AP Biology 2008-2009

DNAThe Genetic Material

Biology---Yippee!

Scientific History Understanding that DNA is the genetic

material T.H. Morgan (1908) Frederick Griffith (1928) Avery, McCarty & MacLeod (1944) Hershey & Chase (1952) Watson & Crick (1953) Meselson & Stahl (1958)

Genes are on chromosomes T.H. Morgan

working with fruit flies Determined: genes are on

chromosomes The question then was: is it

the protein or the DNA of the chromosomes that are the genes? through 1940 proteins

were thought to be genetic material because their structure is more complex

1908 | 1933

Frederick Griffith 1928

was working to find cure for pneumonia (Streptococcus pneumonia bacteria)

harmless live bacteria mixed with heat-killed infectious bacteria causes disease in mice

substance passed from dead bacteria to live bacteria = “Transforming Factor”

The “Transforming Factor”

Transformationsomething in heat-killed bacteria could still transmit disease-causing properties to the harmless bacteria

live pathogenicstrain of bacteria

live non-pathogenicstrain of bacteria

mice die mice live

heat-killed pathogenic bacteria

mix heat-killed pathogenic & non-pathogenicbacteria

mice live mice die

A. B. C. D.

Avery, McCarty & MacLeod purified both DNA & proteins from Streptococcus pneumonia bacteria to determine which will transform non-pathogenic bacteria?

injected protein into bacteria no effect

injected DNA into bacteria transformed harmless bacteria

into virulent bacteria

Concluded DNA is the

transforming factor

1944

What’s theconclusion?

Confirmation of DNA Hershey & Chase

classic “blender” experiment worked with bacteriophage

viruses that infect bacteria grew phage viruses in 2 media,

radioactively labeled with either 35S in their proteins 32P in their DNA

infected bacteria with labeled phages

1952 | 1969Hershey

Blender experiment Radioactive phage & bacteria in blender

35S phage radioactive proteins did NOT enter bacteria

32P phage radioactive DNA did enter bacteria

Confirmed DNA is “transforming factor”

Taaa-Daaa!

Summary Used radioactive labels to identify genetic

material Labeled DNA with radioactive phosphorus (32P) Labeled protein coats with radioactive sulfur

(35S) Blender Examined newly infected host cells (pellet) and

found only radioactive phosphorus label (not sulfur)

***DNA is the genetic material

Hershey & Chase

Alfred HersheyMartha Chase

1952 | 1969Hershey

Nucleic Acids (review) section 10.2

Examples: RNA (ribonucleic acid)

single helix DNA (deoxyribonucleic acid)

double helix

Structure: monomers = nucleotides

RNADNA

Nucleotides3 parts

1. nitrogen base Adenine, guanine, cytosine & thymine

2. pentose sugar (5 Carbon)ribose in RNA

deoxyribose in DNA

3. phosphate (PO4) group

Nitrogen baseI’m the

A,T,C,G or Upart!

Erwin Chargaff DNA composition: “Chargaff’s rules”

varies from species to species Amount of thymine = amount of adenine Amount of cytosine = amount of guanine

1947

Structure of DNA Watson & Crick

developed double helix model of DNA other scientists working on question:

Rosalind FranklinMaurice WilkinsLinus Pauling

1953 | 1962

Franklin Wilkins Pauling

Rosalind Franklin (1920-1958)

Rosalind Franklin & Maurice Wilkins (1950)1. used X-ray crystallography to study the structure

of DNA. In this technique, X-rays are diffracted as they

passed purified DNA. The diffraction pattern can be used to deduce the

three-dimensional shape of molecules.

Watson and Crick1953 article in Nature

CrickWatson

James Watson and Francis Crick

1. Concluded that the structure of DNA is a double helix. (After Watson viewed Franklin’s x-ray diffraction photo)

2. Backbone consists of alternating sugar & phosphate units.

3. Attached to the backbone are four kinds of bases.

I. Adenine III. Cytosine

II. Guanine IV. Thymine

Twist

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 16.5

Copying DNA Replication of DNA

base pairing allows each strand to serve as a template for a new strand

Franklin Stahl

Matthew Meselson

Meselson & Stahl experiment used radioactive nitrogen to determine that DNA replication is semi-conservative (each of the 2 new DNA molecules is half original or parent DNA and half newly made)

Semi-conservative replication 1958

AP Biology 2008-2009

DNA Replication

DNA Replication DNA used to make DNA Making an exact copy of the DNA before

the cell divides original strand serves as a template for

the new strand Each resulting double-stranded DNA

molecule is made of one original and one new strand ( ½ parent template and ½ new DNA) semi-conservative replication

Anti-parallel strands DNA molecule has

“direction” complementary strand runs

in opposite direction Replication only occurs in

the 5’ to 3’ direction

3

5

5

3

Bonding in DNA weak bonds

3

5 3

5

covalentphosphodiester

bonds

hydrogenbonds

strong bonds

Base pairing in DNA Pairing

A : T 2 bonds

C : G 3 bonds

Replication: 1st step Unwind DNA

Helicase enzyme unwinds part of DNA helix stabilized by single-stranded binding proteins

single-stranded binding proteins replication fork

helicase

DNAPolymerase III

Replication: 2nd step Build daughter DNA

strand add new complementary

bases to 3’’ end of growing DNA strand

Enzyme DNA polymerase III

strand only grows 53

Loss of DNA With each replication, small segments at the

end of our chromosomes (called telomeres) are lost This may be part of the aging process When enough DNA is lost the cell can no longer

divide

An enzyme called telomerase is able to add on to the end of chromosomes – but it is inactivated in most of our cells Cancer cells keep the enzyme active and can

divide forever