Unit 1: Genetic Processes
Lesson 2: Mitosis
The Role of Cell Division
When a unicellular organism such as Amoeba divides to form a duplicate organism, the division of a cell reproduces an entire organism. But cell division also enables multicellular organisms, including humans, to grow and develop from a single cell – the fertilized egg. Even after the organism is fully grown, cell division continues to function in renewal and repair, replacing cells that die from normal wear and tear or accidents. In a dividing cell, the mitotic
(M) phase alternates with interphase, a growth period.
The first part of interphase,
G
1
, is followed by the S phase, when the chromosomes replicate; the last phase of interphase is called G
2
. In the M phase, mitosis divides the nucleus and distributes its chromosomes to the daughter nuclei and cyctokinesis divides the cytoplasm, producing two daughter cells. When a cell can no longer divide, it goes through G
0
, whereby apoptosis, or programmed cell death occurs.
Mitotic Phases:
1. PROPHASE
Chromosomes begin to shorten and thicken and become visible under a microscope. In animal cells, small protein bodies in the cytoplasm called centrioles separate and move to opposite poles of the cell. Centrioles are involved in the formation and organization of spindle fibres, which attach to the chromosomes during cell division. The centromere helps anchor the chromosomes to the spindle fibres through a protein structure called a kinetochore. During prophase, the nuclear membrane starts to dissolve.
3. ANAPHASE
Centromeres divide, and the sister chromatids, now referred to as chromosomes, move to opposite poles of the cell. The chromosomes are being pulled by their centromeres, resulting in a distinctive pattern with the ends of the chromosomes trailing behind. If mitosis proceeds correctly, the same number and type of chromosomes will be found at each pole of the cell.
2. METAPHASE
The spindle fibres being moving and aligning the chromosomes. Each chromosome, composed of sister chromatids, is pulled toward the centre of the cell, where its centromere becomes aligned across the middle, or equator, of the cell.
4. TELOPHASE
Chromosomes reach the opposite poles of the cell and being to unwind. As they do, the spindle fibres dissolve and nuclear membranes form around the chromosomes. This results in two daughter nuclei.

MITOSIS:
5. Cytokinesis:
Cytoplasm of a eukaryotic cell is divided to form two new cells. Although the nucleus is divided during mitosis, the division of the cell content into two new daughter cells occurs during cytokinesis. In addition to separating the two new nuclei, cytokinesis separates the other organelles approximately equally. During the growth stage of the cell cycle, additional copies of organelles are formed.

Homework:
1.
Describe the key events involving chromosomes that take place during the following portions of the cell cycle: a) interphase b) prophase c) metaphase d) anaphase e) telophase
2.
Compare the genetic material of cells before and after cell division
3.
Distinguish between a chromosome and a sister chromatid.

Solutions:
1.
2. 3.