5. Meiosis
Meiosis occurs in sexual reproduction when a diploid
germ cell produces four haploid daughter cells that
can mature to become gametes (sperm or egg).
6. Meiosis
Goal: reduce genetic material by half
Why?
n (mom) + n (dad) = 2n (offspring)
from mom
from dad
child
too
much!
meiosis reduces
genetic content
Just
right!
7. Meiosis: cell division in two parts
Sister
chromatids
separate
Chromatids
separate
Meiosis I
(reduction
division)
Meiosis II
(equational
division)
Diploid 2n
Diploid 2n
Haploid n
Result: one copy of each chromosome in a gamete.
8. Meiosis I : the reduction
division
Spindle
fibers
Nucleus
Prophase I
(early)
(diploid)
Prophase I
(late)
(diploid)
Metaphase I
(diploid)
Nuclear
envelope
Anaphase I
(diploid)
Telophase I
(diploid)
9. Prophase I
Late prophase
Early prophase
Chromatids pair.
Crossing over
occurs.
Chromosomes
condense.
Spindle forms.
Nuclear
envelope
13. Meiosis II
Gene X
Sister chromatids carry
identical genetic
information .
Meiosis II produces gametes with
one copy of each chromosome and thus
one copy of each gene.
14. Meiosis II : the equational
division
Prophase II
(diploid)
Metaphase II
Anaphase II
Telophase II
Four
Non-identical
haploid
daughter cells
20. What Meiosis is About
Meiosis allows the creation of unique individuals
through sexual reproduction.
21. Mitosis
Meiosis
Number of divisions
1
2
Number of daughter
cells
2
4
Yes
No
Same as parent
Half of parent
Where
Somatic cells
Sex cells
When
Throughout life
At sexual maturity
Role
Growth and repair
Sexual reproduction
Genetically
identical?
Chromosome #
22. MITOSIS
MEIOSIS
Occurs in somatic (body)
cells
Occurs only in gonads (sex
organs: ovary/testes)
Produces cells for repair,
maintenance, growth, asexual
reproduction
Only produces gametes (sex
cells: egg/sperm)
Results in identical diploid
(2n) daughter cells
Reduction division results in
4 haploid (n) cells
23. KARYOTYPE
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
centromere.
25. •Karyotypes
of embryos are
obtained from:
CVS (chorionic villus sampling) and
amniocentesis.
•Karyotypes
of adults can be
obtained from any growing cells.
•Karyotypes
can not tell if there are
mutations of a gene.
26. Products of MITOSIS: 2 x 2n
cells.
2n
2n
2n
2 diploid cells
Products of MEIOSIS: 4 x n
cells
2n
n(4
n
n
monoploid cells)
n
27. MEIOSIS
is two mitotic divisions in a
row,
(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
in gametes.
- There is no dna replication in the interphase
28.
29. Homologous
Chromosomes
Pair of chromosomes ( maternal and paternal) that
paternal
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
chromosomes.
a.
b.
22 pairs of autosomes
1 pair of sex chromosomes
31. Crossing Over (SYNAPSIS)
Crossing over (variation) may
occur between
nonsister chromatids at the chiasmata.
chiasmata
Crossing over:
over
segments of nonsister
chromatids break and reattach to the other
chromatid.
chromatid
Chiasmata (chiasma) are
crossing over.
over
the sites of
Synapsis increases genetic variability –
and that’s a good thing…
32. Crossing Over - variation
nonsister chromatids
Tetrad
chiasmata: site
of crossing over
variation
34. Cytokinesis:
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
38. Conditions caused by
non-disjunction
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)
= 2n-1
Klinefelter’s syndrome = XXY - male which has
an extra X chromosome = 2n + 1
39. 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
screening methods
(amniocentesis, CVS) could
lead to a five-fold reduction in
the number of miscarriages
linked to an invasive
procedure used to confirm
Down's syndrome.
40. Klinefelter’s syndrome XXY
Normal boy who
develops some
female secondary
sex character-istics
at puberty.
-slightly lower IQ,
infertile, delayed
motor, speech,
maturation
-treated with
testosterone.
41. REFERENCES
Farrellw (2012) MEIO. ACCESS ON 7 MARCH 2014
http://www.slideshare.net/farrellw/meio-11234603
Docsawyer (2010) MEIOSIS. ACCESS ON 07 MARCH
2014
http://www.slideshare.net/docsawyer/meiosis-notes3075315