Meiosis
Chapter 13
Octopus Sex
• Male reaches under his mantle with
tentacle, removes packet of sperm, and
insert it into female’s egg chamber
• Eggs are fertilized and give rise to new
octopuses
Limpet Sex
• Larva can become adult of either sex
– Depends on whether other limpets are
present and what their sex is
• Adults can change sex in response to
new arrivals
Aphid Sex
• Females are produced from unfertilized
eggs in summer
• Males are produced as autumn
approaches
• Females produced by sexual
reproduction can overwinter and begin
producing new females in spring
Sexual Reproduction
• Chromosomes are duplicated in germ
cells
• Germ cells undergo meiosis and
cytoplasmic division
• Cellular descendents of germ cells
become gametes
• Gametes meet at fertilization
Asexual Reproduction
• Single parent produces offspring
• All offspring are genetically identical to
one another and to parent
Sexual Reproduction
• Involves
– Meiosis
– Gamete production
– Fertilization
• Produces genetic variation among
offspring
Homologous Chromosomes
Carry Different Alleles
• Cell has two of each chromosome
• One chromosome in each pair from
mother, other from father
• Paternal and maternal chromosomes
carry different alleles
Sexual Reproduction
Shuffles Alleles
• Through sexual reproduction, offspring
inherit new combinations of alleles,
which leads to variations in traits
• This variation in traits is the basis for
evolutionary change
Gamete Formation
• Gametes are sex cells (sperm, eggs)
• Arise from germ cells
ovaries
testes
anther
ovary
Chromosome Number
• Sum total of chromosomes in a cell
• Germ cells are diploid (2n)
• Gametes are haploid (n)
• Meiosis halves chromosome number
Meiosis: Two Divisions
• Two consecutive nuclear divisions
– Meiosis I
– Meiosis II
• DNA is NOT duplicated between
divisions
• Four haploid nuclei are formed
Meiosis I
Each homologue in the
cell pairs with its partner,
then the partners
separate
Meiosis II
• The two sister chromatids of each
duplicated chromosome are separated
from each other
two chromosomes
(unduplicated)
one chromosome
(duplicated)
Stages of Meiosis
Meiosis I
Meiosis II
• Prophase I
• Prophase II
• Metaphase I
• Metaphase II
• Anaphase I
• Anaphase II
• Telophase I
• Telophase II
Meiosis I - Stages
Prophase I
Metaphase I Anaphase I
Telophase I
Prophase I
• Each duplicated, condensed
chromosome pairs with its homologue
• Homologues swap segments; called
crossing over
• Each chromosome becomes attached
to microtubules of newly forming spindle
Metaphase I
• Chromosomes are pushed and pulled
into the middle of cell
• Sister chromatids of one homologue
orient toward one pole, and those of
other homologue toward opposite pole
• The spindle is now fully formed
Anaphase I
• Homologous chromosomes segregate
from each other
• The sister chromatids of each
chromosome remain attached
Telophase I
• The chromosomes arrive at opposite
poles
• The cytoplasm divides
• There are now two haploid cells
• This completes Meiosis I
Meiosis II - Stages
Prophase II
Metaphase II Anaphase II Telophase II
Prophase II
• Microtubules attach to the kinetochores
of the duplicated chromosomes
• Motor proteins drive the movement of
chromosomes toward the spindle’s
equator
Metaphase II
• All of the duplicated chromosomes are
lined up at the spindle equator, midway
between the poles
Anaphase II
• Sister chromatids separate to become
independent chromosomes
• Motor proteins interact with
microtubules to move the separated
chromosomes to opposite poles
Telophase II
• The chromosomes arrive at opposite
ends of the cell
• A nuclear envelope forms around each
set of chromosomes
• The cytoplasm divides
• There are now four haploid cells
Crossing Over
•Each chromosome
becomes zippered to its
homologue
•All four chromatids are
closely aligned
•Non-sister chromosomes
exchange segments
Effect of Crossing Over
• After crossing over, each chromosome
contains both maternal and parental
segments
• Creates new allele combinations in
offspring
Random Alignment
• During transition between prophase I
and metaphase I, microtubules from
spindle poles attach to kinetochores of
chromosomes
• Initial contacts between microtubules
and chromosomes are random
Random Alignment
• Either the maternal or paternal member
of a homologous pair can end up at
either pole
• The chromosomes in a gamete are a
mix of chromosomes from the two
parents
Possible Chromosome
Combinations
As a result of random alignment, the
number of possible combinations of
chromosomes in a gamete is:
2n
(n is number of chromosome types)
1
Possible
Chromosome
Combinations
or
or
or
2
3
Plant Life Cycle
mitosis
multicelled
sporophyte
zygote
fertilization
Diploid
meiosis
Haploid
spores
gametes
multicelled
gametophytes
mitosis
Animal Life Cycle
mitosis
multicelled
body
zygote
fertilization
Diploid
Haploid
gametes
meiosis
Spermatogenesis
secondary
spermatocytes
(haploid)
spermatogonium
(diploid male
reproductive
cell)
primary
spermatocyte
(diploid)
spermatids (haploid)
Growth
Mitosis I,
Cytoplasmic division
Meiosis II,
Cytoplasmic division
Oogenesis
first polar
body
haploid)
oogonium
(diploid
reproductive
cell)
Growth
three polar
bodies
haploid)
primary oocyte
(diploid)
secondary
oocyte
haploid)
Mitosis I,
Cytoplasmic division
ovum
(haploid)
Meiosis II,
Cytoplasmic division
Fertilization
• Male and female gametes unite and
nuclei fuse
• Fusion of two haploid nuclei produces
diploid nucleus in the zygote
• Which two gametes unite is random
– Adds to variation among offspring
Factors Contributing to
Variation Among Offspring
• Crossing over during prophase I
• Random alignment of
chromosomes at metaphase I
• Random combination of gametes at
fertilization
Mitosis & Meiosis Compared
Mitosis
• Functions
– Asexual reproduction
– Growth, repair
• Occurs in somatic
cells
• Produces clones
Meiosis
• Function
– Sexual reproduction
• Occurs in germ cells
• Produces variable
offspring
Prophase vs. Prophase I
• Prophase (Mitosis)
– Homologous pairs do not interact with each
other
• Prophase I (Meiosis)
– Homologous pairs become zippered
together and crossing over occurs
Anaphase, Anaphase I, and
Anaphase II
• Anaphase I (Meiosis)
– Homologous chromosomes are separated
from each other
• Anaphase/Anaphase II (Mitosis/Meiosis)
– Sister chromatids of a chromosome are
separated from each other
Results of Mitosis and Meiosis
• Mitosis
– Two diploid cells produced
– Each identical to parent
• Meiosis
– Four haploid cells produced
– Differ from parent and one another
Film of Meiosis