DNA, Genes and
Chromosomes
Frederick Griffith 1928
• Are genes made up of DNA or protein?
• Griffith did an experiment with mice and
two different strains of bacteria called
Streptococcus pneumoniae
• One variety strain S looks smooth and
causes pneumonia in mice.
• The other strain R has a rough coat and
does not cause pneumonia in mice
Griffith’s experiment
Now, you answer these questions.
• 1. What happened to the mouse injected
with just strain S?
• 2. What happened to the mouse injected
with just strain R?
• 3. What happened to the mouse injected
with a mix of strain R and strain S which
had been HEATED up?
• 4. What happened to a mix of R and S
when only the S had been heated?
What’s the big idea?
* TRANSFORMATION
Griffith hypothesized that the heatkilled S bacteria had transformed
the live R strain changing it into S
bacteria.
He concluded that the transforming
material was genetic material.
In 1944 Oswald Avery
• Discovered that only DNA from strain S
bacteria was necessary to transform strain
R bacteria into S strain
• The importance- His findings supported
that DNA not protein was the genetic
material
DNA or Protein?????
• In 1952 Alfred Hershey and Martha Chase
conducted and important experiment in
order to prove that DNA was in fact the
genetic material.
• They used special viruses which can infect
bacteria called Bacteriophages.
• A bacteriophage is made up of two
components- DNA and protein. Hmm good
choice!
Hershey and Chase experiment
• Cell Culture 1
• Cell Culture 2
Bacteriophage
With Radioactive
DNA
Bacteriophage
With Radioactive
protein
Experiment summary
• In culture 1 the phages with radioactive
DNA infected the bacteria and the bacteria
BECAME radioactive too! SO, the DNA
went inside the bacteria.
• In culture 2, when the phages with
radioactive protein infected the bacteria,
the bacteria DID NOT become radioactive
because the protein remained on the
outside. SO, DNA is the heredity material!
The next big question was what
does DNA look like?
• DNA is made up of
4 Nitrogenous
bases:
• Adenine& Thymine
• Guanine& Cytosine
• And a Phosphate
and Deoxyribose
(sugar) backbone
DNA nucleotides
• A DNA nucleotide is made up of :
• One of the 4 Nitrogenous bases (A,T,G,C)
• A Phosphate
• And a sugar (Deoxyribose)
The bases are divided into two groups:
Purines- Adenine and Guanine
Pyrimidines- Thymine and Cytosine
The history of the race to the
double helix
• 1952-Edwin Chargaff discovers that in all
living organisms the amount of adenine
equals the amount of thymine and the
amount of cytosine equals the amount of
guanine.
• This discovery about the specific amount
of these bases became known as
Chargaff’s rules.
Maurine Wilkins and Rosalind
Franklin
• Both of these scientists especially the
brilliant Rosalind Franklin took pictures of
DNA using a special technique called
• X-ray Crystallography
• One of Rosalind’s pictures proved to be
the missing link in solving the puzzle of
how DNA looked.
Rosalind Franklin
• Rosalind Franklin and her famous picture
of DNA. Two scientists Watson and Crick
used this to discover the secret of DNA
1953 James Watson and Francis
Crick
• Discovered the structure of DNA and built
the first model of DNA. They used
Rosalind Franklins picture to figure out
that the A and T’s went together and the G
and C’s went together. Each base letter
was bonded to a sugar and a phosphate to
make a sugar-phosphate backbone
• =double helix
• NOBEL prize recipients for their work.
The famous Watson and Crick
Wow, James its
amazing what
you can make
these days
out of tinker toys!!
James Watson
Francis Crick
DNA replication
• Turn in your books to page 162.
• Write down the 3 steps in your notes.
• 1.
• 2.
• 3.
• What does DNA polymerase do?
Linked genes
• Genes that are on the same
chromosome are linked.
• That’s why we can have only 46
chromosomes with thousands of
characteristics!
Refining Mendel’s laws
• Geneticists William Bateson and R.C.
Punnett discovered that some genes are
linked when they experimented with flower
color in sweet pea plants.
Do genes shuffle?
• YES!
• In meiosis in prophase I the tetrads snap
together and the arms of the chromatids
cross allowing for genes to be exchanged.
• The result: LOTS of variation!
Sex Linkage
• Involves Sex chromosomes ONLY
• Traits that are the X chromosome are
found more often in males than females.
• Females are X
•
Males are XY
Sex linked Conditions
•
•
•
•
•
•
•
Color blindness in humans
XNXN= normal female
XNXn= Normal female carrier
XNY= Normal male
XnY= Color blind male
Hemophilia (Bleeder’s disease) Humans
White eyes in fruit flies
Sex –linked cross
N= Normal n= colorblind
XN
XN
Y
Xn
XNXN
XNXn
XNY
XnY
Sex- limited traits
• Traits that are only expressed in the
presence of SEX hormones and are only
observed in one sex or the other.
• Beard growth in males
• Milk production in females
• Differences in female and male bird
feathers.
Sex-influenced traits
• Traits that are expressed in both sexes but
they are expressed differently.
• Baldness in males is dominant and in
females recessive.
• Males
Females
• BB=bald
BB= hair
• Bb= bald
Bb= hair
• bb=hair
bb= bald
• A picture of one’s
chromosomes
taken when they
are lined up in
metaphase.
• An amniocentesis
is done to obtain
the cells from the
amniotic fluid
surrounding the
fetus.
Karyotype
The Human Genome Project
• A project established in 1988 to sequence
all of the DNA in the human body.
• That means all 23 chromosomes pairs
with 90,000 genes and 3 billion bases!
• Goal: Use the knowledge to cure disease
and improve life.
Chromosome Problems
• Nondisjunction- Down’s Syndrome.
• A child with 3 number 21 chromosomes
which total up to 47 in a somatic cell
instead of 46
• Called a Trisomy
Polyploidy
• Nondisjunction occurs in all the
pairs.
• Generally happens only in plants.
• Potatoes, oats, bananas, apples,
strawberries…