Chapter 16

• Transformation


  A change in genotype and phenotype due to assimilation of foreign DNA
• Viruses


  • Bacteriophages (phages)
• Virus


  DNA (sometimes RNA) enclosed by a protective coat, often simply protein
• Phage infection of cells

  1. Phage head attaches to cell

  2. Phage DNA enters cell

  3. Phage protein shell remains outside cell

  4. Centrifugation separates cell contents from outside phage parts
• In 1952, Hershey and Chase showed that DNA is the genetic material of a phage known as T2
• DNA


  A polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group
• The nitrogenous bases can be adenine (A), thymine (T), guanine (G), or cytosine (C)
• Chargaff's rules


  • The base composition of DNA varies between species
  • The number of A and T bases is equal and the number of G and C bases is equal
• Franklin's X-ray crystallographic images of DNA enabled Watson & Crick to deduce that DNA was helical
• Double helix


  The DNA molecule is made up of two strands, forming a double helix
• Antiparallel


  The DNA strands run in opposite directions
• Watson and Crick determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C)
• The Watson-Crick model explains Chargaff's rules: in any organism the amount of A = T, and the amount of G = C
• The significance of the experiments in which 32P and 35S were used is that it demonstrated that radioactively labeled bacteriophages transfer their DNA but not their protein coats to their host bacteria
• Replication fork


  The Y-shaped region where new DNA strands are elongating
• Enzymes involved in DNA replication

  • Helicases - untwist the double helix at the replication forks
  • Single-strand binding proteins - bind to and stabilize single-stranded DNA
  • Topoisomerase - relieves the strain of twisting of the double helix by breaking, swiveling, and rejoining DNA strands
• Replication begins at particular sites called origins of replication, where the two DNA strands are separated, opening up a replication "bubble"
• A eukaryotic chromosome may have hundreds or even thousands of origins of replication
• Replication proceeds in both directions from each origin, until the entire molecule is copied
• Replication bubble


  Region where DNA strands are separated, opening up replication
• Eukaryotic chromosome


  • May have hundreds or even thousands of origins of replication
• DNA replication


  Proceeds in both directions from each origin, until the entire molecule is copied
• Replication fork


  1. shaped region where new DNA strands are elongating
• Helicases


  • Enzymes that untwist the double helix at the replication forks
• Single-strand binding proteins


  • Bind to and stabilize single-stranded DNA
• Topoisomerase


  • Relieves the strain of twisting of the double helix by breaking, swiveling, and rejoining DNA strands
• Synthesizing a new DNA strand

  1. Initial nucleotide strand is a short RNA primer synthesized by primase

  2. DNA polymerases catalyze the synthesis of new DNA at a replication fork

  3. Each nucleotide added is a nucleoside triphosphate
• dATP


  Supplies adenine to DNA and is similar to ATP of energy metabolism, but has deoxyribose instead of ribose
• DNA polymerases


  • Add nucleotides only to the free 3' end of a growing strand, so a new DNA strand can elongate only in the 5' to 3' direction
• Elongating the leading strand

  DNA polymerase synthesizes the leading strand continuously, moving toward the replication fork
• Elongating the lagging strand

  1. DNA polymerase must work in the direction away from the replication fork

  2. Lagging strand is synthesized as Okazaki fragments, which are joined by DNA ligase
• Okazaki fragments


  Short segments that make up the lagging strand
• DNA polymerases


  • Proofread newly made DNA, replacing any incorrect nucleotides
• Mismatch repair


  • Repair enzymes correct errors in base pairing
• Nucleotide excision repair


  • A nuclease cuts out and replaces damaged stretches of DNA
• The error rate after proofreading and repair is low but not zero</b>
• Sequence changes may become permanent and can be passed on to the next generation
• These changes (mutations) are the source of the genetic variation upon which natural selection operates and are ultimately responsible for the appearance of new species
• Bacterial chromosome


  Double-stranded, circular DNA molecule associated with a small amount of protein, found in the nucleoid region of the cell
• Chromatin


  Complex of DNA and proteins in the eukaryotic cell nucleus