Biology / Unit 6: DNA and Protein Synthesis
6.1 DNA Structure
Notes
DNA (deoxyribonucleic acid) stores the instructions for making proteins.
- DNA is a double helix, like a twisted ladder. It was described by Watson and Crick in 1953, using X-ray images by Rosalind Franklin.
- The sides of the ladder are sugar (deoxyribose) and phosphate; the rungs are pairs of nitrogen bases.
- Base-pairing rules: A pairs with T, C pairs with G. The bases are held together by hydrogen bonds.
- The two strands are complementary. Because of base pairing, the amount of A equals T, and C equals G (Chargaff's rule).
- In eukaryotes, DNA wraps around proteins called histones to form chromosomes.
Practice
One DNA strand has the sequence ATCGGA. What is the complementary strand?
A pairs with T and C pairs with G.
A DNA sample is 30% adenine. What percent is guanine?
A = T = 30%, so C + G = 40%, and G = 20%.
What makes up the "sides" of the DNA ladder?
The sugar-phosphate backbone forms the sides; bases form the rungs.
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