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A gene is a sequence, a chromosome is an organised DNA structure, and a genome is the full genetic information. Keep these scales distinct when explaining inheritance and expression.
What you'll be able to do
A gene is a DNA base sequence that contributes to a functional product: a polypeptide or a functional RNA such as rRNA or tRNA. A locus is its position on a particular DNA molecule; different alleles are alternative versions at that locus. A gene is therefore not an entire chromosome, and not every gene codes for a protein.
Eukaryotic nuclear chromosomes contain long linear DNA associated with proteins including histones. A typical bacterial chromosome is a smaller circular DNA molecule without a surrounding nucleus. Mitochondria and chloroplasts also contain DNA, usually organised as smaller circular molecules rather than the nuclear chromosome arrangement.
In a protein-coding sequence, a triplet of bases corresponds to an amino acid through the genetic code. An mRNA triplet used in translation is called a codon. The code is non-overlapping: successive codons are read as separate groups of three rather than a sliding sequence of shared bases.
The code is degenerate because more than one codon can specify the same amino acid. It is treated as universal in the syllabus model: the same code is shared across organisms, with limited biological exceptions outside that simple model. This shared coding system helps explain why a gene transferred between species can sometimes be expressed.
Much eukaryotic DNA does not directly specify an amino-acid sequence. It includes repeated sequences and regions involved in regulation. In many eukaryotic genes, introns interrupt sequences retained in the mature transcript; splicing removes introns from pre-mRNA and joins exons.
Exons are retained in mature RNA, but the introductory syllabus often focuses on their protein-coding portions. Do not infer that every non-coding region is useless or that all RNA is translated. A functional RNA gene is an important reminder that genetic function extends beyond polypeptide coding.
Try each question before opening the worked response. Use precise biological terms, connect each cause to its effect, and check units and assumptions when interpreting data.
Common mistakes to avoid
Key takeaways
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.