Meiosis is a reduction division of a diploid nucleus to form haploid nuclei.
Homologous chromosomes definition
Homologous chromosomes - chromosomes that
have the same genes in the same sequence, but not necessarily the same alleles
of the genes.
Process of meiosis
Meiosis I
Prophase I
Chromosomes shorten/thicken.
Homologous chromosomes pair by synapsis
(forming bivalent), chromosomes have 2 chromatids.
Nuclear envelope disintegrates.
Spindle microtubule forms.
Crossing over occurs where there is
exchange of genetic material between non-sister chromatids (chiasma - point of
crossover).
Metaphase I
Bivalents are attached to individual
microtubules of spindle by centromeres.
Chromosomes move to equator of cell.
Orientation of maternal and paternal
chromosomes are random.
Anaphase I
Homologous pairs separate; one chromosome
moves to opposite poles
Pieces of chromatids exchanged during
crossover
Telophase I
Chromosomes uncoil, during interphase
that follows, no replication occurs; spindle breaks down.
Reduction of chromosome numbers from
diploid to haploid completed.
Nuclear envelope forms, nucleolus forms,
cleavage furrow forms.
Cytokinesis occurs.
Prophase II
Chromosomes condense.
Centrioles move to opposite poles.
Spindle microtubules form at right angle
to previous spindle.
Nuclear envelope disintegrates.
Metaphase II
Chromosomes line up along equator.
Anaphase II
Centromeres separate / Chromatids move to
opposite poles.
Daughter chromosomes separate.
Telophase II
Chromatids reach opposite poles.
Nuclear envelope forms.
4 cells with ½ chromosome number of
original parent cell.
Chromatids are recognized as chromosomes,
they uncoil and disperse.
Cytokinesis occurs.
Non-disjunction and Down syndrome
Non-disjunction occurs when either at
anaphase I or II, the chromosomes fail to separate and go to opposite side of
the poles. This leads to a gamete that has either 45 or 47 chromosomes,
resulting in Down syndrome. Failing to separate during anaphase I will result in
four affected daughter cells and during anaphase II, non-disjunction will lead to
two affected daughter cells. For Down syndrome, one parent can contribute 2
copies of chromosomes 21 and the other can contribute 1 healthy copy, and when
gametes fuse, there are 3 copies.
Non-disjunction (lc.wnlsd.ca)
Karyotyping
In karyotyping, chromosomes are arranged
in pairs according to their size and structure.
Karyotyping in chorionic villus sampling and amniocentesis
Karyotyping is performed using cells
collected by chorionic villus sampling or amniocentesis, for pre-natal diagnosis
of chromosome abnormalities.
Amniocentesis - drawing needle, guided by
ultrasound, to withdraw sample of amniotic fluid from amniotic sac.
Chorionic villus sampling - sample cells
from placenta, chorion.
Analysis of human karyotype to determine gender and non-disjunction
Male Karyotype (science-esl.wikispaces.com)
Male Karyotype (science-esl.wikispaces.com)
Female Karyotype (daviddarling.info)
Non-disjunction Karyotype (php.med.unsw.edu.au)
No comments:
Post a Comment