The cell cycle, mitosis and stem cells
Your body makes new cells for growth and repair by mitosis, which produces exact copies. Most cells then specialise for one job. Stem cells have not specialised yet, which is what makes them so useful in medicine.
Part 1 of 3: Learn it
In short
- Mitosis makes two genetically identical cells, for growth, repair and asexual reproduction.
- Before mitosis the cell grows, copies its DNA and makes more sub-cellular structures.
- Stem cells can divide and turn into other types of cell.
Where this is in your specification
Spec points: AQA 4.1.1.3, 4.1.1.4 and 4.1.2.1 to 4.1.2.3 (Combined Trilogy 4.1.1.3, 4.1.1.4 and 4.1.2), Edexcel Topic 2, OCR B2.1 and B2.2
| Board | Topic: Cell biology |
|---|---|
| AQA 8461 | 4.1 (8461); 4.1 (8464) |
| Edexcel 1BI0 | Topics 1 and 2 |
| OCR J247 | B1 and B2 |
| Eduqas C400 | 1.1 to 1.3, 2.1 |
| Cambridge IGCSE 0610 | 2.1, 2.2, 3.1 to 3.3, 17.2 |
| National 5 C807 75 | Cell biology: 1 Cell structure, 2 Transport across cell membranes; Multicellular organisms: 1 Producing new cells |
Chromosomes
The nucleus holds DNA arranged in chromosomes. Human body cells have 46 chromosomes in 23 pairs; one of each pair comes from each parent.
The cell cycle
- Stage 1: the cell grows, makes more ribosomes and mitochondria, and copies its DNA so each chromosome has two identical halves.
- Stage 2 (mitosis): one set of chromosomes is pulled to each end of the cell and two new nuclei form.
- Stage 3: the cytoplasm and cell membrane divide to give two identical cells.
Specialisation
| Cell | Adapted by |
|---|---|
| Sperm cell | a tail to swim, lots of mitochondria, enzymes in the head to get into the egg |
| Nerve cell | long, with branched ends to connect to other cells |
| Muscle cell | proteins that slide to contract; many mitochondria |
| Root hair cell | a long projection that gives a large surface area for absorbing water |
| Xylem and phloem | tubes for carrying water (xylem) and dissolved sugars (phloem) |
Becoming specialised is called differentiation. Most animal cells differentiate early in life; many plant cells can differentiate throughout their life.
Stem cells
- Embryonic stem cells can become almost any type of cell.
- Adult stem cells in bone marrow can form some types, such as blood cells.
- Meristem tissue in plant shoots and roots can become any plant cell, which lets gardeners clone plants quickly.
- In therapeutic cloning, an embryo is made with the patient's own genes, so its stem cells would not be rejected.
Stem cell treatments might help conditions such as diabetes and paralysis. Concerns include the use of embryos, the risk of passing on viral infections, and that some people feel it is wrong to use embryos at all.
How many cells does mitosis produce, and how do they compare?
Show the answer
Two cells, genetically identical to each other and to the parent cell.
Part 2 of 3: See it worked
Worked examples
Example 1
In a sample of 200 root tip cells, 24 are in mitosis. The whole cell cycle takes 20 hours. Estimate how long mitosis lasts.
- Fraction in mitosis = 24 ÷ 200 = 0.12
- Time in mitosis = 0.12 × 20 hours
Answer: About 2.4 hours.
Example 2
A single cell divides by mitosis five times in a row. How many cells are there at the end?
- Each division doubles the number of cells
- 2⁵ = 32
Answer: 32 cells.
Common mistakes
- Saying mitosis makes gametes. Gametes are made by meiosis.
- Forgetting that DNA is copied before mitosis starts.
- Saying adult stem cells can become any cell. Their range is limited.
- Mixing up xylem (water) and phloem (sugars).
Where are stem cells found in plants?
Show the answer
In the meristems at the tips of shoots and roots.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
How many cells does mitosis produce, and how do they compare?
Two cells, genetically identical to each other and to the parent cell.
Where are stem cells found in plants?
In the meristems at the tips of shoots and roots.
Give two uses of mitosis.
Any two of: growth, repair of damaged tissue, asexual reproduction.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses