CELL
DIVISION
CHROMOSOMES HAPLOID
AND DIPLOID CELLS CELL CONTINUITY INTERPHASE
MITOSIS PROPHASE METAPHASE ANAPHASE TELOPHASE
SUMMARY CHART
OF THE PHASES OF MITOSIS ANIMAL CELL
CYTOKINESIS
PLANT CELL CYTOKINESIS MITOSIS SUMMARY MEIOSIS
MEIOSIS SUMMARY SUMMARY CHART: MEIOSIS-MITOSIS
CELLS AND CANCER INTERNET LINKS
Chromosomes are structures located in the nucleus of the cell. They are made of DNA and protein. Chromosomes are long thin threads called chromatin until cell division occurs.
Then they become visible as rod-like
chromosomes. Chromosomes are composed of genes. All the genes of an organism make up the organism’s genome. Genes control the physical
characteristics of a species. All organisms of the same species contain the
same number of chromosomes in their nuclei.
Haploid
cells contain 1 set of
chromosomes in their nuclei. Diploid
cells contain 2 sets of chromosomes in their nuclei. The human species
contain 46 chromosomes in their nuclei. This is the diploid (2n) number. The sex cells of the species
have 23 chromosomes in their nuclei. This is the haploid (n) number. When fertilisation takes place the 23 chromosomes (n) from the father (called Paternal chromosomes) and the 23
chromosomes (n) from the mother
(called the Maternal chromosomes)
combine to form the diploid (2n=46) number
of chromosomes in the fertilised egg cell.
CELL CONTINUITY
All cells develop from existing cells. This is called cell continuity. Cell continuity
enables organisms to grow, replace dead cells, and reproduce. A cells life can
be described with the cell
cycle. There are three stages of the cell cycle: interphase,
division of the nucleus (mitosis or meiosis) and cytokinesis. (Note that there
are 3 stages in interphase but you are not responsible for this in your
course.)
THE STAGES OF THE CELL CYCLE
This is the part of
the cell’s life process when it does not divide. In
this phase the
cell grows, proteins and enzymes are made, and it increases the number of
cytoplasmic organelles. Near the end of interphase chromosome duplication occurs. When this occurs the single
strand chromosome becomes a double
strand. Each strand has identical genes.
View this animation of interphase
MITOSIS-DIVISION OF THE NUCLEUS
View this introduction to mitosis
Mitosis is the
division of the nucleus. Two daughter
nuclei, genetically identical the original nucleus, are formed. The 2 cells
formed by mitosis are called daughter
cells.
STAGES OF MITOSIS
There are four stages of mitosis. They
are prophase, metaphase, anaphase, and
telophase.
As seen on
the diagram below, the chromatin
condenses forming chromosomes. Each
chromosome is composed of two identical sister chromatids connected at the centromere.
At this stage the nucleolus and the nuclear membrane break down and the spindle fibres form.
View this animation of prophase
As seen on
the diagram below, the chromosomes line up along the middle of the cell. Each
chromosome is connected to both sides of the cell by spindle fibres attached to the centromeres.
View this animation of metaphase
As
seen on the diagram below, the spindle fibres shorten (contract) and split the
pair of chromosomes at the centromeres. The 2 sets of chromosomes are pulled to
the opposite sides (poles) of the
cell.
View this animation of anaphase
As seen on
the diagram below, each chromosome group becomes a nucleus when a nuclear
membrane is formed around it. The chromosomes uncoil to become chromatin and the nucleolus
reforms.
View this animation of telophase
SUMMARY CHART OF THE PHASES OF MITOSIS:
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Interphase This cell is shown, as this is how
all cells look before mitosis. Please be aware that Interphase is a phase of the cell cycle, but NOT a stage of mitosis.
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Mitosis - Early Prophase To begin mitosis, the nuclear
membrane breaks down, while the chromosomes shorten and thicken (here, a
chromosome is two chromatids, bound at a point called the centromere, making
an "X" shape). The other structures important for mitosis are also
forming (i.e. the centrioles). |
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Mitosis - Metaphase The mitotic spindle apparatus has now
formed and lies on the poles of the nucleus (but remember, the nuclear
membrane has broken down, so there is no distinctly delineated nucleus). The
chromosomes are lined up along the cell's equator, also known as the
equatorial plate, and are attached to the mitotic spindle apparatus via
microtubules (to try and visualize the microtubules extending from the poles
to the chromosomes on the equator, think of the Earth - it's as if rope was
extending from the chilly north and south poles to the chromosomes basking in
the sun at the equator). Here's the confusing part - When the individual
chromatids (˝ of the "X") are separated from the chromosome (the
"X"), they are now each referred to as a chromosome (i.e. In
metaphase, the chromosome, composed of two chromatids, separates into the
individual chromatids, which are then renamed chromosomes, even though they
were only one half of a chromosome only moments before!) - Whew! |
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Mitosis - Anaphase The newly formed chromosomes (which
were recently chromatids while they were still ˝ of the "X") are
pulled along the microtubules toward opposite poles of the cell (like Monarch
butterflies migrating back to |
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Mitosis - Telophase The chromosome have finished their
migration to the poles and the mitotic structures breakdown. The plasma
membrane of the cell pinches down along the equator creating two separate
cells (similar to twisting a long balloon and forcing it to pinch in the
middle). At this time, the chromosomes become indistinct (as they are during
Interphase), the nuclear membrane forms again and the nucleolus reappears. |
Cytokinesis
results in 2 daughter cells each
with identical chromatin. The process differs in animal and plant cells.
