dna (outline 14)

chromosomes

DNA + proteins

DNA

4 nucleotides (monomers)

Proteins

20 amino acids

Federick Griffith

1st to show evidence against protein carries genes hypothesis

model: streptocaus pneumiae — 2 strains

• rough → fine

• smooth → bad

Hershey & Chase

used bacteriophage T2

bacteriophadge

virus that infects bacteria

2 main components of bacteriophage

• DNA (or RNA)

• protein coat

grow 1 virus population with radioactive sulfur

proteins radiolabeled

grow 2nd virus with radioactive phosphorous

DNA radiolabeled

Result of experimental design

• allow each to infect bacteria

• separate viruses from bacteria

• Determine the location of radioactivity

• Conclusion: viruses transmit DNA; DNA carries genes

nucleotides

monomers of nucleic acids

each nucleotide contains 3 parts:

• phosphate group

• sugar

• nitrogenous base

nitrogenous base

• purines

• pyrimidines

purines

2 rings

pyrimidines

1 ring

Erwin Chargaff

studied chemical composition of DNA

Chargaff’s rule

• Total purine (A+G) = total pyrimidine (C+T)

• A = T

• C = G

• A+T ≠ C+G

Rosalind Franklin’s discovery

• DNA is double helix

• bases on the inside

• diameter is consistent

• distance b/w turns

• nucleotides per turn

Watson & Crick

explained how DNA could replicate & carry genetic information

Watson & Crick DNA model

1. Double Helix

2. Each strand has 2 parts

3. Strands held together with H-bonds

4. Strands are Antiparallel

5. Base Pairing Rules

1. Double Helix

2 strands of DNA wound around each other

one DNA molecule = one double helix

2. Each strand has 2 parts

• sugar phosphate background

• nitrogenous bases

3. Strands held together with H-bonds

• A and T— 2 hydrogen bonds

• G and C— 3 hydrogen bonds

4. Strands are Antiparallel

• run in opposite directions

• carbons in sugar numbered from 1-5

• Phosphate on each 5’C

• Hydroxyl on each 3’C

• 2 ends of strands differ

  • 3’: hydroxyl

  • 5’: phosphate

5. Base Pairing Rules

• always purine with pyrimidine

  • A-T base pair: 2 h-bonds

  • G-C base pair: 3 h-bonds

• explains constant helix width

Strands are complementary

• if sequence of 1 strand is known, the other can be determined

• provides mechanism for DNA replication (each stand can be a template for a new strand)

Semi-conservative replication

• Each existing strand acts as the template for a new strand

• has 2 parts

Initiation

• occurs at origin of replication

• double helix must be unwinded (done by DNA helicase)

RNA primer added

• short piece of RNA

• made by primase

• required to start adding new DNA nucleotides

• removed later & replaced with DNA

primase

an enzyme that can start a new strand

Elongation

• DNA polymerase adds nucleoside triphosphate

• 5’ phosphate attaches to previous 3’ hydroxyl (5’→ 3’)

• forms new phosphodiester bond

Elongation occurs at…

replication fork

Leading strand synthesis

5’-3’ towards replication fork – continuous elongation

Lagging strand synthesis

5’-3’ away from replication fork

• Elongates in discontinuous segments – Okazaki fragments

• Fragments connected via ligase