Reproduction:
Sexual Reproduction and Meiosis
Objectives
• Compare and contrast sexual vs.
asexual reproduction.
• Summarize and describe the events
of meiosis and gamete formation.
• Compare and contrast mitosis vs.
meiosis
Terms you must know
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Chromosomes
Homologous
Diploid
Haploid
Meiosis
Gametes
Crossing over
-Kinetochore
-Synapsis
-Chiasma
-Disjunction
-Gametogenesis
-Spermatogenesis
-Oogenesis
Mitosis and Meiosis
• Recall that mitosis is a part of cell
division which produces daughter
cells that are genetically identical to
the parent cell.
• All of your body cells reproduce by
mitotic cell divisions (except sex cells
and those that do not divide).
Body cells and sex cells
• In mitosis, a human cell with 46
chromosomes divides and produces two
daughter cells each containing 46
chromosomes.
• Human body cells contain 23 pairs of
chromosomes and are said to be diploid.
• Diploid means 2 sets of chromosomes. If
N is the number of chromosomes in one
set then 2N is the number of
chromosomes in 2 sets
Body cells and sex cells
• 1 set of 23 chromosomes comes from the
father and the other set of 23
chromosomes comes from the mother.
• These 2 sets of chromosomes are said to
be homologous meaning that each of the
23 chromosomes that came from the
father has a corresponding chromosome
that came from the mother.
Body cells and sex cells
• In contrast to body cells, male and female
gametes (sperm and egg) only contain one set
of 23 chromosomes and are said to haploid.
• Haploid: Refers to one set of chromosomes
• Haploid – N
• Diploid – 2N
• Body cells are diploid
• Sex cells are haploid
• Body cells reproduce by mitosis.
• Sex cells reproduce by meiosis.
Questions
1. How many chromosomes are in a human
sperm or egg cell?
2. How many chromosomes are in a human
body cell?
3. A fruit fly body cell has 8 chromosomes,
how many chromosomes in a fruit fly
sperm cell?
4. Is the chromosome number in a fruit fly
sperm cell haploid or diploid?
Meiosis and gamete formation
• How are haploid (N) gamete cells
produced from diploid (2N) cells?
• This is where meiosis comes in.
• During meiosis, the number of
chromosomes per cell is cut in half
through the separation of homologous
chromosomes.
Meiosis and Gamete formation
• Meiosis involves two distinct stages:
• Meiosis 1: The first meiotic cell division.
• Meiosis 2: The second meiotic cell
division.
• By the end of meiosis 2, the diploid cell
that entered meiosis has become four
haploid cells.
1 diploid
cell
4 haploid
cells
Meiosis 1
• Interphase: Before meiosis 1 begins, the
cell grows and each chromosome is
duplicated.
• There are two rounds of cell division in
meiosis beginning with meiosis 1.
• Meiosis one has four phases
– Prophase 1
– Metaphase 1
– Anaphase 1
– Telophase 1
Prophase 1
• Chromosomes become
visible.
• the two chromosomes
combine to form tetrads -synapsis
• Tetrads contain one pairs
of chromosomes (4
chromatids)
• chromosomes cross over
at points called chiasmata.
• Crossing over allows the
chromosomes to exchange
genetic material, allowing
for more different
combinations of genetic
material.
Prophase 1: Synapsis and
Crossing Over
Crossing Over
During crossing over, homologous
chromosomes line up and switch
their genes. When they separate,
they end up with new gene
combinations.
Metaphase 1
• In metaphase I the
tetrads are again
arranged across the
center by the
movements of the
kinetochores (1 per
chromosome rather
than 1 per chromatid).
• Chromosomes are
aligned with the two
centromeres opposite
each other, but this time
sister chromatids will
not be pulled apart as in
mitosis.
Anaphase 1
• The two homologous
chromosmes of each
tetrad are pulled apart
and separated from each
other into separate poles
–disjunction.
• Since the chromosomes
from each parent can go
into either pole this is
another means to
increase genetic
diversity.
• Chromosome # is
beginning to be cut in
half
Telophase 1
• In this phase, like in
mitosis the
chromosomes are
moved into opposite
poles and the nuclear
envelope reforms and
the spindle is broken
down
• Chromosome # is
haploid but each
chromosome is
duplicated.
Kinetochore and Spindle Microtubules
Meiosis 2
• The two cells produced by meiosis 1 now
enter a second meiotic division.
• Unlike meiosis 1, neither cell undergoes
interphase or DNA replication before
undergoing meiosis 2.
• After meiosis 2, the two haploid (N) cells
produced during meiosis 1 form four
haploid (N) daughter cells.
• Meiosis 2 is virtually identical to mitosis
Prophase 2
• In meiosis the cell
goes directly from
telophase I to
prophase II without
the interphase.
• The nuclear
envelope is again
dissolved and the
spindle network is
set up again.
• Prophase II is
identical to prophase
of mitosis except that
there is half the
amount of
chromosomes.
Metaphase 2
• Again (just like
mitosis) spindle
fibers attach to
kinetochores and
move chromosomes
to center of cell
• Now the chromatids
of each
chromosome will be
pulled apart during
anaphase 2.
Anaphase 2
• The
kinetochores
and spindle
fibers pull the
chromosomes
apart towards
the poles,
• The sister
chromatids are
split up and
pulled toward
opposite ends
of the cell.
Telophase 2
• Chromatids concentrate
in the poles, the nuclear
envelope is reformed
and the spindle again is
dissolved.
• The cells divide for the
last time, leaving a total
of four haploid cells,
which have half the
chromosomes of the
diploid mother cell.
• Unlike the daughter
cells from mitosis, the
daughter cells produced
here cannot immediately
cycle back to interphase
Summary of Meiosis
Gametogenesis: the name of the game
• Gametogenesis: The production of four
haploid gametes from one diploid body cell
in the gonads.
• Spermatogenesis: The production of 4
haploid sperm from one diploid body cell in
the testes.
• Oogenesis: The production of 1 haploid
egg (and three haploid polar bodies) from
one diploid body cell in the ovaries.
Spermatogenesis
• Spermatagonium- Diploid body cell in testes
(primordial germ cell). Becomes primary
spermatocyte during prophase 1.
• Primary spermatocyte- Diploid cell formed
during prophase 1
• Secondary spermatocyte- Haploid
daughter cells produced after telophase 1.
• Spertatid-Daughter cells produced after
telophase 2. Sperm without tails yet.
• Spermatozoa- Differentiated spermatids,
have flagella for swimming.
Oogenesis
• Oogonium: Diploid primordial germ cell in
ovary. Becomes primary oocyte during
prophase 1
• Primary oocyte: diploid cell produced during
prophase 1
• Secondary oocyte: haploid daughter cell
produced after telophase 1
• Polar bodies: non functional egg cells,
degenerate to provide nourishment to egg
cell.
• Ootid: Immature egg
• Ovum: Mature egg