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Meiosis and
Sexual
Reproduction
The Process of Meiosis
Sexual Reproduction
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Meiosis and Sexual Reproduction > The Process of Meiosis
The Process of Meiosis
• Introduction
• Meiosis I
• Meiosis II
• Comparing Meiosis and Mitosis
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Meiosis and Sexual Reproduction > The Process of Meiosis
Introduction
• Sexual reproduction is the production of haploid cells and the fusion of two of
those cells to form a diploid cell.
• Before sexual reproduction can occur, the number of chromosomes in a diploid
cell must decrease by half.
• Meiosis produces cells with half the number of chromosomes as the original cell.
• Haploid cells used in sexual reproduction, gametes, are formed during meiosis,
which consists of one round of chromosome replication and two rounds of nuclear
division.
• Meiosis I is the first round of meiotic division, while meiosis II is the second round.
Offspring Closely Resemble Their Parents
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Meiosis and Sexual Reproduction > The Process of Meiosis
Meiosis I
• Meiosis is preceded by interphase which consists of the G1 phase (growth), the S
phase (DNA replication), and the G2 phase.
• During prophase I, the homologous chromosomes condense and become visible
as the x shape we know, pair up to form a tetrad, and exchange genetic material
by crossing over.
• During prometaphase I, microtubules attach at the chromosomes' kinetochores
and the nuclear envelope breaks down.
• In metaphase I, the tetrads line themselves up at the metaphase plate and
homologous pairs orient themselves randomly.
• In anaphase I, centromeres break down and homologous chromosomes separate.
Crossover between homologous chromosomes
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• In telophase I, chromosomes move to opposite poles; during cytokinesis the cell
separates into two haploid cells.
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Meiosis and Sexual Reproduction > The Process of Meiosis
Meiosis II
• During prophase II, chromsomes condense again, centrosomes that were
duplicated during interphase I move away from each other toward opposite poles,
and new spindles are formed.
• During prometaphase II, the nuclear envelopes are completely broken down, and
each sister chromatid forms an individual kinetochore that attaches to
microtubules from opposite poles.
• During metaphase II, sister chromatids are condensed and aligned at the equator
of the cell.
• During anaphase II sister chromatids are pulled apart by the kinetochore
microtubules and move toward opposite poles.
Meiosis I vs. Meiosis II
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• During telophase II and cytokinesis, chromosomes arrive at opposite poles and
begin to decondense; the two cells divide into four unique haploid cells.
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Meiosis and Sexual Reproduction > The Process of Meiosis
Comparing Meiosis and Mitosis
• For the most part, in mitosis, diploid cells are partitioned into two new diploid cells,
while in meiosis, diploid cells are partitioned into four new haploid cells.
• In mitosis, the daughter cells have the same number of chromosomes as the
parent cell, while in meiosis, the daughter cells have half the number of
chromosomes as the parent.
• The daughter cells produced by mitosis are identical, whereas the daughter cells
produced by meiosis are different because crossing over has occurred.
• The events that occur in meiosis but not mitosis include homologous
chromosomes pairing up, crossing over, and lining up along the metaphase plate
Comparing Meiosis and Mitosis
in tetrads.
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• Meiosis II and mitosis are not reduction division like meiosis I because the
number of chromosomes remains the same; therefore, meiosis II is referred to as
equatorial division.
• When the homologous chromosomes separate and move to opposite poles during
meiosis I, the ploidy level is reduced from two to one, which is referred to as a
reduction division.
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Meiosis and Sexual Reproduction > Sexual Reproduction
Sexual Reproduction
• Introduction
• Life Cycles of Sexually Reproducing Organisms
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Meiosis and Sexual Reproduction > Sexual Reproduction
Introduction
• The variation that sexual reproduction creates among offspring is very important
to the survival and reproduction of the population.
• In sexual reproduction, different mutations are continually reshuffled from one
generation to the next when different parents combine their unique genomes; this
results in an increase of genetic diversity.
• On average, a sexually-reproducing population will leave more offspring than an
otherwise similar asexually-reproducing population.
The Sexual Life Cycle
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Meiosis and Sexual Reproduction > Sexual Reproduction
Life Cycles of Sexually Reproducing Organisms
• In the diploid-dominant cycle, the multicellular diploid stage is the most obvious
life stage; the only haploid cells produced by the organism are the gametes.
