Meiosis in Animals - Exercise 13
Objectives
-Understand meiosis and know where it occurs.
-Know why meiosis is so important.
-Be able to explain the phases of meiosis I & meiosis
II.
-Be able to show meiosis using peptides (simulation).
-Know at what three phases rearrangement takes
place in meiosis.
MEIOSIS
• Occurs only in the germ cells.
• Process of gamete production.
• One (2n, diploid) cell divides to form four
(1n,haploid) gametes.
• Divided into two phases analogous to two
divisions without nuclear replication
between them.
• Chromosomes replicate once, the cell divides
twice.
Gametes
• Unlike somatic cells (skin, liver)
–Gametes, sperm and egg cells are
haploid cells, containing only one set
of chromosomes
• At sexual maturity
–The ovaries and testes produce
haploid gametes by meiosis
Most multicellular organism, as well as many unicellular
organism, produce sexually. They utilize specialized SEX cells
called GAMETES. All other types of cells are collectively called
SOMATIC cells. In most organisms, there are two
morphologically distinct types of gametes the EGG cell, and the
SPERM cell.
Meiosis I used in animals to generate (egg and sperm). It is
required for animals to do sexual reproduction. The main
function of meiosis is to reduce the chromosome number in
half. In many ways meiosis is very similar to mitosis, the stages
are the same in general terms and Meiosis II is identical to
mitosis. Chromosome number of nucleus is reduce by one half
from diploid to haploid and chromosome gene carried are
shuffled or recombined so daughter cell has unique array of
genes. 2 round of chromosome separation occurs in gonads. Ovaries
and testis.
• Egg cells are produced by females and sperm
cells are by males. Part of the process of
fertilization involves the fusion of a sperm nucleus
with an egg nucleus to form a single cell, the
ZYGOTE. The zygote has one nucleus containing
the chromosomes from both the sperm cell and
the egg cell.
• Zygote is single cell then divides to form a
human. When fertilization fuses to form single
cell it’s called a zygote, which has one nucleus
containing chromosomes of sperm and egg.
Since sexual reproduction involves the combining of chromosomes from two
different cells, and since the number of chromosomes per cell remains
constant within members of a species from generation to generation, there
must be a reduction of chromosomes number in each generation to offset
the increase at fertilization. In a normal somatic cell of an organism, there
are actually two complete sets of chromosomes. For each chromosome,
there is a “, and other morphological characters. These two
chromosomes apartner” chromosome which is identical in terms of length,
location of kinetochorelso contain genes for the same characteristics, although
not
necessarily for the same form of the characteristic (i.e body size – one gene
may specify tall, the other gene may specify short, but both genes specify body
size). Such chromosomes are said to be HOMOLOGOUS, and one of each of them
was provided by each parent. A cell that has two complete sets of chromosomes
is said to be DIPLOID or to contain the 2N number of chromosomes. A cell that
has only one complete set of chromosomes is HAPLOID and contains the N
number of chromosomes. In animals, normally somatic cells are diploid, while
normal gametes are haploid.
OVERVIEW OF MEIOSIS
• Each gamete has the potential to be genetically
unique due to the processes of crossing over and
independent assortment.
• Independent assortment- Alignment of the tetrads
in Metaphase I is independent of each other, this
allows the reshuffling of genetic material.
• The probablility that all chromosomes from the
same parent assort into the same gamete is equal
to (1/2)n. Where n=number of homologous pairs.
• In humans: P=(1/2)23=1/8,368,608.
Significant Events in the stages of Meiosis
The process of meiosis, like that of mitosis, is a
continuous sequence of events that have been
arbitrarily divided into stages for convenience. Since
there are two rounds of chromosomes separation,
The two divisions are called MEIOSIS I and MEIOSIS
II. Each division stage is further separated into
prophase, metaphase, anaphase, and telophase
followed by the appropriate Roman numeral (e.g.,
prophase I). Prophase I is a very long, complicated
process, and it too is subdivided into several substages.
MEIOSIS I
• “Reductional Division”
• Number of chromosomes (centromeres) is
reduced by ½.
• Homologous chromosomes separate from
each other into separate cells.
• One diploid cell (2n) divides to form two
haploid cells (1n).
Homologous Chromosomes
• Chromosomes that pair up during meiosis
– Contain the same genes
– May have different alleles of these genes
– One came from each parent
• Each is one long DNA molecule
– A gene is a short region of the molecule
– Each chromosome can have > 1,000 genes
The process of MEIOSIS is modified version of the mitotic
cell division process. During meiosis, two highly
significant consequences occur: (a) the chromosome
number of the nucleus is reduced by one half from the
diploid (2N) to the haploid (N) number, and (b) the
chromosomes (and thus the genes that they carry) are
shuffled or recombined so that each daughter cell has a
unique array of genes. It is this latter effect that causes
no individuals (except identical twins) to be exactly alike.
