6.1 DNA & RNA Structure

The molecules of heredity

DNA is a double helix of two strands, using the sugar deoxyribose and the bases A, T, G, C. The strands are antiparallel and held together by hydrogen bonds between complementary bases (A–T, G–C). RNA is usually single-stranded, uses ribose, and replaces thymine with uracil (U).

Purines and pyrimidines

The bases come in two shapes: purines (adenine, guanine) have two rings; pyrimidines (cytosine, thymine, uracil) have one ring. A purine always pairs with a pyrimidine, keeping the helix a uniform width. Each strand has a 5′ end and a 3′ end.

Key terms

DNA double helix

Two antiparallel strands twisted together, with base pairs on the inside.

Antiparallel strands

The two DNA strands run in opposite 5′→3′ directions.

Purines vs. pyrimidines

Purines (A, G) have two rings; pyrimidines (C, T, U) have one.

RNA vs. DNA

RNA is single-stranded, uses ribose, and has uracil instead of thymine.

Base pairing rules

A pairs with T (or U in RNA); G pairs with C.

Try an AP-style question

Which conclusion is best supported?

1. A heritable factor from the dead S cells transformed the R cells
2. The R bacteria were virulent all along
3. The mice's immune cells turned into S bacteria
4. Heat brought the virulent S bacteria back to life