Chapter 13 - DNA Structure and Function

Structure of DNA

chromosomes are composed of DNA and protein
composed of 4 different nucleotides
- 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
2 strands wind 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

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
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 old 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)