ANIMAL CELL CYTOKINESIS
Animal
cell cytokinesis
occurs by a process called cleavage.
As seen in the diagram below, a cleavage
furrow, appears. This furrow becomes deeper until, eventually, it breaks
the 2 halves apart to form 2 daughter
cells.
To view this more than once click your
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As seen in the diagram below, sacs
called vescicles form around the
centre of the cell. These sacs contain cellulose
which will form a cell plate. 2 new cell walls develop along the middle lamella. The cells then break
apart forming 2 daughter cells.
The formation of a multicellular
organism.
Asexual reproduction in organisms such
as amoeba and yeast.
Vegetative reproduction of plants.
Cell replacement and regeneration.
Gamete formation in the flowering
plant.
Faithful copying of genes and their
transfer to the next generation of nuclei or cells.
Maintains the correct chromosome number
in cells.
View this introduction to meiosis
A human body cell contains 23 pairs of chromosomes.
The gametes - sperm or eggs - contain half this number of chromosomes, which is
why meiosis is sometimes called 'reduction division'. (For more on chromosomes
go to DNA and genes)
Before meiosis
begins, the chromosomes are copied exactly. The DNA
of each chromosome is replicated to form two chromatids. They then arrange
themselves into homologous pairs (both coding for the same characteristics),
and prepare for cell division. At this point maternal and paternal chromatids
can exchange bits of DNA to recombine their genetic material and increase the
potential for variation.
The homologous pairs of chromosomes
then separate and move to the poles of the parent nucleus. For each of the 23
pairs there is a 50-50 chance as to which pole the paternal or maternal pair of
chromatids goes. With over 8 million possibilities there are many opportunities
for variation.
The nucleus now divides to form two
daughter nuclei, each with a mixture of paternal and maternal chromosomes but
with half the full complement of genetic material (and no pairs at all). This
division is called Meiosis 1.
Finally the two daughter nuclei
themselves divide to form gametes.
This second division - Meiosis 2 - works just like mitosis. The chromosomes
(really pairs of chromatids) split apart to form the genetic material of the
four new cells. The end result is four sex cells each with a complete but
single set of 23 chromosomes.
On fertilisation the nuclei of the
sperm and the egg join to form a new nucleus, called the zygote.
The zygote contains 23 pairs of chromosomes - 23 single chromosomes from the
sperm, and 23 single chromosomes from the egg.
View
this animation of crossing over
View this animation of meiosis
View another animation of meiosis
SUMMARY ANIMATION:
For meiosis to work, first the chromosomes are doubled, just as
they are for mitosis.
Then the cell splits into two, just as it does for mitosis.
But, it does not stop there. With meiosis, there is an
additional division in order to split up the information so that each cell has
50%. The animation below shows the second division:
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The daughter cells now look like this:
Increases genetic variation in the
population and, as a result, in evolution
Makes sexual reproduction in plants and
animals possible
Gamete formation in animals
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It's
really important that you don't get meiosis
and mitosis
confused! Take some time to look at the table below and make sure you
understand all the differences between the two types of cell division.
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View this comparison of mitosis and meiosis
Cancer results from cells that divide at an
abnormally high rate. As a result a tumour
results. Tumours may be either benign
or malignant.
BENIGN TUMOURS
Malignant tumours can invade parts of the body and,
therefore, damage that part of the body. The movement to other parts is called metastisis.
Benign tumours are not life-threatening. They do not
invade other areas of the body and most can be removed.
CAUSES OF CANCER
Cancer is caused by substances called carcinogens.
Carcinogens include:
Environmental Chemicals – cigarette
smoke, toxic substances in plants, benzene
Physical Agents – nuclear radiation,
X-rays, UV light
Biological Agents – viruses
TREATMENT OF CANCER
Cancer can be treated with surgery (to remove the tumour), radiation (to burn out the tumour), and
chemotherapy (to poison the tumour).
INTERESTING AND INFORMATIVE LINKS:
EXCELLENT INTERACTIVE
ANIMATION OF MITOSIS