MEIOSIS
4.2
Key Concepts
• Gametes have half the number of
chromosomes that body cells have.
• During meiosis, diploid cells undergo two cell
divisions that result in haploid cells.
Haploid and Diploid
• The cells which contain two of each type of
chromosome are called diploid.
• The cells which contain one of each type of
chromosome are called haploid.
• In diploid cells each pair of chromosomes have
the same genes, arranged in the same sequence.
However, they do not usually have the same
alleles of all of these genes. They are therefore
not identical but instead are homologous.
Meiosis
• The number of chromosomes in a cell can be
reduced from diploid to haploid by the
process of meiosis.
• Living organisms that reproduce sexually have
to halve their chromosome number at some
stage in the life cycle because the fusion of
gametes during fertilization doubles it.
Homologous Chromosomes
• In a diploid human cell, the 46 chromosomes can be
grouped into 23 pairs of chromosomes called
homologous chromosomes.
• Homologous means similar in shape and size and it
means that the two chromosomes carry the same
genes. The reason there are two of each is that one
came from the father and the other from the mother.
• Although a pair of chromosomes carry the same genes,
they are not identical, because the alleles for the genes
from each parent could be different.
4.2.1
State that meiosis is a reduction division of a diploid
nucleus to form haploid nuclei.
• Meiosis is a form of cell division which results in the
formation of gametes.
• One characteristic feature which makes meiosis
unique is that each new cell which results from
meiosis has only half the number of chromosomes
that a typical cell in that organism has. For example,
humans have 46 chromosomes in their cells, but in
the gametes (sperm and egg cell) there are only 23
chromosomes.
• Cells with half the number of chromosome number
are called haploid cells.
Homologous Chromosomes
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.
• The phases of meiosis
• Meiosis is a step-by step process by which a diploid parent cell produces
four haploid daughter cells.
• In order to produce four cells, the parent cell must divide two times: the
first meiotic division makes two cells and then each of these divides
during the second meiotic division to make a total of four cells.
• http://www.youtube.com/watch?v=D1_-mQS_FZ0
• Video on meiosis
•
• http://www.johnkyrk.com/mitosis.html
• animation of meiosis and mitosis
•
• http://www.layyous.com/Videoclips/meiosis%20.htm
• meiosis video
Interphase
• Interphase
• Meiosis is preceded by an interphase, during
which the chromosomes duplicate. At the end of
this interphase, each chromosome consists of
two genetically identical sister chromatids
attached together, but at this stage the
chromosomes are not visible under the
microscope except as a mass of chromatin. The
cell’s centrosomes have also duplicated by the
end of this stage.
Prophase I
Prophase I
• Prophase I is the most complex phase of meiosis.
• Early in this phase, the chromatin coils up, so that
individual chromosomes become visible with the
microscope.
• In a process called synapsis, homologous chromosomes,
each composed of two sister chromatids, come together
as pairs. The resulting structure, consisting of four
chromatids , is called a tetrad.
• During synapsis, chromatids of homologous chromosomes
exchange segments in a process called crossing over.
Crossing over rearranges genetic information. Crossing
over can make an important contribution to the genetic
variability resulting from sexual reproduction.
Prophase I
• As prophase I continues, chromosomes
condense further as the nucleoli disappear.
• Now the centrosome move away from each
other, and a spindle starts to form between
them. The nuclear envelope breaks into
fragments. The chromosome tetrad, captured
by spindle microtubule, are moved toward
the center of the cell.
Metaphase -I
• At metaphase I, the chromosome tetrad are
aligned on the metaphase plate, midway
between the two poles of the spindle.
• Each chromosome is condensed and thick, with
its sister chromatid still attached at their
centromeres.
• Spindle microtubules are attached to
kinetochores at the centromeres.
• The homologous chromosomes of each tetrad
are poised to move toward opposite poles of the
cell.
Metaphase I
Anaphase I
Anaphase I
• Anaphase I of meiosis marked by the migration
of chromosomes toward the two poles of the
cell.
• Spindle fibres from the poles attach to
chromosomes and pull them to opposite poles
of the cell.
• The sister chromatids making up each doubled
chromosomes remain attached at their
centromeres. Only the tetrads split up. ( we can
see doubled chromosomes , the haploid number
moving toward each spindle pole.
