Sexual Reproduction
and Meiosis
 Meiosis

≠ sexual reproduction!
Meiosis makes the cells that are responsible
for sexual reproduction
Sexual Reproduction
 Producing
a new organism by combining
chromosomes from 2 parents
gametes
 Specialized sex cells = _____________
 Male = ___________
sperm
 Female = _________
egg
 Union of gametes = _________________
fertilization
which produces a ______________
zygote
Gametes
 Each
sex cell of a multicellular organism
contains the ___________
mono/haploid (n) number of
chromosomes characteristic of that
species (in humans, n = 23)
 These chromosomes are NOT present in
pairs
 Gametes are produced during
gametogenesis in the ___________
gonads
_____________
 Male
gonads = _________
testes
 Female gonads = _________
ovaries
 The process that creates monoploid
gametes is ____________
meiosis
 Organisms that contain both male &
female gonads are called _____________
hermaphrodites

Ex. Earthworms
Meiosis
AKA “Reduction Division”
A
cellular division in which the number of
chromosomes is reduced by half (2n → n)
 Why do gametes need to have a
monoploid number of chromosomes?


In order for fertilization to produce a normal
2n zygote, the sperm & egg must have only
half the # of chromosomes
n
+ n
2n
fertilization
haploid
haploid
diploid
sperm
egg
zygote
 To
establish this monoploid # of
chromosomes, sperm and eggs undergo
meiosis
 Similar to mitosis, but meiosis has one
additional ___________
division
mitosis
Meiosis I
Meiosis II
Laser clip
1st Meiotic Division
 Interphase


Chromatin replicates
Single stranded → double stranded
 Prophase



I:
I:
Chromatin coils → chromosomes
Homologous chromosomes pair up during
synapsis & form tetrad (group of 4
chromatids)
Chromosomes may twist & exchange genes =
crossing over

Metaphase I:



Anaphase I:





Tetrads align @ equator (different than mitosis!! No
tetrads in mitosis)
Spindles attach to centromeres
Tetrads split
Homologous pairs move to opposite poles
(disjunction)
New cells: monoploid & double stranded
Non-disjunction: when tetrads fail to separate leaving
more/less chromosomes in each new cell)
Telophase I:


Cytoplasm divides
2 new daughter cells that are monoploid w/ double
stranded chromosomes
2nd Meiotic Division
Just like mitosis w/o chromosome replication
 Prophase

II:
Spindle fibers form & attach to centromeres
 Metaphase

Chromosomes line up on equator
 Anaphase

II:
Double stranded chromosomes split &
chromatids move to opposite poles & cells are
now single stranded
 Telophase

II:
II:
Both daughter cells divide forming 4
monoploid cells
End Result of Meiosis
1
diploid
primary (1o)
sex cell (2n)
↓
 4 monoploid
gametes (n)
Comparison of Mitosis & Meiosis
Ist step of division
Type of
reproduction
Type & # of
daugher cells
compared to
parent cell
# of divisions
synapsis?
Mitosis
Replication of
chromosomes
Meiosis
same
Asexual & growth
Used to make
gametes for
sexual
reproduction
2; same as parent 4, 1/2
(2n)
chromosomes of
parent (n)
1
2
no
yes
 Spermatogenesis:


The production of
sperm in the testes
4 monoploid sperm
produced from each
primary spermatocyte
 Oogenesis:



The production of
eggs (Latin: eggs =
ova) in ovaries
4 monoploid eggs
produced from each
primary oocyte
BUT…only one is
viable (survives)
The other 3 are very
small & are called
polar bodies. They
degenerate in the
female’s body.
 Unlike
spermatogenesis, oogenesis is
NOT continuous:


Before birth, oogonia divide by mitosis to
produce the limited lifetime supply of oogonia
(which develop into oocytes)
In sexually mature females, one primary
oocyte develops into a viable egg
approximately every 28 days .
Egg vs. Sperm






Egg
Monoploid
Female ovaries
Non-motile (sessile)
Fewer in # (1 per
meiotic division)
Large
Yolk






Sperm
Monoploid
Male testes
Motile
Many in # (4 per
meiotic division)
Small
No yolk
 After
gametes are produced, the next step
in reproduction is FERTILIZATION:



The union of a monoploid (n) sperm w/ a
monoploid (n) egg resulting in a diploid (2n)
ZYGOTE n + n = 2n
Requires fluid medium for sperm to swim to
egg
When a sperm comes in contact with an egg,
the acrosome (covers head of sperm)
releases enzymes that dissolve an opening
into the egg
What type of asexual
reproduction is this?
What’s wrong with this
picture?
Sites of Fertilization




External
The union of sperm & egg
OUTSIDE of the body of
the female
Occurs mainly in aquatic
animals
Requires large #’s of
eggs to insure survival
because there is little
protection for the egg
Ex. Amphibians




Internal
Union of sperm & egg
INSIDE the body of the
female
Occurs in most terrestrial
vertebrate animals
Smaller # of offspring &
fewer # eggs released
because the egg is
protected inside the
mother
Ex. Mammals
 In



vitro fertilization:
Fertilization outside
of the female’s
body
Zygote implanted
into female after
fertilization
Vitro = glass
I say, thank goodness for Veterinarians & IVF:
No more singles’ bars, no more dating!
 Parthenogenesis:




The development of an egg into a mature
organism WITHOUT fertilization by sperm
Therefore all offspring look identical to the
mother
Ex. Bees, artificial stimulation of frog eggs,
sea urchins, rabbits & turkeys
Parthenogenesis is the “asexual reproduction”
in normally sexually reproducing organisms.
mitosis
fertilization
meiosis
diploid
Monoploid/haploid
Chromosome number