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IB DP Biology: SL復(fù)習(xí)筆記1.4.2 Phases of Mitosis

Category: IB課程, 教材筆記, 福利干貨 Date: 2022年8月2日下午5:48

Phases 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)
Significance of mitosis: mitosis occurs whenever the production of genetically identical nuclei are required in eukaryotic cells
- E.g. during embryonic development, growth, tissue repair and asexual reproduction

Embryonic development and growth of multicellular organisms

Unicellular zygotes?divide by mitosis in order to?grow in size
After a certain amount of growth, they then differentiate?into embryos
Growth?of multicellular organisms occurs as the number of new cells increases due to mitosis
This 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, new individuals grow from the parent organism (by cell division) and then detach (‘bud off’) from the parent in different ways
This type of reproduction can be observed in different plant, fungi and animal species
Some examples of these are budding in?Hydra?and yeast and runners from strawberries

Phases of Mitosis

Although mitosis is, in reality, one continuous process, it can be divided into?four main stages or phases
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 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?(consists of two centrioles in animal cells)
The?nuclear envelope?(nuclear membrane)?breaks down?into small vesicles
The nucleolus disappears

Prophase-of-mitosis

Prophase

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
- This attachment involves specific proteins called?kinetochores
Each?sister chromatid?is attached to a spindle fibre originating from?opposite poles

Metaphase-of-mitosis

Metaphase

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-of-mitosis

Anaphase

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
New?nucleoli form?within each nucleus

Telophase-of-mitosis

Telophase

Exam Tip

Make sure you learn the four stages of mitosis and what is happening to the DNA molecules (one chromatid contains one DNA molecule) at each stage – learn?‘PMAT’?(prophase, metaphase, anaphase, telophase) to help you remember the order of the stages!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.

Chromosomes Condense

Condensation of chromosomes

DNA molecules are?very long molecules?(human DNA can be more than 50,000 μm) that need to fit within much smaller nuclei (human nuclei average less than 5 μm)
Prior to mitosis, the DNA molecules are loosely coiled (around?histones?in eukaryotic cells) to form a complex called?chromatin
During?prophase, the chromatin gets?condensed?by?supercoiling?to form?chromosomes
Condensation occurs by the repeated coiling of the DNA molecule (supercoiling)
This supercoiling is aided in eukaryotic cells by the presence of histone proteins and enzymes

Chromatin-and-histone-proteins