CELL CONTINUITY
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Contents
Cell continuity
Chromosome
Sexual reproduction
Mitosis
Meiosis
Cell division – Mitosis
Cancer
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Cell Continuity
Is the ability of cells to divide and survive from
one generation to the next.
It involves growth, synthesis and reproduction.
It can be summarised in the “cell cycle”.
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Cell Cycle – flow chart
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Chromosome
rod like structure composed of DNA and
proteins,
contains the genetic information (genes)
which is passed from one generation of cells
or organisms to the next –
occur in pairs in most plant and animal cell
nucleii.
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Chromosome Structure
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How many chromosomes?
Each species has a characteristic number of
chromosomes.
In humans this is 46 i.e. two sets of 23, you
get one set from each parent.
As a result you have two of each type of
chromosome.
This is called the diploid number (2n).
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Sexual reproduction
(1/2)
Two sex cells fuse to form a new organism.
If two diploid cells were to fuse, what would
happen to the number of chromosomes in the
next generation?
It would double.
And in the next generation what would happen?
Double again.
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Sexual reproduction
(2/2)
If this continued to happen eventually the cell
would be filled by a nucleus.
To prevent this happening, special cells are
produced which contain only half of the
diploid number of chromosomes.
These cells are called gametes and are haploid
(n) i.e. they contain only one set of
chromosomes.
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Cell Division
There are two types of cell division: -
Mitosis,
and
Meiosis
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Mitosis
Is a type of cell division that produces cells
which are identical to the original parent
cell
1 n cell  mitosis  2 x n cells
1 2n cell  mitosis  2 x 2n cells
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The Products of Mitosis
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Function of Mitosis
• In single celled organisms, e.g. amoeba,
bacteria, etc., mitosis allows the organism to
multiply.
• In multicellular organisms, e.g. most plants
and animals (including humans), mitosis is
primarily for growth.
Back to Types of Cell Division
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L.S. Root tip - Onion - mitosis.
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Meiosis
(1/3)
Is a type of cell division that produces cells
which have only half of the genetic
information of the parent cell i.e.
1 2n cell  meiosis  4 x n cells
One diploid (2n) cell divides to produce four
new dissimilar haploid (n) cells.
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Meiosis
(2/3)
Each haploid cell is given one chromosome
from each chromosome pair of the original
cell.
See Products of Meiosis
Chromosomes are passed to the haploid cells
(gametes or spores) independently of each
other.
Large numbers of different haploid cells can
be produced.
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Meiosis
(3/3)
e.g. in the human there are 23 pairs of
chromosomes
 223 possible gametes – for both sperm and
egg production
= 8,388,608
 (223)2 different fertilisation combinations
= 70,368,744,177,664
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The Products of Meiosis
Back to Slide 14
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Functions of Meiosis
• To produce haploid cells – all of which are
different
• To produce a variety of gametes, as a result
of independent assortment
• The variety of gametes leads to variation
from generation to generation by rearrangement of chromosomes and genes
• Variations are the functional units of
evolution
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Products of Mitosis
& Meiosis compared
Mitosis
Meiosis
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Cell Division – Mitosis
The process of mitosis occurs in four stages: -
• Prophase
• Metaphase
• Anaphase
• Telophase
• But between telophase and prophase there is a
preparation stage: -
Go here first
Interphase
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Interphase
• Resting / preparatory stage between
divisions.
• Cells build up a supply of energy to
complete the division process.
• Chromosomes replicate (duplicate).
• Organelles duplicated
Back to the Four Stages
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Mitosis Summary
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Prophase
• Chromosomes thicken and become apparent
- chromosomes have duplicated.
• Each pair of sister chromosomes held by
centromere.
• Nucleolus disappears.
• Centrioles separate and migrate to opposite
ends of cell - form ‘poles’.
• Spindle fibres form from pole to pole.
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• Nuclear membrane disappears.
L.S. Root tip - Onion - mitosis.
prophase & interphase.
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Diagram of Prophase
Nuclear membrane
Sister
SisterChromosomes
Chromosomes
Back to the Four Stages
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Metaphase
• Spindle fully formed.
• Chromosome pairs attached to spindle by
centromere - arrange along 'equator'.
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L.S. Root tip - Onion - mitosis.
metaphase
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Diagram of Metaphase
Sister Chromosomes
Back to the Four Stages
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Anaphase
• Centromeres split.
• Chromosomes separated and pulled towards
opposite 'poles'
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L.S. Root tip - Onion - mitosis.
anaphase
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Diagram of Anaphase
Back to the Four Stages
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Telophase
• Nuclear membrane forms around two new
nuclei - chromosomes begin to disappear
• Cell membrane constricts between both
nuclei – cleavage furrow forms in animal
cells and cell plate forms in plant cells.
• Each new daughter cell now contains the
same number of chromosomes as the parent
cell.
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L.S. Root tip - Onion - mitosis.
telophase
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Diag. of Telophase – Animal
Sister Chromosomes
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Mitosis
Vs
• Can take place in
haploid or diploid
cells.
• Daughter cells have
same no. of
chromosomes as
parent.
• Produces two daughter
cells identical
genetically
Meiosis
(1/2)
• Can take place in
diploid cells only.
• Daughter cells have
half the no. of
chromosomes as
parent.
• Produces 4 daughter
cells - may all be
different genetically.
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Mitosis
• Occurs in somatic
cells.
• One division
(anaphase)
Vs
Meiosis
(2/2)
• Occurs in gamete or
spore producing cells
only.
• Two divisions.
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Cancer
Cancer is the uncontrolled growth of cells.
In all cases the control of cell division is lost.
Cells divide more rapidly than usual and
produce a large mass of cells called a
tumour.
Some parts of this tumour may then spread to
other parts of the body.
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The effects of a cancerous cell
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Cause of Cancer
Cancer is caused by a change (mutation) in
the parts of the DNA of a cell that controls
cell division.
These parts are called the oncogenes,
and when sufficient of these are damaged
control of cell division is lost.
Environmental factors that cause DNA
mutations are called carcinogens.
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Carcinogens
• Risk of cancer increases with age –
mutations build up over time
• Chemicals e.g. smoking
• Radiation e.g. from the sun, x-rays
• Diet – moderate intake of all food types
• Some people may inherit an increased
likelihood of getting a certain small number
of cancers
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END
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