MEIOSIS
Meiosis is a process of reductional division in which the number of
chromosomes per cell is cut in half. In animals, meiosis always results in the
formation of gametes, while in other organism it can give rise to spores. The
word “meiosis” comes from greek world meioun, means “to make small”,
since it results in a reduction of the chromosome number.
The term meiosis was coined by Farmer and Moore (in 1905). The division was
first studied by Van Benedin (1887), Strassburger (1888), Sutton (1900) and
Winiwater (1900). Meiosis I &II were differentiated by Gregoire. In 1911 the
American geneticist Thomas Hunt morgan ( 1866 – 1945) observed cross-over
in Drosophila melanogaster meiosis and provided the first genetic evidence
that genes are transmitted on chromosomes
Meiosis is essential for sexual reproduction and therefore occurs in all
eukaryotes ( including single –celled organisms) that reproduce sexually.
Meiosis does not occur in archaea or bacteria, which reproduce via asexual
process such as binary fission.
During meiosis, the genome of diploid germ cell, which is composed of long
segments of DNA packed into chromosomes, undergoes two rounds of
division, resulting in four haploid cells. Each of these cells contain one
complete set of chromosomes, or half of the genetic content of the original
cell. If meiosis produces gametes, these cells must fuse during fertilization to
create a new diploid cell, or zygote before any new growth can occur. Thus
the division mechanism of meiosis is a reciprocal process to the joining of two
genomes that occurs at fertilization. Because the chromosomes of each
parent undergoes enetic recombination during meiosis, each gamete and thus
each zygote, will have a unique genetic blue print encoded in its DNA.
Together meiosis and fertilization constitutes sexually in the eukaryotes, and
generate genetically distinct individuals in population.
In lower plants, and in many protists, meiosis results in formation of haploid
cells that can divide vegetatively without undergoing fertilization, referred to
as spores. In these groups, gametes are produced by mitosis
Biochemically, meiosis uses some of the same mechanism employed during
mitosis to accomplish the redistribution of chromosomes. There are several
features unique to meiosis, most importantly the pairing and recombination
between homologous chromosomes, which enable then to separate from
each other.
The cells of a particular species have a constant number of chromosomes. In
sexually reproducing organisms male and female gametes fuse together to
form the zygote. If the gamete has the same number of chromosomes number
remains constant from generation to generation. This is because of meiotic
division which reduces the chromosome number to half, and counteracts the
effect of fertilization. Thus fertilization and meiosis are compensating events.
Types of meiosis
The cells in which meiosis takes place are called meiocytes. In animals,
meiocytes are of two types, spermatocytes and oocytes. In higher plants,
meiocytes are differentiated into microsporocytes and megasporocytes.
Depending upon the stage when meiosis occurs, the latter is of three types -
gametic, zygotic and sporic meiosis.
Gametic meiosis
Meiosis is most of the animal take place during the formation of gametes
(gametogenesis). It is termed as genetic meiosis. When two gametes fuse in
fertilization, a diploid zygote is formed. Gametic meiosis results in diplontic
life cycle
Zygotic meiosis
In some lower plants meiosis takes place in the zygote and the resulting
organism are haploid. It is called zygotic meiosis. Organism having zygotic
meiosis have haplontic life cycle.
Sporic meiosis
In plants, meiosis generally occurs at the time of sporogenesis ( formation of
spore or microspores and megaspores) It is called sporic meiosis or
intermediate meiosi. Spores produce a new gametophytic phase in the life
cycle. Gametes are formed by gametophytes. Because of the presence of two
distinct multicelluar phase, diploid and haploid, life cycle of plant is
diplohaplontic .
Meiosis is a process of reductional division in which the number of
chromosomes per cell is cut in half. In animals, meiosis always results in the
formation of gametes, while in other organism it can give rise to spores. The
word “meiosis” comes from greek world meioun, means “to make small”,
since it results in a reduction of the chromosome number.
The term meiosis was coined by Farmer and Moore (in 1905). The division was
first studied by Van Benedin (1887), Strassburger (1888), Sutton (1900) and
Winiwater (1900). Meiosis I &II were differentiated by Gregoire. In 1911 the
American geneticist Thomas Hunt morgan ( 1866 – 1945) observed cross-over
in Drosophila melanogaster meiosis and provided the first genetic evidence
that genes are transmitted on chromosomes
Meiosis is essential for sexual reproduction and therefore occurs in all
eukaryotes ( including single –celled organisms) that reproduce sexually.
Meiosis does not occur in archaea or bacteria, which reproduce via asexual
process such as binary fission.
During meiosis, the genome of diploid germ cell, which is composed of long
segments of DNA packed into chromosomes, undergoes two rounds of
division, resulting in four haploid cells. Each of these cells contain one
complete set of chromosomes, or half of the genetic content of the original
cell. If meiosis produces gametes, these cells must fuse during fertilization to
create a new diploid cell, or zygote before any new growth can occur. Thus
the division mechanism of meiosis is a reciprocal process to the joining of two
genomes that occurs at fertilization. Because the chromosomes of each
parent undergoes enetic recombination during meiosis, each gamete and thus
each zygote, will have a unique genetic blue print encoded in its DNA.
Together meiosis and fertilization constitutes sexually in the eukaryotes, and
generate genetically distinct individuals in population.
In lower plants, and in many protists, meiosis results in formation of haploid
cells that can divide vegetatively without undergoing fertilization, referred to
as spores. In these groups, gametes are produced by mitosis
Biochemically, meiosis uses some of the same mechanism employed during
mitosis to accomplish the redistribution of chromosomes. There are several
features unique to meiosis, most importantly the pairing and recombination
between homologous chromosomes, which enable then to separate from
each other.
The cells of a particular species have a constant number of chromosomes. In
sexually reproducing organisms male and female gametes fuse together to
form the zygote. If the gamete has the same number of chromosomes number
remains constant from generation to generation. This is because of meiotic
division which reduces the chromosome number to half, and counteracts the
effect of fertilization. Thus fertilization and meiosis are compensating events.
Types of meiosis
The cells in which meiosis takes place are called meiocytes. In animals,
meiocytes are of two types, spermatocytes and oocytes. In higher plants,
meiocytes are differentiated into microsporocytes and megasporocytes.
Depending upon the stage when meiosis occurs, the latter is of three types -
gametic, zygotic and sporic meiosis.
Gametic meiosis
Meiosis is most of the animal take place during the formation of gametes
(gametogenesis). It is termed as genetic meiosis. When two gametes fuse in
fertilization, a diploid zygote is formed. Gametic meiosis results in diplontic
life cycle
Zygotic meiosis
In some lower plants meiosis takes place in the zygote and the resulting
organism are haploid. It is called zygotic meiosis. Organism having zygotic
meiosis have haplontic life cycle.
Sporic meiosis
In plants, meiosis generally occurs at the time of sporogenesis ( formation of
spore or microspores and megaspores) It is called sporic meiosis or
intermediate meiosi. Spores produce a new gametophytic phase in the life
cycle. Gametes are formed by gametophytes. Because of the presence of two
distinct multicelluar phase, diploid and haploid, life cycle of plant is
diplohaplontic .
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