6.1 DNA and RNA Structure

Every living cell stores its genetic information in DNA, and passes it on by copying that DNA. Some viruses use RNA instead (examples include influenza and HIV), which is why the topic says hereditary information is stored in DNA and, in some cases, RNA.

How the DNA is packaged differs between the two main cell types:

• Prokaryotes (bacteria and archaea) usually have a single circular chromosome that sits in the cytoplasm in a region called the nucleoid. There's no nucleus.
• Eukaryotes have several linear chromosomes inside a nucleus. Each chromosome is one very long DNA molecule wrapped around proteins called histones, like thread around spools. This DNA–protein mix is called chromatin, and it can be packed loosely or tightly (which matters for gene expression in 6.5).
• Plasmids are small, circular, extra DNA molecules separate from the main chromosome. They're common in bacteria and also found in some eukaryotes such as yeast. Plasmids often carry useful genes, such as antibiotic resistance genes, and they're a key tool in biotechnology (6.8).

DNA and RNA are polymers of nucleotides. Each nucleotide has a five-carbon sugar (deoxyribose in DNA, ribose in RNA), a phosphate group and a nitrogenous base. The sugar of one nucleotide bonds to the phosphate of the next, forming a sugar-phosphate backbone with a 5′ end and a 3′ end.

In DNA, two strands wind into a double helix. The strands are antiparallel: one runs 5′ → 3′ and its partner runs 3′ → 5′. The bases point inward and pair by hydrogen bonds. RNA is usually single-stranded, but it can fold back on itself where bases pair, which gives molecules like tRNA their shape.

The bases come in two shapes. Purines, adenine (A) and guanine (G), have a double-ring structure. Pyrimidines, cytosine (C), thymine (T) and uracil (U), have a single ring. A purine always pairs with a pyrimidine, which keeps the helix the same width all the way along.

The specific pairs are A with T (A with U in RNA) and G with C. G–C pairs form three hydrogen bonds and A–T pairs form two. Because the pairing is so specific, each strand's sequence determines its partner's, so either strand can serve as a template for an exact copy. These pairing rules are the same in all known life, which is part of the evidence for common ancestry.

In double-stranded DNA, every A is paired with a T and every G with a C. So the amount of A equals the amount of T, and the amount of G equals the amount of C. If you know the percentage of one base, you can work out all four. This shortcut works only for double-stranded DNA; a single-stranded RNA molecule has no partner strand, so its A and U don't have to match.