5.1 Replication and Division of Nuclei and Cells

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. Describe the structure of a chromosome, including: DNA, histone proteins, sister chromatids, centromere, and telomeres
  2. Explain the importance of mitosis in producing genetically identical daughter cells during: growth of multicellular organisms; replacement of damaged or dead cells; repair of tissues; asexual reproduction
  3. Outline the mitotic cell cycle, including: interphase (G1, S, and G2 phases); mitosis; and cytokinesis
  4. Outline the role of telomeres in preventing the loss of genes from chromosome ends during DNA replication
  5. Outline the role of stem cells in cell replacement and tissue repair by mitosis
  6. Explain how uncontrolled cell division can result in the formation of a tumour

1. Structure of a Chromosome

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What is a chromosome?

A chromosome is a structure found inside the nucleus of a cell. It is made of a very long molecule of DNA that is tightly coiled and packaged so it fits inside the tiny nucleus.

DNA and Histone Proteins

  • DNA (deoxyribonucleic acid) carries the genetic instructions for the cell. It is arranged into sections called genes, each of which codes for a specific protein.
  • DNA is a very long molecule, so it needs to be organised and compressed. It does this by wrapping around special proteins called histones.
  • Histone proteins are large, positively charged, globular (roughly ball-shaped) proteins. The DNA coils tightly around them, like thread wrapping around a bobbin.
  • The combination of DNA wound around histone proteins forms a material called chromatin. Chromatin is what chromosomes are made of.

Think of it this way: If the DNA in a single human cell were unravelled and laid out in a straight line, it would be about 2 metres long. Histone proteins allow this enormous length of DNA to be squeezed into a nucleus that is only a few micrometres across.

Sister Chromatids and the Centromere

Before a cell divides, the DNA inside the nucleus is replicated (copied). This means each chromosome ends up consisting of two identical copies of the original DNA molecule. Each copy is called a chromatid.

  • The two identical copies that make up one chromosome are called sister chromatids. They are genetically identical to each other because one is an exact copy of the other.
  • The two sister chromatids are held together at a point called the centromere. The centromere is a narrow region that joins the two chromatids together.

Telomeres

  • Telomeres are found at the very ends of each chromatid (and therefore at the ends of chromosomes).
  • They are made of non-coding DNA — this means they do not contain any genes and do not code for proteins.
  • Their base sequence is made up of short sequences of bases repeated many times over.
  • Their job is to protect the ends of the chromosomes (explained fully in Section 4).

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