How is genetic information stored, organised and expressed in living organisms?
Describe the structure of DNA, how genes code for proteins, and how DNA is packaged into chromosomes
A focused answer to the WACE Year 12 Biology dot point on DNA, genes and chromosomes. Covers nucleotide structure, the genetic code, transcription and translation, and how DNA is packaged into chromosomes.
Reviewed by: AI editorial process; not yet individually human-reviewed
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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.
Exam-style practice questions
Practice questions written in the style of SCSA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WACE 20226 marksDescribe the structure of a DNA molecule and explain how the sequence of bases in a gene is able to store the information needed to make a polypeptide.Show worked answer →
A 6 mark answer needs structure plus the structure-to-information link.
Structure. DNA is a polymer of nucleotides, each made of a deoxyribose sugar, a phosphate group and one of four bases (A, T, C, G). Nucleotides join via sugar-phosphate bonds into two strands that run antiparallel and twist into a double helix. The strands are held together by complementary base pairing: A with T (two hydrogen bonds) and C with G (three hydrogen bonds).
Storing information. A gene is a length of DNA. Its information is held in the order (sequence) of bases. The sequence is read in triplets called codons; each codon specifies one amino acid. Because the order of codons sets the order of amino acids, the base sequence determines the polypeptide that is made.
Markers reward nucleotide composition, complementary pairing, the antiparallel double helix, and the triplet-code link from base sequence to amino acid sequence.
WACE 20245 marksAbout two metres of DNA must fit inside a nucleus only micrometres across. Explain how DNA is packaged into chromosomes and why this packaging is important for cell division.Show worked answer →
A 5 mark answer needs the packaging hierarchy plus its function in division.
Packaging. The long DNA molecule is wound around histone proteins; roughly base pairs wrap around a core of eight histones to form a nucleosome (beads on a string). Nucleosomes coil into a thicker fibre that loops and folds repeatedly. When a cell prepares to divide, this fibre condenses tightly into a visible chromosome made of two identical sister chromatids joined at a centromere.
Why it matters. Condensing the fragile DNA threads into compact, discrete chromosomes lets the spindle move them cleanly and distribute one complete copy of the genome to each daughter cell. Without condensation the long strands would tangle and break, causing errors in inheritance.
Markers reward the histone or nucleosome detail, condensation into chromosomes or chromatids, and the link to accurate separation during division.