Chromosomes — AQA GCSE Biology

Chromosomes are linear strands of tightly coiled DNA found in the cell nucleus, carrying hundreds to thousands of genes that encode vital proteins. In human body cells, chromosomes exist in 23 homologous pairs (46 in total), which replicate and segregate precisely during the cell cycle and mitosis.

Chromosomes are structures found in the nucleus of eukaryotic cells, composed of DNA coiled around histone proteins. Each chromosome carries many genes, which are specific segments of DNA providing instructions for synthesizing functional proteins.

Understanding chromosomes is foundational to genetics, cell division, and inheritance. In AQA GCSE Biology, mastering chromosome behavior underpins the cell cycle, mitosis, stem cell biology, meiosis, and inherited disorders.

• →Location and composition: Chromosomes reside in the nucleus and consist of long DNA molecules wrapped around protein scaffolds.
• →Ploidy levels: Normal body cells are diploid (containing pairs of matching homologous chromosomes), while sex cells are haploid (carrying single copies).
• →Genes and alleles: Each chromosome carries hundreds of genes located at specific positions, which determine structural and metabolic characteristics.
• →Replication in the cell cycle: Before any cell division occurs, each single chromosome replicates to form two sister chromatids joined at a centromere.

• States that the nucleus contains the cell's genetic material in the form of chromosomes.
• Describes a chromosome as a structure made of a DNA molecule, typically coiled and associated with protein.
• Links DNA to a code or sequence of bases that carries genetic information.
• Places the structures in order of size: nucleus larger than chromosome, chromosome larger than DNA molecule.
• Uses the human example of 46 chromosomes in a body cell to make the idea concrete.
• Defines a gene as a section of DNA that codes for a protein or characteristic.
• States that one chromosome carries many genes, not just one.
• Explains that genes are arranged along the length of the DNA molecule in a chromosome.
• Links the base sequence of a gene to the protein it codes for.
• Uses a human example, such as many genes on chromosome 1, to show that numbers per chromosome are large.
• States that body cells are diploid, containing two copies of each chromosome.
• Explains that one chromosome of each pair comes from each parent.
• Uses the human example of 46 chromosomes arranged as 23 pairs.
• Distinguishes paired chromosomes from gametes, which carry a single set.
• Links paired chromosomes to alleles, the different versions of a gene that a pair may carry.

• 💡Use the sequence nucleus, chromosome, DNA, gene when explaining where genetic material is found, so each level is clearly placed.
• 💡If asked to describe the relationship, give a size order as well as a location, because both are commonly needed for full credit.
• 💡Check that any diagram is labelled with straight lines and that the label chromosome points to a structure inside the nucleus, not to the nuclear membrane.
• 💡Define gene and chromosome separately before describing their relationship, because muddled definitions lose marks quickly.
• 💡Use the phrase section of DNA when defining a gene, as this signals the correct scale.
• 💡When a question asks about the number of genes, avoid quoting an exact figure unless it is given; say a large number instead.
• 💡Quote the diploid number and the pair number together, for example 46 chromosomes as 23 pairs, to show both facts clearly.
• 💡If a question contrasts body cells with gametes, state the chromosome number in each to make the difference explicit.
• 💡Use the term allele only when referring to different versions of the same gene, so the vocabulary stays precise.
• 💡Use precise biological terms like 'homologous pairs', 'diploid', 'haploid', and 'chromatid' rather than informal descriptions like 'double strands'.
• 💡Remember that the 23rd pair in humans consists of sex chromosomes: XX in biological females and XY in biological males.
• 💡Ensure you distinguish clearly between DNA (the chemical polymer), a gene (a section of DNA), and a chromosome (the entire condensed structure).

• Saying the nucleus contains genes but omitting chromosomes; the correction is to state that genes are carried on chromosomes, which are made of DNA.
• Treating DNA and a chromosome as identical; the correction is that a chromosome is a DNA molecule packaged with protein, so DNA is the molecule and the chromosome is the packaged structure.
• Writing that chromosomes float freely in the cytoplasm; the correction is that in a eukaryotic cell the chromosomes are found inside the nucleus.
• Confusing a gene with a chromosome; the correction is that a gene is a small section of DNA, while a chromosome is the whole packaged DNA molecule containing many genes.
• Saying each chromosome carries one gene; the correction is that each chromosome carries a large number of genes.
• Stating that genes are made of protein; the correction is that genes are made of DNA and code for proteins.
• Saying body cells contain 23 chromosomes; the correction is that they contain 46 chromosomes arranged as 23 pairs.
• Describing paired chromosomes as physically joined; the correction is that they are separate matching chromosomes, not fused structures.
• Assuming the two chromosomes of a pair are identical; the correction is that they carry the same genes but may carry different alleles.
• Thinking humans have 23 chromosomes in total rather than 23 pairs (46 chromosomes) in body cells.
• Assuming that replication doubles the chromosome number from 46 to 92; it doubles the DNA quantity, but the chromatids remain joined as 46 replicated chromosomes until anaphase.
• Believing that all organisms have 46 chromosomes; chromosome count is species-specific (e.g., fruit flies have 8, dogs have 78).

1. 1Day 1: Learn core definitions (chromosome, gene, DNA, diploid, haploid) and draw the hierarchical relationship between them.
2. 2Day 2: Memorise chromosome numbers for humans (46 in somatic cells, 23 in gametes) and review the karyotype of sex chromosomes (XX vs XY).
3. 3Day 3: Connect chromosome behavior to the stages of the cell cycle and mitosis.
4. 4Day 4: Complete past paper exam questions focused on chromosome counts and DNA mass changes during cell division.

• 📋Short definition questions: Defining terms such as 'gene', 'chromosome', or stating the normal human complement.
• 📋Data and calculation questions: Calculating DNA mass or identifying chromosome numbers before, during, and after cell division.
• 📋Comparative extended response: Contrasting the chromosome count and genetic composition between somatic cells and gametes.