.
What is Meiosis?
Meiosis produces daughter cells that have one half the
number of chromosomes as the parent cell. 2N → N
Meiosis enables organisms to reproduce sexually.
Gametes (sperm and eggs) are haploid.
Meiosis involves two divisions producing a total of four
daughter cells.
Phases of Meiosis
A cell undergoing meiosis will divide two times; the first division is
meiosis 1 and the second is meiosis 2. The phases have the same
names as those of mitosis. A number indicates the division number
(1st or 2nd):
meiosis 1: prophase 1, metaphase 1, anaphase 1, and telophase 1
meiosis 2: prophase 2, metaphase 2, anaphase 2, and telophase 2
In the first meiotic division, the number of cells is doubled but the
number of chromosomes is not. This results in 1/2 as many
chromosomes per cell.
The second meiotic division is like mitosis; the number of
chromosomes does not get reduced
Table 1. Salient points of meiotic stages
Stage
Sub-division
Prophase I
Leptotene
Zygotene
Features
DNA replication of each chromosome to give sister chromatids held
together at the centromere
Start of chromosome condensation, formation of the axial elements of
the synaptonemal complex
Chromosome pairing; completion of the formation of the
synaptonemal complex
Pachytene
Chromosomes fully paired, cross-overs/chiasmata fully established
Diplotene
Initiation of separation of the synapsed chromosomes
Chromosomes condense and become fully separated, except at points
of crossing-over/chiasma formation.
Diakinesis
Metaphase I
Homologous chromosomes align on the equatorial plate with the
kinetochores being the point of attachment to the spindles
Anaphase I
Reductional division; homologous pairs separate, but sister chromatids
remain together
Telophase I
Formation of two daughter cells with the haploid chromosome number
Prophase II
Metaphase II
Nuclear envelope dissolves; creation of a new spindle
Chromosomes align on the spindle
Separation of centromeres; migration of sister chromatids to opposite
poles
Further cell division resulting in four potential haploid gametes from
each parent cell
Anaphase II
Telophase II
Oat microsporocyte at diakinesis showing 21 bivalents
with interstitial and terminal chiasmata
Definition:
Meiosis
Mitosis
A type of cellular
reproduction in which the
number of chromosomes are
reduced by half through the
separation of homologous
chromosomes, producing
two haploid cells.
A process of asexual
reproduction in which the
cell divides in two producing
a replica, with an equal
number of chromosomes in
each resulting diploid cell.
Function:
sexual reproduction
Type of Reproduction:
Sexual
Humans, animals, plants,
fungi
different
Yes, mixing of chromosomes
can occur.
Yes
2
Occurs in:
Genetically:
Crossing Over:
Pairing of Homologs:
Number of Divisions:
Number of Daughter Cells
produced:
Chromosome Number:
Cellular Reproduction &
general growth and repair of
the body
Asexual
all organisms
identical
No, crossing over cannot
occur.
No
1
4 haploid cells
2 diploid cells
Reduced by half
Remains the same