and Chase now had two types of bacteriophages: one with a radioactive external protein coat, the other with radioactive DNA.

HERSHEY & CHASE

• The bacteriophages were allowed to infect the bacteria. The phage injects its DNA into the bacterial cell, while the protein coat remains outside.

• Using methods to separate the liquid and used phage protein coats from the bacterial cells…

• In the cultures infected by bacteriophages with radioactive sulfur, the radioactivity was in the liquid with the phages.

HERSHEY & CHASE

• In the cultures infected by bacteriophages with radioactive phosphorus, the radioactivity was in the pellet of infected bacteria.

• Thus, Hershey and Chase discovered that the radioactive protein hadn't entered the bacterial cells, but the DNA had.

• Lets take a look: http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/bio21.swf::Hershey%20and%20Chase%20Experiment

HERSHEY & CHASE

• Maurice Wilkins shared the 1962 Nobel Prize for Physiology and Medicine with Watson and Crick for the discovery of the structure of DNA.

• Why was Rosalind Franklin not honored for her truly essential contribution to this discovery, and could not be recognized by the Nobel Committee in 1962?

WILKINS & FRANKLIN

• In 1952 Maurice Wilkins and Rosalind Franklin were given the task of determining the molecular structure of DNA.

• They investigated DNA through the scientific technique of X-ray crystallography, in which Rosalind was very skilled.

• She took the pictures and made the calculations that led to the discovery of the shape of DNA.

WILKINS & FRANKLIN

• Rosalind Franklin’s X-ray diffraction Photo 51 (B) which clarified the double helix shape of DNA.

WILKINS & FRANKLIN

• In 1953, Francis Crick and James Watson described the structure of DNA.

• Their model of DNA was based on their own DNA research AND the research of several other scientists including:

oLinus Pauling, 1948, who discovered that many proteins take the shape of an alpha helix, spiraled like a spring coil.

WATSON & CRICK

o Biochemist Erwin Chargaff, 1950, found that the arrangement of nitrogen bases in DNA varied widely, but the amount of certain bases always occurred in a one-to-one ratio.

o Maurice Wilkins and Rosalind Franklin’s __________________ photos of DNA double helix, 1952.

WATSON & CRICK

• DNA stands for deoxyribonucleic acid.

• Nucleotides are the units that make-up DNA.

• Nucleotides have three parts: - 5 Carbon sugar – Deoxyribose - a phosphate group - a nitrogen base (one of four) * arranged along the backbone in

5’ (phosphate) to 3’ (OH) direction.

DNA STRUCTURE

DNA STRUCTURE

• Helical shape, Double-strand pairing, Antiparallel (one strand 5’ to 3’, the other 3’ to 5’)

• Four different nitrogen bases: Adenine, Thymine, Cytosine, Guanine• Complementary base-matching: A-T, C-

G• Base-matching achieved by Hydrogen

bonding and geometry• A, G are long, double ring purines• T, C are short, single ring pyrimidines

DNA STRUCTURE

NUCLEOTIDE STRUCTURE

5’

3’

5’

3’

• DNA replication is semi-conservative. Meaning one strand from each of the initial two parent strands ends up in a new daughter strand.

• Each strand serves as a template for a new strand.

• New strand is formed by complementary base-pairing of the correct nucleotides.

DNA REPLICATION

• DNA replication begins when helicase unwinds a segment of the DNA and breaks the hydrogen bonds between the two complementary strands.

• DNA polymerase can only add new nucleotides to a free 3’ end of a growing chain. Synthesis of one strand of the DNA, called the leading strand, proceeds continuously in the 5’ to 3’ direction.

• Synthesis of the complementary strand, called the lagging strand, is more complex.

DNA REPLICATION

• DNA polymerase can add new nucleotides only to a free 3’ OH end.

• To provide a free 3’ OH starting point, RNA primase attaches to the DNA and synthesizes a short RNA primer. DNA polymerase III then adds new nucleotides to the 3’ end of the RNA primer. (Adds nucleotides between primers.)

• DNA polymerase I replaces DNA polymerase III, removes the RNA primer and replaces it with DNA nucleotides.

DNA REPLICATION

• DNA ligase forms a phosphodiester bond between the 3’ OH of the growing strand and the 5’ phosphate in front of it.

• During DNA replication, the leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously.

• Lets take a look: http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120076/micro04.swf::DNA%20Replication%20Fork

DNA REPLICATION

• The pieces of new DNA strand between the RNA primers are called Okazaki fragments.

DNA REPLICATION

DNA

DNA VS. RNA

RNA

Make yourself a chart!

DNA• Deoxyribose

sugar

DNA VS. RNA

RNA•Ribose sugar

DNA• Deoxyribose

sugar• Double Strand

DNA VS. RNA

RNA•Ribose sugar•Single Strand

DNA• Deoxyribose

sugar• Double Strand• Bases: A, C, G,

T

DNA VS. RNA

RNA•Ribose sugar•Single Strand•Bases: A, C, G, U

DNA• Deoxyribose

sugar• Double Strand• Bases: A, C, G,

T• For the

“Storage” of information

DNA VS. RNA

RNA• Ribose sugar• Single Strand• Bases: A, C,

G, U• A

“Messenger” of information