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LECTURE PRESENTATIONS
For CAMPBELL BIOLOGY, NINTH EDITION
Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson
Chapter 13
Meiosis and Sexual
Life Cycles
Lectures by
Erin Barley
Kathleen Fitzpatrick
© 2011 Pearson Education, Inc.
Overview: Variations on a Theme
• living organisms are distinguished by their ability to
reproduce their own kind
• Genetics =the scientific study of heredity and
variation
• Heredity = the transmission of traits from one
generation to the next
• Variation = demonstrated by the differences in
appearance that offspring show from parents and
siblings
Inheritance of Genes
• in a literal sense, children do not inherit particular
physical traits from their parents
– it is genes that are actually inherited
• Genes are the units of heredity
– made up of segments of DNA
– passed to the next generation via reproductive cells
called gametes (sperm and eggs)
– somatic cells – any other cell other than the gamete or
its precursor
• each gene has a specific location called a locus on a
certain chromosome
Comparison of Asexual and Sexual Reproduction
• asexual reproduction - a single individual passes genes to its
offspring without the fusion of gametes
– produces an exact copy of the parent – a clone
– done through fission or mitosis – following duplication of DNA
• clone = a group of genetically identical individuals from the same
parent
• sexual reproduction - two parents give rise to offspring that have
unique combinations of genes inherited from the two parents
Sets of Chromosomes in Human Cells
• a life cycle = generation-togeneration sequence of stages in the
reproductive history of an organism
• a karyotype is an ordered display of
the pairs of chromosomes from a cell
APPLICATION
– human somatic cells have 23 pairs of
chromosomes
• the two chromosomes in each pair
are called homologous
chromosomes, or homologs
– chromosomes in a homologous pair
have the same length, the same
centromere position
– also carry genes at the same loci
controlling the same inherited
characters
TECHNIQUE
Pair of homologous
duplicated chromosomes
Centromere
Sister
chromatids
Metaphase
chromosome
5 m
Some Definitions You Know Already
• sex chromosomes = X and Y
– human females have a
homologous pair of X
chromosomes (XX)
– human males have one X and one
Y chromosome
• gamete (sperm or egg) contains a
single set of chromosomes  haploid
(n)
– humans, the haploid number is 23
(n = 23)
– in an unfertilized egg (ovum), the
sex chromosome is X
– in a sperm cell, the sex
chromosome may be either X or Y
Some Definitions You Know Already
• remaining 22 pairs of chromosomes
are called autosomes
• each pair of homologous
chromosomes includes one
chromosome from each parent
• a diploid cell (2n) has two sets of
chromosomes
– humans, the diploid number is 46
(2n = 46)
– dogs, the diploid number is 72!
(2n = 72)
– Drosophila – 2n = 4
• in a cell in which DNA synthesis has occurred - each
chromosome has been replicated
• each duplicated chromosome consists of two identical
sister chromatids joined at a centromere
Key
2n  6
Maternal set of
chromosomes (n  3)
Paternal set of
chromosomes (n  3)
Sister chromatids
of one duplicated
chromosome
Two nonsister
chromatids in
a homologous pair
Centromere
Pair of homologous
chromosomes
(one from each set)
Let’s Remind Ourselves Shall We?
• chromosome = organized structure of DNA,
protein and RNA found in the nucleus or
nucleoid region
– single piece of coiled DNA containing
genes, regulatory elements and other
nucleotide sequences
– associated with DNA binding proteins
for the packaging of the DNA and
control of gene expression
– true definition – DNA organized into
chromatin
• two forms in interphase – euchromatin
(active form) and heterochromatin (inactive
form)
– replicated DNA condenses during the
early stages of mitosis and meiosis to
form two sister chromatids joined at a
centromere
1. chromatid
2. centromere
3. p arm (short arm)
4. q arm (long arm)
Let’s Remind Ourselves Shall We?
