Unit 6: Gene Expression and Regulation

Thomas Hunt Morgan

Worked with fruit flies

Did not know if the DNA or protein that make up chromosomes are the genes

What did Thomas Hunt Morgan conclude?

Genes are on chromosomes

Frederick Griffith

Studied bacteria to find a cure for pneumonia

What did Griffin discover?

• Harmless live bacteria mixed with heat-killed infectious bacteria caused diseases in mice

• Substance passed from dead bacteria to live bacteria - “transforming factor”

Avery, McCarty, & MacLeod

Purified both DNA and proteins from bacteria

Avery, McCarty, & MacLeod, what results did they get?

• Injected protein into bacteria → no effect

• Injected DNA into bacteria → transformed harmless bacteria into virulent bacteria

Hershey and Chase

Used bacteriophages (viruses that infect bacteria) with radioactive sulfur and phosphorous

Hershey and Chase, results from sulfer and phosphorous.

• Radioactive sulfur did not enter the bacteria

• Radioactive phosphorus did enter the bacteria → DNA is “transforming factor”

Chargaff Discovery

DNA composition varies from species to species but the bases are present in a characteristic ratio

In humans, what are the DNA bases? What are their percentages?

A = 30.9%

T = 29.4%

G = 19.9%

C = 19.8%

Watson and Crick

• Developed the double helix model of DNA

• Used a photograph taken by Rosalind Franklin

Pyramidines

Nitrogen bases with 1 ring (T & C)

Purines

Nitrogenous bases with 2 ring (A & G)

How do the DNA strands run?”

The strands are antiparallel

Which side on a strand has a phosphate and which doesn’t.

• 3’ = no phosphate on end

• 5’ = phosphate on end

What holds the strands together and how many of this thing? Which bases go together?

• Hydrogen bonds hold the two strands together

  • 2 between A & T

  • 3 between G & C

What holds the phosphates and sugars together?

Phosphodiester bonds

DNA in Prokaryotes vs. Eukaryotes

DNA Replication

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

How is DNA replication a semi-conservative process?

Each double helix consists of a parent/template strand and a new DNA strand

When does DNA replication occur?

S phase of the cell cycle

Enzymes involved in DNA Replication

Helicase, Topoisomerase, DNA polymerase, RNA polymerase (primase), Ligase

Helicase

unwinds part of the DNA double helix

Topoisomerase

helps relieve the strain of unwinding by breaking, swiveling, and rejoining DNA strands

DNA polymerase

connects nucleotides together to make a strand

There are multiple DNA polymerases; you are not expected to differentiate between them

RNA polymerase (primase)

adds a few nucleotides of RNA to get the process started

Ligase

connects DNA fragments together

How does DNA replication start?

First DNA helicase unwinds the DNA strands.

Topoisomerase relaxes supercoiling in front of the replication fork.