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Meiosis
Joanna
George-Johnson, Contributor
The
last time, we looked at the process
of Mitosis. This time, we will be
focusing on Meiosis. Both processes
are often confused with each other;
however, there are marked distinctions
between the two.
By
the end of this article, you should
be able to:
- Compare
the processes of mitosis and meiosis
- Describe
how meiosis occurs
- State
the importance of the process
Meiosis
is a special type of cell division
that only occurs in organisms that
undergo sexual reproduction. This
type of cell division produces gametes
that are used in sexual reproduction.
Only specialised cells within the
sexually reproducing organisms can
undergo this process.
Important
reduction
By
the end of the process, the number
of chromosomes within each cell is
decreased by 50 per cent. Considering
that the purpose of this cell division
is to produce gametes (egg or sperm),
this reduction in chromosome number
is important. The reduction of chromosome
number ensures that fertilisation
(meeting of egg and sperm) restores
the complete number of chromosomes
in the resulting zygote.
In
human beings, meiosis occurs at specific
times during our lifetime. For females,
meiosis occurs before birth! This
means that females are born with the
total number of eggs (though in their
immature stage) that they will ever
have. Males, however, start meiosis
at puberty and they continue throughout
their lifetime, barring any medical
problems. Therefore, males will continuously
produce millions of sperm per day
from puberty until they die, even
if they become impotent!
This
means that each parent is responsible
for half of the genes of their offspring.
A gene is a discrete unit of information
on the DNA that codes for one particular
characteristic.
Cells
that are not gametes are known as
somatic: they have two sets of chromosomes,
one set from the mother and one set
from the father. We have two sets
of 23 chromosomes, totalling 46 chromosomes
in each cell. The two chromosomes
in each pair are referred to as being
homologous chromosomes, so we could
say that humans have 23 pairs of homologous
chromosomes.
There
are two important terms that you need
to understand before we start to examine
the process of meiosis in details,
these are:
1)
Diploid: When a cell contains
the full complement of chromosomes.
In human beings, this is 46 chromosomes.
The diploid number is also referred
to as the 2n number.
2)
Haploid: When a cell contains
half of the number of chromosomes.
This would be 23 in humans and is
also referred to as the n number of
chromosomes.
Let
us now examine the process of meiosis.
Unlike mitosis, meiosis occurs in
two stages, with several steps. These
two stages are meiosis I and meiosis
II. Meiosis I is characterised by
reduction, while meiosis II is characterised
by division. Meiosis produces four
haploid cells. It is important to
note that meiosis II is a similar
process to mitosis.
The
table below summarises what occurs
in meiosis I.
| Step |
Description |
| Interphase |
Genetic
material is replicated |
| Prophase
1 |
Duplicated
chromatin condenses. Each chromosome
consists of two, closely associated
sister chromatids.
Crossing over can occur during
the latter part of this stage.
This is the reason offspring have
different characteristics.
(See figure 1 below this table.) |
| Metaphase
1 (See figure 2) |
Homologous
chromosomes align at the equator. |
| Amphase
1 (See figure 2) |
Homologous
pairs seperate with sister chromatids
remaining together. |
| Telophase
1 (See figure 2) |
Two
daughter cells are formed with
each daughter containing only
one chromosome of the homologous
pair. |
Meiosis
2 is where gamete formation occurs.
The process is tabulated below. Remember
that an identical process takes place
as in mitosis.
| Step |
Description |
| Prophase
2 |
Nuclear
envelope disappears and spindle
fibres reform. |
| Metaphase
2 |
Chromosomes
align at the equator. |
| Amphase
2 |
Chromosomes
divide and sister chromatids migrate
seperately to each pole. |
| Telophase
2 |
Cell
division is complete. Four haploid
daughter cells are obtained. |
The
diagram below summarised the process
of meiosis.
One
parent cell produces four daughter
cells. Daughter cells have half the
number of chromosomes found in the
original parent cell and with crossing
over, are genetically different.
Meiosis
differs from mitosis, primarily because
there are two cell divisions in meiosis,
resulting in cells with a haploid
number of chromosomes.
Questions
1.
Where does meiosis occur in: a) males
and b) females?
2.
Compare mitosis and meiosis.
3.
From your knowledge of meiosis, explain
why sexual reproduction produces genetically
dissimilar offspring.
Joanna
George-Johnson teaches at Ardenne
High School.
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