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BiologyGCSE, IGCSE & A Level

What is dna?

DNA is the double-helix molecule that stores genetic information as a sequence of four bases along a sugar-phosphate backbone.

DNA explained

Complementary base pairing — adenine with thymine, cytosine with guanine — holds the two strands together with hydrogen bonds and allows accurate replication.
A gene is a section of DNA coding for a protein; triplets of bases specify amino acids, whose order determines the protein's shape and function.

Key formula

A–T and C–G base pairs

Worked example

The base sequence TAC-GGA codes for two amino acids in the polypeptide chain being built.

Examiner tip

Use the correct terms: base, nucleotide, gene, chromosome. Mixing them up is one of the most heavily penalised errors in genetics.

DNA: common questions

What is a mutation?
A change in the base sequence of DNA. Most have no effect, some are harmful, and rarely one is beneficial.
What is the genome?
The entire genetic material of an organism, including all of its genes and non-coding DNA.

Related terms

Mitosis

Mitosis is nuclear division that produces two genetically identical daughter cells, used for growth, repair and asexual reproduction.

Natural selection

Natural selection is the process by which organisms with advantageous inherited characteristics are more likely to survive, reproduce and pass those alleles on.

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