Major Objectives:
1. Understand heredity and genes and
basic information on DNA
2. Differences between asexual and
sexual life cycles
3. Understand role of meiosis in sexual
life cycles
4. Know the major steps of meiosis
Major Objectives:
5. Know where the major sources of
genetic variability come from during
meiosis
6. Know when the different phases
occur in humans (females and males)
7. Know what can go wrong in meiosis
8. Know major differences between
mitosis and meiosis
What is heredity?
The transmission of traits from one
generation to the next
How are these traits passed on?
By offspring acquiring genes from
parents by inheriting chromosomes
(SEXUAL REPRODUCTION)
Karyotype
Ordered
display of an
individual's
chromosomes
Homologous
Chromosome
ASEXUAL REPRODUCTION
* A single
individual
passes along
copies of all
its genes to
offspring via
binary fission
in prokaryotes
Rod-Shaped Bacterium, E. coli, dividing by binary fission (TEM x92,750). This image is copyright Dennis Kunkel
MITOSIS REVIEW QUESTIONS
1. What type of cells does mitosis occur in?
2. How many times does DNA replicate in
mitosis?
3. How many cellular division(s) occur in
mitosis?
4. How many daughter cells are formed by
mitosis?
5. What is the chromosome number in
daughter cells formed by mitosis from diploid
parent cells?
6. In mitosis, are daughter cells identical to
or different from parent cells?
SEXUAL REPRODUCTION
Reproduction where two parents give rise to
offspring that have unique combinations of
genes inherited from the parental gametes
Gametes need 1) motility to be able to meet
and unite and 2) food to nourish the
developing embryo
Why Meiosis?
Note: The actual process of meiosis is more complicated than
this drawing—we will get to that in a bit
Original
organisms
Gametes
New organism
Diploid (2n)
Haploid (n)
Diploid (2n)
Reduces genetic information that is passed on to
offspring; fertilization of sperm and ovum recombine
genetic information to form new organism
(zygote)=GENETIC VARIABILITY
Why Meiosis?
Genetics Problem—Color of leaves:
G=chlorophyll; g=no chlorophyll
Original
organisms
gg
GG
Diploid (2n)
g
g
G
Gametes
New organism
Gg
G
Haploid (n)
Diploid (2n)
Where does meiosis occur?
Only in Reproductive Organs:
Males-testes-sperm cell(s)
Females-ovaries-egg cell (ovum; ova)
Human gametes only have one set of
chromosomes- 22 autosomes and an X
(female) or an X or Y (male) sex
chromosome= HAPLOID
REVIEW--HAPLOID/DIPLOID
1. If an organism has 2N=24 chromosomes is
this the haploid or diploid number of
chromosomes?
2. How many chromosomes will be found in a
gamete produced by this organism?
3. How many chromosomes will be found in a
zygote produced by this organism?
4. If the zygote of an organism has 12
chromosomes, N= __ chromosomes.
Life CycleGeneration to
generation
sequence of
stages on the
reproductive
history of an
organism—from
conception to
production of
own offspring
Sporotophyte
Gametophyte
Pteridophytes
SPOROPHYTE:
DOMINANT: In
ferns, rhizome with
roots & leaves
(fronds); sporangia
in sori on the
underside of fronds
GAMETOPHYTE: In ferns, INDEPENDENT,
heart-shaped prothallus with antheridia
and archegonia
MeiosisReplicates
once,
divides
twice
(reduces
amount of
genetic
material)
Interphase is an
important stage
preceding meiosis.
Without this
stage meiosis
would not occur.
During this stage,
each individual
chromatid
replicates, similar
to mitosis.
Prophase I is one of
the most important
stages of meiosis.
1) DNA of the
chromosomes begin to
twist and condense,
making the DNA visible
to the microscope.
2) Each chromosome
actively seeks out its
homologous pair (which
also has a sister
chromatid).
Cells in Prophase I
At Metaphase I, the
homologous pairs and
their sister chromatids
prepare for
separation.
During metaphase, the
chromosomes are lined
by the spindle fibers
at the metaphase
plate.
Cell in Metaphase I
Anaphase I pulls
apart the tetrad,
separating each
homologous
chromosome.
It is by random
chance that a certain
chromosome is pulled
to a certain pole.
Cell in Anaphase I
In Telophase I two
nuclear envelopes begin
to surround the
separate chromosomes.
Interkinesis follows,
which is a resting period
from Telophase I to
Prophase II. This
differs from mitosis
because DNA replication
does not occur.
Cell in Telophase I
MAIN EVENTS OF MEIOSIS I
The number of chromosomes has
been cut in half, HOMOLOGOUS
CHROMOSOMES SEPARATE.
End result is two haploid daughter
cells.
During Prophase II,
each dyad (1/2 a
tetrad) is composed of a
pair of sister chromatids
connected by a
centromere.
The centrosomes
(replicated during
Telophase I) produce
the spindle fibers and
start to move toward
the poles of the cell.
Cells in Prophase II
Metaphase II is
similar to
Metaphase I in
that the dyads
(sister chromatids)
are lined up at a
metaphase plate by
the spindle fibers.