• Most fungi and algae employ a haploid-dominant life cycle type in which the
"body" of the organism is haploid; specialized haploid cells from two individuals
join to form a diploid zygote.
• Observed in all plants and some algae, species with alternation of generations
have both haploid and diploid multicellular organisms as part of their life cycle.
Diploid-Dominant Life Cycle
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Appendix
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Meiosis and Sexual Reproduction
Key terms
• asexual reproduction any form of reproduction that involves neither meiosis nor fusion of gametes
• chromatid either of the two strands of a chromosome that separate during meiosis
• crossing over the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes
• diploid of a cell, having a pair of each type of chromosome, one of the pair being derived from the ovum and the other from the
spermatozoon
• equatorial division a process of nuclear division in which each chromosome divides equally such that the number of
chromosomes remains the same from parent to daughter cells
• gamete a reproductive cell, male (sperm) or female (egg), that has only half the usual number of chromosomes
• gametophyte a plant (or the haploid phase in its life cycle) that produces gametes by mitosis in order to produce a zygote
• haploid of a cell having a single set of unpaired chromosomes
• meiosis II the second part of the meiotic process; the end result is production of four haploid cells from the two haploid cells
produced in meiosis I
• ploidy the number of homologous sets of chromosomes in a cell
• reduction division the first of the two divisions of meiosis, a type of cell division
• sexual reproduction Sexual reproduction is the creation of a new organism by combining the genetic material of two
organisms.There are two main processes during sexual reproduction: meiosis, involving the halving of the number of
chromosomes, and fertilization, involving the fusion of two gametes and the restoration of the original number of chromosomes.
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Meiosis and Sexual Reproduction
• sporophyte a plant (or the diploid phase in its life cycle) that produces spores by meiosis in order to produce gametophytes
• tetrad two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis
process
• zygote a diploid fertilized egg cell
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Meiosis and Sexual Reproduction
Offspring Closely Resemble Their Parents
<em>In kind</em> means that the offspring of any organism closely resemble their parent or parents.The hippopotamus gives birth to hippopotamus
calves (a).Joshua trees produce seeds from which Joshua tree seedlings emerge (b).Adult flamingos lay eggs that hatch into flamingo chicks (c).
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Meiosis and Sexual Reproduction
Crossover between homologous chromosomes
Crossover occurs between non-sister chromatids of homologous chromosomes.The result is an exchange of genetic material between homologous
chromosomes.
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Meiosis and Sexual Reproduction
Synapsis holds pairs of homologous chromosomes together
Early in prophase I, homologous chromosomes come together to form a synapse.The chromosomes are bound tightly together and in perfect alignment
by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere.
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Meiosis and Sexual Reproduction
Meiosis I ensures unique gametes
Random, independent assortment during metaphase I can be demonstrated by considering a cell with a set of two chromosomes (n = 2).In this case,
there are two possible arrangements at the equatorial plane in metaphase I.The total possible number of different gametes is 2n, where n equals the
number of chromosomes in a set.In this example, there are four possible genetic combinations for the gametes.With n = 23 in human cells, there are
over 8 million possible combinations of paternal and maternal chromosomes.
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Meiosis and Sexual Reproduction
Meiosis I vs. Meiosis II
The process of chromosome alignment differs between meiosis I and meiosis II.In prometaphase I, microtubules attach to the fused kinetochores of
homologous chromosomes, and the homologous chromosomes are arranged at the midpoint of the cell in metaphase I.In anaphase I, the homologous
chromosomes are separated.In prometaphase II, microtubules attach to the kinetochores of sister chromatids, and the sister chromatids are arranged at
the midpoint of the cells in metaphase II.In anaphase II, the sister chromatids are separated.
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Meiosis and Sexual Reproduction
Complete Stages of Meiosis
An animal cell with a diploid number of four (2n = 4) proceeds through the stages of meiosis to form four haploid daughter cells.
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Meiosis and Sexual Reproduction
Comparing Meiosis and Mitosis
Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions.The four daughter cells
resulting from meiosis are haploid and genetically distinct.The daughter cells resulting from mitosis are diploid and identical to the parent cell.
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Meiosis and Sexual Reproduction
The Sexual Life Cycle
In animals, sexually-reproducing adults form haploid gametes from diploid germ cells.Fusion of the gametes gives rise to a fertilized egg cell, or
zygote.The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring.
