Biomedical Engineering Reference
In-Depth Information
2. The zygote begins with one member of each pair of chromosomes from the
father and one member of each pair from the mother.
3.
Meiosis
assures that the genetic code is passed on to next generation. It assures
that the gametes are prepared with the
n
number of chromosomes (23) so that
sexual reproduction and fertilization will result in the
2n
number (46) having in
each pair of chromosomes one father's and one mother's alel.
Meiosis is division of gametes
-
not somatic cells and it also provides for variation
due to a reshuffling of the genes
—
genetic recombination
(
chiasma
)—
crosssing over
of two non
-
sister chromosomes. In that way
,
genes can be exchanged and variation
of the gametes is increased. By pairing haploid number of chromosomes
(
half
from mother and half from father
),
the zygotis growing into embryo and a new
individual with all the cells at one place in the
- ₒ
blastocyst and later on
,
in uterus
.
There is an
Interphase
before meiosis-similar to mitosis. It means, that preced-
ing meiosis chromosomes are replicated. Result is two sister
chromatids
attached
by a
centromere. Centrioles
also get reproduced.
A.
Meiosis I
—Reduction division
•
Prophase I
: Chromosomes condense,
synapsis
—pairing of homologous
pairs,
chiasma
and
crossing over
,
tetrads
appear, centriole pairs separate,
spindle microtubules for between them, chromosomes begin to migrate to
equator (metaphase plate). Longest stages—90 % of meiosis.
•
Metaphase I
: Homologous pairs (unlike single chromosomes in mitosis)
line up on metaphase plate.
Spindle fibers
from poles attach to a single chro-
mosome of the pair.
•
Anaphase I
:
Spindle apparatus
moves chromosomes towards poles.
Chromatids remain attached. Homologous pairs are pulled apart and towards
opposite poles.
•
Telophase I
: Chomosomes reach poles. Each pole has monoploid (haploid
number).
Cytokineses
occurs and two daughter cells are formed. Result of
meiosis I is two daughter cells with
n
number of chromosomes (Fig.
3.5
).
B.
Meiosis II
: A mitotic-like process
•
Prophase II
: Note-there is little or no interphase before prophase II. A spin-
dle apparatus forms. Chromosomes are pulled to
metaphase plate
.
•
Metaphase II
: Similar to
mitosis
. Individual chromosomes line up on
equator.
•
AnaphaseII
: Centromeres split and chromatids are pulled apart, becoming
separates chromosomes and move towards opposite poles.
•
Telophase II
: Nuclei form and cytokinesis occurs. Result is now four daugh-
ter cells with
n
number.
