Chapter 12 - DNA Structure and Function

DNA as the Hereditary Information

• the identity of the hereditary information was uncertain until 1952
• protein or DNA
• a key experiment by Hershey and Chase confirmed that it's DNA [Figure 12.5]

Structure of DNA

• chromosomes are composed of DNA and protein
• composed of 4 different nucleotides [Figure 12.6]
  • each has phosphate, sugar, and nitrogen-containing base
  • nitrogen-containing base on each nucleotide is different (Adenine, Cytosine, Guanine, Thymine)
• worked out by Franklin and Watson and Crick (1953)
• nucleotides are linked into strands by bonds between the phosphates and sugars [Figure 12.7]
• 2 strands wind together into a double helix that is held together by the attraction of the bases
• the 2 strands run in opposite directions
• complementary base pairing: A attracts T and C attracts G
• same structure in all organisms
• the order of the bases provides the hereditary instructions

DNA Replication

• copying the DNA
• occurs before cell division can occur (in interphase)
• overview: DNA unwinds and each strand acts as a template to make a new strand
• occurs in 4 steps [Figure 12.10]
  1. DNA double helix unwinds.
  2. Free nucleotides are attracted to the unwound strands by complementary base pairing.
  3. Nucleotides are added one by one to the ends of new DNA strands.
  4. The original strand and the new strand wind together.
• result: two new double helices; each with an "old strand" and a "new strand"
• proofreading prevents mutations (changes in the DNA sequence, i.e. mistakes)

More Information

Science Odyssey's DNA Workshop

© Carol L. Wymer, 2003