MEIOSIS
IB BIO II
January 15, 2014
Van Roekel
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.
Stages of Meiosis
• Draw, label, and annotate the stages of Meiosis using
pages 240-241 in green Campbell-Reece Biology books
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
provide ½ of the chromosomes each – so the
offspring has genes from both parents
Meiosis does two things 1) Meiosis takes a cell with two copies
of every chromosome (diploid) and
makes cells with a single copy of
every chromosome (haploid).
In meiosis, one diploid cells produces
four haploid cells.
2) 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
crossing-over.
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 the difference between meiosis I and
meiosis II?
• Meiosis I separates homologous chromosomes
so each daughter cell only has half the genetic
material.
• Meiosis II separates sister chromatids so each
daughter cell has one copy of each chromosome.
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.
• 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.
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
•
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