Chromosomes, genes and DNA: keep the levels distinct
| English | 中文 | Pinyin |
|---|---|---|
| chromosome/ˈkrəʊməsəʊm/ | 染色体 | rǎn sè tǐ |
| gene/dʒiːn/ | 基因 | jī yīn |
What would explain this observation?
- A chromosome 染色体 is not one gene 基因. A single chromosome carries many genes along its DNA molecule, and body cells normally contain chromosomes in pairs.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- The nucleus contains chromosomes made of DNA molecules. A gene is a section of DNA, and each chromosome carries a large number of genes. Chromosome pairs contain corresponding types of genetic information; they are not two halves joined merely because the cell is large.
- chromosome: A DNA-containing structure carrying many genes; gene: A section of DNA associated with genetic information.
Which statement distinguishes a gene from a chromosome?
Keep three levels separate: DNA is the chemical genetic material, a chromosome is an organized DNA-containing structure, and a gene is a section of DNA. A replicated chromosome has copied genetic material ready for division. Replication and the later distribution into daughter cells have different effects.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Keep three levels separate: DNA is the chemical genetic material, a chromosome is an organized DNA-containing structure, and a gene is a section of DNA. A replicated chromosome has copied genetic material ready for division. Replication and the later distribution into daughter cells have different effects.
- Use a labelled chromosome model and a short DNA segment. Identify the chromosome pair and mark several gene positions on each model rather than labelling an entire chromosome as a single gene. For counting exercises, state whether chromosomes are being counted in a body cell, a gamete or a daughter cell.
Which two habits make the investigation or model in this case more defensible?
Use a labelled chromosome model and a short DNA segment. Identify the chromosome pair and mark several gene positions on each model rather than labelling an entire chromosome as a single gene. For counting exercises, state whether chromosomes are being counted in a body cell, a gamete or a daughter cell.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: a model body cell has 12 chromosomes, organized as six pairs. The number of pairs is 12/2=6. After mitosis, each daughter cell normally has the same chromosome number, 12, rather than six. The halving associated with gamete production belongs to meiosis, studied separately.
A model body cell has 18 chromosomes in pairs. How many pairs are present? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A model body cell has 18 chromosomes in pairs. How many pairs are present?
The result is 9 pairs. Known: a model body cell has 12 chromosomes, organized as six pairs. The number of pairs is 12/2=6. After mitosis, each daughter cell normally has the same chromosome number, 12, rather than six. The halving associated with gamete production belongs to meiosis, studied separately.
Check the conclusion and its limits
- DNA, gene and chromosome are not interchangeable terms. A chromosome count alone does not tell how many genes an organism has. Copying DNA before mitosis is not the same as permanently doubling the chromosome number of each daughter cell.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Every chromosome contains only one gene. This claim is false: DNA, gene and chromosome are not interchangeable terms. A chromosome count alone does not tell how many genes an organism has. Copying DNA before mitosis is not the same as permanently doubling the chromosome number of each daughter cell.
Chromosomes, genes and DNA: keep the levels distinct: Keep three levels separate: DNA is the chemical genetic material, a chromosome is an organized DNA-containing structure, and a gene is a section of DNA. A replicated chromosome has copied genetic material ready for division. Replication and the later distribution into daughter cells have different effects.
Every chromosome contains only one gene.
DNA, gene and chromosome are not interchangeable terms. A chromosome count alone does not tell how many genes an organism has. Copying DNA before mitosis is not the same as permanently doubling the chromosome number of each daughter cell.
A DNA-containing structure carrying many genes: write the technical term.
chromosome means A DNA-containing structure carrying many genes.