INTRODUCTION TO PLANT BREEDING

AGRONOMY 815 / COURSE NOTES

P. STEPHEN BAENZIGER, 338 Keim Hall, 472-1538

DEPARTMENT OF AGRONOMY / UNIVERSITY OF NEBRASKA

MITOSIS AND MEIOSIS

Textbook of Cytogenetics. 1972. W. V. Brown, Chapt. 4 and 6.


    Chromosome: a vehicle for carrying genes which are:

  1. Arranged in a linear order (reversed by inversion);
  2. Capable of being transferred from one chromosome to another by crossing-over or translocation.

    Chromatids: half chromosomes.

MITOSIS:

Telophase -- "resting stage" -- actually metabolically can be very active, DNA synthesis for chromatids is going on. 2C to 4C

Prophase -- chromosomes become visible and contract, chromosome are two chromatids, but the chromosomes do not pair. Nucleolus and nuclear membrane disappear. 4C

Metaphase -- doubled chromosomes at the equational plate. Chromatids separate to form two daughter chromosomes, spindle formation. 4C

Anaphase -- daughter chromosomes move to the poles, cytokinesis (cell division). 4C back to 2C

    Key features of Mitosis:

  1. Somatic cells contain two chromosomes of each kind.

  2. Members of a pair do not synapse. Occasionally, there is somatic crossing over (e.g., twin spots, can be seen in developmental tissue, but is rare).

  3. Each chromosome duplicates itself (chromatids).

  4. Duplicate chromosomes are separated from each other (chromatids become chromosomes).

  5. Daughter cells are identical with each other and to the parent cell. Important in cell growth and development, and in vegetative propagation.


MEIOSIS

Cell cycle: S1, G1, S2, G2 occur before prophase 1. S is for DNA synthesis, G is for resting stage between DNA synthesis.

Egg mother cell (megasporocyte) and pollen mother cell (microsporocyte) contain 2 chromosomes of each kind (2N).

Prophase 1: Can be broken down into many distinct stages (leptotene, zygotene, pachytene, diplotene, diakinesis). Nuclear membrane and nucleolus disappear. Chromosomes become visible, homologous pair, formation of chromatids, crossing-over. 2C to 4C

Metaphase 1: Chromosomes at equational plate. 4C
Anaphase 1: Chromosomes (not chromatids) moving poleward. 4C
Telophase 1: Nuclear membrane restored. Each nucleus contains ½ the number of chromosomes. 4C to 2C

Then follows a mitotic like division of prophase 2 (2C), metaphase 2(2C), anaphase 2 (2C), telophase 2 (2C to 1C due to formation of cell wall) to tetrad formation. This the formation of spores.

Key features of meiosis:

1. Egg mother cell (megasporocyte) and pollen mother cell (microsporocyte) contain 2 chromosomes of each kind.

2. In prophase 1, members of a pair usually synapse and crossing-over usually occurs.

3. At anaphase 1 and telophase 1 members of a pair go to opposite poles and into daughter cells now with reduced chromosome number (N).

4. Meiosis includes a mitotic-like division to give 4 cells with "n" chromosome number. These cells are called spores.

5. Recombination and segregation of genetic materials occur.