chapter 12

The physical structure of DNA is a sequence of monomers called nucleotides which form a double .
- • James Watson and Francis Crick studied X-ray diffraction data from Rosalind Franklin to deduce the double helix structure of DNA. They won the Nobel Prize with Maurice Wilkins in 1962.
- • Rosalind Franklin's X-ray diffraction "photo 51" of the B form of DNA provided clues for Watson and Crick to discover the structure of DNA.
- • The nucleotide subunits of DNA are composed of three parts:
  A negatively charged phosphate group.
  A five-carbon deoxyribose sugar, whose carbon atoms are given prime numbers.
  A nitrogenous base, which can be of 4 types: Adenine (A), Guanine (G), Cytosine (C), Thymine (T).
- • The sugar in each nucleotide of DNA is deoxyribose.
  The carbon atoms in the sugar ring are given prime numbers.
  The 1' carbon is attached to the nitrogenous base.
  The 3' carbon is attached to a free OH group.
  The 5' carbon is attached to the phosphate group.
- • DNA is a double helix made of nucleotides.
  The sugar-phosphate backbones are on the outside.
  The 2 strands are oriented in antiparallel directions: the 3' carbon of one nucleotide forms a covalent bond to the phosphate group (on the 5' carbon) of the next nucleotide.
  Complementary pairs of nitrogenous bases are held together on the inside by hydrogen bonds that follow these base pairing rules:
  - Cytosine (C) pairs with Guanine (G) by forming 3 hydrogen bonds.
  - Adenine (A) pairs with Thymine (T) by forming 2 hydrogen bonds.
  Note: RNA can also base-pair with DNA using the same base pairing rules, except A pairs with U.
  Review:
  Base-pairing Exercise:
DNA involves these steps:
- The enzyme unwinds the double helix, and lays down an RNA .
  - •
    1. The enzyme helicase unwinds the DNA double helix, forming a replication fork.
      Another enzyme primase puts down a short piece of RNA called the primer on a parental strand.
- DNA adds nucleotides to each strand.
  - •
    1. DNA polymerase assembles a complementary new strand on each parental strand.
      The two strands are replicated in opposite directions, always 5' -> 3'.
    Review:
  - • Nucleotides are added to the new strand by DNA polymerase, in the 5' -> 3' direction.
    The addition of the nucleotides follows base pairing rules: A pairs with T, C with G.
  - • DNA polymerase builds the leading strand as a continuous strand moving into (5' -> 3') the replication fork.
    The lagging strand is assembled moving away from (also 5' -> 3') the replication fork, in short pieces called Okazaki fragments, each with an RNA primer.
- DNA joins the fragments together.
- •
  1. RNA primer is removed and replaced with DNA.
    Ligase joins the fragments on the lagging strand.
    Helicase unwinds another segment.
    Review:
Summary:
A is a change in the DNA sequence that may affect proteins.
- • A mutation is a change in the DNA sequence, due to errors in DNA replication or to damage from physical or chemical agents called mutagens.
  Fruit flies normally have one pair of wings, extending from the thorax.
  This fly is a bithorax mutant.
  Because of a mutation in a gene regulating a stage of development, it has two thorax segments and two sets of wings.
- • Mutations.
  1. Mutations that change a single base pair are called base substitution mutations.
  2. Insertions and deletions can lead to frame-shift mutations.
- • Base substitution mutation.
  A base substitution mutation (change in a single base pair) can result in a change in a single amino acid (point mutation).
  Here a change in the DNA from G to T results in a change from C to A in the RNA, and the amino acid Proline is changed to Threonine.
  Review:
- • Frame-shift mutations.
  A deletion of one or more nucleotides can cause mis-reading of the RNA message, leading to a frame-shift mutation.
  Here deleting an A in the DNA results in a deleted U in the RNA, and the amino acid Histidine is changed to Glutamine.
  This also affects all amino acids of the protein downstream from the mutation.
  An insertion of one or more nucleotides can also lead to a frame-shift mutation.
  Review:
Some genes can "jump" from one place on the chromosome to another through a process called and result in a mutation.
- • Transposition occurs when a gene (transposon or "jumping gene") moves from one place on the chromosome to another.
  
  If a transposon (B) moves to another location (possibly on another chromosome), it may become inactive.
  
  If a transposon lands within another gene, it can cause a mutation in the latter.