1.1 Essential ideas

1.1.6 Cell division: Mitosis and cytokinesis

A mature eukaryotic cell undergoes cell division a certain number of times in its life. The process of cell division occurs in two stages.

  1. Mitosis divides the nucleus. The resulting daughter nuclei are genetically identical.
  2. Cytokinesis is the splitting up of other cell components equally between two plasma membranes. Cytokinesis is slightly different in plant and animal cells. 

The overall result of cell division is that one parent cell splits to become two identical daughter cells.

Mitosis is the division of the nucleus

Figure 1.1.6a – The phases of mitosisFigure 1.1.6a – The phases of mitosis

Mitosis is a continuous process, but it helps to visualise it in phases. Some textbooks consider more phases, such as ‘early’ and ‘late’ anaphase. Details of four stages are sufficient for the IB. Make sure you can recognise the stages from micrographs!

Stage Description
Prophase

Early – nucleolus disappears and chromosomes condense by supercoiling.

Late (prometaphase) – nuclear membrane breaks down; spindle fibres (made of microtubules) from the poles attach to the centromere of each chromosome.
Metaphase Chromosomes line up at the equator of the cell and are held in place by tension from the mitotic spindle.
Anaphase The centromere of each chromosome divides under pressure from the mitotic spindle. Two chromatids are released from each chromosome and begin to move to the poles.
Telophase Chromatids reach the pole and uncoil to become individual chromosomes; nuclear membrane re-forms around the chromosomes.

 

The role of supercoiling

  • Chromosomes need to be condensed because they are extremely long and tend to fold over onto themselves. Organising them sufficiently for mitosis would be difficult without supercoiling.
  • When chromosomes are condensed, they are wrapped up around specific proteins that act like spools.
  • Proteins and enzymes are involved in the process of supercoiling.

ball and spool of yarnFigure 1.1.6b – How do you organise a tangled mess of yarn?
Wrap it around a spool? (left) How do you organise DNA? Supercoil it around proteins? (right)

Cytokinesis is different in animal and plant cells 

  • Cytokinesis, the division of cytoplasm and organelles, is often already occurring during late-stage telophase.
  • In general, it occurs after mitosis.
  • Sometimes, as is the case of fungal aseptate hyphae (see Page 1.1.1), it does not occur at all and cells are left with many nuclei.

 Cytokinesis in animal cells

  • Contractile proteins, called actin and myosin, form microfilaments that contract the plasma membrane towards the centre of the cell.
  • A cleavage furrow forms. The cleavage furrow is a ring that continues to tighten, eventually pinching the two daughter cells apart.

Cytokinesis in animal cellsFigure 1.1.6c – Cytokinesis in animal cells

Cytokinesis in plant cells

  • Golgi apparatus packages cell wall components into vesicles and moves them towards the equator.
  • Microtubules form a scaffold to help assemble the components into a rudimentary cell wall, called a cell plate.
  • Assembly of the cell plate moves from the inside outwards until a continuous cell wall is made.

Figure 1.1.6d – Cytokinesis in plant cellsFigure 1.1.6d – Cytokinesis in plant cells

Essential idea

  • Cell division is essential but must be controlled.

CytokinesisFigure 1.1.6e – Cytokinesis
These cells have undergone mitosis and are in the process of cytokinesis.

In the lab

Make sure you are able to identify the stages of mitosis from microscope slides or micrographs. Use fixed slides, or practise making squash mounts of onion root tips.

Language hint

Students are often confused by the words chromosome, chromatids, and chromatin.

  • Chromatin refers to the material that makes up the structure of the chromosomes, including DNA and proteins.
  • Chromatin is never expressed in the plural (i.e. chromatins) because it does not define a structural unit. Chromosomes are always made of chromatin arranged in different ways.
  • Chromatids are the identical copies of chromatin that are attached to each other at centromeres during the first three phases of mitosis. Chromatids are often called ‘sister’ chromatids to indicate that they are a product of DNA replication.
  • When attached, two chromatids form one chromosome.
  • When separated, each chromatid is a chromosome on its own.

Chromosome in prophaseFigure 1.1.6f – Chromosome in prophase
A chromosome in prophase is composed of two sister chromatids attached at a centromere. Chromatids are made of supercoiled chromatin.

Course link

HL students will learn details about supercoiling in Page 7.1.1

Sources

  1. Figure 1.1.6c and 1.1.6d by Kelvinsong [CC BY 3.0], via Wikimedia Commons