1. For a species to survive, it must REPRODUCE!
Ch 13 Notes – Meiosis
Genetics Terminology:
Locus Autosomes
Somatic cell Sex chromosomes
Gamete Diploid
Karyotype Haploid
Homologous chromosomes Zygote
Meiosis

2. 1) ASEXUAL REPRODUCTION:
each new organism gets a set of chromosomes identical to parents
DNA from generation to generation remains the same
therefore, no differences or variations occur in the offspring (difficult to adapt!)

3. 2) SEXUAL REPRODUCTION:
fusion of nuclei from 2 special cells: GAMETES
(female gamete = egg; male gamete = sperm)
with 2 parents, offspring receive half of their chromosomes from the female parent and half from the male parent.
therefore, the offspring will
show new combinations of
characteristics

4. (this ensures the survival of a species)
**Sexual reproduction leads to VARIATION!
(this ensures the survival of a species)
SOMATIC CELLS = all cells EXCEPT the gametes
each somatic cell of an organism contains a specific # of chromosomes (humans = 46)

5. the chromosomes occur in “matching pairs” (1 member of each pair came from the egg and 1 member came from the sperm)
these pairs of chromosomes are called HOMOLOGOUS PAIRS

6. Human Gamete
~cells with half the #
of Chromosomes are
HAPLOID (n)
Human Somatic Cell
~cells with full # of
Chromosomes are
DIPLOID (2n)

7. the union of 2 haploid gametes restores the diploid # of chromosomes
MEIOSIS: cell division in which daughter cells receive only ½ of the chromosomes of the parent cell
RESULT: formation of gametes!
occurs only in reproductive tissues
male = testes female = ovaries

8. Meiosis takes place in 2 stages:
Meiosis I
Meiosis II I II

9. 1) Meiosis I preceded by Interphase: DNA is copied
Prophase I: each chromosome pairs up with its homologous chromosome to form a TETRAD (4 chromatids)
Metaphase I: tetrads line up in the center of the cell
Anaphase I: homologous chromosomes are pulled apart
Telophase I

10. Key Difference!

12. *DNA DOES NOT replicate
before the 2nd
round of division

13. 2) Meiosis II
the 2 cells produced in meiosis I undergo a second division (PMAT)
the result: 4 haploid daughter cells!

16. SOURCES OF GENETIC VARIATION:
1) Independent Assortment: random distribution of maternal & paternal homologues to the gametes. (meiosis)
*random distribution of genes located on different chromosomes resulting in the reshuffling of chromosomes because maternal & paternal homologues will carry different genetic information at many of their corresponding loci

18. 2) Crossing Over (chiasmata): occurs in prophase of meiosis I; exchange of genetic material between homologues which produces chromosomes that contain genes from both parents (meiosis)

19. 4 different
cells!

20. 3) Random fusion of gametes during fertilization (meiosis)
4) Mutation
(any time)

21. What is a somatic cell? Any body cell Male sex cells Female sex cells
Cells with a mutation

22. What is a homologous pair?
2 sister chromatids attached at the centromere
The sex chromosomes (XX or XY) of an individual
2 chromosomes (1 maternal and 1 paternal) with the same genes on them
2 genes (1 maternal and 1 paternal) coding for the same protein

23. If an animal’s skin cell has 20 chromosomes, which of the following is it’s diploid number?10 20 n
Both A and C

24. In which stage of meiosis does the chromosomal number become haploid?
Cytokinesis
Anaphase I
Prophase II
Anaphase II

25. What is a tetrad? The 4 haploid cells that result from meiosis
The 4 sister chromatids that contain the same genes
The name for a homologous pair
Both B and C
None of the above

26. What is the end result of meiosis?
2 identical cells which are identical to the parent cell
2 haploid cells that are identical to the parent cell
4 haploid cells that are different than the parent cell
4 haploid cells that are identical to the parent cell

27. Which of the following can occur in ANY type of cell?
Independent assortment of chromosomes
Crossing over
Random fusion of gametes
mutation