Chapter 9DNA: The Genetic Material

Mrs. Cook

Biology

Identifying the Genetic Material

• Griffith’s Experiments- 1928 Frederick Griffith- Worked with Streptococcus pneumoniae- was trying to prepare a vaccine

A substance that is prepared from killed or weakened disease-causing agents, and is introduced into the body to protect the body against future infections by the disease-causing agent.

Identifying the Genetic Material

-Griffith worked with two forms of the S.pneumoniae1st strain:

•has a capsule, which protects the cell from the body’s defenses•it virulent or able to cause disease. • because of the capsule it grows with smooth (S) edged colonies.

2nd Strain: •lacks a capsule•does not cause disease• forms rough (R) colonies

Identifying the Genetic Material

- Griffith knew:• Mice infected with the S bacteria grew sick

and died.• Mice infected with the R bacteria were not

harmed

- Griffith Experiment:• Injected mice with dead S bacteria and

mice remained healthy• Prepared a vaccine by “heat killing”-

raised temperature of bacteria to the point where they were alive but could no longer reproduce.

Identifying the Genetic Material

• Then injected mice with “heat killed” S bacteria, the mice lived.

• Then he mixed the harmless live R bacteria with the harmless “heat killed” S bacteria

- The mice died- The live R bacteria had acquired capsules- The harmless R bacteria had become

virulent.

- We now call what happened TRANSFORMATION- a change in genes when cells take in a foreign genetic material

Identifying the Genetic Material

• Avery’s Experiments

-1944- Avery and his co-worker’s at the Rockefeller Institute in NYC, demonstrated that DNA is the material responsible for transformation.

- Not all scientists accepted this.

- Most thought proteins were the genetic material

- little was known about DNA at this time

Identifying the Genetic Material• Alfred Hershey and Martha Chase Experiment

- 1952- worked at Cold Spring Harbor Laboratory in NY.

- worked with viruses, which are much simpler than cells, are composed of DNA or RNA surround by a protective protein coat.

-A bacteriophage, which is a virus that, infects bacteria and produce more viruses when the bacterial cell ruptures.

Identifying the Genetic Material

- The experiment:• The 1st bacteriophage’s outer protein coat was

labeled with radioactive Sulfur.

• The 2nd bacteriophage’s DNA was labeled with radioactive Phosphorous.

• When they each infected a E.coli bacteria, Hershey and Chase discovered that

- the protein coat of the bacteriophage did not affect the E.coli bacteria

- the DNA of the 2nd bacteriophage had been injected into the E.coli.

Identifying the Genetic Material

Identifying the Genetic Material

• Chargaff’s Observations

- 1949, a biochemist at Columbia University, NY city.

- observed that adenine paired with thymine, cytosine pairs with guanine in the chemical make up of DNA.

Identifying the Genetic Material

• Rosalind Franklin’s Photographs

- developed X-ray diffraction

- photographed DNA

- 1st to theorize that DNA was a helix

Identifying the Genetic Material

• Watson and Crick

- 1953- used finding’s from Chargarff and Franklin

- used their knowledge of chemical bonding

- discovered that DNA is a double helix, a “spiral staircase” of two strands of nucleotides twisting around a central axis

The Structure of DNA

• DNA is a Double Helix• It is a Nucleic Acid made of two

long chains (also called strands) of repeating subunits called nucleotides

• Each Nucleotide is made of three parts:

1. a Five Carbon Sugar2. a Phosphate Group3. a Nitrogenous Base

Phosphate

5 Carbon Sugar

Nitrogen Base

The Structure of DNA

• In DNA, the sugar is Deoxyribose

• The phosphate group consists of a Phosphorous atom bonded to four oxygen atoms

• The Nitrogenous Base contains nitrogen atoms, and carbon atoms and is a BASE- accepts hydrogen ions.

• The alternating sugar and phosphate groups are held together by strong covalent bonds and make up the sides of the staircase/ladder.

Covalent Bonds

Phosphate

5 Carbon Sugar

Nitrogen Base

The Structure of DNA

• Each full turn of Helix has 10 Nucleotide pairs

• The Nitrogenous Bases face toward the center of the DNA Molecule and Bonds to the Bases on the other side with weak hydrogen bonds.

• 4 different Nitrogenous Bases

- Thymine (T)

- Cytosine (C)

- Adenine (A)

- Guanine (G)

The Structure of DNA• A and G have a double ring

of carbon and nitrogen atoms and called Purines.

• C and T have a single ring of carbon and nitrogen atoms and are called Pyrimidines

Chargaff’s Rule: A Purine must always pair with a Pyrimidine!

The Structure of DNA

• Complementary Bases:

- Base Pairing Rules state that

C always bonds with G

A always bonds with T

- These pairs of bases are called Complementary Base Pairs

- The order of the nitrogenous bases on a chain of DNA is called Base Sequence.

The DNA Code

• The sequence of nucleotide pairs controls how an organism will produce its proteins, therefore DNA controls the structure & function of the organism…it is not random!

• All organisms share the same chemical DNA. The difference is in the number, kind and order of the nucleotides!

DNA Replication

• DNA Replication is the process by which DNA is copied in a cell before a cell divides by Mitosis or Meiosis. Also called Duplication.

• Three Major Steps:

1. Enzyme called Helicase separates the DNA strands by breaking the hydrogen bonds between the base pairs.

- The Y-shaped region that is formed is called the Replication Fork.

DNA Replication

2. Enzymes called DNA Polymerases then add complementary nucleotides (which are found floating freely in the nucleus) to the original strands

- at a rate of about 50 nucleotides per second!

3. Bases are added in opposite directions on the strands.

- Synthesis moving away from the replication fork leaves gaps which can later be filled in by the enzyme DNA Ligase.

DNA Replication

Replication Fork- Hydrogen bonds broken by Helicase.

DNA Replication

• While the new bases are being added, new covalent bonds are formed on the outside between the new phosphate and sugar molecules

• New hydrogen bonds are formed between the new complementary base pairs on the original and new strands.

• DNA Polymerase falls off, resulting in two separate and identical DNA molecules that are ready to move to new cells in cell division.

DNA Replication

• After DNA Replication is complete, the end result is the original DNA molecule and an identical new DNA strand.

• This type of replication is called

Semi-Conservative Replication, because each of the new DNA molecules has kept one of the two original DNA strands.

DNA Replication

DNA Replication

DNA Replication

DNA Replication

• In Prokaryotes, remember that there is one circular DNA molecule.

-DNA Replication begins in one spot

- Two replication forks are formed and proceed in different directions until they meet.

DNA Replication

• In Eukaryotic, DNA is very long and straight.

- Replication beings at many points, or origins, along the DNA

- at each origin, two replication forks move in opposite directions

DNA Replication

• Any Error in DNA replication can lead to a mutation.