DNA History Structure & Replication

Griffith 1928 (British) • Studied: Better way to fight pneumonia

• Experiments:

Smooth bacteria=dead mouse.

Rough bacteria = alive mouse.

Heat killed smooth= live mouse

Heat killed smooth + rough=Dead mouse

• Conclusions: Some molecules or group of molecules changed the harmless rough bacteria into deadly smooth bacteria. Called transformation

Avery 1944• Studying: Process of transformation as a

key to finding heredity

• Experiments: Repeated above experiment: Treated heat-killed smooth with enzymes that destroyed either proteins, lipids, carbohydrates, RNA. None affected transformation. When DNA was destroyed transformation was blocked.

• Conclusions: Genes are made of DNA

Hershey & Chase 1950

• Studying: Skeptical of Avery’s conclusion. Wanted more support

• Experiments: Bacteriophage- virus infects bacteria. Knew viruses made of protein & DNA. Used radioactive isotopes to mark them Protein-sulfur-35, DNA-phosphorus-32 Found phosphorus-32 in bacteria

• Conclusions: Genetic material is DNA, not protein

Chargaff 1950• Studying: Studied nucleotide composition in

DNA

• Experiments: Showed that there was the same amount of adenine & thymine, and cytosine & guanine

• Conclusions: Chargaff’s rule: Always!! A-T C-G

• (Ash Tray / Garbage Can)

Franklin 1951• Studying: Studied DNA using x-ray diffraction

• Experiments: Powerful x-ray beams is aimed at a sample, ten the scattering pattern of the x-ray is recorded on the film

• Conclusions: By itself does not reveal the structure of DNA

– SHE WAS CHEATED OUT OF THE PRIZE!!!

Watson & Crick 1953• Studying: What is the structure of DNA? What

held the two strands together?

• Experiments: Analyzed other scientists’ work. Few weeks after Franklin’s x-ray experiments- they solved the puzzle of the structure of DNA. Made sketches & built models

• Conclusions: Two strands twisted around one another Double Helix

• Hydrogen bonds between the bases

Won Nobel Prize in 1962:

*1962-Watson & Crick- discovered structure of DNA

*1960- Brenner-Discovered mRNA

*1977- Gilbert, Maxam, & Sanger- read DNA sequence

*2000- Human Genome Project- sequence all human DNA is nearly complete

DNA MOLECULE

Nucleotides is the monomer of Nucleic Acids

consist of : 1)Phosphate Group

2)5 sugar deoxyribose

3)A nitrogen base

-Weak Hydrogen bonds join nucleotides between nitrogen bases

-Strong Stable Covalent bonds holds each part of the nucleotide together.

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The 4 Bases of DNA• (A)denine• (T)hymine• (C)ytosine• (G)uanine

Only specific pairs of bases bond together

A bonds to T

C bonds G (Ash Tray / Garbage Can)

DNA form a DOUBLE HELIX (like a twisted ladder)

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Pyrimidines

• Purines

Role of DNA

• Chromosomes are formed of genes

• Genes are segments of DNA which are the units of heredity.

• DNA controls the development of traits and cellular activities.

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DNA Sequence

Sequence of nucleotides form unique genetic information

• *Different living organisms all have DNA made of the same nucleotides but in a different order.

» Example: CAT, TAC, ACT it has the same amount of letters, the same letters but they are all different. That is why every living organism can have the same things that make them up but is still different.

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• Which one of the following nucleotide pair bonds would be found in a DNA molecule?

a. cytosine-guanineb. adenine-cytosinec. cytosine-uracild. adenine-guanine

DNA REPLICATIONPage 11

DNA REPLICATION• Before Cells divide, DNA has to copy itself,

which is called… DNA Replication.

• During this process DNA is in the form of Chromatin (DNA wrapped in protein)

• There are many “free” nucleotides found in the nucleus of a cell

• These “free” nucleotides act as building blocks for new DNA

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STEPS OF DNA REPLICATION

1) Double helix untwists, enzyme “Helocase” breaks the H bonds, 2 nucleotide chains begin to separate like a zipper.

2) Each ½ serves as a pattern for the formation of a new DNA chain.

3) Free nucleotides in the nucleus join with the correct bases. (Chargaff’s BASE PAIRING RULE)

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STEPS (cont.)4) 2 new molecules of DNA (each contain an

original & a new strand) become twisted and take the form of a Double Helix

The 2 new DNA molecules are now called “Sister Chromatids” and remain stuck together until the cell divides.

Uncoiling - unzipping -- adding new parts - 2 DNA molecules - recoiling

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DNA RECOMBINATION

• Notice A always joins to T and G always to C

DNA errors in replication

• Mutation- a change in the nucleotide sequence

• Sometimes the mutation allows individuals to survive and reproduce better

• Sometimes the mutations can cause diseases such as cancer.