Meiosis and Sexual Reproduction
Chapter 9
9.1 Genes and Alleles
Genes
• Sequences of DNA that encode heritable traits
Alleles
• Slightly different forms of the same gene
• 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
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
9.2 What Meiosis Does
Meiosis
• Nuclear division mechanism that precedes
gamete formation in eukaryotic cells
• Halves parental chromosome number
Fertilization
• Fusion of two gamete nuclei
• Restores parental chromosome number
• Forms zygote (first cell of new individual)
Fig. 9.12, p.150
gametes gametes
germ cell germ cell
each chromosome
duplicated during
interphase
MEIOSIS II
separation of
sister chromatids
MEIOSIS I
separation of
homologues
diploid number
restored at
fertilization zygote
2n
2n
n
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
9.3 Tour 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)
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
Prophase I (cont.)
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
Metaphase I
Microtubules from both poles position all pairs of
homologous chromosomes at the spindle
equator
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
Telophase I
Two nuclei form
• Typically, the cytoplasm
divides
All chromosomes are still
duplicated
• Each still consists of
two sister chromatids
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
plasma membrane
spindle equator (midway between the two poles)
one pair of homologous chromosomes
Prophase I Metaphase I Anaphase I Telophase I
Meiosis I
Fig. 9.5a, p.142
newly forming microtubules of the spindle
breakup of nuclear envelope
centrosome with a pair of centrioles, moving to opposite sides of nucleus
Chromosomes were duplicated earlier, in interphase.
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.
One of each duplicated chromosome, maternal or paternal, moves to a spindle pole; its homologue moves to the opposite pole.
One of each type of chromosome has arrived at a spindle pole. In most species, the cytoplasm divides at this time.
Fig. 9.5b, p.142
Prophase II Metaphase II Anaphase II Telophase II
Meiosis II
there is
no DNA
replication
between
the two
divisions
Fig. 9.5b, p.142
In each cell, one of two
centrioles moves to the
opposite side of the
cell, and a new bipolar
spindle forms.
By now,
microtubules from
both spindle poles
have finished a tug-
of-war.
The sister chromatids
of each chromosome
move apart and are
now individual,
unduplicated
A new nuclear envelope
encloses each parcel of
chromosomes, so there
are now four nuclei.
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
Key Concepts:
STAGES OF MEIOSIS (cont.)
Meiosis sorts out a reproductive cell’s
chromosomes into four haploid nuclei
Haploid nuclei are distributed to daughter cells
by way of cytoplasmic division
9.4 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 segments at the same place
along their length
Each ends up with new combinations of alleles
not present in either parental chromosome
Crossing Over
Fig. 9.6, p.144
a A maternal chromosome (purple)
and paternal chromosome (blue)
were duplicated earlier, during
interphase. They become visible in
microscopes early in prophase I,
when hey star to condense to
threadlike form.
b Each chromosome
and its homologous
partner zipper
together, so all four
chromatids
are tightly aligned.
mom’s
allele
B
mom’s
allele
A
mom’s
allele
A
mom’s
allele
A
mom’s
allele
B
dad’s
allele
a
dad’s
allele
b
dad’s
allele
b
c Here is a simple way to
think about crossing over.
(Chromosomes are still c
ondensed and threadlike,
and each is tightly aligned
with its homologous
partner.)
d Their intimate contact
promotes crossing over
at different places along
the length of nonsister
chromatids.
e At the crossover
site, paternal and
maternal chromatids
exchange
corresponding
segments.
f Crossing over mixes
up maternal and paternal
alleles on homologous
chromosomes.
Fig. 9.6a, p.144
Fig. 9.6b, p.144
Fig. 9.6c, p.144
Fig. 9.6d, p.144
Fig. 9.6e, p.144
Fig. 9.6f, 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
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
9.5 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 (multicelled
plant bodies in which haploid gametes form)
From Gametes to Offspring
Animals
• Germ cells in the reproductive organs give rise to
sperm or eggs
• Fusion of a sperm and egg at fertilization results
in a zygote
Comparing Plant And Animal Life Cycles
meiosis DIPLOID
fertilization
zygote
gametes spores
HAPLOID
Fig. 9.8a, p.146
meiosis
meiosis
meiosis
a Plant life cycle
(2n)
(2n)
(n)
multicelled gametophyte
multicelled sporophyte
(n) (n)
Fig. 9.8b, p.146
meiosis DIPLOID fertilization
zygote
HAPLOID
meiosis
(2n)
(2n)
(n) gametes
multicelled body
b Animal life cycle
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
9.6 Comparing Mitosis and Meiosis
Both mitosis and meiosis require bipolar spindle
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