In 1882 the German anatomist Walther Flemming published a book on cell substance, the nucleus and cell division. Using newly available dyes, he stained salamander cells and saw thread-like structures in the nucleus that split and moved apart as a cell divided. He named the process mitosis, from the Greek mitos, "thread". In 1888 Wilhelm Waldeyer called the threads chromosomes, "coloured bodies". What Flemming didn't know was what the threads were for. Gregor Mendel's work on inheritance had been published in 1866, but it was largely overlooked until 1900, and the link between chromosomes and genes came in the early 1900s.
This lesson covers cell division and stem cells in AQA GCSE Biology Topic 1 (Cell biology). It's also useful for Combined Science.
Chromosomes
The nucleus of a cell contains chromosomes, each made of a long DNA molecule. Each chromosome carries a large number of genes. In body cells, chromosomes are normally found in pairs: human body cells have 23 pairs.
The cell cycle in three stages
Cells divide in a series of stages called the cell cycle. During the cycle the genetic material is doubled, then divided between two identical cells.
- Growth and DNA replication. The cell grows and makes more sub-cellular structures, such as ribosomes and mitochondria. Its DNA replicates, forming two copies of each chromosome.
- Mitosis. One set of chromosomes is pulled to each end of the cell, and the nucleus divides.
- Division of the cell. The cytoplasm and cell membrane divide, forming two identical cells.
AQA asks for these three overall stages. You don't need the names of the separate phases within mitosis.
Where mitosis happens
Mitosis is how a multicellular organism grows and develops, and how it repairs and replaces cells. In mature animals, cell division is mainly restricted to repair and replacement. Exam questions often describe a situation and ask you to spot mitosis: a growing embryo, a healing cut, bone marrow making blood cells, or a root tip growing longer.
Stem cells and differentiation
As an organism develops, its cells differentiate: they become specialised, gaining the sub-cellular structures they need for one job. Most animal cells differentiate early. Many plant cells keep the ability to differentiate throughout life.
A stem cell is an undifferentiated cell that can make many more cells of the same type, and from which other types of cell can arise by differentiation.
- Embryonic stem cells can be cloned and made to differentiate into most types of human cell.
- Adult stem cells from bone marrow can form many types of cell, including blood cells.
- Meristems at plant root and shoot tips can differentiate into any type of plant cell, throughout the plant's life.
Stem cells in medicine
Treatment with stem cells may be able to help conditions such as diabetes and paralysis. In therapeutic cloning, an embryo is produced with the same genes as the patient, so its stem cells wouldn't be rejected by the patient's body.
There are risks, such as transferring a viral infection, and some people have ethical or religious objections, for example to using human embryos. Exam questions may ask you to weigh these against the possible benefits.
Cloning plants from meristems
Meristem tissue can be used to produce clones of plants quickly and cheaply. Rare species can be cloned to protect them from extinction, and crop plants with useful features, such as disease resistance, can be cloned in large numbers for farmers.
Common exam mistakes
- Halving the chromosomes. Mitosis keeps the number the same. Halving happens in meiosis, when gametes are made.
- Starting with mitosis. The cell grows and copies its DNA first.
- "Identical" without saying to what. The two new cells are genetically identical to each other and to the parent cell.
- Saying adult stem cells can become any cell. Bone marrow stem cells form many types, including blood cells, but not as many as embryonic ones.
- Writing that stem cells "will cure" a disease. AQA's wording is that they may be able to help.
Frequently asked questions
What are the three stages of the cell cycle?
The cell grows and copies its DNA; mitosis pulls one set of chromosomes to each end and the nucleus divides; then the cytoplasm and cell membrane divide.
What is the difference between embryonic and adult stem cells?
Embryonic stem cells can become most types of human cell. Adult stem cells, such as those in bone marrow, form fewer types, including blood cells.
Why is mitosis important?
It produces new, genetically identical cells for growth, development and the repair of damaged tissue.
What is a meristem?
Tissue at plant root and shoot tips that keeps dividing and can differentiate into any type of plant cell.
Try it
Say each answer before you flip the card. Then put yourself in Flemming's place in 1882: you've watched coloured threads split and separate, but nobody has told you what they carry. What would you have tested next?
Braineos is not affiliated with or endorsed by AQA.