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Leading candidates for the chemical identity of the Gene?

1. DNA
2. Proteins (LEADING)

When different strains of bacterium infect the same host, how do the strain interact?

Some benign, other virulent, others transformed
Killed mice.

What was the "transforming factor" in Avery & others experiments?


What is the "stuff" of genes?


Does the transformation factor consist of protein, RNA or DNA

Different experiments in which all but one of the three factors were digested. Result - DNA

Do viral genes consist of DNA or protein?

Viral genes consist of DNA. Viral coats consist of protein.

Who discovered the DNA's structure?

Crick, Watson, Franklin

What is the basic building block of DNA?


Structure of DNA?

Phosphate group
Hydroxyl group
Adenine (A, T, G, C)

Primary structure of DNA? (4)

1. 5' end, 3' end
2. Phosphodiester bonds link nucleotides
3. Nitrogen-containing bases project from the backbone
4. Sugar-phosphate backbone of DNA strand

Specificity of Base pairing is dicated by...

hydrogen bonding

DNA replication is guided by....

Complementary pairing of bases (A with T, C with G) guides the replication of DNA.

DNA replication starts at special ....

"origins of replication"

DNA replication occurs at multiple points in the ....

Eukaryotic chromosomes
(only one in prokaryotes)

Direction of DNA Synthesis

Occurs in the 5' to 3' direction

DNA Synthesis differs between...

The leading and lagging strands.
(lagging strands - Okazaki fragments which need to be spliced together by ligase)

Synthesis of Leading Strand Steps (4)

1. DNA is unwound & opened by Topoisomerase and Helicase
2. Single-strand binding proteins (SSBP's) stabilize single strands
3. Primase lays down an RNA primer
4. DNA Polymerase III synthesizes DNA in 5' to 3' direction

Enzymes of Leading Strand Replication (5)

topoisomerase, helicase, SSBP's, Primase, DNA Polymerase II

Synthesis of Lagging Strand Steps

1. Primase synthesis RNA primer
2. DNA Polymerase II and Primase work together to make first one, then another Okazaki Fragment
3. DNA Polymerase I removes RNA Primer and replaces DNA
4. DNA Ligase closes gap in the sugar-phosphate backbone

Enzymes of Lagging Strand Steps (8)

Helicase, SSBP, Topoisomerase, Primase, DNA Polymerase II, Primase, DNA Polymerase I, DNA Ligase

Helicase Function

1. Breaks hydrogen bonds between base pairs
2. Opens double helix

SSBP function

Stabilize single-stranded DNA

Topoisomerase function

1. Break and rejoin DNA double helix
2. Relieve twisting forces

Primase function

Makes RNA primer (leading strand)
Makes RNA primer on an Okazaki fragment (lagging)

DNA Polymerase III

Extends the leading strand
Extends an Okazaki fragment (lagging)

Sliding clamp

Holds DNA polymerase in place during strand extension (leading & lagging)

DNA Polymerase I

Removes RNA primer and replaces it with DNA (lagging)

DNA Ligase

Joins Okazaki fragements into a continuous strand (Lagging)

What's the dilemma of replicating chromosome ends?

No way to replace RNA primer at the 5' ends of DNA in somatic cells

What results in the shortening of DNA in telomere (end) regions?

The inability of most somatic cells to replace RNA primer at the 5' ends of DNA

Telomeres contain...

non-coding DNA

Telomerase function

Replaces the missing DNA in the telomere region

Telomere replication steps (5)

1. When the RNA primer is removed from the 5' end of the lagging strand, a strand of DNA remains unreplicated
2. Telomerase binds to the overhanging section of ss DNA.
3. Telomerase adds nucleotides to the end of the parent DNA, extending it
4. Telomerase moves down the DNA strand and adds additional repeats
5. Primase, DNA polymerase, and ligase then synthesize the lagging strand in the 5' - 3' direction, restoring the original length of the chromosome

Cells that contain Telomerases?

Germ-line cells & some somatic cells

Loss of telomeric DNA in somatic cells linked to...


What is found in cancerous somatic cells and may be a key factor in carcinogenesis?



Creation of carcinogens

What kind of errors can occur during synthesis?


DNA can be damaged by what?


Chemicals that can damage DNA?

benzo-alpha-pyrenes (smoke)
UV light

DNA polymerase (repair)

Proof-read DNA to reduce errors

Nucleases (repair)

cut out damaged DNA

DNA polymerase & ligase (repair)

replace damaged nucleotides


tars in smoke that damage DNA
Harmless chemical, but converted to potent mutagen by body's own attempt to eliminate BPs

Benzo-alpha-pyrenes mutagen damages..

DNA, appears to damage a DNA repair gene

DNA Polymerase can repair...


Thymine Dimers

formed by UV Radiation Damage

Nucleotide Exision Repair

Cuts and excises damage strand and DNA polymerase fills in gaps, while DNA Ligase links the old with the new.

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