History of DNA Biology AP Todeschini

Race to Discover By the mid 20 th century genetics was well understood but he molecule in which it was conserved and passed was not. Many scientists’ contributions led to the discovery of the molecule of inheritance, DNA

Griffith(1928): Transformation A Scottish microbiologist, Frederick Griffith Discovered that bacteria could give other bacteria the ability to cause disease even after they were dead. Up until 1940 most people believed proteins to be the genetic molecule, wrong!

Avery, McCarthy, MacLeod(1944): Griffith Refined Destroyed various components of virulent bacteria and exposed non- virulent strains to them one at a time. From lipids, proteins, carbohydrates and DNA, only damaged DNA prevented transformation.

Hershey & Chase(1952): Bacteriophages Tagged proteins of viruses with radioactive sulfur and DNA with radioactive phosphorus to which one entered the cell. Only DNA enters the cell and left instructions to make more viruses.

Fig EXPERIMENT Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P)

Fig EXPERIMENT Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P) Empty protein shell Phage DNA

Fig EXPERIMENT Phage DNA Bacterial cell Radioactive protein Radioactive DNA Batch 1: radioactive sulfur ( 35 S) Batch 2: radioactive phosphorus ( 32 P) Empty protein shell Phage DNA Centrifuge Pellet Pellet (bacterial cells and contents) Radioactivity (phage protein) in liquid Radioactivity (phage DNA) in pellet

Chargaff(1950): Base Pairing Rule The amount of adenine, thymine, guanine, and cytosine varies between species All have equal amounts of adenine=thymine, guanine=cytosine. Hmmm…

Franklin & Wilkins: DNA X-Rays Once it was undeniable that DNA was the molecule, the race was on to determine the structure. Rosalind Franklin crystallized DNA and took pictures of it. Maurice Wilkins, her colleague shared them with Watson and Crick, without her permission. (photo 51)

Watson & Crick: DNA structure Watson and Crick were working on solving the “puzzle” by comparing other scientists work on DNA. When they saw Franklin’s photo 51, it clicked! The won the Nobel Prize for discovering the double helix structure of DNA(1962)

DNA Structure By knowing the structure the mechanisms for replication and expression of genes could be elucidated.

DNA Molecule Bases on one side of a strand are covalently(permanently) bonded to each other with phosphodiester bonds. Bases on opposite strands are hydrogen bonded (temporarily) to each other forming base pairs

DNA  Chromatin  Chromosomes Chromosomes are densely packed double stranded DNA molecules (with hundreds of millions of base pairs). Chromosomal proteins help mediate this packing.

DNA Replication Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules Semi-conservative: Each new strand contains one old and one new half.

Fig A T G C TA TA G C (a) Parent molecule DNA Replication

Fig A T G C TA TA G C A T G C T A T A G C (a) Parent molecule (b) Separation of strands DNA Replication

Fig A T G C TA TA G C (a) Parent molecule AT GC T A T A GC (c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand (b) Separation of strands A T G C TA TA G C A T G C T A T A G C DNA Replication: semiconservative

DNA Replication The copying of DNA is remarkable in its speed and accuracy More than a dozen enzymes and other proteins participate in DNA replication Replication begins at special sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble”

Fig Origin of replication Parental (template) strand Daughter (new) strand Replication fork Replication bubble Two daughter DNA molecules (a) Origins of replication in E. coli Origin of replicationDouble-stranded DNA molecule Parental (template) strand Daughter (new) strand Bubble Replication fork Two daughter DNA molecules (b) Origins of replication in eukaryotes 0.5 µm 0.25 µm Double- stranded DNA molecule

Fig a Origin of replication Parental (template) strand Daughter (new) strand Replication fork Replication bubble Double-stranded DNA molecule Two daughter DNA molecules (a) Origins of replication in E. coli 0.5 µm

Fig b 0.25 µm Origin of replicationDouble-stranded DNA molecule Parental (template) strand Daughter (new) strand Bubble Replication fork Two daughter DNA molecules (b) Origins of replication in eukaryotes

DNA Replication Enzymes Helicases are enzymes that untwist the double helix at the replication forks Single-strand binding protein binds to and stabilizes single-stranded DNA until it can be used as a template Topoisomerase corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork An enzyme called primase can start an RNA chain from scratch and adds RNA nucleotides(RNA primer) one at a time using the parental DNA as a template DNA ligase seals together fragments of DNA backbone on leading and lagging strand.

DNA Replication DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides DNA can be damaged by chemicals, radioactive emissions, X-rays, UV light, and certain molecules (in cigarette smoke for example)

Telomeres Each time DNA replicates because of the position of the primer DNA gets shorter each time An enzyme called telomerase catalyzes the lengthening of telomeres in germ cells It has been proposed that the shortening of telomeres is connected to aging (cellular fuse) Over active telomerase activity could be linked to cancer in cells.