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Cell Cycle and Nuclear Division
Chromosome structure
Chromosomes contain DNA (which is tightly packed) and transmits information from
one generation to the next
...

DNA combines with groups of 8 histones forming nucleosomes
...


During cell division; chromosomes are sorted out
...
Before the nucleus divides, a copy
of DNA is produced
...
They consist of two arms that extend out from a region called the
centromere
...

Y = males (shorted than female, lacks some of the genes found on the female
gene)
...

Mutations in DNA results to the production of defective genes
...
They determine
the structural, physiological and biochemical characteristics
...

-

A gene is a piece of DNA which codes for a protein (polypeptide)

-

Alleles are alternative forms of the same gene

A homologous pair of chromosomes has two alleles (1 from the father and 1 from
the mother)
...


The Cell Cycle
1
...
Cell division (cytokinesis): distribution of organelles and cytoplasm into
daughter cells
3
...
Metabolic rate is high
...
Histone proteins are synthesised and
cover each DNA strand
...


G2 (Gap 2)

Mitochondria replicate, energy stores increase
...


The cell cycle describes the series of events the cell goes through from the moment
it is formed by division to the point when the cell itself divides
...

The length of the cycle depends on the type of cell and external factors
(temperature, food and oxygen) and factors produced by the body (hormones)
...

Centrioles and spindle formation
Centrioles occur in pairs and lie at right-angles to each other
...
This consists of material that initiates
the development of the microtubules which make up the spindle fibres
...

The centrioles probably orient the spindle, determining in which plane the cell
divides
...

The events before, during and after nuclear division
1
...
Prophase (first phase of mitosis)
o Chromosomes become shorter and
thicker
o Centrioles move to opposite poles of
the cell
o Microtubules

of

spindle

fibres

originate from the centrioles – asters
o Nucleolus disappears
o Spindle fibres attach to kinetochores (found at the centre of the
chromosomes)
o Nuclear envelope starts breaking down

3
...
Anaphase
o Centromeres split allowing
chromatids to separate
o Spindle fibres contract and
pull sister chromatids apart
dragging them centromere

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foremost towards opposite poles

5
...

6
...

o Microfilaments of actin and myosin are thought to be responsible for
drawing in the cell membrane to form a cleavage furrow along the
outside surface of the cell
...
The vesicles fuse together and form a
cell plate
...


Mitosis in plant cells

Mitosis in animal cells

No centrioles present

Centrioles present

No aster forms

Aster forms

Cell division involves formation of a cell Cell division involves furrowing of the
plate

membrane and cleavage of cytoplasm

Occurs mainly at meristems

Occurs in tissues throughout the body


Significance of mitosis

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Growth - The number of cells within an organism increases by mitosis and this is
the basis of growth in multicellular organisms
...
Cells are
constantly dying and being replaced e
...
skin

Regeneration - some animals are able to regenerate whole parts of the body, such
as legs in crustaceans and arms in starfish

Asexual reproduction – mitosis is the basis of asexual reproduction; the
production of new individuals of a species by one parent organism
...
Daughter cells are genetically identical to the parent
cell and no variation
...

It is sometimes referred to as reduction division since it reduces the number of
chromosomes, from 2n to n
...
Meiosis proceeds with two nuclear
and cell divisions known as meiosis I and meiosis II, resulting in the production of 4
haploid daughter cells
...
These stages occur in both meiosis I and II
...
Prophase I: (longest phase)
o Chromosomes shorten and become visible
o Homologous chromosomes pair up

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This process is called synapsis
...

o The bivalents shorten and thicken and each chromosome and its
centromere can now be seen clearly to consist of two chromatids
o Homologous
chromosomes join
at several points
along their length
called chiasmata
Each chiasmata is the site of
exchange of genetic material
between non-sister chromatids
...
This is called crossing over
...
Metaphase I

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o The bivalents become arranged around the equator of the spindle

3
...


