Meiosis:
The meiosis consists of two successive divisions called the first and second meiotic
divisions.
During the interphase preceding the first division, duplication of the DNA content of
chromosomes takes place as in mitosis.
As a result, another chromatid identical to the original one is formed.
Thus, each chromosome is now made up of two chromatids.
First Meiotic Division:
Prophase: The prophase of the first meiotic division is prolonged and is usually divided into
a number of stages as follows:
Leptotene: The chromosomes become visible (as in mitosis). Although each chromosome
consists of two chromatids, these cannot be distinguished at this stage.
Zygotene: The 46 chromosomes in each cell consist of 23 pairs (the X- and Y-chromosomes
of a male being taken as a pair).
The two chromosomes of each pair come to lie parallel to each other, and are closely
apposed.
This pairing of chromosomes is also referred to as synapsis or conjugation. The two
chromosomes together constitute a bivalent.
Pachytene: The two chromatids of each chromosome become distinct. The bivalent now has
four chromatids in it and is called a tetrad.
There are two central and two peripheral chromatids, one from each chromosome (Fig.
2.18C). An important event now takes place.
The two central chromatids (one belonging to each chromosome of the bivalent) become
coiled over each other so that they cross at a number of points.
This is called crossing over. At the site where the chromatids cross, they become adherent;
the points of adherence are called chiasmata.
Diplotene: The two chromosomes of a bivalent now try to move apart.
As they do so, the chromatids involved in crossing over “break” at the points of crossing and
the “loose” pieces become attached to the opposite chromatid.
This results in exchange of genetic material between these chromatids.
, Metaphase: As in mitosis the 46 chromosomes become attached to the spindle at the
equator, the two chromosomes of a pair being close to each other.
Anaphase: The anaphase differs from that in mitosis in that there is no splitting of the
centromeres.
One entire chromosome of each pair moves to each pole of the Telophase: The anaphase is
followed by the telophase in which two daughter nuclei are formed.
The division of the nucleus is followed by division of the cytoplasm.
Second Meiotic Division
The first meiotic division is followed by a short interphase. This differs from the usual
interphase in that there is no duplication of DNA.
Such duplication is unnecessary as chromosomes of cells resulting from the first division
already possess two chromatids each.
The second meiotic division is similar to mitosis. However, because of the crossing over that
has occurred during the first division, the daughter cells are not identical in genetic
content.
Significance of Meiosis:
In this kind of cell division, there is reduction of the number of chromosomes from diploid to
haploid.
At the time of fertilization, the diploid number (46) is restored.
This provides consistency of chromosome number from generation to generation.
The 46 chromosomes of a cell consist of 23 pairs, one chromosome of each pair being
derived from the mother and one from the father. During the first meiotic division, the
chromosomes derived from the father and those derived from the mother are :
• distributed between the daughter cells entirely at random.
• This, along with the phenomenon of crossing over, results in thorough shuffling of the
genetic material so that the cells produced as a result of various meiotic divisions (i.e.
ova or spermatozoa); all have a distinctive genetic content.
• A third step in this process of genetic shuffling takes place at fertilization when there is a
combination of randomly selected spermatozoa and ova.
• It is, therefore, not surprising that no two persons (except identical twins) are alike.
The meiosis consists of two successive divisions called the first and second meiotic
divisions.
During the interphase preceding the first division, duplication of the DNA content of
chromosomes takes place as in mitosis.
As a result, another chromatid identical to the original one is formed.
Thus, each chromosome is now made up of two chromatids.
First Meiotic Division:
Prophase: The prophase of the first meiotic division is prolonged and is usually divided into
a number of stages as follows:
Leptotene: The chromosomes become visible (as in mitosis). Although each chromosome
consists of two chromatids, these cannot be distinguished at this stage.
Zygotene: The 46 chromosomes in each cell consist of 23 pairs (the X- and Y-chromosomes
of a male being taken as a pair).
The two chromosomes of each pair come to lie parallel to each other, and are closely
apposed.
This pairing of chromosomes is also referred to as synapsis or conjugation. The two
chromosomes together constitute a bivalent.
Pachytene: The two chromatids of each chromosome become distinct. The bivalent now has
four chromatids in it and is called a tetrad.
There are two central and two peripheral chromatids, one from each chromosome (Fig.
2.18C). An important event now takes place.
The two central chromatids (one belonging to each chromosome of the bivalent) become
coiled over each other so that they cross at a number of points.
This is called crossing over. At the site where the chromatids cross, they become adherent;
the points of adherence are called chiasmata.
Diplotene: The two chromosomes of a bivalent now try to move apart.
As they do so, the chromatids involved in crossing over “break” at the points of crossing and
the “loose” pieces become attached to the opposite chromatid.
This results in exchange of genetic material between these chromatids.
, Metaphase: As in mitosis the 46 chromosomes become attached to the spindle at the
equator, the two chromosomes of a pair being close to each other.
Anaphase: The anaphase differs from that in mitosis in that there is no splitting of the
centromeres.
One entire chromosome of each pair moves to each pole of the Telophase: The anaphase is
followed by the telophase in which two daughter nuclei are formed.
The division of the nucleus is followed by division of the cytoplasm.
Second Meiotic Division
The first meiotic division is followed by a short interphase. This differs from the usual
interphase in that there is no duplication of DNA.
Such duplication is unnecessary as chromosomes of cells resulting from the first division
already possess two chromatids each.
The second meiotic division is similar to mitosis. However, because of the crossing over that
has occurred during the first division, the daughter cells are not identical in genetic
content.
Significance of Meiosis:
In this kind of cell division, there is reduction of the number of chromosomes from diploid to
haploid.
At the time of fertilization, the diploid number (46) is restored.
This provides consistency of chromosome number from generation to generation.
The 46 chromosomes of a cell consist of 23 pairs, one chromosome of each pair being
derived from the mother and one from the father. During the first meiotic division, the
chromosomes derived from the father and those derived from the mother are :
• distributed between the daughter cells entirely at random.
• This, along with the phenomenon of crossing over, results in thorough shuffling of the
genetic material so that the cells produced as a result of various meiotic divisions (i.e.
ova or spermatozoa); all have a distinctive genetic content.
• A third step in this process of genetic shuffling takes place at fertilization when there is a
combination of randomly selected spermatozoa and ova.
• It is, therefore, not surprising that no two persons (except identical twins) are alike.