• chromosome = organized structure of DNA,
protein and RNA found in the nucleus or
nucleoid region
– circular (prokaryotes) or linear
(eukaryotes)
• prokaryotic “chromosome” known as a
genophore – not organized as chromatin
– genes organized into operons – no introns
• smaller, circular genophores = plasmids
• circular “chromosomes” found in eukaryotic
mitochondria and chloroplasts
Centromeres
•
centromere = point where the two sister chromatids join
– physical role – act as the site of assembly for the kinetochore
– indirect role – control the separation of chromatids
• kinetochore – complex group of proteins that attaches the centromere to the
spindle microtubules and is responsible for chromatid separation (signals that all
chromosomes are attached, aligned and are ready for separation)
• two kinetochore regions
– inner plate: associates with the centromere DNA sequences
» modified histone proteins that interact with DNA
– outer plate: associates with the microtubules
** the sister chromatids of a
chromosome are linked all along their
length by cohesin proteins
-cutting of cohesins happens during
prophase until metaphase – only point
of connection is the cohesins in the
centromere region
Centromeres
• centromere = point where the two sister
chromatids join
– two types: Point and Regional
• Point: smaller and more compact (e.g. yeasts)
– DNA sequence is necessary for the formation of the
centromere
– bind to specific kinetochore proteins
• Regional: most centromeres (e.g. humans)
– DNA sequence contributes to formation of the
centromere
– not an actual defined sequence of DNA but is an
array of repetitive sequences of “satellite DNA” that
are similar to one another
– DNA of the centromere is heterochromatin form
– heterochromatin form is critical for the adherence
of cohesin proteins in that region
– end up with a high concentration of cohesins in the
centromere region
1. chromatid
2. centromere
3. p arm (short arm)
4. q arm (long arm)
Centromeres
• so what makes up a centromere?
– 1. DNA regions – two strands of DNA interact through base
pairing
• DNA is heterochromatin
– 2. cohesion proteins – high concentration
• interacts with the DNA
– 3. kinetochore – inner and outer plates
• for attachment to the microtubules and chromatin separation
Centromeres
chromosome 1
• centromere positions
– Metacentric: typical X shaped
chromosome
• two arms (p and q) are equal in length
• humans – chr 1 & 3
– Submetacentric: arms are unequal
• p is shorter
– Acrocentric: p arm is so short it is hard to
observe
• humans: chr 13,14,15,21,22 and Y
– Telocentric: centromere is located at the
end of the chromosome
• none in humans
– Holocentric: the entire length of the
chromosome acts as the centromere
• plants and many invertebrates (nematodes)
chromosome 13
Centromeres
• the centromere position in humans is inherited
– daughter chromosomes will assemble their centromeres in
the same positions as the parent chromosome
• epigenetic – nothing to do with DNA sequence
– has to do with interactions with histones
• role of H3 histone ? – H3 in the histones that interact
with the centromere region is replaced with a related
protein called Centromere Protein A/CENP-A (humans)
Centromeres – How’s this for cool?
•
•
the tension receptors in the interzone sense the attachment of the kinetochore to
microtubules
cdc20 promotes the activation of the Anaphase Promoting Complex (APC) – a
group of proteins that initiates anaphase & triggers the separation of chromatids
during anaphase
It gets even cooler! Yes it does!
•
cdc20 promotes the activation of the
Anaphase Promoting Complex (APC) – a
group of proteins that initiates anaphase
& triggers the separation of chromatids
during anaphase
– APC is a ubiquitin ligase – promotes
the attachment of ubiquitin to
protein targets  degradation
– ubiquitin is attached to S and M
cyclins – degradation results in
transition from metaphase to
anaphase
– ubiquitin is also attached to a protein
complex made of proteins called
securin & separase
APC
ubiquination
ubiquitin
Securin
S and M cyclins
Separase
Degradation
of Securin
Separase release
Degradation
Separase
• ubiquitin dependent degradation of
securin “releases” separase from the
complex
• separase now targets the cohesin
protein complexes that link
chromatids
Cohesin degradation
Metaphase  Anaphase
Transition
Sister chromatid release
•
once separated - dynein motor proteins in the outer plate “walk” the separated
chromatids along the microtubule to the opposite centrioles of the cell
Separase
Separase
Separase
GO HAVE LUNCH!
