Ch. 9 How Genes Work

Ch. 9-1 Understanding DNA

How Scientists Identified the Genetic Material

Scientists knew that chromosomes were involved in the transmission of traits

Wanted to know which chemical substance in the chromosomes, DNA or protein, store and transmit the genetic traits

Griffith Studies Bacteria pg.191

Frederick Griffith (British, 1928)Trying to find out HOW bacteria make people sickWanted to prepare a vaccine against pneumonia

Background: Studied 2 forms of the bacteria Pneumococcus:

Isolated from mice, both grow well in culture; only 1 kind caused pneumonia

2 types of Pneumococcus:

Type S – surrounded by a smooth capsulevirulent - disease causing

Type R – no capsule with rough edges

nonvirulent - does NOT cause disease

vaccine – substance prepared from killed or weakened pathogens and introduced into body to produce immunity

Griffith’s Experiments

Type S growsin culture

Type R growsin culture

Nothing

Conclusion: Some “factor” from dead Type S transformed (or changed) Type R into Type S

Griffith discovered:

Transformation - the transfer of genetic material from one organism to another, as when a bacterium takes up foreign DNA

Avery Identifies the Agent of Transformation (1944)

Avery modifed Griffith’s experiment:

Wanted to identify the transforming factor (the substance that made nonvirulent bacteria (Type R) become virulent bacteria (Type S)

Was it DNA or protein?

Experiment 1:

Extracted DNA from smooth (Type S) colonies of bacteria and added it to rough (Type R) colonies of bacteria

Some of the bacteria that grew from this mixture formed smooth (Type S) colonies

Transformation occurred

Experiment 2:

Added protein-destroying enzymes to bacteria and transformation still occurred

Added DNA-destroying enzymes to bacteria and transformation did NOT occur!!!

Conclusion:

Avery concluded that DNA was the genetic material

Hershey and Chase Confirm that DNA is the Genetic Material (1952)

Background: Used bacteria-infecting viruses called bacteriophages

Structure of viruses:

DNA core surrounded bya protein coat

Viruses attach to the surface of bacteria, inject their hereditary information which directs the synthesis of new viruses

What was injected, the DNA or protein?

When new viruses are mature, they burst out of infected bacteria and attack new cells.

Experiment (pg. 192):

Used viruses with radioactive isotope labels One group of viruses labeled with 35S which labels protein (with sulfur) Another group of viruses labeled with 32P which labels DNA (with phosphorus)

Each kind of virus infected 2 separate cultures of bacteria

After allowing enough time for the viruses to infect bacteria (by injecting their genetic material), they used a blender to separate the viruses and bacteria (centrifuge)and then measured for radioactivity.

Results:

No radioactivity in bacteria infected with 35S labeled viruses Viruses still radioactive

Radioactivity in bacteria infected with 32P labeled viruses

Viruses NOT radioactive

Conclusion:

Viral DNA enters bacteria and directs the synthesis of new viruses

DNA is the genetic (hereditary) material

DNA (deoxyribonucleic acid) – nucleic acid that contains the sugar deoxyribose and acts as the hereditary material

- Stores and transmits genetic information from one generation of an organism to the next AND codes for the production of a cell’s proteins

Nucleotide - structural unit of a nucleic acid consisting of 3 parts

3 parts of a nucleotide are: 5-C sugar (deoxyribose) phosphate group nitrogenous base (4 types)

4 types of N bases are: adenine (A)guanine (G)thymine (T)cytosine (C)

How Scientists Determined the Structure of DNA

Chemical components (building block of DNA & RNA):

Adenine Nucleotide

2 Types of nitrogenous bases:

purines - one of the 2 larger nucleotide bases (double ring structure)

ex. adenine and guanine

pyrimidines - one of the 2 smaller nucleotide bases (single ring structure) ex. cytosine and thymine in DNA

(uracil in RNA)

Chargaff’s Rules or base-pairing rules

Analyzed amount of N bases in DNA from different organisms

Found that:

Amount of adenine (A) in a DNA molecule alwaysequals the amount of thymine (T)

Amount of guanine (G) always equals the amountof cytosine (C)

Rosalind Franklin And X-Ray Diffraction (early 1950’s)

The X near the center shows that fibers that make up DNA are twisted to form a double helix

Groups of molecules in DNA are spaced out at regular intervals

Her pictures provided importantclues about the structure of DNA

James Watson and Francis Crick

Double Helix (1953)

They applied clues provided by Chargaff’s rules and Franklin’s X-ray diffraction studies

a. DNA was shaped like a spiral staircase composed of 2 strands of nucleotides whose N bases face each

other

b. Double helix – spiral of 2 strands, as in DNA structure

c. Two strands of nucleotides held together by weak H bonds between N bases

Adenine forms 2 H bonds only with thymineGuanine forms 3 H bonds only with cytosine

d. Overall structure of DNA

2 side chains made of alternating 5-C sugar (deoxyribose) and phosphate groups

Pair of N bases make up the “steps” and are linked by weak H bonds

N bases are attached to the deoxyribose (5-C sugar)

Francis Crick and James Watson were awarded the Nobel Prize in 1962

DNA molecular structure needed to explain HOW DNA could be copied exactly

Within a few weeks Watson and Crick wrote another paper describing the copying mechanism

Pairing Between Bases

Guanine

Cytosine

Adenine

Thymine

Base-pairing rules - rule that states that cytosine pairs with guanine and adenine pairswith thymine in DNA (adenine with uracil in RNA)

Complementary base pairs -

Characteristic of nucleic acids in which the sequence of bases on one strand is paired to the sequence of bases on the other strand.

Replication - copying of DNA before cell division (during Interphase - S)

DNA must be copied precisely every time a cell divides

Watson - Crick model suggested HOW this could occur

1. The 2 stands of the double helix are complimentary to each other

The sequence of bases of the one strand determines the sequence of bases on the other strand

Ex. If one strand is: A T T G C A T the complementary strand is: T A A C G T A

2. When the 2 strands separate, each strand exposes information to build 2 identical strands

Replication fork - area where the double helix (DNA) separates

3. DNA replication requires: several enzymes:

One enzyme: DNA helicase - breaks H bonds between base pairs and untwists strands

Another enzyme: DNA polymerase - attaches the correct complimentary nucleotides; catalyzes the formation of the DNA molecule

Replication usually preserves the sequence of bases in an organism’s DNA however, mutations can occur

Mutations - abrupt change in the genotype (DNA) of an organism

Mutagens - environmental agent that can alter the structure of DNA ex. Radiation, chemicals

4. Checking for errors - DNA polymerase has a “proof-reading” role, checking for errors

The Rate of Replication

Prokaryotes have a circular chromosome;2 replication forks that begin at the same pointand work away from each other

Eukaryotes have multiple chromosomes made of long strands of DNA

Each chromosome has multiple replication forks (may have about 100) and can replicatein 8 hours (If there was 1 replication fork would take33 days!)

DNAReplication