You will be asked to connect the nucleus, chromosomes, DNA and genes, then use that structure to reason about chromosome sets, inheritance and gene activity. A containment check usually turns a list of facts into one clear answer.
Build the hierarchy first
Picture nested levels. The nucleus is a cell compartment that contains chromosomes. A chromosome is one long, coiled DNA molecule, and a gene is a shorter region along that DNA. This gives a reliable size and containment route: nucleus, chromosome, gene. One chromosome can carry many genes, with each gene at a particular position.
| Signal in the question | Reasoning route |
|---|---|
| A location or size comparison | Follow the containment chain from nucleus to chromosome to gene. |
| A human body cell or gamete | A body cell has two sets, giving 46 chromosomes in 23 pairs; a gamete has one chromosome from each pair, giving 23. |
| A matching chromosome pair | Trace one member to each parent rather than treating the pair as a copy and a spare. |
| A gene, protein and characteristic | Use the direction gene code to protein to effect on a characteristic. The gene is neither the finished protein nor the characteristic itself. |
| Different body-cell types | Keep the full set of genes in both cells; explain their differences by which genes are switched on. |
Use whole and part language carefully. The genome is the complete collection of genetic material, not a single chromosome and not only the genes active in one cell. Chromosome number can differ among species, but the count is not a scale of complexity or advancement.
Where the reasoning goes wrong
| Tempting route | How to reject it |
|---|---|
| Putting chromosomes in the cytoplasm, membrane or ribosomes | Those structures have other roles. Follow the hierarchy to the nucleus, the compartment that houses chromosomes. |
| Treating a chromosome as protein, genes without DNA, or one enormous gene | A chromosome is DNA, and genes are shorter regions distributed along it. The material and its named regions are not interchangeable. |
| Using the number of pairs as the total chromosome count | A pair contains two chromosomes. Decide whether the cell has two sets or one before stating a total. |
| Explaining a pair as a random duplicate or two maternal copies | Inheritance supplies one member through each parent, which is why the body cell has two corresponding sets. |
| Shrinking the genome to one active group or ranking species by chromosome count | Genome refers to the whole genetic collection, including inactive genes, while chromosome count does not measure biological advancement. |
Try it
Order these three from largest to smallest as they occur in a cell: gene, nucleus, chromosome.
- Nucleus, chromosome, gene
- Gene, chromosome, nucleus
- Chromosome, nucleus, gene
- Gene, nucleus, chromosome
What does a gene carry?
- The characteristic itself, for example tallness
- The coded instructions for making a protein
- A finished protein, ready for use
- A miniature copy of the whole organism
A skin cell and a muscle cell are taken from the same person. Which statements about these two cells are true? Select all that apply.
- They contain the same genes
- The skin cell contains only skin genes
- Different genes are switched on in each of them
- The muscle cell has thrown away the genes it does not use