1. Sexual Reproduction and Meiosis

2. Meiosis ≠ sexual reproduction!
Meiosis makes the cells that are responsible for sexual reproduction

3. Sexual Reproduction
Producing a new organism by combining chromosomes from 2 parents
Specialized sex cells = _____________
Male = ___________
Female = _________
Union of gametes = _________________ which produces a ______________
gametes
sperm
egg
fertilization
zygote

4. Gametes
Each sex cell of a multicellular organism contains the ___________ (n) number of chromosomes characteristic of that species (in humans, n = 23)
These chromosomes are NOT present in pairs
Gametes are produced during _____________ in the ___________
mono/haploid
gametogenesis
gonads

5. Male gonads = _________ Female gonads = _________
The process that creates monoploid gametes is ____________
Organisms that contain both male & female gonads are called _____________
Ex. Earthworms
testes
ovaries
meiosis
hermaphrodites

6. 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
haploid haploid diploid
sperm egg zygote
fertilization

7. Similar to mitosis, but meiosis has one additional ___________
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

8. 1st Meiotic Division Interphase I: Prophase I: Chromatin replicates
Single stranded → double stranded
Prophase I:
Chromatin coils → chromosomes
Homologous chromosomes pair up during synapsis & form tetrad (group of 4 chromatids)
Chromosomes may twist & exchange genes = crossing over

9. Metaphase I: Anaphase I: Telophase I:
Tetrads equator (different than mitosis!! No tetrads in mitosis)
Spindles attach to centromeres
Anaphase I:
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

10. 2nd Meiotic Division Just like mitosis w/o chromosome replication
Prophase II:
Spindle fibers form & attach to centromeres
Metaphase II:
Chromosomes line up on equator
Anaphase II:
Double stranded chromosomes split & chromatids move to opposite poles & cells are now single stranded
Telophase II:
Both daughter cells divide forming 4 monoploid cells

11. End Result of Meiosis 1 diploid primary (1o) sex cell (2n) ↓
4 monoploid gametes (n)

13. Comparison of Mitosis & Meiosis
Ist step of division
Replication of chromosomes
same
Type of reproduction
Asexual & growth
Used to make gametes for sexual reproduction
Type & # of daugher cells compared to parent cell
2; same as parent (2n)
4, 1/2 chromosomes of parent (n)
# of divisions 1 2
synapsis? no
yes

14. Spermatogenesis: The production of sperm in the testes
4 monoploid sperm produced from each primary spermatocyte

15. 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.

16. 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 .

17. 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

19. 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

20. What type of asexual reproduction is this?

21. What’s wrong with this picture?

22. 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

23. In vitro fertilization:
Fertilization outside of the female’s body
Zygote implanted into female after fertilization
Vitro = glass

24. I say, thank goodness for Veterinarians & IVF:
No more singles’ bars, no more dating!

25. 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.

26. mitosis
fertilization
meiosis

27. diploid
Monoploid/haploid
Chromosome number