Download Chapter 11.4 Meiosis

11.4 Meiosis
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Vocabulary:
Homologous
Diploid
Haploid
Meiosis
Crossing-over
Tetrad
Key Concept:
•What happens during the process
of meiosis?
•How is MEIOSIS different than
mitosis?
Blast from the past
• What is mitosis?
– One of the 3 main stages in cell cycle
– Divides the nucleus into 2 nuclei distributing
the same amount of genetic material to each
cell
– 4 stages in mitosis: prophase, metaphase,
anaphase, and telophase
Vocabulary
• Homologous—chromosome from mom
has corresponding chromosome from
dad
• Diploid—two sets of chromosomes
– Code for same genes (ex: hair color)
• Haploid—one set of chromosomes
– In the sex cells
– These will join with sex cells from the
opposite sex (ex: egg joins with sperm)
So what is meiosis?
• It is a specialized type of cell
division that occurs in the
formation of gametes such as
egg and sperm.
• Meiosis appears much more
complicated than mitosis…
– it is really just two divisions
in sequence
– each one of which has
strong similarities to
mitosis.
INTERPHASE
• Interphase in meiosis is identical to
interphase in mitosis
• There is no way, by simply
observing the cell, to determine
what type of division the cell will
undergo when it does divide.
• Meiotic division will only occur in
cells associated with male or
female sex organs.
MEIOSIS I
• Meiosis I, the first of the two divisions, is often
called reduction division
– it is here that the chromosome number is
reduced from 2N (diploid) to 1N (haploid).
• Ex: Humans: diploid number = 46 (haploid
= 23)
PROPHASE I
• Prophase I is similar to
prophase in mitosis
– involving the appearance
of the chromosomes
– the development of the
spindle apparatus
– breakdown of the nuclear
envelope
Prophase I—Important Events
• Homologous chromosomes pair up to form
a tetrad
• The sister chromatids press together at
points along their length
• It is during this alignment that chromatid
arms may overlap and temporarily fuse
resulting in crossing over
• Result: chomosomes (consisting of 2
sister chromatids) are no longer just
from “mom” or “dad”—the chromosomes
are a mixture!!!
Vocabulary
• Tetrad—each
chromosome pairs with
homologous chromosome
– 4 sister chromatids!
• Crossing over– when
chromosomes are in
tetrads, they may
exchange parts
– This happens in Prophase
I
– Result–increased variety in
offspring (a good thing)
Result: INCREASED genetic variety!!!
METAPHASE I
• Here is where the CRITICAL
DIFFERENCE occurs between
Metaphase I in meiosis and
metaphase in mitosis.
METAPHASE I
• In Mitosis, all the chromosomes
line up on the metaphase plate in
no particular order.
• In Metaphase I (of Meiosis), the
chromosome pairs are aligned on
either side of the metaphase
plate.
• It is during this alignment that
chromatid arms may overlap and
temporarily fuse (chiasmata)
resulting in crossing over
• Crossing over increases genetic
diversity
ANAPHASE I
• During Anaphase I the
spindle fibers contract,
pulling the homologous
pairs away from each
other and toward each
pole of the cell.
TELOPHASE I
• Cleavage is followed by
cytokinesis, but…
• the nuclear membrane
(envelope) usually is not
reformed and the chromosomes
do not disappear.
At the end of Telophase I
• each daughter cell has a single
set of chromosomes
• half the total number in the
original cell where the
chromosomes were present in
pairs.
• While the original cell was diploid (2n),
the daughter cells are now haploid
(1n).
• This is why Meiosis I is often called
reduction division.
– Diploid = 4
– Haploid = 2
It was 4, now it will be 2 in
each cell!!
MEIOSIS II
• Meiosis II is quite simple in that it is
simply a mitotic division of each of the
haploid cells produced in Meiosis I.
• There is no Interphase between Meiosis
I and Meiosis II and the latter begins
with…
PROPHASE II
• A new set of spindle
fibers forms
• the chromosomes
begin to move toward
the equator of the cell.
METAPHASE II
• All the chromosomes
in the two cells align
with the metaphase
plate.
ANAPHASE II
• The centromeres split
• The spindle fibers shorten
– drawing the chromosomes
toward each pole of the cell.
TELOPHASE II
• Cleavage is followed by
cytokinesis
• The nuclear membrane
(envelope) reforms
• The chromosomes begin to
fade, replaced by the
granular chromatin
characteristic of interphase.
• Total of four daughter
cells, each with half the total
number of chromosomes as
the original cell.
• Male structures
– all four cells will
eventually develop into
typical sperm cells.
• Female life cycles in
“higher” organisms
– three of the cells will
typically abort
– leaving a single cell to
develop into an egg cell
• usually much larger than
a typical sperm cell.
Gametes
NOVA video
http://www.pbs.org/wgbh/nova/baby/divi_flash.html