3.2.2 All cells arise from other cells

Eukaryotic organisms
In multicellular organisms not all cells retain the ability to divide. Eukaryotic cells that to retain the ability to divide show a cell cycle. Cell division can occur by mitosis or meiosis. Mitosis produces two daughter cells that have the same number of chromosomes as the parent. We will cover mitosis in this section. Meiosis produces four daughter cells each with half the number of chromosomes of the parent cell. Meiosis is covered in section 3.4.3.

Mitosis can be split into 5 stages:

Interphase
Mitosis is proceeded by a period during which the cell is not dividing (this is interphase). Here, DNA replication occurs. The two copies of DNA remain joined at the centromere.
Prophase

• Chromosomes first become visible by shortening and condensing
• (in animals) centrioles move to opposite ends/the poles of the cell
• spindle fibres develop from each centriole. These span the cell from pole to pole and are collectively called the spindle apparatus.
• the nucleolus disappears and the nuclear envelope breaks down leaving the chromosomes free in the cytoplasm.
• the chromosomes are drawn towards the equator by spindle fibres attached to the equator

Metaphase

• the chromosomes can now be seen to be made up of two chromatids (each an identical copy of the DNA from the parent cell unless mutation occurs)
• chromosomes arrange themselves along the equator of the cell

Anaphase

• centromeres divide in two and spindle fibres pull the individual chromatids apart
• chromatids more to their respective poles and are now referred to as chromosomes (energy provided by mitochondria situated around the centrioles)

Telophase (+ cytokinesis)

• chromosomes reach their respective poles and become longer and thinner and disappear altogether leaving widely spread chromatin
• spindle fibres disintegrate

NOTE: Mitosis is a controlled process. Uncontrolled cell division can lead to the formation of tumours and of cancers. Many cancer treatments are directed at controlling the rate of cell division.

We need to be able to calculate mitotic index:

mitotic index = number of cells in mitosis x 100 / total number of cells

Prokaryotic cells
These divide by binary fission:

• The circular DNA molecule replicates and both copies attach to the cell membrane
• the plasmids replicate (a variable number)
• the cell membrane begins to grow between the two DNA molecules and begins to pinch inward, dividing the cytoplasm in two
• a new cell wall forms between the two molecules of DNA dividing the original cell into two identical daughter cells (each with a single copy of the circular DNA and a variable number of copies of the plasmids).

Viruses

Viruses do not undergo cell division. They inject their nucleic acid into a host cell infecting it. The host cell replicates the viral particles.