PM Lecture
MEIOSIS
Behavior of Chromosome Sets in the
Haploid gametes (n  23)
Key
Human Life Cycle
Haploid (n)
Egg (n)
Diploid (2n)
• Fertilization is the union of gametes the sperm and the egg
• the gametes have one set of
Sperm (n)
chromosome - haploid
MEIOSIS
FERTILIZATION
– produced via meiosis from a diploid
germ cell
– gametes are the only types of
human cells produced by meiosis Ovary
Testis
rather than mitosis
Diploid
– has one set of chromosomes from
zygote
each parent
(2n  46)
• the zygote produces somatic cells by
Mitosis and
mitosis and develops into an adult
development
• fertilization and meiosis alternate in
sexual life cycles to maintain
Multicellular diploid
chromosome number
adults (2n  46)
The Variety of Sexual Life Cycles
• the alternation of meiosis and fertilization is common to
all organisms that reproduce sexually
• the three main types of sexual life cycles differ in the
timing of meiosis and fertilization
Key
Haploid (n)
Diploid (2n)
n
Gametes
n
Mitosis
n
n
MEIOSIS
Diploid
multicellular
organism
(a) Animals
Mitosis
Mitosis
n
Spores
Gametes
FERTILIZATION
n
Diploid
multicellular
organism
(sporophyte)
2n
n
n
Gametes
FERTILIZATION
2n Zygote
Mitosis
(b) Plants and some algae
Mitosis
n
n
FERTILIZATION
MEIOSIS
Zygote 2n
Mitosis
n
n
n
MEIOSIS
2n
Haploid unicellular or
multicellular organism
Haploid multicellular organism
(gametophyte)
2n
Zygote
(c) Most fungi and some protists
Animals
• gametes are the only haploid
cells in animals
– produced by meiosis
– undergo no further cell
division before fertilization
– fuse to form a diploid zygote
that divides by mitosis to
develop into a multicellular
organism
Key
Haploid (n)
Diploid (2n)
n
Gametes
n
n
MEIOSIS
2n
Diploid
multicellular
organism
(a) Animals
FERTILIZATION
Zygote 2n
Mitosis
Plants
Key
Haploid (n)
•
plants and some algae exhibit an
alternation of generations
– their life cycle includes both diploid
and haploid multicellular stages
– diploid organism - called the
sporophyte - makes haploid spores
by meiosis
– the spore grows (via mitosis) into a
haploid organism called a
gametophyte
• gametophyte bears the reproductive
parts of the organism OR is the
reproductive part
– gametophyte makes haploid gametes
by mitosis
– fertilization of gametes produces a
new diploid sporophyte
Diploid (2n)
Haploid multicellular organism
(gametophyte)
Mitosis
n
n
Mitosis
n
n
n
Spores
Gametes
MEIOSIS
2n
Diploid
multicellular
organism
(sporophyte)
FERTILIZATION
2n
Mitosis
(b) Plants and some algae
Zygote
Fungi
Key
Haploid (n)
Diploid (2n)
• in most fungi and some protists,
the only diploid stage is the
single-celled zygote
• there is no multicellular diploid
stage
• the zygote produces haploid
spores by meiosis
• each haploid cell grows by mitosis
into a haploid multicellular
organism - fungus
• the haploid adult produces
gametes by mitosis
Haploid unicellular or
multicellular organism
Mitosis
n
Mitosis
n
n
n
Gametes
MEIOSIS
n
FERTILIZATION
2n
Zygote
(c) Most fungi and some protists
• depending on the type of life cycle, either haploid or diploid cells can
divide by mitosis
– in animals – diploid cells undergo mitosis so that the organism
grows into a diploid organism
• specialized diploid germ cells undergo meiosis to produce gametes
• gamete fusion produces a new diploid organism
– in plants & fungus – haploid spores undergo mitosis so the
organism grows into a haploid organism (e.g. gametophyte)
• haploid organism produces gametes via mitosis
• gamete fusion produces a diploid organism that produces haploid spores
via meiosis (e.g. sporophyte)
• only diploid cells can undergo meiosis
• in all three life cycles, the halving and doubling of chromosomes
contributes to genetic variation in offspring
Meiosis reduces the number of chromosome
sets from diploid to haploid
