DNA

The Molecular Basis of Inheritance

http://player.discoveryeducation.com/index.cfm?guidAssetID=3692BEA5-C588-4A4E-AB35-0296ED8BE7DB

Identifying the Genetic Material 1928 Fredrick Griffith (English Bacteriologist)

Trying to find a vaccine for pneumonia Vaccine: prepared from killed/weakened microorganisms

introduced into the body to produce immunity Griffith worked with 2 strains of Streptococcus pneumoniae

bacteria S strain

Polysaccharide Capsule “Smooth” edged colonies Virulent – able to cause disease

R strain No Capsule “Rough” edged colonies Nonvirulent - does not cause disease

Griffith’s Experiment

Griffith’s Conclusion: Something had passed from heat killed bacteria to the nonvirulent R strain making them virulent… he called this the “transforming principal”

Griffith did not know what it was, but many scientists thought it was proteins

Today we know…

Transformation – cells take up foreign genetic material, changing their own genes (used for genetic engineering)

Heat killed S bacteria – enzymes were denatured therefore the DNA could not be copied

Proteins are denatured at 600C and DNA is denatured at 900C

DNA of heat killed S bacteria survived and transformed DNA of R bacteria

The Search for what caused the Transformation…

1944 – Oswald Avery, MacLeod, & McCarty (American Bacteriologists)

Experiment:1. Added protease to “R and heat-killed S” mixture

Result Mice died

2. Added DNAase to “R and heat-killed S” mixture Result Mice Lived

Conclusion: DNA, not protein, is the transforming factor in

Griffith’s experiment

More Evidence that DNA is the Genetic Material…

1952 – Alfred Hershey & Martha Chase (NY) Used T2 bacteriophages (phage) – virus

that infects bacteria Composed of nucleic acid surrounded by a

protein coat Viruses infect specific host Viruses are not living

Not composed of cells Cannot reproduce on their own Do not grow and develop

Background Info on Viruses

Hershey & Chase Experiment Experiment:

1. Grew T2 w/radioactive Sulfur 35S (protein coat takes in 35S)2. Grew another group of T2 w/ radioactive Phosphorus 32P (DNA

takes in the 32P)3. 35S-labeled and 32P–labeled phages were used to infect E.Coli

bacteria4. Separated phages from bacteria using a blender and a

centrifuge… the bacterial cells at bottom and viral parts at the top

Results: 35S-labels still in viral parts 32P-labels mostly in the bacterial cells, and new phages also

contained 32P DNA Conclusion:

Viral DNA (not protein) enters bacteria and carries instructions on how to make more phages

Without a doubt, DNA is the hereditary material!

Hershey & Chase Experiment

Structure of DNA By 1950’s most scientists were convinced that

Chromosomes carry genetic material Genes are on chromosomes Genes are made of DNA

Basic Structure of DNA Composed of nucleotides Nucleotides made of 3 parts deoxyribose, phosphate, N base 2 types nitrogen bases:

Purines – double ring of C and N Adenine Guanine

Pyrimidines – single ring of C and N Cytosine Thymine

Discovering DNA’s Structure Erwin Chargaff (NYC)

1947 – DNA composition varies among different species

1949 -Chargaff’s Rules- Discovered regularity of ratios: # Adenines = # Thymines

(ie. Humans A =30%, T=30%) # Guanines = # Cytosines

(ie. Humans G = 20%, C = 20%)

1952 Rosalind Franklin & Maurice Wilkins (England) Developed X-ray crystallography photographs of

DNA Suggested “helix” shape of 2-3 chains of

nucleotides

April 25th, 1953 James Watson & Francis

Crick (England) Built the 1st accurate 3D (tin

and wire) model of DNA “Double Helix” – spiral

staircase Purine is always linked by

h-bond to a pyrimidine 2 strands of DNA are

complimentary to each other

2 strands are anti-parallel 1962 Awarded the Nobel

Prize

More on DNA

Ex. If the sequence of bases on one strand is AATGCGCAT, than the complimentary strand will be: ________________

Human DNA has 3 billion base pairs.. Less than 1% of our DNA makes us different from one another!

Origin of Replication Prokaryotic Cell – single origin of replication where proteins

separate the 2 strands and create a replication bubble, replication proceeds in both directions from the replication fork

Eukaryotic Cells – hundreds or thousands of replication bubbles form to speed up the copying process, replication proceeds in both directions from the replication fork

DNA Replication Watson and Crick proposed that the complimentary strand of DNA serves

as a template for which the other strand is built…experiments confirmed this 5 years later

DNA Replication: Process of Synthesizing new molecules of DNA1. Helicase breaks H-bonds and opens up the double helix forming

replication forks (point at which DNA separates)2. At the replication fork, DNA Polymerases continuously adds

complimentary nucleotides to exposed bases 3. Process continues until all DNA has been copied, end result is 2 new

molecules of DNA each identical to the original and composed of one new and one old strand

http://www.youtube.com/watch?v=L9RjNNfgaEQ

DNA Synthesis

Proofreading

DNA polymerase only moves to the next nucleotide if the previous nucleotide was a correct match

If mismatched, DNA Polymerase backs up, removes the mismatched nucleotides and replaces it with the correct one(s).

Only 1 error per 1 billion nucleotides! http://www.youtube.com/watch?v

=oJZH4lV3h6I&mode=related&search=

DNA Replication & Aging Every time DNA is copied,

DNA polymerase cannot complete replication on the ends

Eukaryotic DNA has a non-coding, repeating nucleotide sequence on the ends called telomeres that protects genes from being eroded over successive replications

It is believed that telomeres are directly related to the aging process