13
DNA Structure and
Replication
I. Introduction and Review of Nucleic
Acids and Genes
A. Genes
1. History—Known that chromosomes contain
genetic information by 1920.
2. Genetic studies in the 1930s and 1940s indicate
that DNA is the genetic information.
3. Questions—How does DNA specify a protein?
How is it copied at each cell division?
II. The Watson-Crick Double Helix
A. Identity of Watson and Crick:
B.
Techniques they used to analyze structure of DNA—
x-ray crystallography with aid of Rosalind Franklin
collaborating with Maurice Wilkins:
II. The Watson-Crick Double Helix
Nucleotides and Nucleic Acids:
a) Review three components of
nucleotides—five-carbon sugar,
phosphate, and four types of nitrogencontaining bases.
b) Joined together in covalent bonds
between sugar and phosphate to
make a chain:
The Watson-Crick Double Helix
Components of Model:
Nucleotides :
Nitrogen bases are laid out
in specific and highly
varied order, carrying code
for protein synthesis.
(animation)
Two chains wrapped around each
other like a spiral staircase, with the
sugar-phosphate covalent bonds
making the sides of the staircase.
Two sides of staircase are held together by hydrogen
bonds between the N- bases (A, C, G, and T) like
steps of the staircase.
Each base makes half a stair with the other always
made up of its complement: A = T, and G = C.
III.Replication
2.Each of the two original strands
serves as a template (pattern) for
construction of a new matching
strand.
III. Replication
B. Steps in DNA replication: Figure 13.4
1. Double helix unwinds.
2. Nucleotides on each of two single strands are
now available to form base pairs with their
complement from a new free-floating
nucleotide.
3. Both original strands can be copied, making
two double helices from one original.
Animation
4. The new helices are composed of half old
(original) and half new nucleotides: Figure 13
DNA Replication in Action
New DNA Strands are
Identical
DNA is now copied so mitosis or
meiosis can take place.
5. Replication is complex, but catalyzed by enzymes
a) The addition and matching of new bases is catalyzed
by DNA polymerase.
b) DNA ligase permanently attaches short sections
together to make one long chromosome.
III. Replication
C. Importance of error-free copying
1. Error rate minimized by DNA polymerase
proofreading.
2. Bacterial polymerase brings 10 million bases
together incorrectly, but makes only 1/100,000
mistakes.
IV. Mutations
A.
Mutation = permanent alteration in cell’s
DNA base sequence: Figure 13.6
1.
Point mutations—slight change in chemical
form of base, or incorrect base pairs
2. Chromosomal mutations: Chapter 12
B. Almost all cancers begin as a mutation that is
passed along at replication.
1.
In somatic cells (body cells)
2.
Mutation rate is low, but after decades of
accumulated mutations, cells can become
malignant.
C. Heritable mutations occur in germ-line cells (cells
that divide to make sperm and eggs).
D. Heritable mutations also create genetic diversity.
The End