5.1.1—Chromosome structure and telomeres

A chromosome is a condensed structure made from a long DNA molecule associated with histone proteins. Packaging DNA as chromatin and then condensing it for division makes the genetic material manageable and helps it be distributed in an organised way.

• DNA and chromatin: DNA carries genetic information. Histone proteins help package the DNA; DNA plus associated proteins forms chromatin.
• Chromosome: Further condensation of chromatin produces a chromosome that can be moved and separated during nuclear division.
• Sister chromatids: After DNA has been copied, one replicated chromosome consists of two genetically matching sister chromatids. They carry corresponding genetic information but are still joined as one replicated chromosome.
• Centromere: The centromere joins the sister chromatids and provides the chromosome region used for their coordinated separation during division.
• Telomeres: Telomeres are repeated DNA sequences at the ends of linear chromosomes. Here they are identified as chromosome-end regions; their replication-buffer role belongs to the dedicated telomere card (4563).

Packaging reduces the tangle of long DNA molecules and condensation makes each replicated chromosome a discrete structure. When sister chromatids are separated at the centromere, one matching DNA copy can be allocated to each daughter nucleus; the structure therefore supports orderly distribution without changing the genetic information carried by the copies.

A chromosome, chromatid and sister chromatid are related but not interchangeable terms. DNA replication creates matching sister chromatids within one replicated chromosome; it does not immediately double chromosome number. Telomeres are DNA end regions, not histone proteins or a substitute for the full telomere mechanism.