Assessment Statement
 4.2.1: State that meiosis is a reduction division of a diploid nucleus to
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form haploid nuclei
4.2.2: Define homologous chromosomes
4.2.3: Outline the process of meiosis, including pairing homologous
chromosomes and crossing over, followed by two divisions, which
results in four haploid cells
4.2.4: Explain that non-disjunction can lead to changes in the
chromosome number, illustrated by reference to Down’s syndrome
4.2.5: State that, in karyotyping, chromosomes are arranged in pairs
according to their size and structure
4.2.6: State that karyotyping is performed using cells colleced by
chorionic villus sampling or amniocentesis, for pre-natal diagnosis of
chromosome abnormalities
4.2.7: Analyze a human karyotype to determine gender and whether
non-disjunction has occurred
Meiosis
 Meiosis is a form of cell division which results in
gametes
 Although mitosis is similar to meiosis, there are some
fundamental differences
Meiosis
 One characteristic which makes
meiosis unique is that each new
cell which results from it has
only half the number of
chromosomes that a typical cell
in that organism has.
 Humans have 46 chromosomes
in their cells, but in the sperm
and egg cells, there are only 23
chromosomes in each cell
 Cells which contain half the
chromosome number are called
haploid cells
 Cells with the full chromosome
number are called diploid cells
Meiosis
 This type of cell division is
called a reduction division
because the number of
chromosomes has been
reduced
 This reduction is necessary in
gamete production because
during sexual reproduction,
each parent contributes 50%
of the genetic information
 The cells formed from cell
division are referred to as
daughter cells
Homologous chromosome
 In a diploid 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
Homologous chromosome
 Although a pair of homologous chromosomes carry
the same genes, they are not identical because the
alleles for the genes from each parent could be
different
 We use the letter n to denote the number of unique
chromosomes in an organism
 In eukaryotes, there are n pairs of chromosome
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With two of each, that makes a total of 2n per cell
 Haploid- n
 Diploid-2n
Phases of meiosis
 Meiosis is a step-by-step process by which a diploid
parent cell produces four haploid daughter cells
 Before the steps begins, DNA replication allows the cell
to make a complete copy of its genetic information
during interphase
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This results in each chromatid having an identical copy, or
sister chromatid, attached to it at the centromere
Phases of meiosis
 In order to produce a total of 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
Phases of meiosis
 Another characteristic which
distinguishes meiosis from
mitosis:
 During the first step, called
prophase I, there is an exchange
of genetic material between nonsister chromatids in a process
called crossing over
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This trading of segments of genes
happen when sections of two
homologous chromatids break at
the same point, twist around each
other and each connects to the
other’s initial position
Phases of meiosis
 Crossing over allows DNA from a person’s maternal
chromosomes to mix with DNA from the paternal
chromosomes
 In this way, the recominant chromatids which end up in
the sperm or the egg cells are a mosaic of the parent
cell’s original chromatids
Phases of meiosis
 Prophase I
 Chromosomes become visible as
the DNA becomes more compact
 Homologous chromosomes, also
called homologues, are attracted
to each other and pair up – one is
from the individual’s father, the
other from the mother
 Crossing over occurs
 Spindle fibers made from
microtubules form
Phases of meiosis
 Metaphase I
 The bivalents (another
name for the pairs of
homologous
chromosomes) line up
across the cell’s
equator
 The nuclear
membrane
disintegrates
Phases of meiosis
 Anaphase I
 Spindle fibers from the poles attach to chromosomes and
pull them to opposite poles of the cell
Phases of meiosis
 Telophase I
 Spindle and spindle fibers disintegrate
 Usually, the chromosomes uncoil and new nuclear
membrane form
 Many plants do not have a telophase I state
Phases of meiosis
 At the end of meiosis I, cytokinesis happens: the cell
splits into two separate cells
 The cells at this point are haploid because they contain
only one chromosome of each pair
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Each chromatid still has its sister chromatid attached to it, so
not S phase is necessary
 Now meiosis II takes place in order to separate the sister
chromatids
Phases of meiosis
 Prophase II
 DNA condenses into visible
chromosomes again
 New meiotic spindle fibers are
produced
 Metaphase II
 Nuclear membranes disintegrate
 The individual chromosomes line
up along the equator of each cell
in no special order; this is called
random orientiation
Phases of meiosis
 Anaphase II
 Centromeres of each chromosome split, releasing each
sister chromatid as an individual chromosome
 The spindle fiber pull individual chromatids to opposite
end of the cell
 Because of random orientation, the chromotids could be
pulled towards either of the newly forming daughter
cells
 In animal cells, cell membranes pinch off in the middle,
whereas in plant cells, new cell plates form to demarcate
the four cells
Phases of meiosis
 Telophase II
 Chromosomes unwind their strands of DNA
 Nuclear envelopes form around each for the four haploid
cells, preparing them for cytokinesis
Down Syndrome
 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 this means that an egg cell or a sperm cell might
have 24 instead of 23 chromosomes
 This unexpected distribution of chromosomes is due to a nondisjunction, a process by which two or more homologous
chromosomes stick together instead of separating
Down Syndrome
 In the case of Down’s syndrome, non-disjunction
happens in the 21st pair of chromosome: the child
receives 3 instead of 2.
 Such an anomaly is called a trisomy and Down’s
syndrome is referred to as trisomy 21
 Having an additional chromosome brings about
malformation of the digestive system and causes
differing degrees of learning difficulties
Down Syndrome
 Down Syndrome is the most common
chromosomal anomaly and affects
approximately 1 birth in 800
 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
Karyotypes
 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
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The shape depends mainly
on the position of the
centromere
Karyotypes
 A karyotype is made by the following steps:
 1) The cells are stained and prepared on a glass slide to
see their chromosomes under a light microscope
 2) Photomicrograph images are obtained of the
chromosomes during mitotic metaphase
 3) The images are cut and separated, a process which can
be done using scissors or using a computer
 4) The images of each pair of chromosomes are placed in
order by size and the position of their centromere
Karyotypes
 Obtaining cells for karyotyping
 An unborn baby’s cells can be extracted in one or two
ways: either by a process called amniocentesis or by
removing them from the chorionic villus
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Amniocentesis: Involves using a hypodermic needle to extract
some of the amniotic fluid around the developing baby
 Inside the liquid, some of the baby’s cells can be found and
used for the preparation of a karyotype
Chorionic villus sampling: involves obtaining a tissue sample
from the placenta’s finger-like projections into the uterus wall