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MDC BSC 2010 - Deoxyribonucleic acid

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F. DNA ReplicationDNADeoxyribonucleic acidA. Frederick Griffith identified DNA as the genetic material in 19281. mixed two strains of the bacterium Streptococcus pneumoniaea. the R strain was harmlessb. the S strain was pathogenic2. heat-killed S strain was mixed with live R strain and injected into a mouse, which died of pneumoniaa. transformation- a change in genotype and phenotype due to the assimilation of a foreign substance (DNA) by a cellb. Avery, McCarty, andMacLeod identified the transforming substance asDNA in 1944B. T.H. Morgan identified that genes are located on the chromosomes in 1940.1. Chromosomes are composed of DNA and proteinsC. Hershey and Chase showed that DNA was the genetic material in the bacteriophage (phage) T2 1. T2 infects E. coli2. radio active trace experimenta. radioactive sulfur identified protein and ended up in the supernatantb. radioactive Phosphorus identified DNA(1) ended up inside the bacterium  served as thegenetic material with information to make newvirusesD. Chargaff’s Rules1. DNA composition varies from species to species2. The four bases (ATCG) are found in charasteristic ratios for a species3. The % A = % T The % G = % CEx. 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 crystallography2. diffraction patterns yielded structural data & measurementsa. helical shape- double helix2nm wideb. backbone of sugar phosphatebondsc. 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 apartd. the helix twists every 10 bases at 3.4 nm3. The linear sequence of the four bases variesa. each gene has a unique ordernitrogenous basesF. DNA ReplicationSemi conservative mechanismWhen the double helix replicates each of the daughter molecules will have one old strand and one newly synthesized strand1. complimentary base pairing enables existing DNA strands to serve as templates for new complimentary strands2. the order of bases on one strand is used to add complementary bases and therefore duplicate thepairs of bases exactly. a. The double helix unwinds and the strands separateb. 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 strands3. Meselson & Stahl Experiment toprove semiconservative mechanisma. labeled the nucleotides of the original parental strands with 15N b. then transferred replicating DNA to 14 N medium.c. Separated replicated strandsby density in a centrifugeG. Process of DNA Replication1. starts at several origin of replication sites along the DNAa. the DNA strands separate and form a replication bubble withreplication forks at each endb. the replication bubble elongate as replication proceeds, the bubbles eventually fusec. Helicase, a protein that separates the template strands at the replication forkd. 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' directionb. Later, the RNA Primer is replaced by DNA 3. DNA Polymerases catalyze the elongation of new DNA at the replication forka. nucleotides with complimentary bases alignalong the template strand and are bonded by the DNAPolymeraseb. strands of the double helix are antiparallel. The sugar-phosphate backbones run in the opposite direction. One strand runs from 3' OH →5' PO4 , the other strand runsfrom 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 from5'  3' (3) the other parental strand 5'  3' is used by polymerase as a templatefor the synthesis of the the lagging strand, which is synthesized from a 5'  3' direction, in a serious ofdiscontinous Okazaki fragments which are laterjoined by enzyme DNA Ligase.4.


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