All revision notes topics

Cell division and growthIB MYP Sciences: Revision notes

Section 1

Why cells divide

Cells divide for three main reasons:

  • Growth: an organism gets bigger by making more cells.
  • Repair: damaged cells, such as skin cells in a graze, are replaced.
  • Replacement: cells that wear out, such as red blood cells, are replaced.

In body cells, this type of cell division is called mitosis.

Key termsmitosisdaughter cell

Section 2

Chromosomes and the cell cycle

The nucleus contains chromosomes, which are long coiled molecules of DNA. Human body cells have 46 chromosomes, in 23 pairs. Cells with pairs of chromosomes are diploid.

The cell cycle is the series of stages a cell goes through:

  1. The cell grows and copies its DNA.
  2. Mitosis: the nucleus divides.
  3. The cytoplasm divides, forming two new cells.

The new cells can then start the cycle again.

Key termschromosomediploidcell cycle

Section 3

Mitosis in outline

  1. The DNA is copied, so that each chromosome has an identical copy.
  2. The nucleus divides into two nuclei.
  3. The cytoplasm divides (with the cell membrane) to give two cells.

The result is two genetically identical daughter cells. Each is diploid, with the same number of chromosomes as the parent cell. A cell with 46 chromosomes makes two cells that each have 46.

Common mistake

Mitosis does not halve the chromosome number. The daughter cells have the same number as the parent. Halving happens in meiosis, which makes gametes.

Section 4

Stem cells

A stem cell is an unspecialised cell. It can divide by mitosis to make more cells, and it can become (differentiate into) different types of specialised cell.

Stem cells in bone marrow make new blood cells. Because they can become many cell types, stem cells are being studied for repairing damaged tissue.

Key termsstem celldifferentiate

Section 5

Cancer

Cancer happens when cells divide in an uncontrolled way. The cells form a lump called a tumour. Some tumours stay in one place; others can spread to other parts of the body.

Risk factors that increase the chance of cancer include:

  • ultraviolet (UV) radiation from the Sun or sunbeds
  • smoking tobacco
  • some chemicals
  • ionising radiation, such as X-rays and radioactive materials, in high doses
  • some inherited genes
Key termscancertumour
Exam tip

Say 'uncontrolled cell division' in your answer. 'Cells growing' alone is not precise enough.

Must know

  • Cells divide for growth, repair and replacement
  • Mitosis: two genetically identical diploid cells
  • DNA copied, nucleus divides, cytoplasm divides
  • Chromosomes: coiled DNA; humans have 46
  • Stem cells: unspecialised; can become many types
  • Cancer: uncontrolled cell division, forming a tumour

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Cell division and growth

  1. A student in Lagos grazes her knee while playing football. Within a week, new skin has grown over the wound.
    Explain why the new skin cells have the same DNA as the original skin cells.2 marks
  2. A cell biology class in Chennai builds a model of a dividing animal cell. The model parent cell has 4 chromosomes, shown by lengths of coloured wool inside a paper nucleus.
    Outline what happens to the nucleus and the cytoplasm after the DNA has been copied.2 marks
  3. Researchers in Tokyo study stem cells taken from bone marrow, the soft tissue inside some bones where blood cells are made.
    Outline what is meant by a stem cell.3 marks
See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).