Cells in Metaphase II
Anaphase II
separates the
dyads into individual
chromatids. Each
sister chromatid
ends up on one side
of the cell.
Cells in Anaphase II
In Telophase II, the
nuclear envelopes
forms around each set
of DNA and the
cytoplasm divides.
Four haploid cells have
formed from one
diploid cell. The
chromosomal content
is 1/2 the
chromosomal content
of a diploid cell.
Cells in Telophase II
MAIN EVENTS OF MEIOSIS II
Separate the sister chromatids of the
replicated chromosomes.
End result is four haploid daughter
cells. GAMETES in animals or SPORES
in plants.
MEIOSIS REVIEW QUESTIONS
1. What type of cells does meiosis occur in?
2. How many times does DNA replicate in
meiosis?
3. How many cellular division(s) occur in
meiosis?
4. How many daughter cells are formed by
meiosis?
5. What is the chromosome number in
daughter cells formed by meiosis from diploid
parent cells?
6. In meiosis, are daughter cells identical to
or different from parent cells?
MECHANISMS FOR GENETIC
VARIATION
1. Independent assortment.
2. Crossing over.
3. Random fertilization.
Independent assortment
Independent assortment
*Variation due to the random
orientation of homologous chromosomes
during meiosis I and nonidentical sister
chromatids in meiosis II.
# of possible
combinations when
chromosomes are
independently assorted is
2n, where n=haploid #.
HOW MANY POSSIBLE
COMBINATIONS FOR HUMANS??
2n,2(23 power)= ~8 MILLION
Crossing over
Occurs in
Prophase I;
homologous
portions of two
nonsister
chromatids
trade spaces.
Crossing over
*Occurs in humans, 2-3 times per
chromosome pair.
*Crossing over does
not create new
genes. It simply
recombines existing
genes in new ways.
Random fertilization
HOW MANY POSSIBLE
COMBINATIONS FOR HUMANS??
2n,2(23 power)= ~8 MILLION in ovum
2n,2(23 power)= ~8 MILLION in sperm
Zygote composed of 1 in 70 trillion
possible combinations
Crossing over
and random
fertilization add
more diversity
Gametogenesis-process of sperm
or ovum production via meiosis
MALES
WHAT:Spermatogenesis-production of sperm
WHERE: in testes, 100,000,000/hr/teste
WHEN: puberty until death
END RESULT: 4 haploid spermatids that will
differentiate into 4 mature
sperm cells
Spermatogenesis
Entire process takes ~74 days
FEMALES
WHAT: Oogenesis-production of ova (eggs)
WHERE: in ovaries
WHEN:
Initial steps occur PRIOR to
birth. By week 25, cells develop
into primary oocytes and begin
meiosis until late prophase I and
then STOP (~700,000-2 million).
FEMALES
WHEN: Oocytes recommence at puberty
(~20-40,000 remain). Oocytes
have been in prophase I for 1240 years. During female lifetime
~480 may complete meiosis I
(one/month). Meiosis II will NOT
be completed unless fertilized by
a sperm (cell cycle restarted by
sperm).
END RESULT:
MEIOSIS I: large secondary oocyte and
small polar body; MEIOSIS II: ovum and
three polar bodies
ERRORS IN MEIOSIS
1. Non-disjunction-When homologous
chromosomes fail to separate or disjoin in
Meiosis I. 25-50% of miscarriages result
from zygotes with abnormal chromosome
numbers. 1 in 118 newborns have
chromosomal abnormalities.
Aneuploidy-If effect is limited to one
or a few chromosomes;
one more=TRISOMY,
one less=MONOSOMY
Chromosomal
Abnormality
Trisomy for
1
2
3
4
5
6 - 12
13
14
15
16
17
18
19 - 20
21
Number among
Spontaneous Abortions
Number among
Live Births
0
0
159
0
53
0
95
0
0
0
561
0
128 Paute's Syndrome 17
275
0
318
0
1,229
0
10
0
223 Edward's Syndrome13
52
0
350 Down Syndrome 113
TRISOMY 21-Down Syndrome
*Occurs once in every 650 to 1,000 live
births
*Risk increases with maternal age (above 35
for mother): age 30:1 in 1,000 and at age
40:9 in 1,000
*Possible cause due to "suspended" division
that female gametes undergo
Egg is in center of picture
Many cumulus cells from
the ovary are seen around
the egg. This is a low
quality, oocyte from a
woman 41 years old
Egg is irregularly shaped
and dark.
A "good" egg from a
32 year old woman.
Photos from Advanced Fertility Clinic in Chicago
Sex Chromosomes
XYY
4
46
XXY
4
44
Klinefelter Syndrome
X0
1,350
8
Turner Syndrome
XXX
21
44
Translocations
239
216
Triploidy
1,275
0
Tetraploidy
450
0
Polyploidy
ERRORS IN MEIOSIS
2. Alteration of Chromosome Structure
Four Types of Change:
1) Deletion
2) Duplication
3) Inversion
4) Translocation
REVIEW CELL DIVISION