MEIOSIS
IB BIO II
March 3, 2015
Van Roekel
Observing Mitosis Lab
• Finish observations by looking at both onion root tip and
white fish mitosis cells (10 minutes to finish observations
and questions)
• We will go over questions then turn them in
Meiosis – A Source of Distinction
What are some characteristics you share with your parents? 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
• Type of cell division called reduction division that cuts the
number of chromosomes in half in order to produce sex
cells (or gametes)
Why do we need meiosis?
• Meiosis is necessary to halve the number of
chromosomes going into the sex cells
• This type of cell division is referred to as a reduction
division, because the number of chromosomes has
been reduced
Why halve the chromosomes in gametes?
• At fertilization the male and female sex cells will
combine to provide ½ of the chromosomes each – so
the offspring has genes from both parents and has a
complete set of chromosomes
Meiosis does two things 1) Meiosis takes a cell with two copies
of every chromosome (diploid) and
makes four cells with a single copy of
every chromosome (haploid).
In meiosis, one diploid cells produces
four haploid cells.
2) Meiosis creates a lot of genetic
diversity. This trick is accomplished
through independent assortment,
crossing-over, and random
orientation.
Meiosis scrambles the specific
forms of each gene that each sex
cell (egg or sperm) receives.
Genetic diversity is important for the
evolution of populations and species.
The Stages of Meiosis:
• Occurs in two separate Divisions:
• Meiosis I
• Interphase
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
Stages of Meiosis
• Draw, label, and annotate the stages of Meiosis using
pages 240-241 in green Campbell-Reece Biology books
BILL
• What is meiosis? What two things does meiosis
do?
• Meiosis is a reduction division that produces
gametes (sex cells).
• It cuts the number of chromosomes in half and
turns one diploid cell into 4 haploid cells
• It creates genetic diversity by independent
assortment, random orientation, and crossing
over.
Meiosis
Parent cell –
chromosome pair
Chromosomes
copied
1st division - pairs split
2nd division – produces
4 gamete cells with ½
the original no. of
chromosomes
Meiosis – mouse testes
Parent cell
1st division
2nd division
4 gametes
Meiosis I : Separates
Homologous Chromosomes
• Homologous Chromosomes: pairs of
chromosomes that are similar in size
and shape and carry the same genes
(AKA tetrads/bivalent)
• Interphase
• Each of the chromosomes replicate
• The result is two genetically identical
sister chromatids which remain attached
at their centromeres
Prophase I
• This is a crucial phase for meiosis.
• During this phase each pair of chromatids
match up with their homologous pair and
fasten together (synapsis) in a group of
four called a tetrad.
• Extremely IMPORTANT!!! It is during this
phase that crossing over can occur.
• Crossing Over is the exchange of segments
from homologous chromosomes during
synapsis.
Metaphase I
• The homologous chromosomes, aka the
bivalents, line up at the equator attached
by their centromeres to spindle fibers
from centrioles.
Anaphase I
• The spindle guides the
movement of the chromosomes
towards the poles
• Sister chromatids remain attached
• Move as a unit towards the same
pole
• The homologous chromosome
moves toward the opposite pole
• Contrasts mitosis – chromosomes
appear as individuals instead of pairs
(meiosis)
Telophase I
• This is the end of the first meiotic cell division.
• The cytoplasm divides, forming two new daughter cells.
• Each of the newly formed cells has half the number of
the parent cell’s chromosomes, but each chromosome
is already replicated ready for the second meiotic cell
division
Cytokinesis
• Occurs simultaneously with
telophase I
• Forms 2 daughter cells
• NO FURTHER REPLICATION OF
GENETIC MATERIAL PRIOR TO THE
SECOND DIVISION OF MEIOSIS
• Cells are now considered haploid
because they contain only one copy of
each chromosome
Figure 13.7 The stages of meiotic cell division: Meiosis I
Meiosis II :
Separates sister chromatids
• Proceeds similar to mitosis
• THERE IS NO INTERPHASE II !
Prophase II
• Each of the daughter cells forms a
spindle, and the sister chromatids
move toward the equator
Metaphase II
• The individual
chromosomes (sister
chromatids) line up on
the equator of each cell
in random order. This is
referred to as random
orientation
• Spindle fibers from
opposite poles attach to
each of the sister
chromatids at
centromeres
Anaphase II
• The centromeres of sister
chromatids finally separate
• The sister chromatids of each
pair move toward opposite
poles
• Now considered individual
chromosomes
Telophase II and Cytokinesis
• Nuclei form at opposite poles of the
cell and cytokinesis occurs
• After completion of cytokinesis
there are four daughter cells
• All daughter cells are haploid (n)
Figure 13.7 The stages of meiotic cell division: Meiosis I
Figure 13.7 The stages of meiotic cell division: Meiosis II
Meiosis I & II
• Meiosis I takes place in order to produce two
cells with a single set of chromosomes. In other
words, it separates homologous chromosomes
• Crossing over also occurs during meiosis I,
which allows for an exchange of genetic material
between non-sister chromatids
• Meiosis II takes place in order to separate sister
chromatids, or copies of an individual
chromosome.
Bill
• Explain why meiosis, rather than mitosis is
necessary for gamete production. What is the
difference between meiosis I and meiosis II?
• Because the resulting daughter cells only contain
half the genetic material from the female parent
cells. The number of chromosomes passes from
2n to n.
• Meiosis I is the separation of homologous
chromosomes to create haploid cells, meiosis II is
the separation of sister chromatids.
