2. Sexual Reproduction
►2 parents
► Exchange of DNA
► Offspring genetically different from parents
► Involves specialized sex cells (gametes)
► Creates diversity
3. Sexual Reproduction in Simple
Organisms
► Conjugation –
Cytoplasm bridge forms
between 2 cells and an
exchange of DNA takes place
Occurs between two cells
with different mating types
Mating types are usually
called plus (+) and minus (-)
4. Sexual Reproduction in Animals
► Each parent produces a GAMETE
Female gamete – egg
Male gamete - sperm
► The two gametes combine and produce a zygote
Called fertilization
► Thezygote then divides by mitosis, grows and
process begins again
Zygote has the same number of chromosomes as the
parents
5. Somatic cells vs. Gametes
► Somatic cells are body cells
2 set of chromosomes
Humans have 46 chromosomes or 23 pairs
► Gametes are sex cells
1 set of chromosomes
Humans have 23
chromosomes
6. How are gametes produced?
► Eggs & sperm need to be made and
joined…
► Do we make egg & sperm by mitosis? No!
What if we did, then…
46 + 46 92
egg sperm zygote
Doesn’t work!
7. diploid = 2 copies 2n Human female karyotype
46 chromosomes
23 pairs
XX
8. diploid = 2 copies 2n Human male karyotype
46 chromosomes
23 pairs
XY
9. Paired chromosomes
► Homologous chromosomes
both chromosomes of a pair carry “matching”
genes
►control
same inherited characters
►homologous = same information
diploid
2n eye color eye color
2n = 4 (brown?) (blue?)
homologous double stranded
chromosomes homologous chromosomes
10. How do we make sperm & eggs?
► Must reduce 46 chromosomes → 23
must half the number of chromosomes
haploid
zygote
46 23
23
meiosis egg 46
23
46 23
fertilization
sperm
gametes
11. Meiosis makes sperm & eggs
► 46 chromosomes to 23 chromosomes
half the number of chromosomes
46 23
egg
meiosis
46 23
diploid sperm haploid
12. Meiosis
► Specialcell division in sexually reproducing
organisms
► Makes gametes
Sperm & egg
► Reduction division
Reduce number of chromosomes
2n 1n
diploid haploid
13. Interphase
► DNA replicates
► Cell prepares for
division
14. Prophase I
► Homologous chromosomes
pair up and form a tetrad
This is called synapsis
► Crossing over occurs
during synapsis
Homologous chromosomes
exchange segments of DNA
15. Metaphase I
► Tetrads line up at the
equator.
► Centromeres attach
chromosomes to the
spindle fibers
16. Anaphase I
► Homologous
chromosomes separate
and move to the poles
Called disjunctions
Sister chromatids
remain attached
17. Telophase I
► Cytoplasm divides, forming
two new daughter cells
Cells are haploid
18. Cytokinesis I
► Occurs simultaneously with telophase I
Forms 2 daughter cells
► Plantcells – cell plate
► Animal cells – cleavage furrow
► There is no further replication of
genetic material prior to meiosis II
19. Prophase II
► Spindle forms
► Sister chromatids
move toward the
equator
20. Metaphase II
► Sister chromatids line
up at the equator
Similar to mitosis
► Spindle fibers attach at
the centromere
21. Anaphase II
► Sisterchromatids
separate and move to
opposite poles
22. Telophase II
► Nuclei form at poles
and cytokinesis occurs
► After completion of
cytokinesis there are 4
daughter cells
Each cell is haploid (n)
23. double
Meiosis 1 overview stranded
► 1st Divide 1
division of meiosis
Copy DNA Line Up 1
4 prophase 1 metaphase 1
chromosomes
telophase 1
diploid
2n 2 chromosomes
haploid
gamete 1n
24. Bye Bye 2
Meiosis 2 overview
telophase 2
telophase 1 Line Up 2
4
► 2nd division of meiosis
looks like mitosis metaphase 2
2 chromosomes
haploid
1n gametes
26. Sexual reproduction lifecycle
2 copies
diploid
2n
1 copy 1 copy
fertilization meiosis
haploid haploid
1n 1n
We’re mixing
things up here!
A good thing?
27. Putting it all together…
meiosis → fertilization → mitosis + development
gametes
46 23 46
23 46 46
meiosis egg 46 4646
23 46 46
46
46 23 zygote
fertilization mitosis &
mitosis
sperm
development
28. The value of meiosis 1
► Consistency over time
meiosis keeps chromosome number same
from generation to generation
from Mom
Mom
Dad offspring
from Dad
29. The value of meiosis 2 We’re
mixing things
up here!
► Change over time
meiosis introduces genetic variation
►gametes of offspring do not have
same genes as gametes from parents
►new combinations of traits
from Dad variation
from Mom
offspring
new gametes made by offspring
30. Fertilization
► Joining of the sperm & egg
► Internal Fertilization
Gametes fuse inside the body of a female
Occurs in many land animals
Offspring have a better chance of survival
Fewer offspring are produced
► External Fertilization
Gametes fuse outside the body of a female
Occurs in many water animals
Many offspring produced
Fewer offspring survive
31. Fertilization
► Parthenogenesis
Development of an unfertilized egg into an adult
animal
E.g. rotifers, some insects such as bees, wasps,
E.g. Queen bee can produce fertilized or
unfertilized eggs.
►Fertilized eggs become female workers
►Unfertilized eggs become male drones
32. Independent Assortment
► Chromosomes are separated
and combined independently
of each other during meiosis
E.g. all your mom’s
chromosomes don’t end up
together in your gametes.
They are randomly mixed.
33. Independent Assortment
► Independent
assortment
produces many different
gametes
In humans, n=23 so over 8
million different gametes can
be produced…
34.
35. Crossing Over
► Occurs in prophase I, during synapsis
► Homologous chromosomes twist and exchange
pieces of genetic material
► This results in even more different gametes
36.
37. The Key Difference Between Mitosis and Meiosis is
the Way Chromosomes Uniquely Pair and Align in
Meiosis
Mitosis The first (and
distinguishing)
division of meiosis
38.
39. Problems in Meiosis
► Sometimes chromosomes don’t separate
properly
This is called nondisjunction and result in a cell
with too many or too few chromosomes
41. Problems in Meiosis
Should the gamete with the
chromosome pair be fertilized
then the offspring will not be
‘normal’.
In humans this often occurs
with the 21st pair – producing
a child with Downs Syndrome
42. Trisomy 21– Downs Syndrome
Can you see the extra
21st chromosome?
Is this person male or
female?
•This picture of a person’s
chromosomes is called a
karyotype. The
chromosomes are
organized from largest to
smallest.
•How can karyotypes be
helpful in identifying
diseases?
43. Gametogenesis
► Gametes develop within specialized organs
Female gametes – egg cells
Male gametes – sperm cells
Female gonads – ovaries
Male gonads - testes
44. Gametogenesis
►Gametogenesis – the formation of
gametes in the gonads
Two types:
►Oogenesis – formation of eggs by
meiosis
►Spermatogenesis – formation of sperm
by meiosis