Nucleic acids and hormones

Nucleic acids carry the instructions for life. They are long chains (polymers) of units called nucleotides. Each nucleotide has three parts:

• a base containing nitrogen,
• a five-carbon sugar,
• a phosphate group.

A base joined to a sugar, without the phosphate, is called a nucleoside.

There are two nucleic acids, DNA and RNA, and they differ in three ways:

In DNA's double helix, the two strands are held together by hydrogen bonds between bases, and the bases always pair the same way: A with T, and G with C. So the two strands are complementary: if you know one, you know the other. This is what allows DNA to copy itself (replication) exactly when a cell divides, passing on heredity.

RNA's main job is to make proteins. There are three kinds: messenger RNA (mRNA) carries the code from the DNA, ribosomal RNA (rRNA) forms part of the ribosome where proteins are built, and transfer RNA (tRNA) brings the right amino acids.

Every person's DNA is unique (except identical twins), so a pattern of DNA can identify a person, as fingerprints do. This DNA fingerprinting is used in forensic science, to settle questions of parentage, and to identify bodies.

Hormones are the body's chemical messengers. Glands release them into the blood, and they carry signals to other parts of the body. By chemical type:

• Steroids: the sex hormones, such as testosterone and the estrogens, and the hormones of the adrenal cortex.
• Polypeptides: insulin, which controls blood sugar (too little causes diabetes), and glucagon.
• Amino acid derivatives: thyroxine from the thyroid (too little causes goitre) and adrenaline (epinephrine).