Chapter 11
Sexual Reproduction and Meiosis
Rev Question
Meiosis and Gametes
1
When and where does meiosis occur in humans?
Why is meiosis necessary?
“reductive division”
Meiosis and Gametes
2
What are the stages of
Meiosis?
McGraw-Hill Meiosis
Meiosis unique features
Meiosis I vs Meiosis II
Prophase I
Metaphase I
Anaphase I
Telophase I
Meiosis and Gametes
3
How does Prophase I differ from Prophase of Mitosis?
1) Bivalents form
-- “synapsis”
-- homologous chromosome pairs
2) Crossing over occurs
-- exchange of DNA segments
-- “chiasmata”
During Metaphase 1,
bivalents are aligned at equator
not individual chromosomes
Question
Meiosis and Gametes
4
During Meiosis I parental homologous
chromosomes are independently
sorted to daughter cells
Chromosome reduction occurs
during Meiosis I
McGraw-Hill sorting
Interkinesis
What happens during Meiosis II?
Chromosome reduction
---- sister chromatids separate
Gametes produced
Question
Meiosis and Gametes
5
Comparison of
Meiosis and Mitosis
Metaphase looks different
for each process
Question
2N=4
Question
Meiosis and Gametes
6
Homework assignment:
10 points: due next class
For a cell where 2N=6, draw a series of diagrams showing the chromosomes moving
through the stages of meiosis.
In one of the diagram clearly label:
-- a pair of homologous chromosomes
-- a bivalent
Label all stages
Make diagrams large, clear and neat
Drawings must be neatly done
and chromosomes drawn with
adequate detail. Chromosomes,
bivalents and spindle fibers
must be recognizable as these
structures.
Also answer the following questions:
For an organism with a chromosome number of 2n = 56:
A. How many pairs of homologous chromosomes does each normal body cell possess? ___
B. How many chromosomes does each cell possess after mitosis? ____
C. How many pairs of sister chromatids are present at the beginning of meiosis? ___
D. How many bivalents will form during prophase-I? ____
E. How many chromosomes does each cell possess after meiosis? ___
Meiosis and Gametes
7
Why does meiosis in females yield
only 1 viable gamete?
Why does spermatogenesis in males
assure 50:50 ratio of males to females?
Sex of the sperm determines
sex of the offspring
Meiosis and Gametes
8
How can errors during meiosis cause
Congenital birth disorders?
Non-disjunction of chromosomes
e.g., Down’s syndrome
What is karyotyping?
“trisomy 21”
-- an example of “aneuploidy”
Many effects
-- reduced intellectual development
-- heart and kidney defects
-- often fatal
-- early onset Alzheimer's
Meiosis and Gametes
9
How does meiosis create
genetic diversity?
1) crossing over
2) random assortment of chromosomes
and later …
3) random pairing of gametes
Meiosis and Gametes
10