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Meiosis and Sexual Reproduction
Alternation of Generations
Plants have a life cycle that alternates between a multicellular haploid organism and a multicellular diploid organism.In some plants, such as ferns, both
the haploid and diploid plant stages are free-living.The diploid plant is called a sporophyte because it produces haploid spores by meiosis.The spores
develop into multicellular, haploid plants called gametophytes because they produce gametes.The gametes of two individuals will fuse to form a diploid
zygote that becomes the sporophyte.
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Meiosis and Sexual Reproduction
Diploid-Dominant Life Cycle
In animals, sexually-reproducing adults form haploid gametes from diploid germ cells.Fusion of the gametes gives rise to a fertilized egg cell, or
zygote.The zygote will undergo multiple rounds of mitosis to produce a multicellular offspring.The germ cells are generated early in the development of
the zygote.
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Meiosis and Sexual Reproduction
Haploid-Dominant Life Cycle
Fungi, such as black bread mold (Rhizopus nigricans), have haploid-dominant life cycles.The haploid multicellular stage produces specialized haploid
cells by mitosis that fuse to form a diploid zygote.The zygote undergoes meiosis to produce haploid spores.Each spore gives rise to a multicellular
haploid organism by mitosis.
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Meiosis and Sexual Reproduction
Which of the following explains why meiosis is an integral part of
sexual reproduction?
A) Meiosis produces diploid cells from haploid cells.
B) Meiosis fertilizes a sperm and an egg cell.
C) Meiosis doubles the number of chromosomes in a cell.
D) Meiosis results in a reduction of chromosomes.
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Meiosis and Sexual Reproduction
Which of the following explains why meiosis is an integral part of
sexual reproduction?
A) Meiosis produces diploid cells from haploid cells.
B) Meiosis fertilizes a sperm and an egg cell.
C) Meiosis doubles the number of chromosomes in a cell.
D) Meiosis results in a reduction of chromosomes.
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Meiosis and Sexual Reproduction
How is genetic material exchanged during meiosis?
A) Segments of homologous chromosomes recombine, creating
recombinant chromosomes
B) Following telophase, cytokinesis seperates the cell into two daughter
cells.
C) Microtubules attach to fused homologous chromosomes.
D) Chromosomes are replicated and produce identical copies during DNA
replication.
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Meiosis and Sexual Reproduction
How is genetic material exchanged during meiosis?
A) Segments of homologous chromosomes recombine, creating
recombinant chromosomes
B) Following telophase, cytokinesis seperates the cell into two daughter
cells.
C) Microtubules attach to fused homologous chromosomes.
D) Chromosomes are replicated and produce identical copies during DNA
replication.
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Meiosis and Sexual Reproduction
At what stage do homologous chromosomes separate?
A) Anaphase I
B) Metaphase I
C) Telophase I
D) Prometaphase I
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Meiosis and Sexual Reproduction
At what stage do homologous chromosomes separate?
A) Anaphase I
B) Metaphase I
C) Telophase I
D) Prometaphase I
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Meiosis and Sexual Reproduction
Which of the following is a result of meiosis II?
A) Homologous chromosomes separate and four haploid cells are
created.
B) Sister chromatids separate and four haploid cells are created.
C) Homologous chromosomes separate and four diploid cells are
created.
D) Sister chromatids separate and two diploid cells are created.
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Meiosis and Sexual Reproduction
Which of the following is a result of meiosis II?
A) Homologous chromosomes separate and four haploid cells are
created.
B) Sister chromatids separate and four haploid cells are created.
C) Homologous chromosomes separate and four diploid cells are
created.
D) Sister chromatids separate and two diploid cells are created.
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Meiosis and Sexual Reproduction
Which of the following occurs in both mitosis and meiosis?
A) crossing over between sister chromatids
B) formation of two diploid cells
C) movement of sister chromatids to opposite poles
D) two nuclear cell divisions
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Meiosis and Sexual Reproduction
Which of the following occurs in both mitosis and meiosis?
A) crossing over between sister chromatids
B) formation of two diploid cells
C) movement of sister chromatids to opposite poles
D) two nuclear cell divisions
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Meiosis and Sexual Reproduction
Sexual reproduction is the most successful form of reproduction
within eukaryotes because:
A) in many animals, fragmentation is the sole mode of reproduction.
B) genetic diversity offers offspring a greater change of survival.
C) a sexually-reproducing population will leave less descendants with
decreased genetic diversity.
D) they are able to reproduce within an unpredictable environment
without the requirement of a partner.
