Chapter 16 The Molecular Basis of Heredity

DNA = Genetic Material Chromosomes were determined to carry genes, but was is genetic material DNA or Proteins? 1952 Alfred Hershey and Martha Chase solved it using bateriophages (viruses that infect bateria) They radioactively tagged DNA and protein and waited to see who infected the bacteria…it was DNA

DNA = Double Helix James Watson & Francis Crick (with the help of Franklin and Wilkins) used X-ray crystallography to figure out the structure of DNA X-ray crystallography – is a technique of viewing molecules in 3-D

Watson & Crick Model Watson and Crick determined 3 major features of DNA 1)DNA is a double helix, which is a twisted ladder. The side is made of alternating phosphate and sugar while the rungs are made of pairs of nitrogenous bases 2)The nitrogenous bases are Adenine, Guanine, Cytosine, and Thymine (note: A-T and G-C) 3)The 2 “strands” are antiparallel, meaning one strand is rideside-up while the other is upside-down

DNA Replication Replication is the process of making DNA from pre- existing DNA It is semi-conservative, which means that at the very end the 2 identical DNA contains a parent and a daughter strand

DNA Replication 1.Replication begins at sites called origins of replication 2.Initiation proteins bind to origins of replication and separate the 2 strands (creating a replication bubble) 3.Group of enzymes called DNA polymerases facilitate elongation of new DNA at the replication fork

DNA Replication 4.DNA polymerases add nucleotides to growing chain one by one working in a 5’ to 3’ direction, matching A-T and C-G 5.Being antiparallel, the 5’ to 3’ growth is called the leading strand, while the segment of 3’ to 5’ is made in sections and called the lagging strand 6.the lagging strand is synthesized in pieces called Okasaki fragments which are sealed together by DNA ligase (forming a continuous DNA

Replication Accuracy Several factors to ensure that DNA is replicated correctly The specificity of base pairing (A-T and G-C Mismatch repair – special enzyme fixes incorrectly paired nucleotides Nucleotide excision repair – incorrectly paired nucleotides are excised (cut out) by enzymes called nucleases and the leftover gap is filled in with correct nucleotides NOTE: because nucleotides can only be added to 3’ end, that mean there will be leftover unmatched nucleotides at the 5’ end. This is solved by telomeres which create a cap at the end by adding short repeptitive nucleotides which hold no genes

DNA types There are differences between bacterial DNA and eukaryotic DNA Bacteria has a double stranded circular DNA (called plasmid) Eukaryotes have double stranded linear DNA

DNA & Chromatin In eukaryotes, the DNA is packed tightly into chromatin There are 4 levels of packing DNA 1.Involves DNA wrapped around proteins called histones. This resembles beads on a string and is termed nucleosome 2.the string of nucleosomes fold to form a 30nm fiber 3.Futher folding results in looped domains 4.As the looped domains fold, the chromosome is formed