Biology 106: Lecture # 2 Meiosis

I. Evolutionary Significance

II. Discovery of Meiosis

A. 1883: Different cells in Asceris had different chromosome number

B. Sexual Reproduction

gametes (egg or sperm) - contain single complement of chromosomes

zygote (formed from fusion or Syngamy of egg with sperm)-

zygote consist of 2 or more pairs of homologous chromosomes

Homologous chromosomes- chromosomes identical in gene content and which associate in Prophase

III. Meiosis- DEFN: (to diminish) process in which number of chromosomes is halved during gamete formation.

IV. Stages of Meiosis

a. Similar to Mitosis but with 3 important differences:

1. Homologous chromosomes pair lengthwise and chromatids exchange genetic material by x-over.

2. Sister chromatids do not separate from one another in the first nuclear division.

3. Chromosomes do not replicate between the two nuclear divisions.

b. First Meiotic Division (Reduction Division)

1. Prophase I: Synapsis occurs

What is the evolutionary importance of this paired chromosome state (Homologous Chromosomes) ?

2. Metaphase I: HOMOLOGS LINE UP RANDOMLY !! (in regards to which pole they migrate towards)

3. Anaphase I: Chromatids no longer identical (x-over)

4. Teleophase I: Cytokinesis (cell division)

c. Second Meiotic Division (Equational) is a mitotic division with products of meiosis 1.

V. Abnormalities

VI. Outcome of Mitosis and Meiosis

Mitosis	Meiosis
somatic cells	reproductive (germ) cells
N and 2N cells	only in 2N cells
nucleus divides once	nucleus divides twice
Prophase -no synapse	Prophase I-synapse, tetrads form, x-over
Metaphase- sets of sister chromatids line up singly	Metaphas I- sets of sister chromatids lineup in tetrads
	Held to homologue by chiasmata
Anaphase-sister chromatids separate	Anaphase I- homologs separate
	Prophase II- no replication of chromo
	Metaphase II- sets of sister chromatids line up singly
	Anaphase II- sister chromatids separate

VII. Evolutionary Consequences of Meiosis

A. Reassortment of genetic material produces a tremendous amount of genetic variability.

B. Reassortment occurs in two ways: (1) x-over , and (2) random alignment of chromo. before division.

C. Potential Genetic Variability? GREAT!!!!!

VIII. TAKE HOME MESSAGE

A. Sexual reproduction depends on meiosis

1. reduction of chromosome content of gametes to 1/2 of zygote

2. generation of genetic diversity: a. crossing over, b. random assortment of parent's chromosomes

3. Cannot generate new genes- just rearrange those already there.

**Mutation is the agent for new genes**

THINK ABOUT:

** Evolutionary implications of sex & meiosis

** Why did meiosis and sex evolve?

** Why should organisms become asexual?