Meiosis: Important Vocabulary
Diploid- contains 2
sets of chromosomes
(2n = 46)
Haploid- contains 1 set
(n = 23)
Fertilization- fusion of two haploid gametes forms
a diploid zygote
Zygote- fertilized egg cell
The first cell of a new individual
What would happen if gametes were diploid?
Meiosis occurs in sexual reproduction when a diploid
germ cell produces four haploid daughter cells that
can mature to become gametes (sperm or egg).
Goal: reduce genetic material by half
n (mom) + n (dad) = 2n (offspring)
Meiosis: cell division in two parts
Result: one copy of each chromosome in a gamete.
Meiosis I : the reduction
Chromatid pairs align
along the equator of the
move to opposite
Attached at their
cell into two.
Sister chromatids carry
Meiosis II produces gametes with
one copy of each chromosome and thus
one copy of each gene.
Meiosis II : the equational
along equator of cell.
and move to opposite
cell into two.
Results of meiosis
One copy of
What Meiosis is About
Meiosis allows the creation of unique individuals
through sexual reproduction.
Number of divisions
Number of daughter
Same as parent
Half of parent
At sexual maturity
Growth and repair
Occurs in somatic (body)
Occurs only in gonads (sex
Produces cells for repair,
maintenance, growth, asexual
Only produces gametes (sex
Results in identical diploid
(2n) daughter cells
Reduction division results in
4 haploid (n) cells
1. Photograph a cell in metaphase.
2. Cut out the chromosomes from the picture.
3. Paste homologous pairs together
according to size, banding pattern and
1. If there is an
in number or
of embryos are
CVS (chorionic villus sampling) and
of adults can be
obtained from any growing cells.
can not tell if there are
mutations of a gene.
Products of MITOSIS: 2 x 2n
2 diploid cells
Products of MEIOSIS: 4 x n
is two mitotic divisions in a
(meiosis I and II) except you will see
the homologous chromosomes coming
together . - -- in prophase I the double
homologous chromosomes come together to
form a tetrad
- Crossing-over (synapsis ) occurs in the
tetrad in prophase to increase the variability
- There is no dna replication in the interphase
Pair of chromosomes ( maternal and paternal) that
are similar in shape and size.
Homologous pairs (tetrads) carry genes
controlling the same inherited traits.
Each locus (position of a gene) is in the same
position on homologues.
Humans have 23 pairs of homologous
22 pairs of autosomes
1 pair of sex chromosomes
Crossing Over (SYNAPSIS)
Crossing over (variation) may
nonsister chromatids at the chiasmata.
segments of nonsister
chromatids break and reattach to the other
Chiasmata (chiasma) are
the sites of
Synapsis increases genetic variability –
and that’s a good thing…
Crossing Over - variation
of crossing over
XX chromosome - female
XY chromosome - male
cytoplasm divides. Total
product is 4 cells, each with only a mom or Dad’s
chromosome, in other words, half the normal
chromosome number, or 4 monoploid (n) cells,
or 4 gametes.
Uneven cytokinesis in females: In
oogenesis, get one big egg and 3 nonfunctional polar bodies:
( who die off) and 1 big functional gamete
There are equal
Conditions caused by
Downs syndrome = Trisomy 21 - 3 copies of
chromosomes 21 equalling a total of 47
chromosomes. = 2n + 1(chance of occurring in
oogenesis increases with maternal age)
Turners syndrome = Monosomy X - has only one
X chromosomes totalling only 45 chromosomes in
her body cells. (monosomy X)
Klinefelter’s syndrome = XXY - male which has
an extra X chromosome = 2n + 1
Down’s syndrome – trisomy 21
A simple check for the
presence of a bone in the nose
could more accurately test
unborn babies for Down's
syndrome, scientists say.
Researchers say combining
the nose test with existing
(amniocentesis, CVS) could
lead to a five-fold reduction in
the number of miscarriages
linked to an invasive
procedure used to confirm
Klinefelter’s syndrome XXY
Normal boy who
-slightly lower IQ,
Farrellw (2012) MEIO. ACCESS ON 7 MARCH 2014
Docsawyer (2010) MEIOSIS. ACCESS ON 07 MARCH