Meiosis
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Meiosis
In sexual reproduction each parent who has two genes for nearly every
trait will pass one gene to the offspring. If the each pair of genes would
be identical then the offspring would be identical to the parents, in
other words it would be a clone. However, the genes for a particular trait
can be different . Different forms of the same gene are called alleles Sexual reproductions
will create new combinations of alleles in the offspring, which is the
basis of evolutionary change.
Meiosis is a series of chromosomal divisions after replication. It is a
process which has some similarities to mitosis. Meiosis is restricted to
gametes , the egg and sperm
cells. Gametes have a diploid
number of chromosomes, i.e. each chromosome as an almost identical
copy. In human germ cells (gametes) are the 22 pairs of chromosomes and 2X
chromosomes (females) and 22 pairs of chromosomes and 1X and 1Y chromosome
in males. (total of 46 chromosomes). During meiosis the diploid number of
chromosomes is reduced by half to the haploid number: 2N
(diploid) ---------- N (haploid).
After meiosis human gametes have only 23 chromosomes.
Stages of meiosis
Like mitosis, meiosis is preceded by DNA replication. The almost identical
pair of chromosomes to be copied consist of a maternal and a paternal
chromosome (i.e. one originates from the mother, the other one originates
from the father). The duplicated chromosomes are joined by their centromere. The
replication below is shown for a single pair of chromosomes (out of 23
pairs)
Meiosis I
Meiosis is divided into two stages: meiosis I and meiosis II. Meiosis I begins with a prophase I, in which the
replicated chromosomes pair with each other in a parallel arrangement.
The close proximity of the two parental chromatids allows the exchange of
corresponding segments of the two chromatids in a process called
crossing over. Crossing over is essentially a process by which
genetic information is exchanged between the parental chromosomes.
The next stage of meiosis is metaphase I. A spindle apparatus forms which
attaches microtubules
to the kinetochores
and starts pulling apart the parallel aligned chromosomes . During
anaphase I the chromosomes have moved toward the spindle poles. The
movement of the chromosomal pair is random, i.e. there is no preference
which pair (paternal or maternal) is moving to a particular pole. You can
calculate the number of possible combinations for the movement of e.g. 4
chromosomal pairs (= 8 possible combinations of maternal and paternal
chromosomal pairs) The possible combinations for 23 chromosomal pairs is
over 8 million. Thus, it is highly unlikely that the mixing of the traits
will result in identical or nearly identical offspring.
Anaphase is followed by
cytokinesis and telophase. The two newly formed cells will have a haploid
number of still replicated chromosomes
Meiosis
II
During meiosis II the two sister chromatids of the replicated chromosomes
are separated. The mechanism is similar to mitosis. First two pairs of
centrioles forming a spindle apparatus are moving to oppsite sides of the
nucleus. The microtubules of the assembly make contact to the
kinetochores and through elongation and shortening of the microtubules the
chromosomes are lined up between the two poles (metaphase II). During the
anaphase II the sister chromatids are separated and each individual
chromosome is pulled toward opposite poles. In the telophase II the
nuclear envelope is formed and cyctokinesis causes the formation of
daughter cells.
A summary of the events in meiosis I and II is shown below. Only two pairs
of chromosomes (out of the 23 pairs of human chromosomes) are considered.
All possible alignments of paternal and maternal chromosomes are shown,
leading to the formation of 16 gametes. Among the 16 possible gametes, 4
have different combinations of paternal and maternal chromosomes. Crossing
over was not considered in the figure below.
Fertilization
Egg: The eggs of non-mammalian animals are huge cells which contain
large amounts yolk to sustain the development of the embryo. Mammalian
eggs are much smaller (human egg : 0.1mm diameter) since the nutrition for
the embryo comes from the mother. A developing egg is called oocyte,
whereas a mature egg is called ovum. Oocytes in mammals are formed between
the 3 rd and 8 th month of gestation. During this early development, after
crossing over has occurred they remain in prophase I. After the female
becomes sexually mature, the replicated DNA of the oocytes divides into
two daughter nuclei followed by cyctokinesis. This first meiotic division
is asymmetric. One of the cells produced is called the secondary oocyte,
whereas the other cell is very small and is called polar body. The
secondary oocyte undergoes meiosis II . The developmental stage in which
the oocyte is released from the ovaries is different for different
species. In human eggs the development stops at metaphase II and continues
only after fertilization. At this stage meiosis II will be completed and
a second polar body as well as the mature egg are formed.
Sperm: While the eggs are considered non motile the sperm is motile. Immature diploid sperm cells (spernatogonia) begin to mature into spermatocytes during puberty . The spermatocytes enter meiosis I , become secondary spermatocytes and continue through meiosis II to produces haploid spermatids. Spermatids develop into mature sperms.
The scheme below shows the formation of sperm in male animals, the
formation of an egg in female animals and fertilization . For clarity only
one chromosome is considered.
