Mutation, cell division and cancer, at a glance

A substitution changes one position; an insertion or deletion can shift every downstream codon unless the change preserves the reading frame.

• Substitution: silent, missense or nonsense possibilities.
• Non-multiple-of-three insertion/deletion can shift the frame.
• Effect depends on the affected sequence and function.

Keep in mind: The genetic code is degenerate, so some substitutions are silent at the amino-acid level.

Chromosome aberrations change dosage or arrangement at a larger scale than a single gene sequence.

• Aneuploidy: altered numbers of particular chromosomes.
• Deletion/duplication change segment amount.
• Inversion/translocation change arrangement.

Keep in mind: It means three copies of chromosome 21, a chromosome-number change affecting many genes.

A beta-globin substitution changes intermolecular interactions, linking altered protein behaviour to red-cell and tissue effects.

• GAG to GUG in mRNA changes glutamate to valine.
• Non-polar substitution promotes deoxygenated HbS association.
• Rigid, damaged cells impair circulation and oxygen delivery.

Keep in mind: It involves a beta-globin gene variant; trisomy 21 is a different chromosome-number condition.

Screening estimates risk; informed, voluntary decisions need clear uncertainty, privacy and respect for people living with genetic conditions.

• Screening risk is not a certain diagnosis.
• Consent should be informed and voluntary.
• Privacy, access and disability respect matter.

Keep in mind: Ethical analysis should support informed voluntary choices and explain trade-offs, not impose one outcome.

DNA replication makes sister chromatids; mitosis separates them so daughter nuclei retain the parental chromosome complement.

• S phase doubles DNA, not the centromere count.
• Mitosis separates sister chromatids.
• Daughter nuclei normally retain chromosome number.

Keep in mind: It doubles DNA content while the number of homologous chromosome sets remains the same.

Homologues pair and can exchange segments, then separate in meiosis I while sister chromatids remain together.

• Prophase I: homologous pairing and crossing-over.
• Metaphase I: independent bivalent orientation.
• Anaphase I: homologues separate, sisters stay joined.

Keep in mind: The variation-producing exchange described here is between non-sister chromatids of homologous chromosomes.

Meiosis II separates sisters without another S phase; random gamete fusion adds another layer of genetic variation.

• No S phase between the two divisions.
• Meiosis II separates chromatids.
• Random fertilisation restores diploidy and combines variation.

Keep in mind: A second S phase would disrupt the normal reduction process. Meiosis II uses the chromatids made before meiosis I.

Loss of tumour-suppressor function removes restraint; gain of proto-oncogene function can provide persistent growth-promoting signals.

• Tumour suppressor: loss removes restraint.
• Proto-oncogene: gain can create oncogenic signalling.
• p53 and ras illustrate different mechanisms.

Keep in mind: Proto-oncogenes have normal cellular roles; particular activating changes make them oncogenic.

Cancer develops through accumulated changes and selection within cell populations; invasion and spread require additional capabilities.

• Predisposition and exposure alter risk, not certainty.
• Several cooperating changes accumulate.
• Angiogenesis supplies; metastasis spreads.

Keep in mind: Angiogenesis is new blood-vessel formation; metastasis is establishment of cancer growth at a distant site.