Download MEIOSIS

Mitosis quick review
• https://www.youtube.com/watch?v=VlN
7K1-9QB0
• https://www.youtube.com/watch?v=2W
wIKdyBN_s&feature=related
• For a more thorough review, see the
Mitosis Review PPT on my website.
MEIOSIS
Meiosis – A Source of Distinction
Why do you share some but not all characters of each parent?
What are the rules of this sharing game?
At one level, the
answers lie in
meiosis.
Meiosis quick video
introduction
• https://www.youtube.com/watch?v=D1_
-mQS_FZ0&NR=1
This may help you remember these terms:
• MITosis takes the cell and Makes It Two (diploid)
• Meiosis has to do with sex
• From the cell’s point of view:
– mITosis results in Identical Twins
– mEioSis results in Egg and Sperm (haploid)
Clyde Freeman Herreid – Dept. of Biological Sciences: Buffalo State Univ. of New York
Organisms that reproduce Sexually are
made up of two different types of cells.
1. Somatic Cells are “body” cells and contain the
normal number of chromosomes ….called the
“Diploid” number (the symbol is 2n). Examples
include … skin cells, brain cells, etc.
2. Gametes are the “sex” cells and contain only ½
the normal number of chromosomes…. called the
“Haploid” number (the symbol is n)….. Sperm
cells and ova are gametes.
Meiosis
is the process by which “gametes” (sex cells), with half
the number of chromosomes, are produced.
During Meiosis diploid cells are reduced to haploid cells
Diploid (2n)

Haploid (n)
If Meiosis did not occur the chromosome
number in each new generation would
double…. The offspring would die.
Why do we need meiosis?
• Meiosis is necessary to halve the
number of chromosomes going into the
sex cells
Why halve the chromosomes in gametes?
• At fertilization the male and female sex
cells will provide ½ of the
chromosomes each – so the offspring
has genes from both parents
Why do we need meiosis?
Meiosis scrambles the specific forms of
each gene that each sex cell (egg or
sperm) receives.
This makes for a lot of genetic diversity.
This trick is accomplished through
independent assortment and crossingover.
Meiosis
Parent cell –
chromosome pair
Chromosomes
copied
1st division - pairs split
2nd division – produces
4 gamete cells with ½
the original number of
chromosomes
Autosomes
(The Autosomes code for most of the offspring’s traits)
In Humans the
“Autosomes”
are sets 1 - 22
Sex Chromosomes
The Sex Chromosomes code for the sex of the offspring.
** If the offspring has two “X” chromosomes it will be a female.
** If the offspring has one “X” chromosome and one “Y”
chromosome it will be a male.
In Humans the
“Sex
Chromosomes”
are the 23rd set
XX chromosome - female
XY chromosome - male
Boy or Girl? The Y Chromosome “Decides”
Y chromosome
X chromosome
Boy or Girl? The Y Chromosome “Decides”
Interphase I
• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two
identical sister chromatids attached at their
centromeres.
Interphase I
• Nucleus and nucleolus visible.
chromatin
nuclear
membrane
cell membrane
nucleolus
Meiosis I (four phases)
• Cell division that reduces the chromosome
number by one-half.
• four phases:
a. prophase I
b. metaphase I
c. anaphase I
d. telophase I
Prophase I
• Longest and most complex phase (90%).
• Chromosomes condense.
• Homologous chromosomes come together to
form a tetrad.
• Tetrad is two chromosomes or four
chromatids (sister and nonsister chromatids).
Prophase I – the Tetrad
Homologous chromosomes
sister chromatids
Tetrad
sister chromatids
Homologous Chromosomes
• 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 chromosomes.
a. 22 pairs of autosomes
b. 1 pair of sex chromosomes
Homologous Chromosomes
eye color
locus
eye color
locus
hair color
locus
hair color
locus
Paternal
Maternal
Humans have 23 Sets of Homologous Chromosomes
Each Homologous set is made up of 2 Homologues.
Homologue
Homologue
Prophase I -- Crossing Over
• Crossing over may occur between nonsister
chromatids.
• Crossing over: segments of nonsister
chromatids break and reattach to the other
chromatid.
Crossing Over – creates
variation
nonsister chromatids
Tetrad
variation
One Way Meiosis Makes Lots of Different
Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of
different gamete types produced by independent
assortment.
Prophase I
spindle fiber
centrioles
Metaphase I
• Shortest phase
• Tetrads align on the metaphase plate.
• INDEPENDENT ASSORTMENT OCCURS:
1. Orientation of homologous pair to poles is random.
2. Second way to create Variation
3. Formula: 2n
Example: 2n = 4
then n = 2
thus 22 = 4 combinations
Metaphase I
OR
metaphase plate
metaphase plate
Independent assortment, n=2
Question:
• In terms of Independent Assortment how many different combinations of
sperm could a human male produce?
Answer
• Formula: 2n
• Human chromosomes:
2n = 46
n = 23
• 223 = ~8 million combinations
Anaphase I
• Homologous chromosomes separate and
move towards the poles.
• Sister chromatids remain attached at their
centromeres.
Anaphase I
Telophase I
Cells split into two cells.
• Cytokinesis occurs and two daughter cells
are formed.
Telophase I and Cytokinesis
Meiosis II
• No interphase II
(or very short - no more DNA replication)
• Meiosis II is similar to mitosis
– four phases:
a.
b.
c.
d.
prophase II
metaphase II
anaphase II
telophase II
Prophase II
• Chromosomes are coiled up, and
spindle fibers are present.
Metaphase II
• Chromosomes line up at the equator,
spindle fibers align chromosomes.
metaphase plate
metaphase plate
Anaphase II
• sister chromatids separate
Telophase II
• Nuclei form.
• Cytokinesis occurs.
• Remember:
four haploid daughter cells
produced.
gametes = sperm or egg
Telophase II
Meiosis, with 2 chromosomes
n=2
sex cell
n=2
sperm
n=2
2n=4
haploid (n)
diploid (2n)
n=2
n=2
n=2
meiosis I
meiosis II
Meiosis in males is called
spermatogenesis and produces sperm.
Meiosis in females is called oogenesis
and produces ova.
Oogenesis
The polar bodies die… only one ovum (egg)
is produced from each primary oocyte.
Spermatogenesis
n=23
human
sex cell
sperm
n=23
n=23
2n=46
haploid (n)
n=23
diploid (2n)
n=23
n=23
meiosis I
meiosis II
Fertilization
• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg
n=23
egg
sperm
n=23
2n=46
zygote
Variation in Meiosis
• Important to population as the raw
material for natural selection.
• Question:
What are three sexual sources of
genetic variation?
Answer:
1. crossing over (prophase I)
2. independent assortment (metaphase I)
3. random fertilization
Remember: variation is good!
Question:
• A diploid cell containing 20 chromosomes at
the beginning of meiosis would, at its
completion, produce cells containing how
many chromosomes?
Answer:
• 10 chromosomes (haploid)
Question:
• A cell containing 40 chromatids at the
beginning of meiosis would, at its completion,
produce cells containing how many
chromosomes?
Answer:
• 10 chromosomes
Crash Course Meiosis
• https://www.youtube.com/watch?v=qCL
mR9-YY7o&NR=1#t=207.175104
Sources
• www.ursulinehs.org/powerpoint/meiosi
s.ppt
• www.biology4teachers.com/Cell%20Divi
sion/MEIOSIS.ppt
• http://www.iteachbio.com/Life%20Scien
ce/LifeFunctionsandTheCell/Meiosis.ppt