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Meiosis and Sexual Reproduction
Chapter 12
Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole,
Cengage Learning 2011.
Why Sex?
Why Sex?
 Sexual reproduction
• Reproductive mode by which offspring arise from two
parents and inherit genes from both
• ½ of each parent’s genetic information is passed to
offspring
• Variation in the forms and combinations of heritable
traits  diversity
• Increase chance of surviving in a changing environment
 Asexual reproduction
• All offspring are clones of their parents
• Adapt same way to an environment  equally vulnerable
to changes
Why Sex?
 An adaptive trait tends to spread more quickly
through a sexually reproducing population than
through a asexually reproducing one.
 Asexual reproduction
• New combinations of traits can arise only by
mutation and then the mutation is passed along
 Sexual reproduction
• Mixes up the genes of individuals that often have
different forms of traits
• It generates new combinations of traits in far
fewer generations than does mutation alone.
Genes and Alleles
 Genes
• Sequences of DNA that encode heritable traits
 Alleles
• Forms of a gene the encode slightly different
forms of the gene’s product
• Each specifies a different version of gene product
Sexual and Asexual Reproduction
 Asexual reproduction (1 parent)
• Offspring inherit parent’s genes
• Clones (identical copies of parent)
 Sexual reproduction (2 parents)
• Offspring differ from parents and each another
• Different combinations of alleles
• Different details of shared traits
Sexual Reproduction
 Meiosis, gamete formation, and fertilization
occur in sexual reproduction
 Meiosis and fertilization shuffle parental alleles
• Offspring inherit new combinations of alleles
Where Gametes Form
anther (where
sexual spores
that give rise to
sperm form)
ovules inside an
ovary (where sexual
spores that give
to eggs form)
Flowering plant
Fig. 9.3a, p.140
testis
(where sperm
originate)
Human male
ovary
(where eggs
develop)
Human female
Fig. 9.3b-c, p.140
Key Concepts:
SEXUAL VS. ASEXUAL REPRODUCTION
 By asexual reproduction, one parent alone
transmits its genetic information to offspring
 By sexual reproduction, offspring typically inherit
information from two parents that differ in their
alleles
 Alleles are different forms of the same gene;
they specify different versions of a trait
What Meiosis Does
 Meiosis  Basis of sexual reproduction
• Nuclear division mechanism
• Halves parental chromosome number
• Starts in germ cells (immature diploid reproductive cells)
AND leads to the formation of gametes
• Gametes  mature, haploid reproductive cells
(egg/sperm)
 Fertilization
• Fusion of two gamete nuclei
• Restores parental chromosome number
• Forms zygote (first cell of new individual)
Meiosis and Fertilization
Homologues
 Sexual reproducers inherit pairs of chromosomes
• 1 from maternal parent, 1 from paternal parent
 The pairs are homologous (“the same”)
• Except nonidentical sex chromosomes (X and Y)
• Same length, shape, genes
 All pairs interact at meiosis
• One chromosome of each type sorts into gametes
Homologous Chromosomes
The Process of Meiosis
 All chromosomes are duplicated during
interphase, before meiosis
 Two divisions, meiosis I and II, divide the
parental chromosome number by two
 Each forthcoming gamete is haploid (n)
• Haploid  having one of each type of
chromosome
Meiosis I
 The first nuclear division
 Each duplicated chromosome lines up with its
homologous partner
 The two homologous chromosomes move apart,
toward opposite spindle poles
Prophase I
 Chromosomes condense and align tightly with their
homologues
 Each homologous pair undergoes crossing over
 Microtubules form the bipolar spindle
 One pair of centrioles moves to the other side of
the nucleus
 Nuclear envelope breaks up
• Microtubules growing from each spindle pole
penetrate the nuclear region
 Microtubules tether one or the other chromosome
of each homologous pair
Prophase I
Metaphase I
 Microtubules from both poles position all pairs of
homologous chromosomes at the spindle
equator
Metaphase I
Anaphase I
 Microtubules separate each chromosome from
its homologue, moving to opposite spindle poles
 Other microtubules overlap midway between
spindle poles, slide past each other to push
poles farther apart
 As anaphase I ends, one set of duplicated
chromosomes nears each spindle pole
Anaphase I
Telophase I
 Two nuclei form
• Typically, the cytoplasm divides
 All chromosomes are still duplicated
• Each still consists of two sister chromatids
Telophase I
Meiosis II
 The second nuclear division
 Sister chromatids of each chromosome are
pulled away from each other
 Each is now an individual chromosome
Prophase II
Metaphase II
Anaphase II and Telophase II
 In anaphase II, one chromosome of each type is
moved toward opposite spindle poles
• Occurs in both nuclei formed in meiosis I
 By the end of telophase II, there are four haploid
nuclei, each with unduplicated chromosomes
Anaphase II
Telophase II
Meiosis I
one pair of
homologous
chromosomes
newly forming
microtubules of
the spindle
spindle equator
(midway between the
two poles)
plasma
membrane
breakup
of nuclear
envelope
centrosome with
a pair of centrioles,
moving to opposite
sides of nucleus
Prophase I
Chromosomes
were duplicated
earlier, in
interphase.
Metaphase I
Prior to metaphase I, one
set of microtubules had
tethered one chromosome
of each type to one spindle
pole and another set tethered
its homologue to the other
spindle pole.