As in mitosis, meiosis is preceded in the cell cycle by
chromosome duplication; however, in meiosis there are
two rounds of chromosomes separation or two cell
divisions called meiosis I and meiosis II. In animals, the
meiotic process occurs in the gonads. The gonads are
called the OVARIES in females and TESTES in males.
PROPHASE I
• Homologous chromosomes find each other and
associate along their entire length-SYNAPSIS.
• Synapsis causes the Tetrad stage of meiosis to form
where each homologous pair consists of 4
chromatids in close association
• Non sister chromatids in the tetrad undergo
crossing over and exchange pieces.
• Chiasmata are the areas where the chromatids
cross over.
Meioses I
Prophase I:
• The chromosomes are duplicated during the S stage of the cell cycle, before
meiosis begins, and each of them exists a pair of chromatids joined at their
kinetochores. During prophase I the homologous pairs of chromosomes
come together and align themselves precisely, from end to end, gene for
gene. This process is called SYNAPSIS. Because each member of the
homologous pair is composed of two chromatids, there is a total of four
strands (chromatids) in a synapsed pair. This set is called a TETRAD. There
will be as many tetrads in the cell as there are chromosomes in haploid set
(e.g., 23 tetrads in human cells). As prophase I proceeds, the chromatids in
a tetrad become twisted around one another. Occasionally, two
homologous chromatids break in the same relative location and exchange
places with one another. This is called CROSSING OVER, and it produces
two recombinant chromatids. These recombinant chromatids have some
genes from one chromosome and some from another (a mix of some
maternal and some paternal genes). This is the first way in which new
gene combination may be produced during meiosis.Homeolgous
chromosomes associate and crossing over can occur.
METAPHASE I
• The homologous chromosomes (tetrads) line
up on the center line of the cell.
• Differs from mitosis in that centromeres do
not duplicate.
• Independent assortment occurs here.
Meioses I
• Metaphase I
• The tetrads remain intact throughout prophase I,
and the chromosomes attached to spindle fibers
and migrate to the equator of the cell (metaphase
plate) as tetrads. Spindle attachment is random –
i.e., which chromosome of a tetrad is facing which
pole is determine purely by chance.
ANAPHASE I
• Maternal (mom) and paternal (dad)
homologues of the tetrad separate
and go to opposite poles of the cell.
• This process is known as
disjunction.
Meioses I
• Anaphase I
• During anaphase I, homolgous chromosomes are
pulled away from one another intact (the chromatids
are not separated). Thus one homolog form a tetrad
will go to one pole, and the other homolog will go to
the other pole. Only one half of the total number of
chromosomes will go to each pole. The chromosomes
number will have been reduced from the diploid to
the haploid count in the resultant nuclei. Meiosis I is,
therefore, called the REDUCTION DIVISION. Because
the spindle attachment is random, the tetrads will
separate independently of one another. This is called
INDEPENDENT ASSORTMENT, and it is the second way
in which meiosis produces genetic recombination.
TELOPHASE I
• Maternal and paternal homologues reach
opposite poles of the cell.
• Two haploid cells are formed which are not
identical.
Meioses I
• Telophase I
• Once the chromosomes reach the poles and
telophase I and cytokinesis I have been completed,
the two daughter cells are haploid, but the
chromosomes are still composed of two
chromatids stuck at their kinetochores. Without
any further chromosomal duplication, the two cells
will go through a second stage of division called
Meiosis II.
• 2 cells are formed that ARE NOT
identical !!!
MEIOSIS II
• “Equational Division”
• Number of chromosomes (centromeres) is
equal before and after the division.
• Sister chromatids separate from each other
into separate cells.
• Two haploid cells (1n) divide to form four
haploid gametes (1n).
MEIOSIS II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
PROPHASE II
• May or may not occur, no significant events
here.
METAPHASE II
• Chromosomes arrange themselves on the
center line of the cells.
• Centromeres duplicate.
ANAPHASE II
• Sister chromatids separate to opposite poles
of the cells.
TELOPHASE II
• Chromatids reach the opposite poles of the
cells. Nuclear membrane reforms etc.
• Four haploid (1n) gametes result.
Meioses II
• Meiosis is mechanically just like mitosis. The individual chromatids
are separated at anaphase II. Meiosis II is called EQUATIONAL
DIVISION. Once meiosis II and cytokinesis II are completed, there
will be four haploid daughter cells. Each cell will have one complete
set of chromosomes, and each cell will be genetically unique
because of crossing over and independent assortment. The third
mechanism of recombining genes occurs during fertilization when
two gametes fuse at random to form a zygote.