• meiosis is preceded by the
replication of chromosomes
– just like mitosis
• meiosis takes place in two sets
of cell divisions, called meiosis I
and meiosis II
• the two half as many
chromosomes as the parent cell
• cell divisions result in four
daughter cells, each with
Meiosis - sperm
The Stages of Meiosis
• after chromosomes duplicate,
two divisions follow
– Meiosis I (reductional
division): homologs pair
up and separate,
resulting in two haploid
daughter cells with
replicated chromosomes
– Meiosis II (equational
division): sister
chromatids separate
• the result is four haploid
daughter cells with half the
number of unreplicated
chromosomes as the parental
cell
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair
of homologous
chromosomes
Sister
chromatids
Chromosomes
duplicate
Diploid cell with
duplicated
chromosomes
Meiosis I
1 Homologous
chromosomes separate
Haploid cells with
duplicated chromosomes
Meiosis II
2 Sister chromatids
separate
Haploid cells with unduplicated chromosomes
Anaphase I
Metaphase I
Prophase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Telophase I and
Cytokinesis
Sister chromatids
remain attached
Centromere
(with kinetochore)
Metaphase
plate
Homologous
chromosomes
Fragments
of nuclear
envelope
Duplicated homologous
chromosomes (red and blue)
pair and exchange segments;
2n  6 in this example.
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Each pair of homologous
chromosomes separates.
Chromosomes line up
by homologous pairs.
• Division in meiosis I occurs in four phases:
– Prophase I
– Metaphase I
– Anaphase I
– Telophase I and cytokinesis
Cleavage
furrow
Two haploid
cells form; each
chromosome
still consists
of two sister
chromatids.
Prophase I
• prophase I typically occupies more than 90% of the time required for meiosis
• many similarities with mitosis prophase
– chromosomes begin to condense within the nucleus
– the centrioles migrate and the spindle begins to form
– chromosomes attach to the spindle
• BUT a unique event happens – synapsis = pairing of two homologous
chromosomes
– ends of the chromosomes attach to proteins in the nuclear envelope
– other proteins in the nuclear matrix help the 2 chromosomes align gene by
gene
– the paired chromosomes are called a tetrad
– synapsis is followed by crossing over
http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__stages_of_meiosis.html
Crossing Over
– synapsis in Prophase I is followed by crossing over
crossing over meiosis:
https://www.youtube.com/watch?v=rqPMp0U0HOA
Metaphase I
• the nuclear envelope is gone and the spindle is completing its formation
• tetrads line up at the metaphase plate - with one chromosome facing each pole
– microtubules from one pole are attached to the kinetochore of one
chromosome of each tetrad
– microtubules from the other pole are attached to the kinetochore of the other
chromosome
– different from mitosis – spindle attached to both kinetochores of one
chromosome
Outer Plate
Microtubules
Inner Plate
MITOSIS
Anaphase I
• pairs of homologous chromosomes separate
• one chromosome moves toward each pole - guided by the
spindle apparatus
• sister chromatids remain attached at the centromere
and move as one unit toward the pole
Telophase I and Cytokinesis
• in the beginning of telophase I - each half of the cell has a
haploid set of chromosomes
– BUT each chromosome still consists of two sister
chromatids
•
•
•
•
cytokinesis forms two haploid daughter cells
in animal cells - a cleavage furrow forms
in plant cells - a cell plate forms
no chromosome replication occurs between the end of
meiosis I and the beginning of meiosis II because the
chromosomes are already replicated
Prophase II
Metaphase II
Anaphase II
Telophase II and
Cytokinesis
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.
