The DNA Story• Once the chromosomal theory of

inheritance (which we will study later) was widely accepted, scientists turned their attention to studying the chromosome.

Histone = protein

• The work of Mendel and Morgan allowed scientists to know genes are located on chromosomes.– Chemical composition of chromosomes

• DNA• Protein

• What was the genetic material?...believed to be protein!

• DNA was too simple and too uniform• DNA consists of 4 different nucleotides• Proteins were diverse…consist of 20 different

amino acids• Proteins have a wide variety of specific

functions• What organisms were initially studied?• Bacteria and viruses, since they were so simple

Experiments involved in DNA discovery!!

• Scientists involved:– Griffith (1928)– Avery– Hershey and Chase– Chargaff– Wilkins and Franklin– Watson and Crick (1953 paper published)

(received Nobel Prize in 1962)

-- Meselson and Stahl

1928 - Frederick Griffith

• Streptococcus pneumoniae, a bacteria that causes pneumonia in mammals was studied

• Transformation: change in genotype and phenotype due to the assimilation of external DNA

Oswald Avery

• Heat killed pathogenic bacteria and put individual molecules into non-pathogenic cells…only DNA transformed the non-pathogenic cells into pathogenic bacteria.

Evidence that Viral DNA can Program Cells

• Hershey and Chase used bacteriophage viruses that infect bacteria.

• Viruses made of protein and DNA• Tagged viral protein and DNA with

radioactive isotopes…inserted into E.coli • Found only the radioactive DNA within the

E.coli• Conclusion DNA (rather than protein) is

the hereditary material …at least for viruses

Circumstantial Evidence for DNA

• Prior to mitosis, eukaryotic cell doubles its DNA

• This DNA is distributed exactly evenly to the two daughter cells formed at the end of mitosis

• An organism’s diploid cells have twice the DNA as its haploid gametes

Experimental Evidence For DNA structure:

• Chargaff (1947) analyzed DNA composition of different organisms…discovered base composition. Equivalences between A,T and C,G are known as Chargaff’s rules

• Franklin and Wilkins X-ray crystallography pictures gave evidence for double helix

• Franklin had determined sugar-phosphate backbone of DNA

• Based on x-ray crystallograph of DNA, Watson and Crick determined the double stranded, helical nature of DNA.

• Width of the double helix and previously discovered Chargaff’s rules allowed them to surmise the complementary base pairing rules…A double H-bonds to T; C triple H-bonds to G.

Model of DNA suggests mechanism of its replication

• Watson and Crick proposed semi-conservative replication (rather than conservative or dispersive fashions)

• Late 1950’s Meselson and Stahl performed experiments to prove semi-conservative replication

DNA Replication

• When is the DNA replicated?• During the S phase of interphase.

Messelson and Stahl devised experiments to test how DNA replicates itself

Vocabulary of Replication

• Origin of replication = site on DNA molecule where replication begins (100’s to 1000’s of sites in eukaryotic cells)

• Replication fork = site on DNA strand where DNA “bubbles” and elongation occurs

• Leading strand = complementary strand of DNA copied in the 5’-3’ direction

• Lagging strand = synthesized by series of Okazaki fragments (built individually in the 5’-3’ direction)

• DNA polymerase = enzyme that adds nucleotides to new and growing strand of DNA (rate in humans: 50/sec)

• Helicase = enzyme that untwists and unwinds the DNA

Vocabulary continued

• Primase = enzyme that joins RNA nucleotides to make the primer the initial nucleotide chain

• Single Stranded binding Protein = molecules that stabilize untwisted DNA strands that have not started replicating yet

• Ligase = adds the okazaki fragments together on the lagging strand…adds the DNA that replaces the RNA primer on the leading strand

• Topoisomerase = relieves strain created by untwisting of DNA at replication forks on remaining twisted portion

Rules for Replication

• DNA polymerase can only add nucleotides to an existing polynucleotide (either RNA or DNA)

• DNA polymerase adds new nucleotides to the 3’ end of the growing strand

• The helicase continues to untwist and unwind the DNA in one direction at the replication fork.

• All of these rules combine to create a dilemma…

DNA Strands are antiparallel

Review the enzymes of replication!

1. Lagging strand2. Ligase3. DNA pol. I4. DNA pol. III5. Primase6. Single stranded binding protein7. Helicase8. Parent DNA9. DNA pol III10. Okazaki fragment11. Leading strand12. Overall direction of replication (5’3’)

DNA synthesis/replication and proofreading

• DNA polymerase I and III remove and replace misplaced nucleotides (mistakes 1/100,000 nucleotides, but most get “found” to make it only 1/10 billion)

• Mismatch repair special enzymes fix incorrectly paired nucleotides (like the delete key on a keyboard)

• Nucleotide excision repair Nucleases (enzymes) excise the damaged segment…remaining gap filled in by DNA pol. and ligases

• UV damage causes “thymine dimers”…xeroderma pigmentosa inherited disorder where enzymes are missing that would normally remove thymine dimers and replace them, results in hypersensitivity to UV exposure perhaps leading to skin cancer

The ends of the DNA molecules of eukaryotic cell cause problems on the leading strand

• Why is this problem only seen in eukaryotes?

• Prokaryotic DNA is circular so there is no end.

• Eukaryotes solve this problem with the special ends called telomeres

• Telomeres have no genes so if they are not replicated no important material is lost

Telomerase(an enzyme) is found in germ lines

that produce gametes, and in cancerous

cells. It catalyzes

lengthening

of telomeres, protecting gametes.