Page 71 - Anatomy and Physiology of Farm Animals, 8th Edition

56 / Anatomy and Physiology of Farm Animals

Prophase. Prophase is the first of the begin to unwind into filaments. Ultimately
the chromosomes lose their visible
active phases of mitosis. It is characterized
VetBooks.ir by condensation of chromatin into twisted identity and become the chromatin of the
interphase period.
filamentous threads (chromatids). (The
term mitosis comes from the Greek The cell itself must then divide into two
word mitos, meaning thread.) Also during daughter cells. The division of the cyto­
prophase, the nuclear envelope and the plasm is called cytokinesis. It starts with
nucleolus begin to break down and invagination of the plasma membrane
disappear, and the two pairs of centrioles around the equator of the cell and ends by
move to opposite poles of the cell. pinching off the two halves, with a nucleus
Microtubules become organized and in each half, creating the daughter cells.
arranged in a fan shape, radiating outward Each centriole is also replicated, and each
from the centrioles to the equator of the daughter cell is now a replica of the parent
cell. This arrangement of microtubules cell. Mitosis is now complete.
extending between the two centrioles is
the beginning of the mitotic spindle.
Meiosis
Prometaphase. Chromosomes continue
to condense during prometaphase as Meiosis (reduction division) differs from
microtubules from the asters attach mitosis in a number of ways. It occurs
themselves to the paired chromatids. during gametogenesis, the formation of
Tubules from the mitotic spindle then ova in the female (oogenesis) and sper­
begin to pull the chromatids towards each matozoa in the male (spermatogenesis).
pair of centrioles located on opposite These processes are discussed in detail in
poles.
Chapters 25 and 27. The number of chro­
Metaphase. Metaphase is the period mosomes varies with species and when
when the nuclear envelope and nucleolus discussed related to mitosis and meiosis
totally disappear and the mitotic spindle the number of chromosomes is abbreviated
is fully formed. The chromatids move and simply as “n.” Since fertilization results in
line up across the cell’s equator in the an equal number of chromosomes being
middle of the spindle, and the spindle contributed by the male and female gam­
microtubules are attached to the ete, meiosis is the mechanism by which
centromere region of the chromatids. the somatic, or diploid (2n), number of
chromosomes in each gamete is reduced
Anaphase. Anaphase is the stage in to the haploid (1n) number of chromo­
which each centromere divides, separating somes prior to fertilization.
the two chromatids, now properly called Meiosis not only reduces the diploid
chromosomes again. The cell now contains number of chromosomes to the haploid
twice as many chromosomes as it had number, it also increases the genetic
originally. Half of the chromosomes begin
to migrate toward one centriole at a pole of variability of the offspring by crossing
over. Homologous chromosomes in the
the spindle, and the other half migrates to primary sex (germ) cells pair up during
the other centriole.
prophase of meiosis. Homologous chro­
Telophase. Telophase begins when half mosomes are similar chromosomes that
of the chromosomes have been drawn by were contributed by the two parents of
the microtubules to each pole of the cell. the individual. These paired homologous
A nuclear envelope forms around each set chromosomes may then cross over and
of daughter chromosomes, and a nucleolus exchange similar areas, resulting in two
appears in each new nucleus. The spindle chromosomes that are different from
tubules disappear, and the chromosomes either parent chromosome.