DNA

History of DNA In 1868, Miescher first isolated deoxyribonucleic acid, or DNA, from cells in pus and from fish sperm. No one knew its function Linus Pauling discovered the helical structure of proteins in 1951 In 1953, Watson and Crick discovered the structure of the master molecule of life—DNA.

History of DNA - Transformation In 1928, Fred Griffith was working with S (virulent) and R (nonvirulent) strains of a pneumonia-causing bacterium. He performed four experiments: a. Inject mice with R cells; mice lived b. Inject mice with S cells; mice died c. S cells were killed then injected into mice; mice lived. d. Live R cells plus heat-killed S cells were injected into mice; mice died; live S cells were found in the blood.

History of DNA - Transformation Some substance from the S cells had transformed the R cells. Both proteins and nucleic acids were candidates Oswald Avery showed that the substance was DNA

History of DNA Viruses called bacteriophages use bacterial cells for reproduction. Because they consist of only a protein coat and a nucleic acid core, these viruses were used in experiments (Hershey and Chase) to prove which of these was the hereditary material.

History of DNA 35S-labeled proteins in the bacteriophage coat did not enter the bacteria and thus were not participating in providing directions for new virus assembly. 32P-labeled DNA (the phosphates) in the viral core did enter the bacteria and direct new virus assembly.

The Structure of DNA Double helix as discovered by X-ray crystallography by Rosalind Franklin Each nucleotide is made up of: Deoxyribose (sugar) A phosphate group A nitrogenous base Adenine, guanine, cytosine, thymine A, G = purines 2 carbon rings C, T = pyrimidines 1 carbon ring

DNA Bonding Hydrogen bonds connect bases across from each other Phospho-diester bonds connect the phosphate of one nucleotide to the carbon of the sugar on another

The Structure of DNA Base-Pairing Rules: (Chargaff’s Rules) Guanine pairs with cytosine Thymine pairs with adenine DNA strands are antiparallel They run in opposite directions 5’ and 3’ ends

DNA Replication Big Picture: Occurs in the nucleus A new and identical molecule of DNA is made, using the old one as a template Occurs in the nucleus

Semi-Conservative Replication DNA replication is semi-conservative Each new DNA molecule has one old strand and one new strand

DNA Replication DNA replication begins at the origin of replication, a special sequence of DNA 2 strands are separated by helicase, which breaks apart hydrogen bonds, forming a replication bubble A Replication fork is formed at each end of the replication bubble

DNA Replication At replication fork, nucleotides “line up” with their complementary mates, according to the base-pairing rules DNA polymerase III attaches the nucleotides to the exposed bases of the DNA strand

DNA Replication: A Summary

Leading Strand DNA replication is different on the 2 strands DNA polymerase can only add nucleotides to the 3’ end of a DNA strand Along one template strand, the leading strand, DNA polymerase III just follows the replication fork (replicates continuously in one strand)

Lagging Strand On the other strand of DNA, the lagging strand – DNA polymerase must work in the opposite direction of the replication fork Short segments of DNA– Okazaki fragments – are made These start by RNA Primase setting down a short RNA priming sequence which Polymerase can add onto for DNA Okazaki fragments are joined by DNA ligase

DNA Proofreading DNA polymerase I proofreads each nucleotide as it is added to the DNA strand If there’s a mistake… it backs up removes the wrong nucleotide adds the right nucleotide

Enzymes & Their Job in Replication Helicases- unwind the DNA strand Single strand binding protein- holds the single strands apart for replication. Primase- inserts RNA primer to begin replication process. DNA Polymerase III- adds complementary bases to 3’ end of primer or new DNA strand. DNA Polymerase I- removes RNA primer & inserts DNA nucleotides. (also proofreads) DNA Ligase- “sews” Okasaki fragments of lagging strand together with covalent bonds.