• The significance of meiosis and sexual production lies in the
production of genetic variability within the species. The variety
affords the species as a whole a wilder opportunity to successfully
meet new environmental challenges to its survival. The genetic
variety is introduced by (1) crossing over; (2) independent
assortment; and (3) random union of gametes.
Synapsis and crossing over
–Homologous chromosomes physically
connect and exchange genetic information
Tetrads n the metaphase plate
–At metaphase I of meiosis, paired
homologous chromosomes (tetrads) are
positioned on the metaphase plates
Separation of homologues
–At anaphase I of meiosis, homologous
pairs move toward opposite poles of the
cell
–In anaphase II of meiosis, the sister
chromatids separate
Summary of Meioses II
• Prophase II: NO crossing can occur.
• Telophase II:
-Each new cell has ½ the amount of DNA as the original parent
cell.
-Each has only one copy of each chromosomes (haploid)
-These cells will become either egg or sperm cells.
• In males, gamete formation will result in 1 parent cell producing
4 viable sperm cells and in females, it will result in 1 parent cell
producing one 1 viable egg cell , which both are haploid cells.
• No interphase in meiosis, it goes straight to prophase II (probably
didn’t condense much because didn’t have an interphase stage) –
people say there’s not much difference between telephase I and
Prophase II.
Main difference between Meiosis I & Meiosis II
• Meiosis I- homologous chromosomes
separate.
• Meiosis II- sister chromatids separate.
• Homologous chromosomes- are genetically
equivalent but not identical. One inherited
from mother, one from father.
• Three events are unique to meiosis, and all three
occur in meiosis l:
– Synapsis and crossing over in prophase I: Homologous
chromosomes physically connect and exchange genetic
information
– At the metaphase plate, there are paired homologous
chromosomes (tetrads), instead of individual replicated
chromosomes
– At anaphase I, it is homologous chromosomes, instead
of sister chromatids, that separate and are carried to
opposite poles of the cell
Genetic Variation
• Independent assortment of chromosomes
– Each pair of homologous maternal and paternal
chromosomes are arranged independently of the other
pairs allowing for many possible combinations
chromosomes (2n, where n = haploid number)
• Humans = 8 million possible gametes
• Crossing over
– Homologous portions of two nonsister chromatids trade
places (2-3 crossovers/chromosome)
– Recombinant chromosomes are produced which combine
genes from both parents
• Random fertilization
– The combination of egg (8 million combinations) and
sperm (8 million combinations) produce a zygote with
any of about 64 trillion diploid combinations
Three mechanisms contribute to genetic
variation:
1. Independent assortment of
chromosomes
2. Crossing Over
3. Random fertilization
INDEPENDENT ASSORTMENT
• Homologous pairs of chromosomes orient
randomly at metaphase I of meiosis.
• The purpose of meiosis I is to separated the
maternal and paternal homologues.
• The number of combinations possible is 2n,
where n is the haploid number.
• For humans (n = 23), there are more than 8
million (223) possible combinations of
chromosomes.
CROSSING OVER
• Crossing over produces recombinant
chromosomes, which combine genes inherited
from each parent.
• Crossing over begins very early in prophase I, as
homologous chromosomes pair up gene by gene.
• In crossing over, homologous portions of two
nonsister chromatids trade places.
• Crossing over contributes to genetic variation by
combining DNA from two parents into a single
chromosome.
Crossing Over
Meiosis
Before Meiosis
Gametes produced
• Homologous chromosomes line up during meiosis
• Parts of maternal and paternal chromosomes migrate
RANDOM FERTILIZATION
• Random fertilization adds to genetic variation
because any sperm can fuse with any ovum
(unfertilized egg).
• The fusion of human gametes produces a
zygote with any of about 70 trillion diploid
combinations.
• Crossing over adds even more variation.
• Each zygote has a unique genetic identity.
Simulation of Meiosis
• The simulation of meiosis, using pipe cleaners as
chromosomes.
• The simulation of meiosis will illustrate all of the major events
of meiosis except crossing over.
• The simulation uses different colors and different sizes of
pipe cleaners to represent chromosomes.
• If the pipe cleaners (chromosomes) are the same size,
they are defined as homologous.
• If they (chromosomes) are the same size and the same
color, they are identical and therefore, sister
chromatids.
• The male and female snaps on the pipe cleaners
represent the kinetochore (centromere).
• The sheet of paper represents the cell. The simulation
is for an imaginary species of animal that has an N
number of 2 and a 2N number of 4 (2 homologous
pairs).
• Spermatogenesis will be simulated, not oogenesis.