4
...
Crossing over has also occurred so these
chromatids are not genetically identical and must be separated in a
second meiotic division

o Spindle fibres usually disappear in animal cells and some plants
o Nuclear envelope reforms around each set of chromosomes – each set
contains one member of each pair of homologous chromosomes
o Nucleoli reappear
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o Chromosomes usually uncoil and a nuclear envelope reforms at each
pole and the nucleus enters interphase II
o No further DNA replication occurs during interphase II

Cytokinesis (cleavage or cell wall formation) then occurs in mitosis
...


5
...
The spindle is arranged at rightangles to the spindle of meiosis I

6
...
Anaphase II
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o Centromeres split allowing chromatids to separate
o Spindle fibres contact and pull sister chromatids apart dragging them
from their centromere towards opposite poles

8
...
Meiosis
occurs during gamete formation in animals and during spore formation in plants
...
This leads to genetic variation produced by fusion of the
gametes
...
Independent assortment of chromosomes: which chromosomes face which
pole during metaphase I depends purely on chance
...

2
...
Alleles are exchanged between maternal and
paternal chromosomes at chiasmata
...

3
...






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Mitosis vs Meiosis
Mitosis
Prophase

-

Meiosis I

Homologous chromosomes
remain separate
No formation of chiasmata
No crossing over

-

Homologous chromosomes pair
up
Chiasmata form
Crossing over occurs

Metaphase

-

Pairs of chromatids line up on
the equator

-

Pairs of chromosomes line up
on the equator

Anaphase

-

Centromeres divide
Chromatids separate
Separating chromatis identical

-

Centromeres do not divide
Whole chromosomes separate
Separating chromosomes and
their chromatids may not be
identical due to crossing over

Telophase

-

Same number of chromosomes
present in daughter cells as
parent cells
Both homologous chromosomes
present in daughter cells if
diploid

-

Half the number of
chromosomes present in
daughter cells
Only one of each pair of
homologous chromosomes
present in daughter cells

-

-

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Mitosis

Meiosis II

DNA replication

Before mitosis

Does not occur

Sister chromatids

identical

Not identical – crossing
over

Number of chromosomes

Equal to the number in

Half the number in the

on equatorial plate

parent cell

mitotic nucleus

End product

2 identical diploid nuclei

4 different haploid nuclei

that are identical to parent
nucleus

Mitosis
-

Meiosis
-

Only occurs in diploid & polyploid

polyploid
-

May occur in haploid, diploid &

-

Occurs during animal gamete

Occurs during somatic cell
formation

-

formation
-

Produces only 4 progeny cells,

Repeats itself such that a single

which usually undergo further

cell may give rise to a cast

duplications

number of other cells

Chromosomal Mutations
Involve changes in the number or structure of chromosomes
...
These changes may involve:
-

The loss or gain of single chromosomes: aneuploidy

-

The increase in entire haploid sets of chromosomes: polyploidy

Aneuploidy
Some daughter cells have an extra chromosome whilst others have a missing one
...
Both homologous chromosomes of a pair
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pass to the same pole of the cell and separation of the chromatids during anaphase
II may lead to the formation of gamete cells containing either one or more
chromosomes too many or too few
...
Fusion of either of these gametes with a normal
haploid gamete produces a zygote with an odd number of chromosomes
...
g
...
For example, non-disjunction of chromosome 21
produces Trisomy 21 (Down’s syndrome) (2n = 47)
Non-disjunction of the male and female sex chromosomes may occur:
XX – normal female
XO – abnormal, Turner syndrome, sterile
XXY – abnormal, Klinefeltes syndrome, sterile
XY – normal male
Polyploidy (euploidy)
Gametes and somatic cells containing multiples of the haploid number of
chromosomes are called polyploids (3n, 4n, 5n)
...

It does not prevent mitosis but it prevents meiosis
...

Polyploidy is often associated with advantageous features such as increased size,
hardiness and resistance to disease – hybrid vigour
...
Autopolyploidy: this may arise naturally or artificially as a result of an increase
in number of chromosomes within the same species
...
g
...
Allopolyploidy: this arises from hybridization between different species
followed by duplication of the genome of the hybrid as a result of an error
occurring during cell division – hybrid sterility

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Allopolyploidy does not occur in animals because there are fewer instances of crossbreeding between species
...


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