Chapter 12

Genetic facts in 1900:

• Both female and male organisms have identical chromosomes except for one pair.

• Genes are located on chromosomes

• All organisms have two types of chromosomes:• Sex chromosomes

• Autosomes

Male vs Female

• MALE

• The Y chromosome.

• Y is usually smaller

• Male genotype = XY

• FEMALE

• The X chromosome.

• Larger than the Y

• Female genotype XX

Except BirdsMale = XX

Female = XY

Frederick Griffith

• 1928 = designed and performed experiment on rats and bacteria that causes pneumonia.

• 2 strains of the bacteria

• Type S = causes severe pneumonia (grew smooth colonies)

• Type R = relatively harmless (grew rough edged colonies)

Griffith’s Rats

1. First he injected living harmful Type S bacteria into rats:

• Second he injected dead Type S into the rats (meaning it was heated which killed the virus).

• Next he injected living harmless type R bacteria

• Finally he injected a mixture of heat-killed, disease-causing bacteria (dead S) with live, harmless ones (live R) and injected into the mouse.

Results of experiments:

• Because the dead rat tissue showed living Type S bacteria, something “brought the Type S back to life”

• Griffith called this transformation because one strain of bacteria (the harmless strain) had apparently been changed permanently into another (the disease-causing strain).

• Confirmed by Avery, MacLeod, and McCarty in 1944

In other words……

• Since the ability to cause disease was inherited by the transformed bacteria’s offspring, the transforming factor might be a GENE (or DNA).

Oswald Avery

• Canadian biologist (1877-1955)

• Discovered DNA in 1944 with a team of scientists.

Avery Continued…..

• Avery and his team used enzymes to repeat Griffith’s experiment to determine what caused the transformation.

• The enzymes would break down DNA.

• Conclusion: DNA was the transforming factor.

DNA…..

• Is a nucleic acid that stores and transmits the genetic information from one generation of an organism to the next.

Hershey and Chase

• 1952

• They collaborated in studying viruses, nonliving particles smaller than a cell that can infect living organisms.

The Hershey-Chase Experiment

• They used a bacteriophage (a virus which attacks bacteria) to prove that DNA was definitely the genetic material.

End result: Genetic material of the bacteriophage was DNA, not protein.

Scientists then wondered…..

• How DNA, or any molecule could do the 3 critical things that genes were known to do?

1.Genes had to carry info from one generation to another.

2.Genes put that info to work by determining the heritable characteristics of organisms.

3.Genes had to be easily copied

Phoebus A. LevenePhoebus A. Levene

• Russian born; immigrated to America, moves to Europe.

• 1920’s discovered nucleotides (building blocks of DNA)

1. Sugar (5 Carbon)2. Phosphate group3. Nitrogenous base

Composition of DNA

Components and structure of DNA

• A very long molecule. 4 nitrogenous bases:

DNA Structure

Chargaff’s rules

• The relative amounts of adenine and thymine are the same in DNA

• The relative amounts of cytosine and guanine are the same.

• Named after Erwin Chargaff

Rosalind Franklin

• Used X-Ray diffraction to get information about the structure of DNA:

Structure of DNA

• Discovered in 1953 by two scientists:

• James Watson (USA)

• Francis Crick (GBR)

• Known as the double-helix model.

The double-helix (twisted ladder)

• Watson and Crick’s model of DNA based off the evidence of Franklin’s X-ray demonstrated a double helix. 2 strands were wound around each other.

Base Pairing: Hydrogen bonds can form only between certain base pairs. This is why every adenine in a double-stranded

DNA molecule had exactly one thymine molecule and every cytosine molecule had one

guanine molecule.

How long is the DNA molecule?

Chromosomes & DNA replication

• The nucleus of one human cell contains approximately 1 meter of DNA.

• Chromatin consists of DNA that is tightly coiled around proteins called histones

Together, the DNA and histone molecules form a beadlike

structure called a nucleosome.

DNA replication

• Must occur before a cell divides.

Steps of DNA replication

1. DNA unzips by breaking the hydrogen bonds.

2. Matching nitrogen bases are added to each old strand.

3. DNA then seals back together (H bonds form).

4. The product of replication is 2 identical molecules of DNA, each with 1 old strand and one new strand.

Practice

1. What DNA sequence is complementary to the following DNA sequence?

ATCGCA

2. What DNA sequence is complementary to the following DNA sequence?

GGCAAT