Chapter 13
Meiosis and
Sexual Life
Cycles
Chromosomes
Chromosomes
Chromosomes
Chromosomes
Karyotype
Chromosomes
Nucleosome
Chromosome
DNA
double
helix
Coils
Supercoils
Histones
Chromosomes
• Genes are units of hereditary information that
are passed down from parent to offspring.
• The tens of thousands of genes (alleles) you
inherit from you mother and father make up your
genome.
• A gene’s specific location
on a chromosome is
called the gene’s locus.
Asexual Life Cycles
• Asexual reproduction – A single parent
passes copies of all its genes to its offspring.
• Examples in animals: hydra (budding),
starfish (regeneration), Daphnia
(parthenogenesis)
Asexual Life Cycles
• Asexual reproduction
• Examples in plants: underground stem
(tubers and bulbs), leaves (form tiny plantlets
that fall off and root), roots (aspen send up
shoots from their roots – largest organism on
Earth)
Sexual Life Cycles
• Sexual reproduction – two parents give rise
to offspring that have unique combinations
of genes (alleles) inherited from the two
parents
• Results in greater variation because there is
a combination of genes (alleles).
• Can increase adaptation and success
because some genes (alleles) can be
“better” than others.
Can You Match the Offspring With
Their Siblings and Parents?
The Human Life Cycle
• Somatic cells – Any cell that is not sperm or
ovum. All humans have 46 chromosomes in
each somatic cell.
• Homologous chromosomes (homologues) –
Chromosomes that make up a pair; have the
same length, centromere position, and staining
pattern.
• Sex chromosomes – X and Y chromosomes
• Autosomes – all other chromosomes
• Our 46 chromosomes are actually two sets of
23 chromosomes – one set from mom, one set
from dad
The Human Life Cycle
• Gametes – sex cells or reproductive cells;
have 22 autosomes and one sex
chromosomes
• Haploid cell – a cell with a single chromosome
set (sperm and egg). For humans, the haploid
number is 23 (n = 23)
• Fertilization – the fusion of egg and sperm
• Zygote – the fertilized egg; contains two sets of
chromosomes.
• Diploid cell – a cell with two sets of
chromosomes. For humans, the diploid
number is 46 (2n = 46)
The Human Life Cycle
• Gametes (sperm and egg) are the only
cells not produced by mitosis.
• Instead, gametes are formed by
meiosis.
• In meiosis, the number of chromosomes
is halved so each new gamete receives
half the original number of
chromosomes.
• In humans, 2n = 46, n = 23
The Human Life Cycle
A Variety of Sexual Life Cycle
• In most animals,
meiosis only occurs
during the production
of gametes.
• After fertilization, the
diploid zygote divides
by mitosis producing a
multicellular, diploid
organism.
A Variety of Sexual Life Cycle
• In most fungi and some
protists, gametes fuse
to form a diploid zygote
and then meiosis
occurs to form a
multicellular, adult
haploid organism.
• The haploid organism
then produces gametes
by mitosis to form a
diploid zygote.
A Variety of Sexual Life Cycle
• Plants and some algae
exhibit alternation of
generation.
• The multicellular, diploid
stage is the sporophyte –
produces haploid cells
called spores.
• The spore divides
mitotically to produce the
multicellular stage called
gametophyte – makes
gametes by mitosis which
fuse to produce the zygote.
Sexual Life Cycles of Plants
Gametophyte (N)
Sporophyte (2N)
Bryophytes
Ferns
Seed plants
Meiosis
• Meiosis reduces chromosome number from
diploid to haploid.
• Occurs in two divisions called meiosis I and
meiosis II.
• Divisions result in 4 daughter cells, each with
half the number of chromosomes as the
parent.
• Homologous chromosomes – individual
chromosomes that were inherited from
different parents.
Chromosomes
Meiosis
Meiosis I
Meiosis II
Meiosis – Interphase
• Chromosomes replicate
• Sister chromatids remain
attached at the
centromere
• Centrosomes replicate
Meiosis – Prophase I
• Chromosomes begin to
condense
• Homologous chromosomes pair
up forming a tetrad
• Chiasmata hold homologous
pairs together – crossing over
usually occurs
• Centrosomes move to opposite
poles
• Spindle microtubules begin to
form and attach to the
centromere of the chromosomes
• Nuclear membrane and nucleoli
disappear
Meiosis – Metaphase I
• Chromosomes are arranged
on the metaphase plate, still
in homologous pairs
• Kinetochore microtubules
from one pole of the cell are
attached to one
chromosome of each pair,
while microtubules from the
opposite pole are attached
to the homologue.
Meiosis – Anaphase I
• Sister chromatids remain
attached at their centromere
and move as a single unit
toward the same pole
• The homologous
chromosome moves
towards the opposite pole.
Meiosis – Telophase I & Cytokinesis
• Chromosomes continue to move to
opposite poles
• Each pole now has a haploid set of
chromosomes
• Each chromosome still has sister
chromatids attached at the
centromere
• During cytokinesis, a cleavage
furrow occurs in animal cells and a
cell plate occurs in plant cells.
• No new genetic material is made
before the beginning of meiosis II.
Meiosis – Prophase II
• A spindle apparatus
forms
• The chromosomes move
towards the metaphase II
plate.
Meiosis – Metaphase II
• Chromosomes are
positioned on the
metaphase plate with the
kinetochore of sister
chromatids of each
chromosome pointing
toward opposite poles.
Meiosis – Anaphase II
• The centromere of sister
chromatids finally
separate
• Sister chromatids of each
pair, now individual
chromosomes, move
toward opposite poles of
the cell.
Meiosis – Telophase II and Cytokinesis
• Nuclei form at opposite
poles of the cell and
cytokinesis occurs.
• At the completion of
cytokinesis, there are four
haploid daughter cells
• Each new cell is NOT
identical.
Videos
Comparing Mitosis and Meiosis
Comparing Mitosis and Meiosis
Origins of Genetic Variation
Independent Assortment of
Chromosomes
• Each gamete that a human produces
23
contains one of 8 million (2 )possible
assortments of chromosomes inherited
from that individual’s parents.
Independent Assortment of Chromosomes
Origins of Genetic Variation
Crossing Over
• Crossing over produces recombinant
chromosomes which combine genes
inherited from both parents.
• Occurs early in prophase I
• Homologous portions of two non-sister
chromatids trade places
Crossing Over
Crossing Over
Video
Origins of Genetic Variation
Random Fertilization
• A human egg cell (ovum) represents about
8 million possible chromosome
combinations.
• A human sperm cell represents about 8
million possible chromosome
combinations.
• Any two parents will produce a zygote with
about 64 trillion different combinations
(that’s not even counting the variation from
crossing over!)
Origins of Genetic Variation
• Sexual life cycles produce genetic
variation among offspring
• Evolutionary adaptation depends on a
population’s genetic variation
• Consider the story of the Daphnia…