Therefore, the results of this simulation will be the
formation of four haploid sperm cells. As you go
through the simulation, try to relate the behavior of the
pipe cleaners (chromosomes) to the ultimate
consequences of meiosis (the reduction of the
chromosome number and genetic recombination).
A COMPARISON OF MITOSIS & MEIOSIS
• Mitosis conserves the number of chromosome sets,
producing cells that are genetically identical to the
parent cell.
• Meiosis reduces the number of chromosomes sets from
two (diploid) to one (haploid), producing cells that differ
genetically from each other and from the parent cell.
• The mechanism for separating sister chromatids is
virtually identical in meiosis II and mitosis.
EVOLUTIONARY SIGNIFICANCE OF
GENETIC VARIATION (MEIOSIS)
Reshuffling of genetic material in meiosis
-Produces genetic variation
-Essential to evolution
• Natural selection results in accumulation of genetic
variations favored by the environment.
• Sexual reproduction contributes to the genetic
variation in a population, which ultimately results
from mutations.
Summary
MITOSIS
2 genetically identical diploid cells
Diploid cell with
duplicated
chromosomes
MEIOSIS
4 genetically different haploid cells
• Events unique to meiosis
– Synapsis: homologous chromosomes pair
– Chiasmata: crossing over of nonsister chromatids
to exchange genetic information
– Meiosis I separates homologous chromosomes
• Meiosis II is physically identical to mitosis
Gametes
NORMAL
DISJUNCTION
A
N (haploid)
A
N (haploid)
A
A
A
A
a
1st division
2nd division
Meiosis I
Meiosis II
a
Tetrad Stage
of Meiosis
a
N (haploid)
a
a
a
N (haploid)
Chromosomal Life Cycle
Key
• In animals
– Meiosis occurs during
gamete formation
– Gametes are the only
haploid cells
Haploid gametes (n = 23)
Haploid (n)
Ovum (n)
Diploid (2n)
Animal Life Cycle
Sperm
Cell (n)
Key
FERTILIZATION
MEIOSIS
Haploid
Diploid
n
Ovary
Testis
Mitosis and
development
Multicellular diploid
adults (2n = 46)
Diploid
zygote
(2n = 46)
n
Gametes
n
MEIOSIS
2n
Diploid
multicellular
organism
FERTILIZATION
Zygote
2n
Mitosis
(a) Animals
Human Life Cycle
Meiosis
46
23
23
Somatic
cell
Sperm
cell
Egg cell
Meiosis
46
Zygote
Mitosis
46
Somatic
cell
Page 6 – Lab Book
• 1. Distinguish between the haploid and diploid
chromosome numbers of a species.
• 3.
a) Which meiotic division is the reduction division?
b) Which meiotic division is the equational
division?
Page 6 – Lab Book
• 1. Distinguish between the haploid and diploid
chromosome numbers of a species.
• The diploid condition has homologous pairs.
• Haploid = 23 (half) (n)
• Diploid = 46 (all) (2n)
• 3.
• a) Which meiotic division is the reduction division? Meiosis I
• b) Which meiotic division is the equational division? Meiosis II
Page 6 – Lab Book
• 4. What is the difference between a reduction
division and an equational division in terms of
chromosome number?’
• 6.
a) During which stage of meiosis does synapsis occur?
b) Does synapsis occur during mitosis? If so, when?
c) Why is it important for synapsis to occur in meiosis
II?
Page 6 – Lab Book
• 4. What is the difference between a reduction division and an
equational division in terms of chromosome number?’
• Reduction division reduces the chromosomes number by half.
• The equational division the same chromosome number as the
original parent cell.
• 6.
a) During which stage of meiosis does synapsis occur? Prophase I
b) Does synapsis occur during mitosis? If so, when? No
c) Why is it important for synapsis to occur in meiosis II? There are
no homolgous pairs. The cells going through meiosis II are haploid.
There’s no homologus pairs to line up.
Page 6 – Lab Book
• 8. Describe a tetrad.
• 9. During what stage of meiosis does crossing over occur?
• 10. During what stage of meiosis does independent
assortment occur?
• 14. List 3 ways by which meiosis and/or sexual reproduction
can increase genetic variability in a species.
a)
b)
c)
Page 6 – Lab Book
• 8. Describe a tetrad.
-A synapsed homologous pair of chromosomes
-4 sister chromtides of a particular n chromosome.
• 9. During what stage of meiosis does crossing over occur? Prophase I
• 10. During what stage of meiosis does independent assortment occur?
Anaphase I
• 14. List 3 ways by which meiosis and/or sexual reproduction can increase
genetic variability in a species.
a) Crossing over
b) Independent assortment
c) Union of different gametes (unique fertilization)