DNA: The Chemical Basis of Heredity

Chapter Goals History of how DNA’s structure and function were discovered DNA replication Role of DNA in directing RNA synthesis (transcription) Role of DNA in directing protein synthesis (translation)

The “Transforming Principle” Frederick Griffith (1920s) (Serendipity is important in science!) By 1920s… Chromosomes made up of proteins and DNA But…most scientists did not think DNA was the hereditary material.

Sequentially destroyed in S bacteria: Proteins Lipids Carbohydrates From Griffith’s work… Some substance causes a heritable change between the S and R cells. What is it? Avery & MacLeod, 1944: Sequentially destroyed in S bacteria: Proteins Lipids Carbohydrates DNA Then asked: Which component of S bacteria, when NOT destroyed, was able to “transform” the R bacteria to become virulent?

Very important experiment…but not well received and generally disregarded by the scientific community. “DNA is not complex enough to be the material of heredity.” “Bacteria are so simple, it is unlikely that they have DNA.”

1952: Hershey and Chase Used T2 bacteriophages to show that DNA was the genetic material in viruses

Making use of viruses: The T2 bacteriophage Viruses, in general: Are parasites! Not considered to be living organisms Insert their genetic material into host cell (Ex. bacteria, mucosa, T helper cells) Hijack host cell nutrients to make more virus particles May or not lyse (blow up) the host cell Fairly simple…contain a shell of protein and a strand of DNA inside

“Lytic”

Hershey & Chase (1952): Which part of the virus enters the bacteria Hershey & Chase (1952): Which part of the virus enters the bacteria? The DNA or the PROTEIN? Phosphorus is found in DNA…NOT in proteins Use 32P to label DNA Sulfur is found in proteins…not in DNA Use 35S to label protein

DNA is indeed the hereditary material in things as simple as viruses! (Just 60 years ago!)

X-Ray Crystallography The Structure of DNA X-Ray Crystallography (Rosalind Franklin) DNA is helical Chargoff’s Rule Amount of A = Amount of T Amount of C = Amount of G Quantity of Pyrimidines = Quantity of Purines Previous Modeling Experiments DNA is likely made up of 2 chains These 2 chains run antiparallel This is the information that Watson & Crick started with…

Watson & Crick’s Model

Anatomy of a Nucleotide 5 P P P-O- C Nitrogenous Base (A, C, T, G) 1 4 C C Pentose Sugar C C 3 2

5’ 3’ S P P S 3’ 5’ Nucleotide is made of: Nitrogenous base (A,C,G,T) Deoxyribose Sugar Phosphate 5’ 3’ S P S P 3’ 5’

YES! NO! NO! How Does DNA Replicate? 3 possibilities Semiconservative Replication: Each new molecule has 1 old strand and 1 new strand. YES! NO! NO!

Meselson-Stahl Experiment (Proof that DNA Replication is Semiconservative)

Rules of DNA Replication All chromosomes have at least one “origin of replication” (ori) Prokaryotes have only 1. Eukaryotic chromosomes have many. A huge protein complex, call the REPLICATION COMPLEX, binds to the “ori” 3. Each new DNA strand grows ONLY IN THE 5’ to 3’ DIRECTION

What does 5’ to 3’ mean? C #5 C #3

Steps Involved in DNA Replication Helicase unwinds the double helix (Helicase hacks!) Primase lays down an RNA primer (Primase primes!) DNA polymerase pastes in nucleotides (Polymerase pastes!) Ligase joins the smaller Okazaki fragments and rips up the RNA primer (Ligase links!)

What Happens at the End of the Line? Part of the end of each chromosome is “lost” following each round of replication (anywhere from 50-200 base pairs). Telomeres: The end portions of a chromosome made up of a repeating sequence of DNA—does NOT contain a gene.

Telomerase: An Enzyme to the Rescue But...Is telomerase always a good thing?

What happens when there is a mistake? DNA polymerase: 1,000 base pairs per second 1:1,000,000 error rate (1,000 errors every time DNA is replicated

Only 1 error in 1010 bases remains