Stages of Meiosis
• Draw, label, and annotate the stages of Meiosis using
pages 240-241 in green Campbell-Reece Biology books,
or 232-233 in red Campbell-Reece Biology Books (15
minutes)
Genetic Variety in Gametes
• Meiosis produces sex cells which result in
offspring that show genetic diversity
• Due to the following:
• Crossing Over
• Random orientation of Chromosomes
• Law of independent Assortment
Crossing Over
• Occurs during Prophase I
• Results in the exchange of genetic information
between paternal and maternal chromosomes.
• Resulting chromosomes contain sections of
genetic material which originated in two different
people
• Sister chromatids are no longer identical, so
when they are separated, different alleles will be
present in each gamete
Crossing Over
• Must occur between homologous chromosomes
• Occurs when chromatids from homologous
chromosomes intertwine and break at the same
positions
• Place where chromatids connect to each other is
referred to as a chiasma (plural, chiasmata)
Another Way Meiosis Makes Lots of
Different Sex Cells – Crossing-Over
Crossing-over multiplies the already huge number of different gamete types
Random Orientation
• Occurs during
Metaphase I & II
• Random order of
chromosomes as they
line up at the equator
of each cell
• Results in random
alleles in each gamete
Independent Assortment
• The separation of one pair of alleles is
independent of the separation of another pair of
alleles.
• Meaning that one trait, such as flower color, is
passed from parent to offspring does not depend
on any other traits, such as see color.
• Results because of random orientation of
chromosomes
Independent Assortment & Meiosis
• The genes and, in turn, alleles that will be passes
to daughter cells depends on orientation of
chromosomes during metaphase I
• Produces 2n distinct gametes, where n is the
number of chromosomes, and 2 represents the
number in each homologous pair
• In humans, n=23, so 223= 8,388,608 possible
chromosome orientations per gamete
Boy or Girl? The Y Chromosome “Decides”
Y chromosome
X chromosome
Boy or Girl? The Y Chromosome “Decides”
Division Error
• Occasionally, chromosomes do not separate as
expected, resulting in unequal distribution of
chromosomes
• Non-disjunction occurs when two or more
homologous chromosomes stick together instead
of separating
• Results in gametes with 24 chromosomes
Non-disjunction & Trisomy
• Different results based on what chromosome is
affected
• Trisomy is when a child receives 3 chromosomes
instead of 2
• When this happens on the 21st chromosome
results in Down’s syndrome
• Risk increase with age of the mother, especially
over the age of 35
Karyotypes
• Karyotypes is a photograph of chromosomes
found in a cell, arranged based on size and
shape.
• Photo is taken during metaphase of mitosis
• Obtain cells by:
• Amniocentesis: use a hypodermic needle to extract
some amniotic fluid around the developing baby
• Chorionic Villus Sampling: obtain tissue sample from
the placenta in the uterus wall
Checkup – 20 minutes to work
•
Describe, with the aid of a diagram, the behavior of
chromosomes in the different phases of meiosis.
• (Total 5 marks)
•
Explain how meiosis results in great genetic variety among
gametes.
• (Total 8 marks)
• Why is meiosis referred to as reduction division?
• (Total 2 marks)
•
• Draw and label the stages of meiosis I & II
(Total 8 marks)
• When completed, answer questions 7 & 9 on pg. 90 and
questions 1-3 on pg. 271
Describe, with the aid of a diagram, the behavior of chromosomes in
the different phases of meiosis.
(Total 5 marks
• Chromosomes condense and become shorter/more
visible in prophase I. At this time, homologous
chromosomes will pair together and crossing over can
occur. The homologous pair moves to the middle of the
cell during metaphase I and separate during anaphase I,
creating two haploid daughter cells in telophase I. The
chromosomes condense in prophase II, and this time
individual chromosomes move to the middle of the cell in
metaphase II. Sister chromatids separate during
anaphase II and decondense/uncoil in telophase II.
Explain how meiosis results in great genetic variety
among gametes. (8 marks)
• In meiosis, homologous chromosome pair together to form
bivalents, which allows for the process of crossing over to
occur during prophase I. In crossing over, sister chromatids
from opposite parents exchange genetic information when they
join together at a site called the chiasma. This results in a new
combination of alleles on the sister chromatids. Additionally, the
homologues will randomly orient their positions during
metaphase I, which results in random distribution of alleles to
the gametes, as they separate from each other. Lastly, the law
of independent assortment states that alleles will separate
independently of each other. This leads to the possibility of 223
different gametes forming. This results in a wide range of
possible combinations of alleles in each gamete, based on how
the chromosomes are separated during meiosis.
Why is meiosis referred to as reduction division?
(Total 2 marks)
• Meiosis is referred to as a reduction division because it is
a type of cell division that reduces the number of
chromosomes in each cell in half. The result of meiosis is
that one diploid parent cell will create four genetically
unique haploid daughter cells.
Draw and label the stages of meiosis I & II (Total 8
marks)
7 & 9 page 90
• 7. Gamete (sperm/egg cell)
• 9 Risk of having a baby with Down’s syndrome
increase with age because of the increased
possibility of damage to the DNA of egg cell, so
mothers over the age of 35 are encouraged to
have a karyotype done.
1-3 pg. 271
• 1 Natural selection must have a variety of
offspring to allow for the survival of the fittest. If
all organisms had same genetics, all would be
susceptible to same diseases/predators, so
genetic diversity is required for survival of
species.
• 2.a. Prophase I
b.
• 3.a Metaphase I
• 3.b