GCSE Biology Revision — Chromosomes & Genes
Revise Chromosomes & Genes for GCSE Biology with a topic explanation, worked example and common mistakes. Check the board notes for specification differences.
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What is Chromosomes & Genes?
Chromosomes are long, coiled molecules of DNA found in the nucleus of eukaryotic cells, carrying a large number of genes. Humans have 23 pairs of chromosomes in each body cell. A gene is a section of DNA that contains the code for making a specific protein.
Board notes: Covered by all major boards (AQA, Edexcel, OCR). The relationship between DNA, genes, and chromosomes is a fundamental concept.
Step-by-step explanationWorked examples
Worked example
A typical human body cell, like a skin cell, has 46 chromosomes in its nucleus, arranged as 23 pairs. Pair 23 determines sex (XX for female, XY for male), while the other 22 pairs are autosomes. Each chromosome contains hundreds or thousands of genes, each coding for a specific protein.
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Common mistakes
- 1Confusing chromosomes and chromatids. A chromosome can consist of one or two identical sister chromatids, which are copies made during DNA replication before a cell divides.
- 2Thinking that the number of chromosomes indicates an organism's complexity. Chromosome number varies widely between species and is not a reliable indicator of complexity.
- 3Forgetting that chromosomes in body cells come in pairs. Humans have 23 homologous pairs – one chromosome in each pair is inherited from each parent.
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Frequently asked questions
How many chromosomes do humans have?
Human body cells have 46 chromosomes, arranged as 23 pairs. Gametes (sperm and egg cells) are haploid and have only 23 single chromosomes.
What is the relationship between a gene and a protein?
A gene is a sequence of DNA that provides the instructions for assembling amino acids in a specific order to create a particular protein. This protein then carries out a specific function in the body.