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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 Birds
Male = 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 (18771955)
• 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. Levene
• Russian born; immigrated to America,
•
1.
2.
3.
moves to Europe.
1920’s discovered nucleotides
(building blocks of DNA)
Sugar (5 Carbon)
Phosphate group
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 o
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