Lesson 14 Cell Division Unit 3

During the stages when the cell is actively dividing, the chromosomes undergo a great deal of change. The set of changes taking place in the chromosomes during cell division is called mitosis. Mitosis has four phases: prophase, metaphase, anaphase, and telophase. Following telophase, the rest of the cell divides in a process known as cytokinesis.

What are the five phases of the cell cycle? G1, S, G2, mitosis, and cytokinesis.
What happens to chromosomes during the S portion of interphase? Chromosomes are replicated.

During its life, a cell undergoes a cycle that allows for growth and reproduction. This cell cycle includes interphase, a phase that prepares the cell for division, and is further divided into three phases—G1, S, and G2. Mitosis is the next phase, which is the division of the nucleus. The last phase of the cell cycle is cytokinesis, which is the division of the cytoplasm.

Interphase
- G1
  - It is sometimes referred to as the growth phase.
  - mRNA synthesis occurs.
  - Protein synthesis occurs.
  - It may pause and enter G0 phase.
- S
  - It is typically referred to as the synthesis phase.
  - Copies of DNA are synthesized.
- G2
  - Chromatin coils and condenses into chromosomes.
  - Organelles required for cell division are produced.
Mitosis
- Duplicated chromosomes are divided into two equivalent sets.
Cytokinesis
- The cytoplasm and organelles are physically divided.
- Two new daughter cells are formed.

Mitosis

Mitosis was discovered when biologists began looking at cells through microscopes. They could see distinct differences between neighboring cells in the same tissue. The differences were found to be the result of chromosomes changing shapes and positions in the cells.

Mitosis is a cell division process that only occurs in somatic cells or body cells.

Prophase is the first and longest phase of mitosis.

The condensing of the chromosomes marks the beginning of prophase.
Identical copies of chromosomes are called sister chromatids and are attached at a central point called the centromere.
The mitotic spindle appears, which facilitates the separation of the sister chromatids.
The nuclear envelope disappears.
Another set of spindle fibers grows toward the sister chromatids.

Metaphase

Two key events occur during metaphase.

Spindle fibers attach to the kinetochores on each sister chromatid.
Chromosomes move to the center of the cell with the centromere aligned to the central plane of the cell.

Anaphase

Anaphase is the shortest of the four phases.

Sister chromatids separate at the centromere.
Chromosomes are pulled to opposite sides of the cell.
The cell takes on an elongated shape due to the lengthening of some of the spindle fibers.

Telophase

Telophase is the final phase of mitosis.

The mitotic spindle disappears.
A nuclear envelope forms around the chromosomes.
The chromosomes unwind.
There is an equal number of chromosomes in each new nucleus.

Cytokinesis

At the end of mitosis, the cytoplasm is equally divided between each of the new cells. This division of the cytoplasm and its organelles is called cytokinesis. In animal cells, it begins with the appearance of a cleavage furrow, which is a groove that encircles the cell. The cleavage furrow begins forming during telophase and continues to deepen until the two daughter cells are pinched apart. The furrow is created by fibers that tighten around the cell, effectively cutting the two cells apart.

Since plant cells have a rigid cell wall that can’t be squeezed in half, a cell plate is formed between the two nuclei. The plate is built from vesicles that line up in the center of the cell. The cell plate grows outward as more material is added, eventually dividing the plant cell into two new cells. Cellulose is then added between the layers of the cell plate, completing the new piece of the cell wall.

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