Haploid daughter
Sister chromatids
cells forming
separate
• Division in meiosis II also occurs in four phases
– Prophase II
– Metaphase II
– Anaphase II
– Telophase II and cytokinesis
• Meiosis II is very similar to mitosis
Prophase II
• the spindle apparatus re-forms
• in late prophase II, chromosomes move toward the metaphase plate
• line up like mitosis
Metaphase II
• the sister chromatids are arranged at the metaphase plate
• BUT - because of crossing over in meiosis I - two sister
chromatids of each chromosome are no longer genetically
identical
• kinetochores of sister chromatids attach to microtubules
extending from opposite poles
– like mitosis
Anaphase II
• the sister chromatids separate – like mitosis
• the sister chromatids of each chromosome now move as two newly
individual chromosomes toward opposite poles – like mitosis
• sister chromatid cohesion allows sister chromatids of a single
chromosome to stay together through meiosis I
• protein complexes called cohesins are responsible for this cohesion
– in mitosis: cohesins are cleaved at the end of metaphase
– in meiosis I (anaphase I): cohesins are cleaved along the
chromosome arms in anaphase I  separation of homologs
and at the centromeres
– in meiosis II (anaphase II)  separation of sister chromatids
Anaphase II & Cohesins
• sister chromatid cohesion allows sister chromatids of a single chromosome to stay
together through meiosis I
• protein complexes called cohesins are responsible for this cohesion
• cohesins – complex of 4 protein subunits (SMC1, SMC3, SSC1 & SSC3)
– 4 protein subunits form a ring-like structure that encircles both chromatids
• forms during the S phase of interphase
– functions:
• 1. holds sister chromatids together during metaphase and regulates their
separation during anaphase of mitosis or meiosis
• 2. facilitates chromatid attachment to spindle – interacts with the
kinetochore and helps position the chromosomes at the metaphase plate
• 3. facilitates recombination (crossing-over)
• 4. other functions – e.g. transcriptional regulation
Anaphase II & Cohesins
• cohesins are cleaved by the enzyme called separase
• in mitosis: all cohesins are cleaved at the end of metaphase
• in meiosis I (anaphase I): cohesins are cleaved along the chromosome arms in
anaphase I but not at the centromere  separation of homologs
• in meiosis II (anaphase II)  separation of sister chromatids just like mitosis
• mechanism:
– in meiosis I – the cohesin complex is protected from separase when in
tetrad formation
• only one kinetochore of the two per chromosome is attached to a microtubule at
metaphase I  protection of cohesins from separase & chromosome separation
– in meiosis II & mitosis – both kinetochores in the chromosome contact a
microtubule and cohesins are susceptible to separase action
Metaphase I
chromosomes
Telophase II and Cytokinesis
• just like mitosis
– the chromosomes arrive at opposite poles
– nuclei form, and the chromosomes begin decondensing
– Cytokinesis separates the cytoplasm
• at the end of meiosis, there are four daughter cells - each with a
haploid set of unreplicated chromosomes (half the number as
the parent cell)
• BUT - each daughter cell is genetically distinct from the others
and from the parent cell
A Comparison of Mitosis and Meiosis
• Mitosis conserves the number of chromosome sets, producing cells that are
genetically identical to the parent cell
• Meiosis reduces the number of chromosomes sets from two (diploid) to one
(haploid), producing cells that differ genetically from each other and from the
parent cell
SUMMARY
Property
Mitosis
Meiosis
DNA
replication
Occurs during interphase before
mitosis begins
Occurs during interphase before meiosis I begins
Number of
divisions
One, including prophase, metaphase,
anaphase, and telophase
Two, each including prophase, metaphase, anaphase,
and telophase
Synapsis of
homologous
chromosomes
Does not occur
Occurs during prophase I along with crossing over
between nonsister chromatids; resulting chiasmata
hold pairs together due to sister chromatid cohesion
Number of
daughter cells
and genetic
composition
Two, each diploid (2n) and genetically
identical to the parent cell
Four, each haploid (n), containing half as many
chromosomes as the parent cell; genetically different
from the parent cell and from each other
Role in the
animal body
Enables multicellular adult to arise from
zygote; produces cells for growth, repair,
and, in some species, asexual reproduction
Produces gametes; reduces number of chromosomes
