Meiosis
(in testes) puberty
(2n) mitosis
(2n)
(2n) mitosis
Meiosis
(in ovaries) puberty

Section of an anther pollen grains (n) meiosis
Anther cells (2n)
Ovum
(n)

Gonads
(reproductive organs)
Animals Male gonads = Testes
Gametes
(reproductive cells)
Male gametes = sperms
Plants
Female gonads = Ovaries Female gametes = eggs or ova
(singular: ovum)
Male gonads = Anthers Male gametes = pollen grains
Female gonads = Ovules Female gametes = eggs/ova

Mitosis
1 Parent cell (2n)
Meiosis
2 identical daughter cells (2n)
1 Parent cell (2n) 4 gametes (n)

18.2 Mitosis chromatin centrioles nuclear envelope
Interphase
• = ‘resting’ or non-dividing stage of cell.
• Cells spend most of their time at interphase.
• Cells still carry out absorbing nutrients and building up protoplasm.
• Chromosomes appear as long thin threads called chromatin. The chromatin threads cannot be distinguished individually.
• Centrioles divide ( animal cell only).
• Chromatin threads replicate.
cytoplasm
An animal cell at interphase
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Chromatin threads (DNA) replicates
Sister chromatids
Chromatin thread
DNA replication
Chromatin threads
Coil & shorten centromere
One chromosome

DNA replication and mitosis must be precisely controlled
DNA replication
• copies all information in the chromosomes to ensure daughter cells produced are genetically identical cells.
• If an error occurs during DNA replication, it will be passed to the daughter cells. This leads to harmful changes to the genes and affect how the cells function.
Mitosis
• Uncontrolled cell division leads to cancer.
• The tissue will not perform its proper function eg
• cancerous growth in alveoli – reduce surface area for gas exchange

The Cell Cycle mitosis
The cell cycle :
• is the order of events cytokinesis between one cell division and the next.
interphase
• consists of :
1. Interphase or ‘resting stage’
2. Mitosis , a nuclear division - the daughter nuclei produced contain the same number of chromosomes as the parent nucleus.
3. Cell division, which is division of the cytoplasm or cytokinesis
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The Cell Cycle
Stages of mitosis
• Mitosis can be divided into four stages:
1. Prophase
2. Metaphase
3.
Anaphase
4.
Telophase
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Stages of Mitosis sister chromatids
(one chromosome) aster centrioles centromere nuclear envelope
1 Prophase
• Chromatin threads condense, coil and shorten to become chromosomes.
• Each chromosome consists of two sister chromatids attached at the centromere.
• The chromosomes are now visible under a microscope.
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aster
The Cell Cycle centrioles
1 Prophase:
- Early prophase
• In animal cells, structures called asters form around the centrioles.
Asters are made of microtubules .
• The two pairs of centrioles move to opposite poles or ends of the cell.
• The nucleolus disappears.
sister chromatids
(one chromosome) centromere nuclear envelope
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nuclear envelope breaks up
The Cell Cycle
1 Prophase
- Late prophase
• The nuclear envelope disappears.
• A spindle forms with the spindle fibres extending from one pole of the cell to the other.
spindle fibres forming
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Stages in Mitosis centromere
2 Metaphase
• Chromosomes line up around the equator of the spindle.
• The centromere of each chromosome is attached to a spindle fibre.
spindle fibres attach to centromere
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Stages in Mitosis daughter chromosomes
3 Anaphase
• Each centromere splits.
• The spindle fibres pull the chromatids apart to opposite poles of the cell.
• Once the chromatids are separated, they are called daughter chromosomes .
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Stages in Mitosis chromatin threads nucleolus nuclear envelope furrow
4 Telophase
• Spindle fibres break down.
• A nuclear envelope forms around the chromosomes at each pole of the cell.
• A nucleolus forms and the chromosomes uncoil and lengthen to become thin chromatin threads.
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Stages in Mitosis identical daughter cells
Cytokinesis
• This is the division of the cytoplasm.
• In animal cells, cleavage or furrows appear in the cytoplasm between the two nuclei.
• The furrows deepen and two identical cells are finally produced.
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