Anaphase I
One of each duplicated
chromosome, maternal
or paternal, moves to a
spindle pole; its homologue
moves to the opposite pole.
Telophase I
One of each type
of chromosome has
arrived at a spindle
pole. In most species,
the cytoplasm divides
at this time.
Fig. 9.5a, p.142
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
Stepped Art
Fig. 9-5a, p.142
Meiosis II
there is
no DNA
replication
between
the two
divisions
Prophase II
In each cell, one of two
centrioles moves to the
opposite side of the
cell, and a new bipolar
spindle forms.
Metaphase II
By now,
microtubules from
both spindle poles
have finished a tugof-war.
Anaphase II
The sister chromatids
of each chromosome
move apart and are
now individual,
unduplicated
Telophase II
A new nuclear envelope
encloses each parcel of
chromosomes, so there
are now four nuclei.
Fig. 9.5b, p.142
Haploid Daughter Cells
 When cytoplasm divides, four haploid cells result
 One or all may serve as gametes or, in plants,
as spores that lead to gamete-producing bodies
Key Concepts:
STAGES OF MEIOSIS
 Diploid cells have a pair of each type of
chromosome, one maternal and one paternal
 Meiosis, a nuclear division mechanism, reduces
the chromosome number
 Meiosis occurs only in cells set aside for sexual
reproduction
 Meiosis sorts out a reproductive cell’s
chromosomes into four haploid nuclei
 Haploid nuclei are distributed to daughter cells
by way of cytoplasmic division
Meiosis Introduces Variation in Traits
 Two events in meiosis cause variation in traits in
sexually reproducing species
• Crossing over during prophase I of meiosis
• Chromosome shuffling during metaphase I of
meiosis
Prophase I: Crossing Over
 Nonsister chromatids of homologous
chromosomes undergo crossing over
• They exchange corresponding segments during
prophase I of meiosis
 Each ends up with new combinations of alleles
not present in either parental chromosome
Crossing Over
Fig. 9.6d, p.144
Metaphase I: Chromosome Shuffling
 Homologous chromosomes align randomly
during metaphase I
 Microtubules can harness either a maternal or
paternal chromosome of each homologous pair
to either spindle pole
 Either chromosome may end up in any new
nucleus (gamete)
Chromosome Shuffling:
Random Alignment
1
2
3
combinations possible
or
or
or
Stepped Art
Fig. 9-7, p.145
Key Concepts: CHROMOSOME
RECOMBINATION AND SHUFFLING
 During meiosis, each pair of maternal and
paternal chromosomes swaps segments and
exchanges alleles
 Pairs get randomly shuffled, so forthcoming
gametes end up with different mixes of maternal
and paternal chromosomes
From Gametes to Offspring
 Multicelled diploid and haploid bodies are typical
in life cycles of plants and animals
 Plants
• Sporophyte: A multicelled plant body (diploid) that
makes haploid spores
• Spores give rise to gametophytes  a haploid,
multicelled plant body in which gametes form
during the life cycle of plants
From Gametes to Offspring
 Animals
• Germ cells in the reproductive organs give rise to
sperm or eggs
• Sperm  mature male gamete
• Egg  mature female gamete, or ovum
• Fusion of a sperm and egg at fertilization results
in a zygote
• Zygote  first cell of a new individual
Comparing Plant And Animal Life Cycles
meiosis
zygote
(2n)
fertilization
multicelled
sporophyte
(2n)
DIPLOID
meiosis
HAPLOID
gametes
(n)
meiosis
multicelled
gametophyte
(n)
spores
(n)
meiosis
a Plant life cycle
Fig. 9.8a, p.146
meiosis
zygote
(2n)
fertilization
multicelled
body
(2n)
DIPLOID
meiosis
HAPLOID
gametes
(n)
b Animal life cycle
Fig. 9.8b, p.146
Introducing Variation in Offspring
 Three events cause new combinations of alleles
in offspring:
• Crossing over during prophase I (meiosis)
• Random alignment of maternal and paternal
chromosomes at metaphase I (meiosis)
• Chance meeting of gametes at fertilization
 All three contribute to variation in traits
Sperm Formation in Animals
Egg Formation in Animals
Key Concepts: SEXUAL
REPRODUCTION IN LIFE CYCLES
 In animals, gametes form by different
mechanisms in males and females
 In most plants, spore formation and other events
intervene between meiosis and gamete
formation
Comparing Mitosis and Meiosis
 Both mitosis and meiosis require spindles to
move and sort duplicated chromosomes
 Some mechanisms of meiosis resemble those of
mitosis, and may have evolved from them
• Example: DNA repair enzymes function in both
Differences in Mitosis and Meiosis
 Mitosis maintains parental chromosome number
• Duplicates genetic information
• Occurs in body cells
 Meiosis halves chromosome number
• Introduces new combinations of alleles in
offspring
• Occurs only in cells for sexual reproduction
Comparing Mitosis and Meiosis
Comparing Mitosis and Meiosis
Comparing Mitosis and Meiosis
Key Concepts:
MITOSIS AND MEIOSIS COMPARED
 Recent molecular evidence suggests that
meiosis originated through mechanisms that
already existed for mitosis and, before that, for
repairing damaged DNA
Animation: Comparing mitosis and
meiosis
Animation: Crossing over
Animation: Egg formation
Animation: Generalized life cycles
Animation: Meiosis I and II
Animation: Meiosis step-by-step
Animation: Random alignment
Animation: Sperm formation