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Transcript
DNA Deoxyribonucleic acid A. Frederick Griffith identified DNA as the genetic material in 1928 1. mixed two strains of the bacterium Streptococcus pneumoniae a. the R strain was harmless b. the S strain was pathogenic 2. heat-killed S strain was mixed with live R strain and injected into a mouse, which died of pneumonia a. transformation- a change in genotype and phenotype due to the assimilation of a foreign substance (DNA) by a cell b. Avery, McCarty, and MacLeod identified the transforming substance as DNA in 1944 B. T.H. Morgan identified that genes are located on the chromosomes in 1940. 1. Chromosomes are composed of DNA and proteins C. Hershey and Chase showed that DNA was the genetic material in the bacteriophage (phage) T2 1. T2 infects E. coli 2. radio active trace experiment a. radioactive sulfur identified protein and ended up in the supernatant b. radioactive Phosphorus identified DNA (1) ended up inside the bacterium served as the genetic material with information to make new viruses D. Chargaff’s Rules 1. DNA composition varies from species to species 2. The four bases (ATCG) are found in charasteristic ratios for a species 3. The % A = % T The % G = % C Ex. Human DNA 30.9% A, 29.4% T, 19.9% G, 19.8 % C E. James D. Watson and Francis Crick elucidated the three-dimensional structure of DNA in 1953. 1. Rosalind Franklin and Maurice Wilkins created diffraction patterns of DNA with X-ray crystallography 2. diffraction patterns yielded structural data & measurements a. helical shape- double helix 2nm wide b. backbone of sugar phosphate bonds c. nitrogenous bases, one from each strand, are connected by H-bonds to form pairs in the middle (1) A=T (2) G≡C (3) the bases are 0.34 nm apart d. the helix twists every 10 bases at 3.4 nm 3. The linear sequence of the four bases varies a. each gene has a unique order nitrogenous bases F. DNA Replication Semi conservative mechanism When the double helix replicates each of the daughter molecules will have one old strand and one newly synthesized strand 1. complimentary base pairing enables existing DNA strands to serve as templates for new complimentary strands 2. the order of bases on one strand is used to add complementary bases and therefore duplicate the pairs of bases exactly. a. The double helix unwinds and the strands separate b. one at a time, nucleotides line up along the template strand according to the base pairing rules. c. Then the nucleotides are linked to form new strands 3. Meselson & Stahl Experiment to prove semiconservative mechanism a. labeled the nucleotides of the original parental strands with 15N b. then transferred replicating DNA to 14 N medium. c. Separated replicated strands by density in a centrifuge G. Process of DNA Replication 1. starts at several origin of replication sites along the DNA a. the DNA strands separate and form a replication bubble with replication forks at each end b. the replication bubble elongate as replication proceeds, the bubbles eventually fuse c. Helicase, a protein that separates the template strands at the replication fork d. Single-strand binding Proteins keep template strands apart. 2. To start synthesis an RNA Primer is required. a. Primase, an RNA polymerase, that can start synthesis from a single strand, catalyses the reaction to add the RNA Primer leaving a 3' OH end, to which DNA Polymerase can add nucleotides in a 5'3' direction b. Later, the RNA Primer is replaced by DNA 3. DNA Polymerases catalyze the elongation of new DNA at the replication fork a. nucleotides with complimentary bases align along the template strand and are bonded by the DNA Polymerase b. strands of the double helix are antiparallel. The sugarphosphate backbones run in the opposite direction. One strand runs from 3' OH → 5' PO4 , the other strand runs from a 5' PO4 →3' OH direction (1) DNA Polymerase adds nucleotides to the 3' end. the new DNA strand elongates in the 5' 3' (2) at the replication fork, the parental strand 3'5' is used by polymerase as a template for the synthesis of the leading strand which elongates continuously from 5' 3' (3) the other parental strand 5' 3' is used by polymerase as a template for the synthesis of the the lagging strand, which is synthesized from a 5' 3' direction, in a serious of discontinous Okazaki fragments which are later joined by enzyme DNA Ligase. 4. Summary: