describe the structure of a chromosome, limited to: DNA, histone proteins, sister chromatids, centromere, telomeres

Replication and Division of Nuclei and Cells

Objective

Describe the structure of a chromosome, focusing on DNA, histone proteins, sister chromatids, centromere, and telomeres.

Key Components of a Chromosome

• DNA – the genetic material that carries the instructions for protein synthesis.
• Histone proteins – basic proteins around which DNA winds to form nucleosomes.
• Sister chromatids – two identical copies of a DNA molecule joined together after DNA replication.
• Centromere – the constricted region that links sister chromatids and serves as the attachment point for spindle fibres.
• Telomeres – repetitive DNA sequences at the ends of linear chromosomes that protect them from degradation.

Detailed Description

1. DNA double helix

   The DNA molecule is a double‑stranded helix composed of nucleotides (adenine, thymine, guanine, cytosine). Each strand runs antiparallel, and base‑pairing follows the rule \$A\!-\!T\$ and \$G\!-\!C\$.

2. Histone octamer and nucleosome

   Each nucleosome consists of an octamer of histone proteins (two each of H2A, H2B, H3, and H4) around which \overline{147} base pairs of DNA wrap \overline{1}.65 turns.

3. Sister chromatids

   After S‑phase, each original DNA molecule has been duplicated, producing two identical DNA strands called sister chromatids. They remain attached along their length by cohesin complexes.

4. Centromere

   The centromere is a specialized region of DNA that is less transcriptionally active and contains specific DNA sequences (e.g., \$\alpha\$‑satellite DNA in humans). It is the site where kinetochore proteins assemble, allowing attachment to spindle microtubules.

5. Telomeres

   Telomeres consist of short tandem repeats (e.g., TTAGGG in vertebrates). They form a protective cap and are maintained by the enzyme telomerase, which adds repeats to the 3′ end.

Chromosome Overview Table