Chapter 9:
Sexual
Reproduction
Sexual Reproduction: Meiosis
Due to meiosis, two individuals can
create off-spring that are genetically
different not only from themselves but
also from their siblings
Meiosis is a special type of nuclear
division that results in 4 gamete cells
•
•
Spermatogenesis in males
Oogenesis in females
Meiosis serves 2 functions:
1.
2.
•
Reducing the chromosome
number
Shuffling the chromosomes and
gens to produce genetically
different gametes
Fertilization by the fusion of the
sperm and egg allows even more
shuffling of genes
Homologous Chromosomes
Chromosomes occur in pairs
One pair contains sex chromosomes
(X and Y)
The larger chromosome is the X and
the smaller is the Y chromosome
Autosomes are all the pairs of
chromosomes not including X and Y
The 23 pairs of chromosomes (46
altogether) are called the diploid (2n)
number
Haploid (n) number is half of the
diploid number
Homologous chromosomes
(homologues) are members of a
chromosome pair
Alternate versions of a gene for a
particular trait are called alleles.
Human Life Cycle
The human life cycle involves both mitosis and
meiosis
Mitosis occurs during development and
after birth as part of growth and tissue repair
Somatic (body) cells have the diploid
number due to mitosis
Meiosis occurs during sexual reproduction
and the number of chromosomes are
reduced from diploid to haploid
Spermatogenesis is the production of sperm in
males by the process of meiosis
Occurs in the testes
Oogenesis is the production of eggs in females
via meiosis
This occurs in the ovaries
The fertilization of an egg by a sperm gives rise
to a zygote (a diploid)
The zygote has homologous pairs of
chromosomes
Overview of Meiosis
Meiosis consist of 2 divisions: Meiosis I and Meiosis II
In meiosis I, the homologous chromosomes of each pair
come together and line up side-by-side in an event called
synapsis
A tetrad (2 homologous chromosomes consisting of 2
chromatids each) arise as a result
One chromosome from each homologous pair goes to
each daughter nucleus
2 daughter cells are produced with half the number of
chromosomes (n) but the chromosomes are still
duplicated
Chromosomes composed of 2 sister chromatids are
called dyads
There is no duplication of chromosomes between meiosis I
and meiosis II
During meiosis II, the sister chromatids of each dyad
separate and are daughter chromosomes
4 daughter cells arise at the end of meiosis II
Because no rules restrict which chromosome goes to which
daughter nucleus…all possible combinations of
chromosomes may occur with the gametes
The Importance of Meiosis
Meiosis keeps the chromosome
number constant by producing
haploid daughter cells that later
become gametes
Meiosis introduces genetic variations
through
1.
Crossing-over where the nonsister
chromatids from the tetrad
exchange genetic material
Crossing-over allows shuffling of
alleles and brings about genetic
recombination
Crossing-over increases the
diversity of the gametes and of
the offspring
2.
Many possible combination of
chromosomes can occur in the
daughter cells
Phases of Meiosis
Meiosis I: diploid to haploid
Prophase I: nuclear envelope
fragments and nucleolus
disappears
Spindle fibers appear
Chromosomes condensed and
homologues undergo synapsis to
produce tetrads
Crossing-over of nonsister
chromatids and shuffling of
alleles between chromosomes
Metaphase I: tetrads align at the
spindle equator with each
homologue facing opposite spindle
poles
Anaphase I: Homologous separate
and dyads move to the poles
Telophase I: daughter nuclei are
haploid because each daughter
received one duplicated
chromosome from each
homologous pair
Cytokinesis follows
Interkinesis: The period between
meiosis I and meiosis II where no
replication of DNA occurs
Meiosis II: haploid to haploid
Prophase II: nuclear envelope
fragments, nucleolus disappears,
spindle appears
Each dyad attaches to the
spindle
Metaphase II: Dyads lined up at
the spindle equator and sister
chromatids are facing opposing
poles
Anaphase II: Sister chromatids
separate from the dyad and move
to the poles
Telophase II: Spindle fibers
disappear and nuclear envelopes
are formed
4 genetically different haploid
daughter cells arise after
cytokinesis
Meiosis I
Meiosis II
Meiosis Compared to Mitosis
Meiosis requires 2 nuclear divisions
mitosis requires only 1 nuclear
division
Meiosis produces 4 haploid daughter
cells (half the chromosome number of
the parent cell)
Mitosis produces 2 diploid
daughter cells (same as the parent
cell)
Meiosis produces 4 daughter cells that
are genetically different to each other
and to the parent cell
Mitosis produces 2 daughter cells
that are genetically similar to each
other and to the parent cell
Meiosis v Mitosis
Abnormal Chromosome
Inheritance
Nondisjunction is the failure of homologous
chromosomes or daughter chromosomes to
separate during meiosis I or meiosis II
Meiosis I: both members of a homologous
pair go into the same daughter cell
Meiosis II: sister chromatids fail to separate
and both daughter chromosomes go into the
same gamete
Trisomy: one type of chromosome is present in 3
copies
Monosomy: one type of chromosome is present
in a single copy
Down syndrome (trisomy 21): an individual has
3 copies of chromosome 21
Characteristics: short stature, an eyelid fold,
stubby fingers, a wide gap between the first
and second toes; large, fissured tongue,
round head; mental disabilities
Abnormal Sex Chromosome
Number
Nondisjunction during oogenesis or
spermatogenesis can lead to abnormal sex
traits due to too many or too few X and Y
Normal females have only one
functioning X chromosome (the other X
chromosome becomes an inactive mass
known as a Barr body)
The presence of a Y chromosome almost
always determine maleness
Turner syndrome (45, XO): the
individual is a female
The O indicates the lack of a second
sex chromosome
Characteristics: short female with
broad chest and webbed neck, small
and underdeveloped ovaries,
oviducts, and uterus
Turner females do not undergo
puberty or menstruate
They have normal intelligence
Klinefelter syndrome (47, XXY):
individual is a male
The extra X is a Barr body
Testes and prostate gland are
underdeveloped, no facial hair, large
hands and feet, long arms and legs
Slow to learn but not mentally
handicapped unless there are more
than two X chromosomes