Telophase I and Cytokinesis
Telophase I
• In telophase I, the chromosomes arrive at the poles of the cell.
• Each pole of the cell has a haploid chromosome set, though it is
duplicate form that is each chromosome consists of two sister
chromatids. Usually cytokinesis occurs along with Telophase I, and
two haploid daughter cells are formed.
• Spindle and spindle fibre disintegrates
• Usually, the chromosomes uncoil and new nuclear membrane
forms and there is an interphse before meiosis II begins and this
happens in some species and in other species daughter cells
produced in the first meiotic division immediately begins
preparation for the second meiotic division.
• In either case no chromosome duplication occurs between
Telophase I and the onset of meiosis II
• Now meiosis II takes place to separate the sister chromatids.
Metaphase I to Telophase I
Meiosis II
Meiosis II
Prophase II and Metaphase II
•
•
•
•
•
•
Prophase II
DNA condenses into visible chromosomes again.
New meiotic spindle fibres are produced
Metaphase II
Nuclear membrane disintegrate.
The individual chromosomes line up along the
equator of each cell in no special order; this is called
random orientation.
• Spindle fibres from opposite poles attach to each of
the sister chromatids at the centromere.
•
Anaphase II and Telophase II
• Anaphase II
• Centromeres of each chromosome split, relasing each
sister chromatid as an individual chromosome.
• The spindle fibre pull individual chromatids to opposite
ends of the cell.
• In animal cells, cell membranes pinch off in the middle,
whereas in plant cells new cell plates form to demarcate
the four cells.
• Telophase II
• Chromosomes unwind their strands of DNA
• Nuclear envelopes form around each of the four haploid
cells preparing them for cytokinesis.
4.2.4 Explain that non-disjunction can lead to changes
in chromosomes number, illustrated by reference to
Down syndrome (trisomy 21)
• Sometimes chromosomes do not separate the way they
are expected to during the first or second meiotic division.
This results in an unequal distribution of chromosomes. In
humans, it means that an egg cell or sperm cell might
have 24 instead of 23 chromosomes.
• This unexpected distribution of chromosomes is due to a
non-disjunction, a process by which two or more
homologous chromosomes stick together instead of
separating.
• In the case of Down’s syndrome, non-disjunction happens
in the 21st pair of chromosomes: the child receives 3
instead of 2. Such an anamoly is called trisomy and
Down’s syndrome is also referred to as trisomy 21.
Down’s Syndrome
• Having an additional chromosome brings about
malformation of the digestive system and causes differing
degrees of learning difficulties.
• Children with Down’s syndrome follow specialized
education programmes adapted for their needs.
• The risk of Down’s syndrome increases as the age of the
mother increases, particularly over the age of 35.
• Non-disjunction can happen with other chromosomes,
and all of them can have a major impact on a child’s
development.
• Some developmental consequences are so severe that the
fetus may not survive beyond a few weeks or months.
Non-disjunction
Analysis of the human karyotype
(above
To determine if the individual is a male or female, the last
pair of chromosomes must be examined. In this case,
there is a big x chromosome and a small Y
chromosome to indicate that it is a male.
• To see if non-disjunction has occurred, the pairs of
chromosomes must be checked to see if there are
more or fewer than two for any pair. In this case, the
21st pair has three chromosomes indicating Down’s
syndrome.
• What cannot be determined from this information is
how severely the boy will be affected.
•
Down’s Syndrome (Additional
Information)
• Most children with Down syndrome have some of the
following physical traits:
• Short stature. A child often grows slowly and, as an
adult, is shorter than average.
• Weak muscles (hypotonia) throughout the body. A
child may seem to have less strength than other
children of the same age. Weak abdominal muscles
also make the stomach stick out. Normally, children's
stomach muscles gradually strengthen around age 2.
• A short, wide neck with excess fat and skin. Usually,
this trait is less obvious as the child gets older.
• Short, stocky arms and legs. Some children also have a wide
space between the big toe and second toe.
• A single crease across the center of the palms of the hands.
This is called a transverse palmar crease or simian line.
Facial Features
• Down syndrome often results in distinct facial features,
such as:
• Small, low-set ears.
• Irregularly shaped mouth and tongue. The child's tongue
may partly stick out. The roof of the mouth (palate) may
be narrow and high with a downward curve.