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Meiosis and Sexual Reproduction
Sexual reproduction is the most successful form of reproduction
within eukaryotes because:
A) in many animals, fragmentation is the sole mode of reproduction.
B) genetic diversity offers offspring a greater change of survival.
C) a sexually-reproducing population will leave less descendants with
decreased genetic diversity.
D) they are able to reproduce within an unpredictable environment
without the requirement of a partner.
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Meiosis and Sexual Reproduction
Which is true about the alternation of generations life cycle?
A) All of these answers are true.
B) It is utilized by fungi and some types of algae.
C) It is a combination of the haploid-dominant and diploid-dominant
cycles.
D) Specialized cells of the gametophyte will undergo meiosis and
produce haploid spores.
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Meiosis and Sexual Reproduction
Which is true about the alternation of generations life cycle?
A) All of these answers are true.
B) It is utilized by fungi and some types of algae.
C) It is a combination of the haploid-dominant and diploid-dominant
cycles.
D) Specialized cells of the gametophyte will undergo meiosis and
produce haploid spores.
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Meiosis and Sexual Reproduction
A diploid cell has_______ the number of chromosomes as a
haploid cell.
A) twice
B) one-forth
C) half
D) four times
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Meiosis and Sexual Reproduction
A diploid cell has_______ the number of chromosomes as a
haploid cell.
A) twice
B) one-forth
C) half
D) four times
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Meiosis and Sexual Reproduction
Attribution
• Connexions. "Introduction." CC BY 3.0 http://cnx.org/content/m44468/latest/?collection=col11448/latest
• Wiktionary. "diploid." CC BY-SA 3.0 http://en.wiktionary.org/wiki/diploid
• Wiktionary. "gamete." CC BY-SA 3.0 http://en.wiktionary.org/wiki/gamete
• Wiktionary. "haploid." CC BY-SA 3.0 http://en.wiktionary.org/wiki/haploid
• Connexions. "The Process of Meiosis." CC BY 3.0 http://cnx.org/content/m44469/latest/
• Wikipedia. "crossing over." CC BY-SA 3.0 http://en.wikipedia.org/wiki/crossing%20over
• Connexions. "The Process of Meiosis." CC BY 3.0 http://cnx.org/content/m44469/latest/?collection=col11448/latest
• Wiktionary. "chromatid." CC BY-SA 3.0 http://en.wiktionary.org/wiki/chromatid
• Wiktionary. "tetrad." CC BY-SA 3.0 http://en.wiktionary.org/wiki/tetrad
• Connexions. "The Process of Meiosis." CC BY 3.0 http://cnx.org/content/m44469/latest/?collection=col11448/latest
• Wikipedia. "meiosis II." CC BY-SA 3.0 http://en.wikipedia.org/wiki/meiosis%20II
• Connexions. "The Process of Meiosis." CC BY 3.0 http://cnx.org/content/m44469/latest/?collection=col11448/latest
• Boundless Learning. "Boundless." CC BY-SA 3.0 http://www.boundless.com//biology/definition/equatorial-division
• Wiktionary. "ploidy." CC BY-SA 3.0 http://en.wiktionary.org/wiki/ploidy
• Wiktionary. "reduction division." CC BY-SA 3.0 http://en.wiktionary.org/wiki/reduction+division
• Wikipedia. "sexual reproduction." CC BY-SA 3.0 http://en.wikipedia.org/wiki/sexual%20reproduction
• Connexions. "Reproduction Methods." CC BY 3.0 http://cnx.org/content/m44836/latest/
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• Connexions. "Sexual Reproduction." CC BY 3.0 http://cnx.org/content/m44470/latest/?collection=col11448/latest
• Connexions. "Sexual Reproduction." CC BY 3.0 http://cnx.org/content/m45465/latest/
• Wiktionary. "asexual reproduction." CC BY-SA 3.0 http://en.wiktionary.org/wiki/asexual+reproduction
• Connexions. "Sexual Reproduction." CC BY 3.0 http://cnx.org/content/m44470/latest/?collection=col11448/latest
• Wiktionary. "sporophyte." CC BY-SA 3.0 http://en.wiktionary.org/wiki/sporophyte
• Wiktionary. "gametophyte." CC BY-SA 3.0 http://en.wiktionary.org/wiki/gametophyte
• Wiktionary. "zygote." CC BY-SA 3.0 http://en.wiktionary.org/wiki/zygote
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