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Meiosis
Heredity and Variation
• Heredity
– Is the transmission of traits from one generation to
the next
• Variation
– Shows that offspring differ somewhat in
appearance and other traits from parents and
siblings
• Genetics
– Is the scientific study of heredity and hereditary
variation
Breeding in English Shepherds
Mother
Father
Breeding in English Shepherds - Offspring
Genes – A simple definition
• Genes
– Are the units of heredity
– Are segments of DNA
• Each gene in an organism’s
DNA has a specific locus on
a certain chromosome
• Sexual organisms inherit
one set of chromosomes
from the mother and one set
from the father
The Shallow End of the Gene Pool
Asexual Reproduction
• For example – budding
in Hydra is asexual
reproduction that forms
clones
Asexual Reproduction
• Strawberry reproducing by putting out
asexual runners – also forming clones
Haploid gametes (n  23)
Key
Haploid (n)
Diploid (2n)
Egg (n)
Sperm (n)
MEIOSIS
Ovary
FERTILIZATION
Testis
Diploid
zygote
(2n  46)
Mitosis and
development
Multicellular diploid
adults (2n  46)
Chromosomes
• Homologous chromosomes
– Are the two chromosomes composing a pair
– Have the same characteristics
– Autosomes are the non-sex determining
chromosomes
• Sex chromosomes
– Are distinct from each other in their characteristics
– Are represented as X and Y in mammals
– Determine the sex of the individual, XX being
female, XY being male; X and X are homologous
in female
• A diploid cell
– Has two sets of each of its chromosomes
– In a human has 46 chromosomes (2n = 46)
Pair of homologous
duplicated chromosomes
Centromere
Sister
chromatids
Metaphase
chromosome
5 m
Figure 13.x3 Human female karyotype shown by bright field G-banding of
chromosomes
Figure 13.x5 Human male karyotype shown by bright field G-banding of
chromosomes
Hairy Ears – a sex linked trait
• Square symbol = male
• Round symbol = female
• Dark square = hairy ear trait
Chromosome number in humans if there was no
meiosis prior to reproduction
• First Generation - Sperm and
Egg with 46 chromosomes each
• Second generation 92
chromosomes
• Third generation 184
chromosomes
• Fourth generation 368
chromosomes
• Etc.
Sexual life cycles
Key
Haploid (n)
Diploid (2n)
n
Gametes
n
Mitosis
n
n
MEIOSIS
FERTILIZATION
n
Diploid
multicellular
organism
(a) Animals
Zygote 2n
Mitosis
n
Mitosis
n
Mitosis
n
Spores
Gametes
MEIOSIS
2n
Haploid unicellular or
multicellular organism
Haploid multicellular organism
(gametophyte)
n
n
n
n
Gametes
FERTILIZATION
2n Zygote
Mitosis
(b) Plants and some algae
n
FERTILIZATION
MEIOSIS
2n
Diploid
multicellular
organism
(sporophyte)
Mitosis
2n
Zygote
(c) Most fungi and some protists
Interphase
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair
of homologous
chromosomes
Sister
chromatids
Chromosomes
duplicate
Diploid cell with
duplicated
chromosomes
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
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
MEIOSIS I: Separates sister chromatids
MEIOSIS I: Separates homologous chromosomes
Prophase I
Metaphase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Telophase I and
Cytokinesis
Anaphase I
Duplicated homologous
chromosomes (red and blue)
pair and exchange segments;
2n  6 in this example.
Anaphase II
Telophase II and
Cytokinesis
Centromere
(with kinetochore)
Metaphase
plate
Cleavage
furrow
Fragments
of nuclear
envelope
Metaphase II
Sister chromatids
remain attached
Spindle
Homologous
chromosomes
Prophase II
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Chromosomes line up
by homologous pairs.
Each pair of homologous
chromosomes separates.
During another round of cell division, the sister chromatids finally separate;
four haploid daughter cells result, containing unduplicated chromosomes.
Sister chromatids
separate
Two haploid cells
form; each chromosome
still consists of two
sister chromatids.
Haploid daughter
cells forming
Prophase I
Centrosome
(with centriole pair)
Sister
chromatids
Chiasmata
Spindle
Telophase I and
Cytokinesis
Anaphase I
Metaphase I
Sister chromatids
remain attached
Centromere
(with kinetochore)
Metaphase
plate
Fragments
Homologous
chromosomes of nuclear
envelope
Homologous
chromosomes
separate
Microtubule
attached to
kinetochore
Cleavage
furrow
Each pair of homologous
chromosomes separates.
Chromosomes line up
Duplicated homologous
chromosomes (red and blue) by homologous pairs.
pair and exchange segments;
2n  6 in this example.
Two haploid
cells form; each
chromosome
still consists
of two sister
chromatids.
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.
Sister chromatids
separate
Haploid daughter
cells forming
MEIOSIS
MITOSIS
Parent cell
MEIOSIS I
Chiasma
Prophase
Prophase I
Duplicated
chromosome
Chromosome
duplication
2n  6
Chromosome
duplication
Homologous
chromosome pair
Metaphase
Metaphase I
Anaphase
Telophase
Anaphase I
Telophase I
Daughter
cells of
meiosis I
2n
Daughter cells
of mitosis
2n
Haploid
n3
MEIOSIS II
n
n
n
n
Daughter cells of meiosis II
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
Independent Assortment
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Independent Assortment
Possibility 2
Possibility 1
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Independent Assortment
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
• Mitosis and meiosis have several key differences.
– The chromosome number is reduced by half in
meiosis, but not in mitosis.
– Mitosis produces daughter cells that are genetically
identical to the parent and to each other.
– Meiosis produces cells that differ from the parent
and each other.
• Three events, unique to meiosis, occur during the first
division cycle.
1. During prophase I, homologous chromosomes pair
up in a process called synapsis.
– A protein zipper, the synaptonemal complex, holds
homologous chromosomes together tightly.
– Later in prophase I, the joined homologous
chromosomes are visible as a tetrad.
– At X-shaped regions called chiasmata, sections of
nonsister chromatids are exchanged.
– Chiasmata is the physical manifestation of crossing
over, a form of genetic rearrangement.
– Prophase I is longest and most important phase
2. At metaphase I homologous pairs of chromosomes,
not individual chromosomes are aligned along the
metaphase plate.
• In humans, you would see 23 tetrads.
3. At anaphase I, it is homologous chromosomes, not
sister chromatids, that separate and are carried to
opposite poles of the cell.
– Sister chromatids remain attached at the centromere
until anaphase II.
• The processes during the second meiotic division are
virtually identical to those of mitosis.
The synaptonemal complex binding together
four homologous chromosomes
Chiasmata and crossing over
Prophase I
of meiosis
Pair of homologs
Nonsister chromatids
held together
during synapsis
Prophase I
of meiosis
Pair of homologs
Chiasma
Centromere
TEM
Nonsister chromatids
held together
during synapsis
Prophase I
of meiosis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Nonsister chromatids
held together
during synapsis
Prophase I
of meiosis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Nonsister chromatids
held together
during synapsis
Prophase I
of meiosis
Nonsister chromatids
held together
during synapsis
Pair of homologs
Chiasma
Centromere
TEM
Anaphase I
Anaphase II
Daughter
cells
Recombinant chromosomes
Human Female vs Male Meiotic Timelines
Sexual reproduction leads to
genetic variation via:
• Independent assortment
during meiosis
• Crossing over during
meiosis
• Random mixing of
gametes (sperm and egg)