• A nasal bridge that looks pushed in. The nasal bridge is
the flat area between the nose and eyes.
• Tissue buildup on the colored part of the eye (iris). These
areas are known as Brushfield's spots and do not affect
the child's vision.
• Irregular and crooked teeth that often come in late and
not in the normal sequence.
-• A child may have other medical conditions related to Down
syndrome, such as:
• Cognitive disability (mental retardation). Most children with
Down syndrome have mild to moderate cognitive disability.
• Heart defects. About half of children with Down syndrome are
born with a heart defect.1 Most defects are diagnosed at birth or
shortly thereafter.
• Diseases such as hypothyroidism, celiac disease, and eye
conditions.
• Children with Down syndrome are also prone to developing other
health problems. For example, respiratory infections, hearing
problems, and dental problems are common.
http://www.youtube.com/watch?v=AZSfAJEpElc
(Down’s syndrome)
4.2.5 State that, in karyotyping, chromosomes are
arranged in pairs according to their size and structure.
• A karyotype is a photograph of the
chromosomes found in a cell arranged according
to a standard format. The chromosomes are
placed in order according to their size and
shape. The shape depends mainly on the
position of the centromere.
• The number and appearance of the
chromosomes in an organism is called the
karyotype. Living organisms that are members
of the same species usually have the same
karyotype.
Karyotype of Human
Karyotype
• A karyotype is made by the following steps
• The cells are stained and prepared on a glass slide to
see their chromosomes under a light microscope.
• Photomicrograph images are obtained of the
chromosomes during mitotic metaphase.
• The images are cut out and separated, a process
which can be done using scissors or using a computer.
• The images of each pair of chromosomes are placed
in order by size and the position of their centromeres.
State that karyotyping is performed using cells collected by chorionic
villus sampling or amniocentesis, for pre-natal diagnosis of chromosome
abnormalities.
-
• http://video.about.com/pregnancy/Amnioce
ntesis.htm
Amniocentesis
Amniocentesis
• From a karyotype, the gender of a person can be
deduced and chromosome abnormalities can be
detected. The most useful time to do this before
birth. Cells have to be obtained from the fetus. There
are two ways of doing this.
• Amnicentesis
• A sample of amniotic fluid is removed from the
amniotic sac around the fetus. To do this, a
hypodermic needle is inserted through the wall of the
mother’s abdomen and wall of the uterus. Amniotic
fluid is drawn out into a syringe. It contains cells from
the fetus.
Chorionic villus sampling
• Cells are removed from the fetal tissues in the placenta called
chorionic villi.
• As with amniocentesis a hypodermic needle, inserted through the
mother’s abdomen and uterus wall, is used to obtain the cells.
•
• Once fetal cells have been obtained, they are incubated with
chemicals that stimulate them to divide by mitosis. Another
chemical is used which stops mitosis in metaphase of mitosis.
Chromosomes are most easily visible in metaphase. A fluid is used
to burst the cells and spread out the chromosomes. The burst cells
are examined using a microscope and a photograph is taken of the
chromosomes from one cell. The chromosomes in the photograph
are cut out and arranged into pairs according to their size and
structure. This is called karyotyping.
4.2.7Analyse a human karyotype to determine gender and whether nondisjunction has occurred.
• http://www.biology.arizona.edu/human_bio
/activities/karyotyping/karyotyping.html
• (karyotyping activity)
• http://www.biology.arizona.edu/human_bio
/activities/karyotyping/patient_c/patient_c.
html
• (karyotyping activity)
• The gender of the fetus can be determined
form the sex chromosomes.
Chorionic Villi sampling
• The preparation of a karyotype is an
expensive and invasive procedure. It is
usually used for seeing if an unborn baby has
any chromosomal anomalies: 45 or 47
chromosomes instead of 46. If the parents or
doctors are concerned about the
chromosomal integrity of an unborn child (for
example, if an expected mother is over the
age of 35), a karyotype is recommended.
• Karyotypes can be analysed to find out whether
a fetus has chromosomal abnormalities..
sometimes chromosomes that should separate
and move to opposite poles during meiosis do
not separate and instead move to the same
pole. This can happen in either the first or the
second division of meiosis. Non-separation of
chromosomes is called non-disjunction. The
result is that gametes are produced with either
one chromosome too many or too few.