Biomedical Engineering Reference
In-Depth Information
Meiosis I
is called the reduction division, since it is during this phase
that the chromosome numbers are halved. It consists of the follow-
ing stages:
Prophase-1
is where each chromosome has completed its replication
and is made up of two sister chromatids connected at the centro-
mere. Prophase-1 consists of fi ve periods:
1 .
Leptotene,
where the chromosomes are condensed to a point
where they are visible under the light microscope. At this stage,
separate sister chromatids of each chromosome are not yet
visible.
2 .
Zygotene,
where the homologous chromosomes, one maternal
and the other paternal, pair along the synaptonemal complex.
3 .
Pachytene
, where the chromosomes condense still further and are
seen as bivalent. At this stage the process of recombination starts
between individual chromatids of homologous chromosomes.
The recombinational event is termed 'crossing over', because
it results in the formation of chromatids that have recombined
maternal and paternal chromosomal segments.
4 .
Diplotene
, where the paired homologous chromosomes sepa-
rate. Under the light microscope, one sees at this stage the sites
where crossing over has occurred, in cross-like structures called
chiasmata
.
5 .
Diakinesis,
where the chromosomes reach their maximum
condensation.
Metaphase-1
is where the bivalents line up at the metaphase plate
and each homologous chromosome connects to spindle fi bers via its
centromere.
Anaphase-1
is when the chromosomes of each of the bivalents separate
and travel to the opposite poles of the cell. During this stage, the sis-
ter chromatids of each chromosome stay attached at the centromere
and do not divide. It is important to remember that the homologous
chromosomes of each pair segregate independently to the opposite
poles of the cell during this stage. The movement of the maternal and
paternal chromosomes of each homologous pair, to travel to one pole
or the other, is entirely random. This event, along with the crossing
over phenomenon of Prophase-1, is the basis for the genetic variabil-
ity that is produced in the gametes during each round of meiosis.
Finally,
telophase-1
is where the chromosomes arrive at the opposite
poles and the cytoplasm divides.
Meiosis II
is preceded by a short interphase, which does not have a
round of DNA replication. This division of the chromosomes is similar
