Chromosome

In the syllabus and the exam

Chromosome sits at the point where cell structure, cell division and genetics all meet, and candidates need to describe it correctly as a thread-like structure made of tightly coiled DNA and protein, visible under a microscope specifically during cell division, and carrying the genes that determine an organism's characteristics. A frequent exam focus is chromosome number: candidates should know that body cells are normally diploid, containing chromosomes in homologous pairs, while gametes produced by meiosis are haploid, containing only one chromosome from each pair, and should be able to state or interpret a given species' chromosome number correctly in both states. A common error is confusing 'chromosome' with 'chromatid', particularly during mitosis and meiosis diagrams, where a chromosome that has already replicated consists of two identical sister chromatids joined at a centromere; describing each chromatid as a separate chromosome before they have actually separated at anaphase is a frequent source of lost marks. Candidates are also expected to distinguish autosomes, which carry genes for general body characteristics, from the sex chromosomes (X and Y in humans), which determine an individual's biological sex, a distinction commonly tested in questions on sex determination and sex-linked inheritance. Candidates should also recognise that an abnormal chromosome number, such as an extra copy of one chromosome, can arise from an error during meiosis and lead to a genetic condition, a link the syllabus uses to connect chromosome structure to human health and inheritance. Candidates should also practise counting chromosomes correctly from a diagram showing only one member of each homologous pair, since exam diagrams do not always draw every chromosome to avoid overcrowding the image.

Example

Human body cells normally contain 46 chromosomes arranged in 23 pairs, with one chromosome of each pair inherited from the mother and one from the father.

Related terms

Deoxyribonucleic acid (DNA)DiploidHaploid

Source:SRC-DSKP-EN