What is Meiosis

Meiosis is a very specialized process of cell division that produces gametes (eggs and sperm), and is quite distinct from the mitotic cycle of normally dividing cells

The purpose of meiosis
a) is to reduce the normal diploid cells (2 copies of each chromosome / cell) to haploid cells, called gametes (1 copy of each chromosome per cell).
In humans, these special haploid cells resulting from meiosis are eggs (female) or sperm (male).
In yeast cells, they are spores.

To carry out this specialized process, the cells duplicate their DNA but follow this by two rounds of division, instead of one. The first division separates the duplicated homologues from each other. This essentially reduces the number of chromosomes in each cell. Thus, we call this the reductional, or Meiosis I division.

The second division operates similarly to the mitotic division, and separates the sister chromatids from one another; we call this the equational, or Meiosis II division. The offspring from meiosis have half the number of chromosomes as their parent cell, because they receive just one copy of each chromosome, rather than two. (Compare the offspring in the meiosis diagram to those in mitosis).

Chapter 5 Summary - Meiosis

r Prophase 1

Prophase I
b. lasts much longer than Prophase of mitosis
(1) can last for days
(2) accounts for 90% of time of meiosis
c. much more complex than Prophase
d. chromosomes condense
e. synapsis occurs
(1) homologous chromosomes come together to form "tetrad"
(2) each gene brought into juxtaposition with its homologue
(3) segments of nonsister chromatids break/rejoin
g. crossing over = exchange of genes between nonsister chromatids of homologous chromosomes during synapsis
-number of crossover events varies with length of the chromosomes
-might get 1 with shorter chromosome
-might get 2 with a longer chromosome
gf. chromosomes thicken more but tetrad stays together
h. spindle forms
i. chromosomes move toward metaphase I plate
 

Metaphase I
 a. graphic summary

b. "tetrads" align on metaphase plate (NOTE the difference between metaphase I and metaphase II)
c. both kinetochores of sister chromatids face same pole d. centromeres of the homologous chromosomes face opposite poles







Anaphase I

a. graphic summary
b. attachment between the homologous chromosomes breaks down
c. homologous chromosomes are separated
-Note:
-it is not the chromatids that separate -it is the duplicated chromosomes of the homologous pair that are separated
d. kinetochores pull duplicated chromosomes toward poles

Telophase I

graphic summary
b. duplicated chromosomes reach the poles
(1) each pole now has a haploid set of duplicated chromosomes
(2) each chromosome still has two chromatids
c. cytokinesis occurs
(1) two daughter cells form
(2) daughter cells are haploid with duplicated chromosomes
(3) spindle may or may not break down
d. may be an Interkinesis
(1) period of time between meiosis I and meiosis II
(2) no further replication of genetic material occurs
-recall: chromosomes still in duplicated condition
(3) nuclear envelope may reform briefly

Stages of Meiosis II



Prophase II

a. graphic summary
b. if interkinesis-then NE breaks down
c. spindle reappears
d. duplicated chromosomes move toward metaphase plate





Metaphase II

a. graphic summary
b. duplicated chromosomes align at metaphase plate just like mitosis
c. kinetochores of sister chromatids directed to opposite spindle poles
4. Anaphase II

a. graphic summary
b. centromeres break down
c. kinetochores pull chromatids toward opposite poles


Telophase II

a. graphic summary
b. chromatids reach poles
c. NE reforms
d. cytokinesis occurs

6. now have:

a. 4 daughter cells
b. haploid
c. chromosomes are unduplicated


http://course1.winona.edu/sberg/241f08/Lec-note/Meiosis.htm

Chapter 5 Meiosis

Chapter 5 Mitosis