Describe the structure of DNA, how genes code for proteins, and how DNA is packaged into chromosomes

What this dot point is asking

SCSA wants you to connect three scales: the molecule (DNA), the functional unit of heredity (the gene), and the package that carries genes through cell division (the chromosome). A strong answer moves smoothly between these levels and explains how base sequence becomes protein.

The structure of DNA

DNA (deoxyribonucleic acid) is a polymer made of repeating units called nucleotides. Each nucleotide has three parts: a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases - adenine (A), thymine (T), cytosine (C) or guanine (G).

Nucleotides join through phosphodiester bonds between the phosphate of one and the sugar of the next, forming a sugar to phosphate backbone. Because the sugar has distinct carbon positions, each strand has a direction: a 5 prime end and a 3 prime end.

Two strands then pair to make the famous double helix:

• A pairs with T using two hydrogen bonds.
• C pairs with G using three hydrogen bonds.

This is complementary base pairing. The two strands are antiparallel, meaning they run in opposite directions (one 5 prime to 3 prime, the other 3 prime to 5 prime). The bases sit on the inside of the helix and the backbones on the outside.

Genes and the genetic code

A gene is a segment of DNA that contains the instructions to make a polypeptide (or a functional RNA). The base sequence of a gene is read in groups of three bases called codons. With four bases arranged in triplets, there are 64 possible codons, which is more than enough to specify the 20 amino acids plus start and stop signals.

The genetic code has important features:

• It is degenerate (redundant): most amino acids are coded by more than one codon.
• It is non-overlapping: each base belongs to only one codon.
• It is nearly universal: the same codons specify the same amino acids in almost all organisms.

From gene to protein

Gene expression happens in two stages.

Transcription occurs in the nucleus. The enzyme RNA polymerase unwinds the DNA at the gene and synthesises a single strand of messenger RNA (mRNA) complementary to the template strand. RNA uses the base uracil (U) in place of thymine. The mRNA then leaves the nucleus through a nuclear pore.

Translation occurs at a ribosome in the cytoplasm. The ribosome reads the mRNA codons in sequence. Transfer RNA (tRNA) molecules carry specific amino acids and have an anticodon that pairs with the matching mRNA codon. As codons are read, amino acids are joined by peptide bonds into a growing polypeptide chain. The chain then folds into a functional protein.

This is the central dogma: DNA is transcribed to RNA, and RNA is translated to protein.

Packaging DNA into chromosomes

A single human cell contains about two metres of DNA, which must fit inside a nucleus only micrometres wide. The DNA is wound around proteins called histones. About 147 base pairs wrap around a core of eight histones to form a nucleosome, often described as beads on a string. Nucleosomes coil further into a more compact fibre, which loops and folds repeatedly.

When a cell prepares to divide, this fibre condenses tightly into the visible structures we call chromosomes. A duplicated chromosome consists of two identical sister chromatids joined at a centromere. Each chromatid carries one copy of the DNA molecule produced by replication.

Humans have 46 chromosomes arranged as 23 homologous pairs: 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). Homologous chromosomes carry the same genes in the same order, though they may carry different alleles (versions) of those genes.

Why packaging matters for inheritance

Organising DNA into chromosomes is not just about saving space. It allows the entire genome to be sorted and distributed evenly to daughter cells during mitosis and meiosis. Without condensation into discrete chromosomes, the long, fragile DNA threads could not be moved cleanly by the spindle, and errors in distributing genetic material would be common.