1. Meiosis Formation of Gametes (Eggs & Sperm)

2. Important Terminology
Diploid (2N): “two sets”; a cell that has 1 set of chromosomes from its female parent and 1 set from its male parent (=2 copies of each chromosome)
Homologous: each chromosome from one parent has a corresponding chromosome from the other parent.
Haploid (N or 1N):
the number of chromosomes in a gamete (sex cell – egg or sperm)
Only one copy of each chromosome

3. Polypoidy What would this mean???
More than diploid (more than 2 sets of chromosomes)
Triploid (3N)
Tetraploid (4N)
Not viable (“survivable”) in humans
Seedless watermelons are triploid, formed from a cross between diploid and tetraploid watermelons.
Because the 3 copies of the chromosomes won’t line up correctly during meiosis, meiosis is halted, so no gametes (seeds) are formed.

4. Definition of Meiosis
The process of reduction division in which the number of chromosomes per cell is cut in half (reduction) through the separation (division) of homologous chromosomes in a diploid cell
Homologous chromosomes (“homologs”): “matched” chromosomes (not necessarily identical); both before and after replication!

5. Facts About Meiosis
Preceded by interphase which includes chromosome replication
Two meiotic divisions --- Meiosis I and Meiosis II
Called reduction division
Original cell is diploid (2n)
Four daughter cells produced are monoploid (1n) (AKA, haploid)

6. Facts About Meiosis
Daughter cells contain half the number of chromosomes as the original cell.
Produces gametes (eggs & sperm)
Occurs in the
testes in males (Spermatogenesis)
ovaries in females (Oogenesis)

7. More Meiosis Facts
Human cells start with 46 double stranded chromosomes (2n).
After 1st division - 23 double stranded chromosomes (n) in each of 2 cells
After 2nd division - 23 single stranded chromosomes (n) in each of 4 cells
  Occurs in our germ cells that produce gametes

8. Why Do We Need Meiosis?
It is the fundamental basis of sexual reproduction.
Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote.

9. Fertilization – “Putting it all together”

10. Replication of Chromosomes
Replication is the process of duplicating a chromosome
Occurs prior to division (interphase)
Replicated copies are called sister chromatids.
Held together at centromere
Occurs in Interphase
Replication is the process of duplicating chromosome. The new copy of a chromosome is formed by DNA synthesis during S-phase. The chromosome copies are called sister chromatids. Sister chromatids are held together at the centromere.

11. A Replicated Chromosome
Gene X
Sister Chromatids
(same genes,
same alleles)
Homologs
(same genes, different alleles)
Homologs separate in meiosis I and therefore different alleles separate.

12. Meiosis Forms Haploid Gametes
Meiosis must reduce the chromosome number by half.
Fertilization then restores the 2n number.
from mom
from dad
child
too
much!
meiosis reduces
genetic content
The right number!

13. Meiosis: Two Part Cell Division
Sister
chromatids
separate
Meiosis I
Meiosis II
Homologs
separate
Diploid
Diploid
Haploid