Nuclear Division (SL IB Biology)

• There are two processes by which the nucleus of a eukaryotic cell can divide. These are:
  • Mitosis
  • Meiosis
• It is important for the nucleus of a cell to divide before cell division to avoid the production of anucleate cells
  • An anucleate cell is one without a nucleus
• Mitosis gives rise to genetically identical cells and is the type of cell division used for growth, repair of damaged tissues, replacement of cells and asexual reproduction
  • Mitosis maintains the chromosome number and genome of cells
  • Cells produced during mitosis will often be diploid (2n)
• Meiosis gives rise to cells that are genetically different from each other and is the type of cell division used to produce gametes (sex cells)
  • This is because the daughter cells produced during meiosis will only have half the number of chromosomes of the parent cell
  • These cells are said to be haploid (n)
  • Meiosis is important for generating genetic diversity

DNA replication as a prerequisite for mitosis and meiosis

• During interphase (the period before nuclear division) of the cell cycle, the DNA replicates to create two identical strands of DNA called chromatids
  • The chromatids are joined together by a narrow region called the centromere
• The two chromatids that make up the double structure of a chromosome are known as ‘sister chromatids’
  
  • During anaphase, one chromatid from each chromosomes ends up in one daughter cell while the other chromatid ends up in the other daughter cell
  • After the centromere is split apart at the start of anaphase, the chromatids are referred to as individual chromosomes again

Chromosome structure and DNA replication diagram

Diagram illustrating the structure of a chromosome at different stages of mitosis

Condensation of chromosomes

• DNA molecules are very long molecules (human DNA can be more than 50,000 µm) that need to fit within much smaller nuclei (human nuclei average less than 5 µm)
• Prior to mitosis, the DNA molecules are loosely coiled (around histones in eukaryotic cells) to form a complex called chromatin
  • Histones package DNA into structures called nucleosomes
  • Each nucleosome consists of a strand of DNA coiled around eight histone proteins
• During prophase, the chromatin gets condensed by supercoiling to form chromosomes
• Condensation occurs by the repeated coiling of the DNA molecule (supercoiling)
• This supercoiling is aided in eukaryotic cells by the presence of histone proteins and enzymes

Supercoiling of DNA around histones diagram

DNA is coiled around histone proteins to make chromatin

Movement of chromosomes

• Microtubules and microtubule motors are responsible for the movement of chromosomes during cell division
  • Microtubules are tubulin fibres that form part of the cytoskeleton of the cell
  • They are able to lengthen and shorten in order to enable chromosome movement
  • Two types of tubulin, α-tubulin and β-tubulin form dimers which can be added or removed at the ends of the microtubules to change the length of the tubule
    • A dimer is a compound made up of two subunits
• Chromosome movement is facilitated by motor proteins
  • These carry the chromosomes along the microtubules to the equator of the cell in preparation for nuclear division

Microtubule formation diagram

Microtubules are responsible for the movement of chromosomes during cell division