DNA: The Genetic Material

Chapter 9 Section 1

Who Was the First Person To Isolate DNA?

• Friedrich Meischer• 1870’s

Griffith’s Experiment

• 1928• Fredrick Griffith• Bacteriologist• Trying to prepare a

vaccine against pneumonia

Griffith’s Experiment

• Two types, or strains, of S. pneumoniae• First strain is enclosed in a capsule composed of

polysaccharides.– Capsule protects the bacterium from the body’s

defense system.– Forms smooth-edges (S) when grown in a petri

dish• Helps make the microorganism virulent (able to

cause disease).

Griffith’s Experiment

• Second strain lacks the polysaccharide capsule and does not cause disease.– Forms rough-edges (R) when grown in a petri

dish

Griffith’s Experiment

Griffith’s Discovery

• The harmless R bacteria had changed and became virulent S bacteria.

• Transformation is a change in genes caused when cells take up foreign material.– Genes: sections of DNA in a chromosome that

code for traits

Avery’s Experiment

• 1944• Oswald Avery

– Along with Colin MacLeod & Maclyn McCardy

• Rockefeller Institute in New York

• Repeated Griffith’s experiment to determine which molecule in heat-killed bacteria was most important for transformation.

Avery’s Experiment

• Made an extract, or juice, from the heat-killed bacteria.

• Treated the extract with enzymes that destroyed proteins, lipids, carbohydrates, and other molecules, including RNA.

• Transformation still occurred

Avery’s Discovery

• Repeated the experiment using an enzymes that would break down DNA.

• Transformation did not occur.

• DNA was the transforming factor!

What Scientists Knew

• Avery’s experiment clearly indicated genetic material is composed of DNA

• Many scientist remain skeptical• Proteins are important to many aspects of

the cell structure & metabolism, so most suspected that proteins were the genetic material

• Scientist knew very little about DNA

What Scientists Knew

• Viruses are composed of DNA or RNA surrounded by a protective protein coat.

• Bacteriophage (phage) is a virus that infects bacteria.

• When phages infect bacterial cells, the pages are able to produce more viruses– Released when the bacterial cells rupture.

What Scientists Didn’t Know

• How the bacteriophage reprograms the bacterial cell to make viruses.

• Does the bacteriophage DNA, the protein, or both issue instructions to the bacteria?

The Hershey-Chase Experiment

• 1952• Alfred Hershey & Martha

Chase• Scientists at Cold Spring

Harbor Laboratory, in New York

• Used the bacteriophage T2 to answer this question.

The Hershey-Chase Experiment

• Knew the only molecule in the phage that contains phosphorus is its DNA.

• The only phage molecules that contain sulfur are the proteins in its coat.

The Hershey-Chase Experiment• Grew T2 with E. coli bacteria in a nutrient medium

that contained radioactive sulfur (35S)– The protein coat would incorporate the 35S

• Grew T2 with E. coli bacteria in a nutrient medium that contained radioactive phosphorus (32P)– The radioactive phosphorus would become part of

the cell’s DNA

The Hershey-Chase Experiment

• 35S-labeled & 32P-labeled phages were used to infect two separate batches of E. coli bacteria

The Hershey-Chase Experiment

• They waited a few minutes for the viruses to inject their genetic material

• Next, they separated the viruses from the bacteria & tested the bacteria for radioactivity

Hershey-Chase Discovery

• Nearly all the radioactivity in the bacteria was from phosphorus (32P), the marker found in DNA.

• Concluded that the DNA of viruses is injected into the bacterial cell, while most of the viral proteins remained outside.

• Causes bacterial cells to produce more viral DNA and proteins.

• DNA is the hereditary material.

The Structure of DNA

Chapter 9 Section 2

Structure of DNA

• Double helix- two strands twisted around each other, like a winding staircase.

• Each strand is made of linked nucleotides.

Nucleotides

• 1920’s• The subunits that make up

DNA.• 3 parts

– Phosphate group– A 5-Carbon sugar molecule

(deoxyribose)– Nitrogen-containing base

• Any one of 4 different bases

Purines & Pyrimidines

• Purines are nitrogen bases made of 2 rings of carbon & nitrogen atoms– Adenine– Guanine

• Pyrimidines are nitrogen bases made of a single ring of carbon & nitrogen atoms– Thymine– Cytosine

Nitrogen Bases

How was the actual structure of DNA

discovered?

Chargaff’s Observation

• 1947• Erwin Chargaff• The amount of adenine (A) always equaled the

amount of thymine (T)– A = T

• The amount of guanine (G) always equaled the amount of cytosine (C)– G = C

Wilkins & Franklin’s Photographs

• 1952• Maurice Wilkins & Rosalind Franklin• King’s College in London• Developed high-quality X-ray diffraction

photographs of strands of DNA• Suggested DNA molecule resembled a

tightly coiled helix & was composed of 2 or 3 chains of nucleotides

James Watson & Francis Crick

• 1953• Developed the first

3-D model of DNA• Had to take into

account both Chargaff’s findings & Frankin and Wilkins’s X-ray diffraction data

Base-pairingsWatson & Crick determined:

1. A purine on one strand of DNA is always paired with a pyrimidine on the opposite strand.

2. An adenine on one strand always pairs with a thymine on the opposite strand.

3. A guanine on one strand always pairs with a cytosine on the opposite strand.

Complementary Base Pairs

What is the complementary base pair?

TCGAACTAGCTTGA

The Replication of DNA

Chapter 9 Section 3

Objectives

• Summarize the process of DNA replication.• Describe how errors are corrected during

DNA replication.• Compare the number of replication forks in

prokaryotic and eukaryotic DNA.

Key Terms

• DNA Replication• DNA Helicase• Replication Fork• DNA Polymerase

DNA Replication

• DNA replication is the process of making a copy of DNA.

• Watson & Crick proposed that one DNA strand serves as a template, or pattern, on which the other strand is built.

DNA Replication

• The double helix unwinds, caused by an enzyme (DNA helicase).

• DNA helicases open the double helix by breaking the hydrogen bonds that link complementary base pairs.

• Once separated additional proteins attach to the ends to keep them apart.

Replication Forks

DNA Replication

• At the replication fork, enzymes known as DNA polymerases move along each of the DNA strands

• DNA polymerases add nucleotides to the exposed nitrogen bases, according to the base-pairing rules.

• Two new double helixes are formed.

DNA Replication

• Once DNA polymerase have begun adding nucleotides to a growing double helix, the process continues until all of the DNA has been copied & the polymerase is signaled to detach.

Checking for Errors

• DNA polymerase has a “proofreading” role.• It can only add a new nucleotide if the

previous nucleotide was correct.• If it is incorrect, the polymerase will go

back and remove the incorrect nucleotide & replace it with the correct one.

• Reduces errors in DNA replication to 1 error per 1 billion nucleotides!

Rate of Replication

• The replication of a typical human chromosome with one pair of replication forks spreading from a single point, would take 33 days!

• Each human chromosome is replicated in about 100 sections that are 100,000 nucleotides long, each section with its own starting point.

• As a result, an entire human chromosome can be replicated in about 8 hours.

Replication Forks