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 heat-killed 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

BacteriophageWith 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

James WatsonFrancis Crick

Wow, James its amazing what you can make

these daysout of tinker toys!!

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 crossN= Normal n= colorblind

XNXN XNXn

XNY XnY

XN Xn

XN

Y

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

Karyotype• 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.

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…