7.8 Nucleic Acids

Many of our traits are determined or influenced by genetics. Each of our proteins has a unique sequence, and so do our genes; the sequence of a gene directly determines the sequence of the protein it codes for.

DNA stands for deoxyribo nucleic acid, a double-stranded polymer made of a sugar-phosphate backbone held together by base pairs. RNA or ribonucleic acid is very similar in structure to DNA, but performs a different function in the cell. The RNA backbone consists of alternating monomers of ribose and phosphate, joined on the C5 and C3 oxygens of the ribose. DNA's backbone differs only in having deoxyribose as the sugar, with no oxygen atom on C2.

The end where the first phosphate is attached to the C5 atom is called the 5' terminus, while the end where the last sugar has just an unreacted hydroxy group on its C3 is the 3' terminus.

[skeletal formula of the sugar-phosphate chains of RNA and DNA]
Sugar-phosphate polymers of the same types as the backbones of RNA (left) and DNA (right).

Both DNA and RNA have bases attached to the C1 position of their sugar monomers. DNA uses the bases adenine (A), thymine (T), guanine (G), and cytosine (C); RNA substitutes uracil (U) in place of thymine, but otherwise uses the same bases. These nucleobases are derivatives of purine (A and G) and pyrimidine (T, U, and C) that fit together by means of hydrogen bonds; A mutually pairs with T or U, and G mutually pairs with C.

Adenine A

Thymine T

Guanine G

Cytosine C

Uracil U

The sequence of nucleobases on the strand is the sequence of that strand, and can be represented as letters, read from the 5' end to the 3' end, for example ATGGCATATGATCGATATGTGGCCATCTGCCGC. The two strands fit in opposite directions, and their sequences complement each other in reverse, so e.g. the other strand of the aforementioned sequence would be GCGGCAGATGGCCACATATCGATCATATGCCAT.

Be careful not to confuse the letters of the nucleic acid sequences with the letters of protein sequences.

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