Chromosomes — AQA GCSE Biology
Test yourself on Chromosomes with AQA GCSE practice questions.
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Chromosomes explained
Think of the nucleus as the cell's control room, and inside it the genetic material is packaged into chromosomes.
Read the full explanation
Each chromosome is one very long DNA molecule wound tightly around proteins, so a molecule far too thin to see becomes a compact, visible structure during cell division. DNA itself is a polymer built from four bases whose order forms a code. That code is read in sections, and each section is a gene. In a human body cell there are 46 chromosomes, arranged as 23 pairs, and every one of them is DNA. A useful method is to zoom in mentally: cell, then nucleus, then chromosome, then DNA molecule, then gene. This hierarchy matters because it explains why the nucleus controls cell activities and why DNA can be copied and passed on.
Your focus
- Identify the nucleus as the location of chromosomes in a cell.
- Explain that chromosomes are made of DNA molecules.
- Sequence the nucleus, chromosome, DNA and gene in order of decreasing size.
Chromosomes exam tips
Quick Revision Summary (Key Takeaway)
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.
Topic Overview
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.
Key Concepts
- →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.
Marking Points
- 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.
Examiner Tips
- 💡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.
- 💡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).
Common Mistakes
- 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.
- 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).
Revision Plan
- 1Day 1: Learn core definitions (chromosome, gene, DNA, diploid, haploid) and draw the hierarchical relationship between them.
- 2Day 2: Memorise chromosome numbers for humans (46 in somatic cells, 23 in gametes) and review the karyotype of sex chromosomes (XX vs XY).
- 3Day 3: Connect chromosome behavior to the stages of the cell cycle and mitosis.
- 4Day 4: Complete past paper exam questions focused on chromosome counts and DNA mass changes during cell division.
Exam Question Types
- 📋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.
Command Word Expectations (AQA)
State the key characteristics or structural features of chromosomes without needing to explain why they occur.
Provide scientific reasons and cause-and-effect links, such as explaining why chromosomes must replicate before cell division.
How Students Lose Marks (Examiner Pitfalls)
Step-by-Step Worked Solutions
Question: Describe the arrangement of chromosomes in human body cells compared to human gametes, and explain why this difference is essential for sexual reproduction. (4 marks)
- 1.Step 1: State the chromosome number and arrangement in human somatic cells (46 chromosomes / 23 pairs, diploid).
- 2.Step 2: State the chromosome number in gametes (23 single chromosomes, haploid).
- 3.Step 3: Explain the fusion of gametes during fertilisation (23 + 23 = 46).
- 4.Step 4: Conclude how this maintains the species chromosome number across generations without doubling.
Question: A cell biologist measures the mass of DNA in a human skin cell at different stages. The mass of DNA in a newly formed skin cell is 6.6 picograms (pg). Calculate the mass of DNA present in the cell right before mitosis begins, and state the number of chromosomes present at that point. (2 marks)
- 1.Step 1: Identify that before mitosis, during the interphase stage of the cell cycle, DNA replicates to double its quantity.
- 2.Step 2: Calculate the new mass: 6.6 pg x 2 = 13.2 pg.
- 3.Step 3: Determine chromosome count: although DNA has replicated into sister chromatids, it is still counted as 46 individual chromosomes (each consisting of two identical chromatids joined at a centromere).