Chromosomes, genes and proteins
Different versions of the same gene are called alleles. For example, a gene might determine flower color, while different alleles determine whether the flower is purple or white.
Correction: A gene is specifically the functional unit (the code) for a protein. The chromosome is the physical structure that holds many genes together. Do not say a gene is a chromosome; say a gene is on a chromosome.
Why examiners accept this: The markscheme requires precise terminology. For a gene, you must mention DNA and that it codes for a protein. For an allele, you must state it is an alternative form of a gene.
Example: 'Define allele.' Answer: 'An alternative form of a gene.' (Do not just say 'a version of a gene' without context; 'alternative form' is the key phrase).
Humans have 23 pairs of chromosomes. The first 22 pairs are called autosomes because they are not involved in sex determination. The 23rd pair determines the biological sex and are called sex chromosomes.
- Females have two X chromosomes (XX).
- Males have one X and one Y chromosome (XY).
During gamete formation (meiosis), the chromosome pairs separate. Therefore:
- All egg cells (female gametes) carry one X chromosome.
- Sperm cells (male gametes) carry either an X or a Y chromosome.
Correction: Sperm cells are haploid. They contain only one sex chromosome, either X or Y, never both. Only body cells (diploid) have the XY pair.
Why examiners accept this: You must distinguish between the constant X in eggs and the variable X/Y in sperm. The markscheme looks for 'X' in eggs and 'X or Y' in sperm.
Example: 'Describe the sex chromosomes in human sperm.' Answer: 'Sperm contain either an X chromosome or a Y chromosome.'
Different sequences of amino acids fold into different 3D shapes. The shape of a protein determines its function. For example:
- Enzymes: Have an active site with a specific shape to bind substrates.
- Receptors: Have a binding site with a specific shape to bind neurotransmitters or hormones.
DNA controls cell function by controlling the production of these proteins.
Correction: The sequence of bases determines the sequence of amino acids, which determines the shape, which determines the function. You must include the intermediate step of 'shape' or 'structure'.
Why examiners accept this: You must link structure to function. Use phrases like 'complementary shape' or 'specific binding site'.
Example: 'Explain why the shape of an enzyme is important.' Answer: 'The specific shape allows it to bind to its substrate/complementary shape.'
Proteins are made in a two-stage process involving the nucleus and the cytoplasm:
- In the Nucleus: The gene coding for the protein remains in the nucleus (DNA never leaves). A copy of the gene is made called messenger RNA (mRNA). This mRNA molecule moves out of the nucleus into the cytoplasm.
- In the Cytoplasm: The mRNA passes through ribosomes. The ribosome reads the sequence of bases in the mRNA and assembles amino acids into a protein chain. The specific sequence of amino acids is determined by the sequence of bases in the mRNA.
Correction: DNA stays in the nucleus. Only mRNA leaves the nucleus and goes to the ribosomes.
Why examiners accept this: You must mention the key locations and molecules: Nucleus, mRNA, Cytoplasm, Ribosome, and Amino acids. Do not describe the chemical mechanism of transcription/translation; just the flow.
Example: 'Explain how a protein is made.' Answer: 'The gene remains in the nucleus. mRNA is made as a copy. mRNA moves to the cytoplasm. Ribosomes assemble amino acids into protein based on the mRNA sequence.'
- Gene remains in nucleus.
- mRNA is a copy of the gene.
- mRNA moves to cytoplasm/ribosome.
- Ribosome assembles amino acids into protein.
Most body cells in an organism contain the same genes (the same DNA). However, different cells have different functions because they express different genes.
- A cell only makes the specific proteins it needs for its function.
- Many genes are 'switched off' or not expressed in a particular cell type.
- For example, insulin is only made in pancreatic beta cells, not in skin cells, even though both have the insulin gene.
Correction: All body cells have the same genes. They differ only in which genes are expressed (turned on).
Why examiners accept this: You must state that not all genes are expressed and that cells only produce the proteins they need.
Example: 'Explain how body cells can have different functions.' Answer: 'Not all genes are expressed/switched on. Cells only produce the specific proteins they need.'
Cells can be classified by the number of chromosome sets they contain:
- Diploid: A nucleus containing two sets of chromosomes. In humans, this is 23 pairs (46 total). Body cells are diploid.
- Haploid: A nucleus containing a single set of chromosomes. In humans, this is 23 single chromosomes. Gametes (sperm and egg) are haploid.
| Term | Number of Chromosome Sets | Human Number |
|---|---|---|
| Diploid | Two sets (pairs) | Body cells (somatic) |
| Haploid | Single set | Gametes (sperm, egg) |
Correction: Human diploid cells have 23 pairs (46 total). Human haploid cells have 23 single chromosomes. Do not say haploid cells have 46.
Why examiners accept this: Use the exact phrase 'single set' for haploid and 'two sets' (or 'pairs') for diploid. Mentioning '23 pairs' for humans is also accepted.
Example: 'Describe a diploid nucleus.' Answer: 'A nucleus containing two sets of chromosomes.'