DNA, genes and the genetic code: WACE Year 12 Human Biology Unit 4
“Describe the structure of DNA, the relationship between genes, alleles, genotype and phenotype, and how the genetic code underpins variation”
DNA is a double helix of two strands held together by complementary base pairs (A with T, C with G). A gene is a length of DNA coding for a protein; an allele is a version of a gene; the locus is its position on a chromosome. The genotype is the alleles an individual carries, and the phenotype is the observable result. Differences in alleles, read through the genetic code into different proteins, are the basis of inherited variation.
What this dot point is asking
WACE wants you to have the vocabulary and the molecular basis of inheritance secure, because every later topic (variation, selection, evidence, human evolution) rests on it. This page is the foundation: what DNA is, how it is organised into genes and alleles, and how the code links a sequence to a trait.
The structure of DNA
DNA (deoxyribonucleic acid) is the molecule that stores genetic information. It is a double helix: two strands twisted around each other. Each strand is a chain of nucleotides, and each nucleotide has a sugar (deoxyribose), a phosphate group, and one of four nitrogen bases: adenine (A), thymine (T), cytosine (C) and guanine (G). The two strands are held together by complementary base pairing: A always pairs with T, and C always pairs with G. This base pairing means each strand carries the information to rebuild the other, which is the basis of accurate DNA replication.
Complementary base pairing is the rule that adenine pairs only with thymine, and cytosine pairs only with guanine, across the two strands of DNA. Because the pairing is fixed, the sequence of one strand determines the sequence of the other, allowing DNA to be copied accurately during replication and read reliably during protein synthesis.
Genes, alleles and loci
A gene is a length of DNA that codes for a particular protein (or part of one), and proteins build and run the body, so genes ultimately determine traits. The position of a gene on a chromosome is its locus. Most genes come in alternative versions called alleles, which differ slightly in base sequence and so may produce slightly different proteins. For example, a gene may have one allele that produces a working enzyme and another that does not. Because humans are diploid, each person carries two alleles of most genes, one inherited from each parent.
Genotype and phenotype
The genotype is the combination of alleles an individual carries for a trait. The phenotype is the observable characteristic that results, such as eye colour or blood group. An individual may be homozygous (two identical alleles) or heterozygous (two different alleles) for a gene. The phenotype depends on which alleles are present, how they interact (dominant, recessive, codominant), and often on the environment as well.
Different alleles arise originally through mutation, a change in the DNA base sequence. Meiosis and fertilisation then shuffle existing alleles into new combinations, but they do not create new alleles. So the ultimate source of all the genetic variation that natural selection acts on is mutation, which links this page to the sources-of-variation topic.
The genetic code
The genetic code is the set of rules by which the base sequence of a gene is translated into the amino acid sequence of a protein. Bases are read in groups of three, called codons or triplets, and each triplet specifies one amino acid. The order of bases therefore determines the order of amino acids, which determines the protein's shape and function. A change in the base sequence can change an amino acid and so change the protein, which is how a mutation can alter a phenotype.
From mutation to trait
Suppose a single base is substituted in a gene coding for an enzyme. Because the code is read in triplets, this can change one codon and therefore one amino acid in the protein. If that amino acid is at an important site, the enzyme may fold differently and lose function. The phenotype changes because the trait depends on the protein, and the protein depends on the base sequence. This is the chain a marker wants you to trace: DNA base sequence determines the amino acid sequence determines the protein determines the phenotype, so a new allele (from mutation) can produce a new phenotype that selection may then act on.
How this maps to the exam
Expect questions that ask you to define and use the terms gene, allele, locus, genotype and phenotype correctly, describe the base-pairing structure of DNA, or explain how the genetic code links DNA to a protein and so to a trait. This vocabulary is assumed in every later Unit 4 question, so precision here pays off throughout the paper.
The most common error is treating gene and allele as interchangeable. A gene is the section of DNA coding for a particular protein; an allele is one specific version of that gene. Everyone has the same genes at the same loci, but different individuals may carry different alleles. Also avoid saying DNA itself is a trait; DNA codes for proteins, and the proteins (with the environment) produce the trait.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 marksDescribe the structure of a DNA molecule, and explain how complementary base pairing allows the genetic code to be copied accurately.Show worked answer →
A 6 mark response needs the structural detail plus the base-pairing rule and why it matters.
- Structure
- DNA is a double helix made of two strands. Each strand is a chain of nucleotides, and each nucleotide has a deoxyribose sugar, a phosphate group and one of four nitrogenous bases (adenine, thymine, guanine, cytosine). The sugar and phosphate form the backbone, and the bases project inward.
- Base pairing
- The two strands are held together by hydrogen bonds between complementary bases: adenine pairs with thymine and guanine pairs with cytosine.
- Accurate copying
- Because pairing is specific, each original strand acts as a template that specifies exactly one complementary strand. This means a copied molecule has the same base sequence as the original, so genetic information is conserved when cells divide.
Markers reward the nucleotide components, the A-T and G-C rule, and the template idea linking pairing to faithful copying.
WACE 2023-style4 marksDefine the terms gene, allele, genotype and phenotype, and use an example to show how genotype relates to phenotype.Show worked answer →
A 4 mark answer needs four correct definitions plus a worked example.
- Gene
- A length of DNA at a particular locus that codes for a polypeptide or characteristic.
- Allele
- An alternative form of a gene that produces a variation of the characteristic.
- Genotype
- The combination of alleles an organism carries for a trait.
- Phenotype
- The observable characteristic that results.
- Example
- For a gene with a dominant allele B (brown eyes) and recessive allele b (blue eyes), a genotype of Bb produces the phenotype brown eyes because the dominant allele is expressed.
Markers reward four correct definitions and a genotype-to-phenotype example.