BIO 101 1st Edition Lecture 20 Outline of Last Lecture I Chromosomal basis of sex II Genetics Problems a X linked diseases b Chromosomal alterations i Change in Chromosome number ii Changes in the structure of the chromosome Outline of Current Lecture I DNA as the genetic material a Transformation Experiments b Bacteriophage experiments II DNA Structure III DNA Replication Chapter 16 Search for the Genetic Material 1940 known that chromosomes carry the genetic information and that chromosomes are made of DNA and protein Most scientists thought that PROTEIN was the genetic material Two Lines of Evidence that DNA is actually the genetic material NOT PROTEIN 1 Transformation Experiments means take up of external genetic information by the cell 1928 Frederick Griffith did transformation experiments with bacteria 2 strains of bacteria were used S Strain smooth R strain rough Grew S strain bacteria and then killed them and broke open the cells S Soup which contained everything in the cell including the genetic material Put R strain bacteria into S Soup Some R bacteria took up genetic material and acquired the S phenotype transformation 1940 Avery and colleagues spent 10 years to identify the transforming chemical in S Soup it was DNA still not accepted by most people at that time 2 Bacteriophage experiments virus that infects bacteria Virus is made of nucleic acid DNA or RNA and protein it gets into a cell and makes more virus i e it carries genetic information Is the genetic info in viruses nucleic acid or protein 1952 Alfred Hershey and Margaret Chase experiment used a bacteriophage called T2 made of DNA and protein They made 2 kinds of T2 1 T2 with radioactive protein 2 T2 with radioactive DNA Found only DNA gets into the bacteria Therefore DNA not PROTEIN must be genetic material BIO 101 1st Edition DNA Structure Review DNA has 2 sugar phosphate chains Each sugar has one of 4 nitrogenous bases covalently attached to it A T C and G The two sugar phosphate chains are held together by hydrogen bonds between the nitrogenous bases on one strand and the nitrogenous bases on the other strand The hydrogen bonds are always A T 2 hydrogen bonds or C G 3 hydrogen bonds Each sugar phosphate chain has one 5 end and one 3 end o The 5 end has a free phosphate on carbon 5 of the sugar o The 3 end has a free OH on carbon 3 of the sugar The overall structure of the molecule is like a ladder The 2 sugar phosphates run in opposite directions called ANTI PARALLEL The hydrogen bonded nitrogenous bases are like the rungs of the ladder The whole molecules is then twisted into a helix spiral o Sugar phosphate chains are on the outside of the molecule o The hydrogen bonded bases are on the inside of the molecule The 2 strands are said to be COMPLEMENTARY to each other paired up by base pairing rules C G and A T Semi Conservative DNA Replication Overview 1 Starts at special places on the DNA called origins of replication bacteria have 1 origin eukaryotes have 100 s to 1000 s of origins 2 The 2 strands of DNA come apart like a zipper at the H bonded nitrogenous bases This requires 3 proteins Helicase unwind the spiral DNA Single stranded binding protein keeps 2 strands apart Topoisomerases untangle kinks 3 Each strand acts as a template to make a new complementary strand by the base pairing rules 4 Nucleotides building blocks for DNA for the new strand line up along the template strand bases pairing rules C G and A T BIO 101 1st Edition 5 An enzyme called DNA Polymerase III hooks the lined up nucleotides together Process requires a small RNA primer and a protein called primase New nucleotides are added to this RNA primer only in 5 3 direction i e can ONLY add a nucleotide to an existing 3 OH The Leading Strand is made as a single polymer moving toward the replication fork The Lagging Strand made by more complicated process Has to be made in short pieces called Okazaki Fragments because it goes in opposite direction from replication fork Each Okazaki fragment starts with an RNA primers RNA primers are removed and replaced with DNA by DNA POLYMERASE I The DNA pieces are hooked together by DNA Ligase Semi Conservative Replication each new molecules has one strand from parent molecule and one newly made strand Semi Conservative DNA Replication Complex process highly regulated Is very rapid 500 nucleotides added per second Is accurate o Initially 1 base pair mistake per 10 000 o Proof reading 1 base pair mistake per 1 billion
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