4.2 MEIOSIS
•State that meiosis is a reduction division of a diploid nucleus to form a
haploid nulcei.
•Define homologous chromosomes
•Outline the process of meiosis, including pairing of homologous
chromosomes and crossing over, followed by two divisions, which reults
in four haploid cells.
•Explain non-disjunction, and how it leads to Down’s Syndrome
•State that karyotyping , chromosomes are arranged in pairs according
to their size and structure
•State that karyotyping is performed using cells collected by chorionic
villus sampling or amnioccentesis, for prenatal diagnosis of chromosome
abnormalities.
•Analyze a human karyotype to determine gender and whether nondisjunction occurred.
http://www.youtube.com/watch?v=lJzZ7p-47P8&feature=related
Meiosis is a type of cell
division that produces
gametes (sex cells). In
animals they are known as
sperm and egg.
Each cell that is produced as
a result of meiosis has half
the number of chromosomes
of other cells in the body.
Eukaryotic body cells have a diploid nucleus, which contains two
copies of each chromosome, in homologous pairs.
Homologous chromosomes- a pair of chromosomes with the same
genes but not necessarily the same alleles of those genes.
Humans have a diploid number of 46 chromosomes in 23 pairs,
mangos have 40 chromosomes in 20 pairs, and camels have 70 in 35
pairs.
During sexual reproduction, two gametes (sex cells) fuse
together so, in order to keep the chromosome number
correct in the offspring, each gamete must contain only one
of each chromosome pair.
It must contain half the diploid number of chromosomes,
which is called haploid.
During gamete formation, meiosis reduces the diploid
number to the haploid number. At the moment of
fertilization, the normal diploid number is restored as the
gametes fuse.
In humans, the haploid sperm (23 chromosomes) and the haploid
egg (23 chromosomes) fuse at fertilization to form the diploid
zygote, with 46 chromosomes (23 pairs).
CHROMOSOMES
Homologous Chromosomes
Chromatid
The Process of Meiosis
http://www.youtube.com/watch?v=kVMb4Js99tA
Meiosis occurs in a series of stages, which results in
the production of four cells.
Mitosis, used to replace or repair cells, which is
done by one cell division, meiosis involves two
divisions.
The first reduces the number of chromosomes by
half and the second produces four gametes each
containing the haploid number of chromosomes.
The first division is very similar to mitosis and the
second division is exactly the same as mitosis.
http://www.youtube.com/watch?v=iCL6d0OwKt8
MEIOSIS
Meiosis begins with one diploid cell
containing two copies of each
chromosome (one from the
organism's mother and one from its
father) and produces four haploid
cells containing one copy of each
chromosome.
Each of the resulting chromosomes
in the gamete cells is a unique
mixture of maternal and paternal
DNA, ensuring that offspring are
genetically distinct from either
parent.
This gives rise to genetic diversity in
sexually reproducing populations,
which provides the variation of
physical and behavioral attributes
upon which natural selection acts.
MEIOSIS I
Meiosis I separates homologous
chromosomes, producing two haploid
cells (23 chromosomes in humans),
so meiosis I is referred to as a
reductional division.
A regular diploid human cell contains
46 chromosomes and it contains 23
pairs of homologous chromosomes.
However, after meiosis I, although the
cell contains 46 chromatids, it is only
has 23 chromosomes.
This is because later, in Anaphase I,
the sister chromatids will remain
together as the spindle fibres pull the
pair toward the pole of the new cell.
PROPHASE I
The chromosomes, which have replicated during interphase,
now supercoil. Each one consists of two sister chromatids
joined by the centromere.
Although the genes carried by each chromosome pair are
identical, the alleles may not be.
RECOMBINATION
Prophase I
Sister chromatids may become entangled, break and rejoin so
that alleles are exchanged between them during this process
called crossing over.
New combinations of alleles are formed and genetic variety in
the resulting gametes increases.
Prophase I
The final step in the prophase I is the formation of spindle
microtubles and the breakdown of the nuclear envelope.
METAPHASE I
Chromosomes line up on the equator at the center of the cell.
Each one attached by a centromere to the spindle
microtubules. The alignment of the chromosomes is random
so that maternal and paternal chromosomes can appear on
either side of one another on the equator.
This also increases the genetic variety in the gametes.
ANAPHASE I
The microtubules now contract towards opposite
poles.
The pair of sister chromatids remain together but
homologous pairs are separated.
This is the reduction division where the
chromosome number is halved from diploid to
haploid.
