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Pearson Edexcel BiologyCell biology

Cell division and stem cells

Understand mitosis, growth and the potential uses of stem cells.

Start here

The key idea

Mitosis produces two genetically identical cells. It supports growth, repair and asexual reproduction.

Mitosis makes identical cellsFollow one copied set of chromosomes into two daughter cells.
Mitosis makes identical cellsFollow one copied set of chromosomes into two daughter cells.parentidenticalidentical
Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

1

The cell cycle

The cell cycle is the series of stages a cell passes through to divide.During the long first stage (interphase) the cell grows, makes more ribosomes and mitochondria, and the DNA replicates so each chromosome forms two identical strands.The cell cycle produces two genetically identical daughter cells and is used for growth, development and repair of damaged tissue.

2

Mitosis

Mitosis is the second stage of the cell cycle where the nucleus divides.The chromosomes line up at the centre (equator) of the cell, and the two strands of each chromosome are pulled apart to opposite poles by cell fibres.The cytoplasm and cell membrane then divide (cytokinesis), producing two diploid cells that are genetically identical to the parent cell.

3

Stem cells

A stem cell is an undifferentiated cell that can divide to make more stem cells or differentiate into specialised cell types.Embryonic stem cells can form any type of cell, while adult stem cells (for example in bone marrow) form only a limited range.Stem cells in plant meristems remain active throughout the plant's life, letting plants make clones cheaply and quickly.

4

Uses and issues of stem cells

Stem cells offer treatments for conditions such as diabetes and paralysis, and therapeutic cloning can produce cells with the same genes as the patient to avoid rejection.Risks include transfer of viral infection and the ethical objection that embryos are potential human lives.In medicine, stem cells must be used carefully because they could become contaminated or divide uncontrollably to form a tumour.

Key terms

Definitions to learn

Mitosis

Cell division producing two genetically identical diploid daughter cells, used for growth and repair.

Chromosome

A coiled-up molecule of DNA carrying a large number of genes.

Differentiation

The process by which a cell becomes specialised for a particular function.

Stem cell

An undifferentiated cell that can divide to produce more of the same cell or differentiate into other cell types.

Meristem

Plant tissue found at growing tips and roots containing stem cells that divide throughout life.

Therapeutic cloning

Producing an embryo with the same genes as a patient so stem cells from it are not rejected.

Worked example

A skin cell divides by mitosis. State what happens to its chromosomes and describe the result.

1

The genetic material is copied before division.

2

One set of chromosomes is pulled to each end.

3

The cell divides once.

Final answer

Two genetically identical daughter cells are produced.

Exam habit

Say 'the genetic material is copied' not 'it splits in two'.For stem cell questions, name the stem cell type and state what it can differentiate into.

Watch out

Do not say mitosis produces gametes. Gametes are formed by meiosis.

Examiner tips

How to score full marks

  • 1Mitosis makes genetically IDENTICAL cells — never use the word 'mutation' or 'variation' in a mitosis answer unless asked.
  • 2When describing the cell cycle, mention DNA replication and growth in interphase BEFORE describing mitosis, or you lose marks for sequence.
  • 3For evaluate questions on stem cells, give a balanced answer with at least one benefit AND one risk or ethical issue before a conclusion.
Quick-fire quiz

Test yourself

Pick an answer — you'll see instantly if it's right.

What type of cell division produces two genetically identical daughter cells?

Which stage comes BEFORE cell division in mitosis?

Why can embryonic stem cells treat more conditions than adult stem cells?

Which organism is commonly used as a source of therapeutic stem cells to avoid ethical issues?

A student says 'mitosis produces gametes'. What is wrong with this statement?

Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1Give two uses of mitosis in multicellular organisms.
Mark scheme
  1. 1.Think about when more identical cells are needed.
Growth and repair of damaged tissue.
2Why can embryonic stem cells treat a wider range of conditions than adult stem cells?
Mark scheme
  1. 1.Compare the cell types each can become.
Embryonic stem cells can differentiate into more types of specialised cell.
3Give one risk of using stem cells in medicine.
Mark scheme
  1. 1.Consider uncontrolled division or rejection.
Possible risks include tumour formation, infection or immune rejection.
4Name the type of cell division that produces two genetically identical cells.[1 mark]
Mark scheme
  1. 1.Recall the division used for growth and repair.
Mitosis (1).
5State two purposes of mitosis in the body.[2 marks]
Mark scheme
  1. 1.Think about why an organism needs new identical cells.
Any two from: growth (1); development (1); repair of damaged tissue / replacement of cells (1).
6Describe what happens to the DNA before a cell divides by mitosis.[2 marks]
Mark scheme
  1. 1.Consider why each daughter cell needs a full set of chromosomes.
The DNA replicates (1) so that each chromosome forms two identical copies/strands joined together (1).
7Explain why stem cells from a patient's own body are useful in therapeutic cloning.[3 marks]
Mark scheme
  1. 1.Think about the genes in the cloned cells and the immune system.
The cloned cells have the same genes as the patient (1), so they are not recognised as foreign / are not rejected by the immune system (1), and they can differentiate into the cell type needed to treat the condition (1).
8Evaluate the use of embryonic stem cells to treat human diseases such as paralysis.[6 marks]
Mark scheme
  1. 1.Give benefits, then risks/ethical issues, then a judgement.
Benefit: embryonic stem cells can differentiate into any cell type (1) so could replace damaged nerve cells and restore function (1). Risk/issue: using embryos is considered unethical by some because the embryo is a potential life (1); there is a risk of transferring viral infection (1) or the cells dividing uncontrollably to form a tumour (1). Conclusion: a justified judgement linked to the points made, e.g. the potential to cure currently untreatable conditions may outweigh the risks if cells are screened carefully (1).
9State where adult stem cells are found in the human body and name one condition they can be used to treat.[2 marks]
Mark scheme
  1. 1.Recall specific tissues that contain adult stem cells.
Bone marrow (1); used to treat leukaemia / blood diseases (1).
10Explain why cells in a multicellular organism need to differentiate.[3 marks]
Mark scheme
  1. 1.Link specialisation to the specific function each cell must carry out.
Different cells in the body carry out different functions (1); differentiation allows each cell to develop the structures and characteristics needed for its specific job (1), so the organism can function efficiently (1).
11A student claims that all cells in the human body contain identical DNA. Explain whether this statement is correct.[3 marks]
Mark scheme
  1. 1.Consider what mitosis produces.
  2. 2.Consider whether mutations might occur.
The statement is broadly correct (1) because all body cells are produced by mitosis from a single fertilised egg, so they contain the same DNA sequence (1); however mutations can occur randomly during DNA replication, meaning some cells may have slight differences (1).
12Compare the properties of embryonic stem cells and adult stem cells.[4 marks]
Mark scheme
  1. 1.State what each type can differentiate into.
  2. 2.Consider availability and ethical issues.
Embryonic stem cells are pluripotent and can differentiate into any type of cell (1); adult stem cells can only differentiate into a limited range of cell types (1); embryonic stem cells raise greater ethical concerns because their use involves destroying an embryo (1); adult stem cells can be taken from the patient's own body, reducing rejection risk (1).
13Describe the sequence of events during the cell cycle, from interphase to the production of daughter cells.[4 marks]
Mark scheme
  1. 1.Include DNA replication, growth, then the stages of division.
  2. 2.End with the two daughter cells.
During interphase the cell grows, new organelles are made, and the DNA replicates to produce two identical copies of each chromosome (1); during mitosis the chromosomes line up at the equator of the cell and are pulled apart to opposite poles (1); the cytoplasm divides (cytokinesis) (1); producing two genetically identical diploid daughter cells (1).
14Scientists are investigating using stem cells to treat Type 1 diabetes by replacing the insulin-producing cells in the pancreas. Suggest two scientific problems that would need to be overcome before this treatment could be used.[2 marks]
Mark scheme
  1. 1.Think about rejection, safety and controllability of stem cell division.
Any two from: ensuring the stem cells differentiate into the correct type of insulin-producing beta cell (1); preventing the immune system from rejecting transplanted cells (1); preventing the cells from dividing uncontrollably and forming a tumour (1); risk of viral contamination being transferred to the patient (1).
15Explain why plant stem cells (meristematic cells) are commercially important to horticulturalists.[3 marks]
Mark scheme
  1. 1.Link meristems to cloning and the properties of the clones.
Meristematic cells can divide and differentiate into any plant cell type (1); this allows plants with desirable characteristics to be cloned quickly and cheaply (1); producing large numbers of genetically identical plants for sale (1).
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