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Edexcel A (SNAB) A Level Biology:复习笔记3.2.2 Mitosis

Category: A-level课程, 国际课程, 教材笔记, 福利干货 Date: 2022年11月8日下午4:46

The Stages of Mitosis

Mitosis is the process of nuclear division by which two genetically identical daughter nuclei are produced that are also genetically identical to the parent cell nucleus (they have the same number of chromosomes as the parent cell)
Although mitosis is, in reality, one continuous process, it can be divided into four main stages
These stages are:
- Prophase
- Metaphase
- Anaphase
- Telophase
Most organisms contain many chromosomes in the nuclei of their cells (eg. humans have 46) but the diagrams below show mitosis of an animal cell with only four chromosomes, for the sake of simplicity
The different colours of the chromosomes are just to show that half are from the female parent and half from the male parent

Prophase

Chromosomes condense and are now visible when stained
The chromosomes consist of two identical chromatids called sister chromatids (each containing one DNA molecule) that are joined together at the centromere
The two centrosomes (replicated in the G2 phase just before prophase) move towards opposite poles (opposite ends of the nucleus)
Spindle fibres (protein microtubules) begin to emerge from the centrosomes (which consist of two centrioles in animal cells)
The nuclear envelope (nuclear membrane) breaks down into small vesicles

Prophase stage of mitosis where chromosomes condense into visible structures

Metaphase

Centrosomes reach opposite poles
Spindle fibres (protein microtubules) continue to extend from centrosomes
Chromosomes line up at the equator of the spindle (also known as the metaphase plate) so they are equidistant to the two centrosome poles
Spindle fibres (protein microtubules) reach the chromosomes and attach to the centromeres
Each sister chromatid is attached to a spindle fibre originating from opposite poles

Metaphase, where chromosomes line up along the equator of the cell

Anaphase

The sister chromatids separate at the centromere (the centromere divides in two)
Spindle fibres (protein microtubules) begin to shorten
The separated sister chromatids (now called chromosomes) are pulled to opposite poles by the spindle fibres (protein microtubules)

Anaphase, where chromosomes are pulled to the poles of the cell

Telophase

Chromosomes arrive at opposite poles and begin to decondense
Nuclear envelopes (nuclear membranes) begin to reform around each set of chromosomes
The spindle fibres break down

Telophase, where the nuclei reform and the cell begins to split into two

Exam Tip

Make sure you learn the four stages of mitosis. Cytokinesis is often mistaken as a stage of mitosis but remember this is a separate part of the cell cycle.

The acronym PMAT can be helpful to remind you what happens during each stage of mitosis:

P = Prophase, where the cell Prepares to divides

M = Metaphase, where the chromosomes align along the Middle

A = Anaphase, where the chromosomes move Away from each other

T = Telophase, where Two nuclei reform

The chromosome number is important too; after interphase but before the parent cell undergoes mitosis, the human parent cell nucleus actually contains 92 DNA molecules! This is because during interphase (S phase), the 46 DNA molecules in the parent cell have replicated to form sister chromatids. As human cells have a diploid number of 46 this replication results in 92 molecules. This ensures the two daughter cells will be diploid (have 46 chromosomes each) when mitosis occurs. Remember to read the questions carefully as only human diploid cells have 46 chromosomes so if the question refers to another organism, its diploid number will be different.

The Significance of Mitosis

The process of mitosis is of great biological significance and is fundamental to many biological processes:

Growth of multicellular organisms

The two daughter cells produced are genetically identical to one another (clones) and have the same number of chromosomes as the parent cell
This enables unicellular zygotes (as the zygote divides by mitosis) to grow into multicellular organisms
Growth may occur across the whole body of the organism or be confined to certain regions, such as in the meristems (growing points) of plants

Replacement of cells & repair of tissues

Damaged tissues can be repaired by mitosis followed by cell division
As cells are constantly dying they need to be continually replaced by genetically identical cells
In humans, for example, cell replacement occurs particularly rapidly in the skin and the lining of the gut
Some animals can regenerate body parts, for example, zebrafish can regenerate fins and axolotls regenerate legs and their tail amongst other parts

Asexual reproduction

Asexual reproduction is the production of new individuals of a species by a single parent organism – the offspring are genetically identical to the parent
For unicellular organisms such as Amoeba, cell division results in the reproduction of a genetically identical offspring
For multicellular organisms (as seen with many plant species) new individuals grow from the parent organism (by cell division) and then detach (‘bud off’) from the parent in different ways. Some examples of these are budding in Hydra and yeast and runners from strawberries

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