Transcript File
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Chapter 18: Cell Division Mitosis Meiosis
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)
Where are the gametes formed in animals and plants?
Animals Gonads (reproductive organs) Male gonads = Testes Gametes (reproductive cells) Male gametes = sperms Plants Female gonads = Ovaries Male gonads = Anthers Female gonads = Ovules Female gametes = eggs or ova (singular: ovum) Male gametes = pollen grains 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.
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Chromosomes appear as long thin threads called chromatin. The chromatin threads cannot be distinguished individually.
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Centrioles divide ( animal cell only).
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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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 29 April 2020
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The Cell Cycle
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Stages of mitosis
Mitosis can be divided into four stages: 1.
Prophase
2.
Metaphase
3.
Anaphase
4.
Telophase Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 29 April 2020
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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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 29 April 2020
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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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 29 April 2020
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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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