TELOPHASE I
Now spindle s break down and a new nuclear envelope
forms.
Cytokinesis follows and the cell splits into two cells, each
containing only one chromosome of each homologous pair.
Each chromosome, however, still consists of two sister
chromatids at this point.
The second division (Meiosis II will separate the two sister
chromatids.
MEIOSIS II
Meiosis II is the second part of the process. Mechanically, the process is similar to
mitosis, though its genetic results are fundamentally different. The end result is
production of four haploid cells (23 chromosomes, in humans) from the two
haploid cells (23 chromosomes, each of the chromosomes consisting of two sister
chromatids) produced in meiosis I.
The four main steps of Meiosis II are: Prophase II, Metaphase II, Anaphase II,
and Telophase II
PROPHASE II
In each of the two cells resulting in Meiosis I, new spindle
fibers start to form, the chromosomes recoil and the
nuclear envelope begins to break down.
METAPHASE II
The nuclear envelope is broken down and the
individual chromosomes line up on the equator of
each cell.
Spindle fibers from opposite ends of the cell attach to
each chromatid at the centromere.
ANAPHASE II
Sister chromatids are separated as the centromere splits
and the spindle fibers pull the chromatids to opposite ends
of the cell.
TELOPHASE II
Nuclear envelopes form around the four new haploid
nuclei and the chromosomes now uncoil.
A second cytokinesis occurs, resulting in four cells.
MITOSIS VS MEIOSIS
Mitosis
Meiosis
Occurs to replace and repair
cells.
Occurs in gamete formation
Chromosomes line up
Chromosomes line up in
individually on the spindle fibers homologous pairs on the spindle
fibers at metaphase I
Produces two cells with the same Produces four haploid cells
number of chromosomes as the
original cell, the diploid number
The two daughter cells are
genetically identical
The four daughter cells are
usually genetically different
NON-DISJUNCTION
Non-disjunction is a failure of the homologous pairs of
chromosomes to separate properly during meiosis.
Non-disjunction results in gametes that contain either one too few
or one too many chromosomes.
Those with too few seldom survive , but in some cases a gamete
with an extra chromosome does survive, which produces a zygote
with three chromosomes of one type, known as trisomy.
Trisomy in chromosome 21 results in Down’s Syndrome.
A gamete, usually the female one, receives 24 chromosomes
instead of 23 and a baby with 47 instead of the usual 46
chromosomes in each cell.
DOWN’S SYNDROME
The incidence of Down syndrome is estimated at 1 per 733 births,
although it is statistically more common with older parents due to
increased mutagenic exposures upon some older parents' reproductive
cells.
KARYOTYPING
Chromosomes have unique banding patterns that are
revealed if they are stained with specific dyes during
prophase.
Each chromosome has a characteristic length and has its
centromere at a fixed place, and each one has a homologous
partner
In a karyogram, chromosomes are stained and photographed.
The image is then manipulated to arrange the chromosomes in
order of their size.
Karyograms indicate the sex of an individual (X,Y)
Also used in prenatal diagnosis to check for chromosome
abnormalities.
In the procedure called karyotyping, cells from an unborn child
are collected in one of two ways: chorionic villus sampling (CVS) or
amniocentesis.
The cells are grown in the lab and a karyogram is prepared.
This is checked for extra or missing chromosomes.
The procedures are used when there is concern about potential
chromosome abnormalities, such as a mother over 35 since down’s
Syndrome is more common in babies of older mother.
CVS
Involves taking a sample of cells from the chorionic villi, which
are fine projections of the placenta embedded in the lining of the
uterus.
This is done 8-10 weeks into pregnancy.
AMNIOCENTESIS
Takes a sample of amniotic fluid from the mother between 14 and
16 weeks for t he pregnancy
Both methods carry a small risk of damaging the fetus or
even causing miscarriage.
Once the results are known, the parents may choose to
terminate the pregnancy if abnormalities are discovered.
The tests may reveal an abnormality but cannot give
indication about the severity.
Thoughts to Consider
Who should make the decision to carry out the procedure?
The parents or the health care officials? How important
are legal and religious arguments?
Both procedures carry risks of miscarriage. How can this
potential risk to the unborn child be balanced with the
parents’ desire for information?
Does the information that can be obtained from the
karyogram outweigh the risk to the unborn child?
If the karyogram indicates a genetic abnormality, should
the parents be permitted to consider a termination of the
pregnancy?