By the end of this lesson
- Describe chromosome organisation.
- Locate genes on chromosomes.
- Relate homologous chromosomes to gene loci.
BIG QUESTION
How is a vast DNA molecule packaged and inherited accurately?
DNA associates with proteins, condenses into chromosomes and carries genes in a linear order.
A chromosome contains one long DNA molecule
A eukaryotic chromosomeA long thread of DNA, carrying many genes, found in the nucleus. consists of a long DNA molecule associated with packaging proteins. Between divisions it is less condensed; before and during division it becomes visibly compact.
After DNA replication, a chromosome consists of two sister chromatids joined at a centromere until they separate.
Genes occupy loci
Genes lie in a linear sequence along chromosomes. A gene’s position is its locusThe particular position on a chromosome where a gene is found; both alleles of a gene occupy the same locus on a pair of chromosomes..
Different chromosomes carry different sets of genes, while homologous chromosomes carry the same genes at corresponding loci.
Homologous pairs can carry different alleles
One homolog is normally inherited from each parent. The pair has matching gene loci but may contain different alleleOne of the different forms of the same gene, found at the same locus on a chromosome. of those genes.
Homologous chromosomes are not identical to sister chromatids: homologues can differ in allele sequence.
Chromosome relationships
Use structure and inheritance to distinguish the terms.
| Structure | Relationship | Key feature |
|---|---|---|
| Unreplicated chromosome | One DNA molecule. | Carries many genes. |
| Sister chromatids | Replicated copies joined together. | Initially nearly identical. |
| Homologous chromosomes | Maternal and paternal pair. | Same loci, potentially different alleles. |
| Gene locus | Position on a chromosome. | Corresponds between homologues. |
Language that earns precision
- chromosome
- Long DNA molecule associated with proteins and carrying genes.
- locus
- Specific position of a gene on a chromosome.
- homologous chromosomes
- Chromosome pair carrying the same genes at corresponding loci.
- sister chromatids
- Replicated chromosome copies joined before separation.
Common misconceptions
Claim: Every chromosome contains the same genes.
Correction: Different chromosome types carry different gene sets.
Claim: Homologous chromosomes are identical copies.
Correction: They have corresponding genes but may carry different alleles.
Claim: A replicated chromosome counts as two chromosomes immediately.
Correction: Joined sister chromatids are generally counted as one replicated chromosome until separation.
Match the answer to the command word
Genes occur at loci along chromosomes in the nucleus.
Homologues share loci; sister chromatids are replication products.
Recall, apply, then justify
Explain the relationship between DNA, chromosomes and genes.
Hint
Material → packaged molecule → functional sections.
Suggested marking points
- DNA is the genetic material.
- A chromosome is a long packaged DNA molecule.
- Genes are functional DNA sequences at loci on chromosomes.
Distinguish homologous chromosomes from sister chromatids.
Hint
Origin, joining and allele identity.
Suggested marking points
- Homologous chromosomes are a maternal and paternal pair with the same gene loci.
- Homologues may carry different alleles.
- Sister chromatids are joined copies produced by replication of one chromosome.
Predict how a chromosome deletion could affect several traits.
Planning hint
A chromosome region can contain many genes.
What a strong design includes
- A deletion removes a segment of DNA.
- The segment may contain several genes and regulatory regions.
- Cells may lose one copy of multiple genes.
- Gene dosage or expression can change.
- Several proteins or pathways may therefore be affected.
- The phenotype depends on deletion size, position and whether remaining alleles compensate.