Mutation, cell division and cancer, at a glance

A gene mutation changes a DNA base sequence. A substitution can change a codon; an insertion or deletion not in a multiple of three can shift downstream codon grouping.

• Substitution can be synonymous, missense or nonsense.
• Non-multiple-of-three indels can shift the reading frame.

Keep in mind: Deleting a multiple of three preserves the downstream triplet frame, although amino acids can be lost and protein function can change.

Chromosomal aberrations alter chromosome number or large-scale structure. Aneuploidy changes the number of particular chromosomes; structural changes rearrange or change chromosome segments.

• Numerical and structural aberrations are different categories.
• Deletion, duplication, inversion and translocation describe different segment changes.

Keep in mind: It refers to an extra copy of chromosome 21. Aneuploidy affects particular chromosomes, unlike an extra complete set.

A substitution in the beta-globin gene changes an amino acid, altering haemoglobin interactions and potentially red-cell shape and survival.

• The example replaces glutamic acid with valine in beta-globin.
• Protein aggregation links the sequence change to cell shape.
• Rapid cell loss contributes to anaemia.

Keep in mind: A substitution changes a base without adding or removing one, so the reading frame remains. The important effect is the changed amino acid and protein interaction.

A balanced discussion weighs useful information against uncertainty, burdens and discrimination, while protecting informed choice, privacy and respect for people with genetic conditions.

• Screening is not automatically diagnosis.
• Discuss autonomy, welfare, privacy and fairness.
• Respect people, not just abstract test results.

Keep in mind: Screening and diagnosis answer different questions. A screening result requires interpretation and may lead to an offer of further testing.

Interphase includes growth, DNA replication and preparation. Mitosis separates duplicated genetic material and cytokinesis divides the cell.

• DNA replication occurs in S phase.
• Sister chromatids are copied versions of one chromosome.
• Mitosis and cytokinesis are distinct processes.

Keep in mind: Interphase includes active metabolism, growth, DNA replication and preparation. It occupies much of the cycle.

Replicated chromosomes condense, align and separate as sister chromatids, then new nuclei form around corresponding sets.

• Metaphase aligns duplicated chromosomes; anaphase separates sisters.
• Nuclear envelopes re-form in telophase.
• Cytokinesis differs between animal and plant cells.

Keep in mind: Mitosis normally preserves the chromosome complement in daughter nuclei. Reduction of sets is a role of meiosis.

Cell-cycle control normally prevents inappropriate division. Mutations and damaging exposures can disrupt control, increasing the chance of cancerous growth.

• Checkpoints coordinate safe progression.
• Genetic susceptibility, chemical carcinogens and ionising radiation can raise risk.
• Risk is not certainty.

Keep in mind: Repair is regulated to meet tissue needs. Cancer involves loss of normal controls and other acquired properties.

Cancer commonly develops as mutations accumulate in a cell lineage. Further changes can support blood supply, invasion and spread to distant sites.

• Cancer development is often a multi-step process.
• Angiogenesis supplies; metastasis spreads.

Keep in mind: Angiogenesis supports blood supply. Metastasis requires tumour-cell spread and establishment at another site.

Meiosis produces haploid cells with varied genetic combinations. Fertilisation restores the diploid number and adds further variation through random gamete combinations.

• Meiosis reduces chromosome sets; fertilisation restores them.
• Meiosis and random fertilisation generate combinations.
• Detailed meiotic stages are outside this H1 requirement.

Keep in mind: Meiosis mainly rearranges existing alleles into new combinations. Mutations generate new sequence variants.