by half and introduces genetic variability among the
gametes
MITOSIS
MEIOSIS
Parent cell
MEIOSIS I
Chiasma
Prophase I
Prophase
Duplicated
chromosome
Chromosome
duplication
Chromosome
duplication
Homologous
chromosome pair
2n  6
Metaphase
Metaphase I
Anaphase
Telophase
Anaphase I
Telophase I
Haploid
n3
http://highered.mc
grawhill.com/sites/0072
495855/student_vi
ew0/chapter28/ani
mation__how_mei
osis_works.html
Daughter
cells of
meiosis I
2n
MEIOSIS II
2n
Daughter cells
of mitosis
n
n
n
n
Daughter cells of meiosis II
SUMMARY
Meiosis: A summary
•
Three events are unique to meiosis, and all three
occur in meiosis l
– Synapsis and crossing over in prophase I:
Homologous chromosomes physically connect and
exchange genetic information
– At the metaphase plate, there are paired
homologous chromosomes (tetrads), instead of
individual replicated chromosomes
– At anaphase I, it is homologous chromosomes,
instead of sister chromatids, that separate
Genetic variation produced in sexual life
cycles contributes to evolution
• Mutations (changes in an organism’s DNA) are the original source
of genetic diversity
– mutation is a change in the DNA sequence that lasts through
rounds of mitosis or meiosis (i.e. perpetuates in the daughter
cells)
• mutations create different versions of genes called alleles
• gametes are produced through meiosis – each with a unique
complement of alleles
• reshuffling of alleles during sexual reproduction produces genetic
variation
Origins of Genetic Variation Among
Offspring
• the behavior of chromosomes during meiosis and
fertilization is responsible for most of the variation that
arises in each generation
• three mechanisms contribute to genetic variation
– Independent assortment of chromosomes
– Crossing over
– Random fertilization
Independent Assortment of Chromosomes
• Homologous pairs of chromosomes orient randomly at metaphase
I of meiosis
– red chromosomes from “Dad”; blue chromosomes from “Mom”
– when you undergo meiosis – multiple possibilities exist
– see possibility 1 vs. 2
• in independent assortment - each pair of chromosomes sorts
maternal and paternal homologs into daughter cells
independently of the other pairs
– so there is a 50% chance a daughter cell at the end of meiosis will get
the maternal chromosome and a 50% chance it will get the paternal
homolog
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Daughter
cells
Combination 1 Combination 2
Combination 3 Combination 4
• the number of combinations possible when chromosomes assort
independently into gametes is 2n, where n is the haploid number
– for humans (n = 23), there are more than 8 million (223) possible
combinations of chromosomes
Prophase I
of meiosis
Pair of homologs
Crossing
Over
Nonsister chromatids
held together
during synapsis
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes
• crossing over produces recombinant
chromosomes, which combine DNA
inherited from each parent
• begins very early in prophase I, as
homologous chromosomes pair up
gene by gene
– homologous portions of two
non-sister chromatids trade
places
• contributes to genetic variation by
combining DNA from two parents
into a single chromosome
• in crossing over the nonsister chromatids exchange DNA segments
– also known as genetic recombination
– produces recombinant chromosomes
• the segments that are exchanged are similar in sequence and are on the
same locations on either chromosome
• the crossed region is called a chiasma – region where the two
chromosomes are physically joined
– each tetrad usually has more than one
• requires an enzyme called a recombinase
• major source of genetic variation
http://www.tokyomed.ac.jp/genet/anm/mimov.gif
• crossing over can be unequal
• major source of gene duplication and deletions between
chromosomes
“equal”
cross over
site
Random Fertilization
• random fertilization adds to genetic
variation because any sperm can fuse
with any ovum (unfertilized egg)
– the fusion of two gametes (each with
8.4 million possible chromosome
combinations from independent
assortment) produces a zygote with
any of about 70 trillion diploid
combinations
• crossing over adds even more variation
• each zygote has a unique genetic identity
Random Fertilization
The Evolutionary Significance of Genetic
Variation Within Populations
• Natural selection results in the accumulation of genetic
variations favored by the environment
• Sexual reproduction contributes to the genetic
variation in a population, which originates from
mutations