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Chapter 5
Growth and Division
I.
Cell Growth - Cells do not grow indefinitely, they divide to keep their DNA intact,
and to ease the movement of nutrients and wastes across the cell membrane.
Bigger animals have more cells, not bigger cells.
Cell size is limited.
Volume increases faster than surface area. Why do cells stay small?
A. DNA Overload
1. The information contained in the DNA of a cell is sufficient for the
needs of small cells.
B. Exchanging materials
1. Food, oxygen, and water entering the cell, and wastes leaving the
cell depend on the surface area of the cell membrane.
2. The rate at which food and oxygen are used up and waste
products produced depends on the cell’s volume.
3. As volume increases in the cell, there is a resulting decrease in
the cell’s ratio of surface area to volume, making the moving of
materials in, and wastes out, more difficult for the cell.
C. Cell Division
1. A growing cell forms 2 daughter cells through the process of cell
division.
2. Before cell division occurs, the cell replicates (copies) all of its
DNA. Each daughter cell receives a complete copy.
3. Cell division also solves the problem of increasing size by
reducing cell volume.
II.
Cell Division
Eukaryotic cell division is more complex than prokaryotic.
A. Chromosomes – each organism has a certain number
1. Made up of DNA and proteins.
2. They are not visible in the cell until cell division.
3. The chromosomes replicate and condense before cell division.
They become visible at the beginning of cell division, and each
chromosome consists of 2 identical sister chromatids.
4. Each pair of sister chromatids is attached by a centromere.
The cell cycle –
1. the series of events a cell goes through as it grows and divides.
2. A cell grows, prepares for division, and divides to form 2 daughter
cells. These start a new cell cycle.
The cell cycle has four main stages:
A. Interphase
- G1- growth
- S – synthesis of DNA (replication)
- G2 – organelles and molecules required for cell division are produced
B. M phase – mitosis and cytokinesis
- prophase
- metaphase
- anaphase
- telophase
- cytokinesis
III.
IV.
Mitosis
1.
2.
3.
4.
V.
Cytokinesis
1.
2.
3.
VI.
Prophase – the chromosomes condense and become visible,
centrioles (2 tiny structures located in the cytoplasm near the nuclear
envelope) appear, spindle forms, nucleolus disappears, nuclear
envelope breaks down.
Metaphase – chromosomes line up across the center of the cell. Each
is connected to a spindle fiber at its centromere.
Anaphase – sister chromatids separate into individual chromosomes
and move apart.
Telophase – the chromosomes gather at the opposite ends of the cell
(poles) and lose their distinct shape. Two new nuclei form.
Usually occurs at the same time as telophase
In most animal cells, the cell membrane is drawn inward until the
cytoplasm is pinched into 2 parts.
In plant cells, the cell plate forms midway between the dividing nuclei.
The cell wall begins to appear in the cell plate.
Regulating the Cell Cycle
Not all cells divide at the same rate, and some cells do not divide at all (muscle, nerve)
A. Controls on cell division
1. Controls on cell growth and division can be turned on and off.
2. Cells in a Petri dish will continue to grow until they come into contact
with other cells.
3. During the healing process, the cells at the edge of the cut or bone
break divide rapidly, producing new cells for healing. When the healing
is almost complete, the rate of cell division slows, and growth controls
are restored.
B. Cell Cycle Regulators
1. The timing of the cell cycle is regulated by a protein called cyclin.
2. Internal regulators – proteins that respond to events inside the cell
Ex. Regulatory proteins that does not let a cell enter mitosis until all its
chromosomes have been replicated.
3. External regulators – proteins that respond to events outside of the
cell.
Ex. Growth factors that stimulate growth and division of cells during
embryonic development and wound healing. Molecules on the surfaces
of cells may cause cells to slow down to prevent excessive cell growth.
C. Uncontrolled Cell Growth
1. Cancer – a disorder in which some of the body’s own cells lose the
ability to control growth
2. Cancer cells do not respond to signals that would normally stop them
from dividing.
3. Some cancer cells will no longer respond to external growth regulators.
4. Some cancer cells fail to produce the internal regulators that ensure
orderly growth.