4.2 Meiosis
4.2.1 State that meiosis is a reduction division of a diploid nucleus to form haploid
nuclei.
Diploid ( 2N) means you have two homologous copies of a chromosome.
Haploid (1N) is having only one copy of each chromosome. Humans are diploid the
majority of their life cycle. Not all organisms are exclusively diploid. Kelp is a
haploid organism. The diploid organism is considerably smaller and lives among the
holdfast, or root of the giant kelp. Other organisms, such as ferns, have polyploidy,
or lots of sets of chromosomes.
The point of meiosis is to make sure that each gamete gets a full set of
chromosomes.
4.2.2 Define homologous chromosomes.
Each human has two copies of their chromosomes. The two sets may differ
slightly in each gene. This is what provides variation in the species. You got one set
from your mom, and one set from your dad.
4.2.3 Outline the process of meiosis, including pairing of homologous chromosomes
and crossing over, followed by two divisions, which results in four haploid cells.
[Limit crossing over to the exchange of genetic material between non-sister
chromatids during prophase 1. Names of the stages are required.]
S-phase of interphase: DNA is copied. Exact copies of each chromosome are
connected together by proteins called centromeres.
Prophase 1: DNA becomes visible because of super-coiling, nuclear envelope
dissolves, spindle apparatus starts to form. Homologous pairs form tetrads, this is
where crossing over, or exhange of DNA fragments can occur. The net effect is
further mixing of the DNA information. A chromosome from your mom can end up
with a few genes from your dad on the end of it, and visa-versa.
Metaphase 1: Chromosomes are pulled to line up along the middle of the cell
Anaphase 1: Chromosomes are separated to opposite sides of the cell. Note
that the centromeres do not separate. Mom’s chromosome and its copy goes to one
side, while Dad’s homologous chromosome and its copy goes to the other.
Independent assortment: Each chromosome has an equal chance of going to either
side. This is another source of variation. You could end up with all of mom’s
chromosomes, or any combination of mom’s and dad’s. The possibilities are very
large….23!
Telephase 1: Chromosomes reach the opposite sides of the cell and the
nuclear envelope reforms.
Cytokinesis: the cell actually divides into two new cells.
Interphase: No copying of DNA during this interphase, it is very short, the
daughter cells immediately begin the second set of divisions.
Prophase II : nuclear envelope dissolves, a new spindle begins to form.
Metaphase II: spindle fibers pull the chromosomes to the middle of the cell.
Anaphase II: Centromeres separate the two identical chromatids. Individual
chromatids are now referred to as chromosomes.
Telephase II: Chromosomes reach the spindle ends, the nuclear envelope
reforms.
Cytokinesis: the cell divides into two new cells. Each cell has only one copy of
the chromosomes and is a gamete. Females create only one gamete, the other three
haploid cells are called polar bodies and are re-absorbed into the ovary. This allows
the female to make one big healthy egg by sacrificing 3 others.
4.2.4 Explain that non-disjunction can lead to changes in chromosome number,
illustrated by reference to down syndrome ( trisomy 21)
[The characteristics of down’s syndrome are not required.]
During anaphase, the spindle fibers can break, rather than having the
centromeres break. This results in an extra chromosome going to one cell, and
another cell is missing the chromosome. Down’s syndrome, Turner syndrome,
Kleinfelters syndrome can be the result. Most of the time, the fetus is aborted.
4.2.5 State that, in karyotyping, chromosomes are arranged in pairs according to
their size and structure.
Karyotyping involves inducing cells to undergo mitosis, which causes
chromosomes to become visible. They stain specific base pairs, such as
Thymine/Adenine… which results in a characteristic banding pattern. The stained
cells are photographed during prophase. The photos are cut up matching banding
and size of the homologous chromosomes. Trisomy, and gender can easily be
determined by this process.
4.2.6 State that karyotyping is performed using cells collected by chorionic villus
sampling or amniocentesis, for pre-natal diagnosisi of chromosome abnormalities
[ There are ethical and social issues associated with karyotyping of unborn
fetuses because this procedure allows parents to abort fetuses with a chromosome
abnormality. There is also evidence that, in some parts of the world, abortion on the
basis of gender is carried out.]
There are risks associated with getting the sample of the fetus cells. Infection
of the fetus can occur. In the US, pregnant women over the age of 40 are usually
required by their doctors to have the procedure done.
4.2.7
Analyse a human karyotype to determine gender and whether non-disjunction
has occurred.