Lesson 10: Title: Cell Division (Mitosis)

Lesson 10: Title: Cell Division (Mitosis)

Mr. Harwood

Updated: February 2015
Course: SNC2D1
Unit: Biology

Lesson 10: Title: Cell division (mitosis)

Preliminaries:

Lesson:

Q. What is asexual reproduction? No sex, male or female. The children are identical to the parents.

Bacterial fission,
budding (a piece of an algae or hydra breaks off and starts a new organism),
runners (spider plants, strawberries),
fragmentation (worms, starfish)

Mitosis: asexual reproduction of individual cells.

Meiosis : sexual reproduction of cells  only to produce sperm and egg cells. Not identical to parents!

Cells divide for 3 reasons:

1. Growth – as multicellular organisms grow larger, they need more cells

2. Repair – when cells are damaged, they must be replaced

3. Reproduction – in order to produce more organisms

Life span of a cell.

The cell’s life is customarily defined at the time between two successive cell divisions.

For example: when cell A divides, it produces two identical cells. We could call one of these cell A still, but which one?
Instead, we’ll call them cell B and C.
Similarly, when C divides into two cells, we say that C no longer exists. It is now D and E.

* A cell can also end its lifestyle if it dies.

Definitions:

(show video and image from Wikipedia to illustrate this)

chromatin: DNA and the histone proteins that it wraps around. Looks like beads on a string. This is the normal state of DNA in the nucleus

chromosome: aside from being one of the 46 pieces that our DNA is divided into, “chromosome” more commonly refers to each piece of DNA when it has coiled up so tightly that it is visible under a light microscope. Chromosomes then look like an X

chromatid: a chromatid is one side of the X. A chromosome has two identical pieces of DNA that are attached at a centromere. Each side is called a sister chromatid.

Life cycle of a cell:

Most of the cell’s life is spent in normal life, not dividing. This is called Interphase.

There are three parts to interphase

  • G1: gap1. The cell grows. Organelles duplicate.
  • S: synthesis: the DNA duplicates
  • G2: enzymes needed for mitosis are produced.

The S stage looks simple. Read the final paragraph on this page for some idea of what really happens.

Mitosis

The process by which the cell divides is called mitosis.

The crazy thing is that the nucleus and DNA cannot direct the cell as to how to do this since all of the DNA is wrapped up into chromosomes. How does the cell know how to proceed through the phases?

There are four stages of mitosis:

1. ProphaseYou can remember this by PMAT (play me a tune)

2. Metaphase

3. Anaphase

4. Telophase

Prophase

  • DNA is compacted from chromatin until the chromosomes look like Xs
  • In animal cells, the nuclear membrane dissolves
  • Centrioles begin to migrate to opposite ends of the cell, and send out long microtubules or spindle fibres.

Metaphase

  • The spindle fibres are attached to the centromeres.
  • The spindle fibres line up all the chromosomes in the middle of the cell.

Anaphase

  • The chromosomes separate into sister chromatids
  • They are pulled to oppsite ends of the cell.

Telophase

  • The chromosomes unwind to the chromatin form.
  • The nucleus reforms.

The cell physically divides in two. This is called ‘cytokinesis’.

To remember what happens in each stage:
P=prepare
M=middle
A=apart
T=two

Homework:

Complete handout on identifying mitosis

Interesting quote: from “Programming of Life”, Donald Johnson, p. 24.

Biology professor Jerry Bergman uses a good analogy for the DNA replication process during cell division to create two cells from a single cell. Since all life starts as a single cell, this description applies to every organism. Scale-up DNA by a factor of one million to the equivalent of two 125-mile long strands of fisherman’s monofilament line wrapped together to form a double-helix structure, neatly folded and packed to fit into a basketball (nucleus equivalent). Envision the engineering problem of creating an exact duplicate of each 125-mile long line to form two identical sets of those pairs of twisted lines, each packed into a new basketball! During cell division, the entire length of DNA must be split apart, duplicated, and repackaged for each daughter cell. There are about 25 million protein spools around which the DNA is wrapped, organized into an extremely complex hierarchical set of protein structures. Once an initiator protein locates the correct place to begin copying, a helicase “unzipper” unwinds the strands at approximately 8000 rpm, forming a fork area, without tangling the DNA strands as they separate. An “untwister” enzyme (topo-isomerase) systematically cuts and repairs resulting strands to prevent tangling as each DNA strand is formed. Other enzymes copy the flat, untwisted sections of DNA, which are then connected together via DNA ligases into one continuous strand. There are over 30 specific functional proteins required for cell replication, each manufactured according to its own implemented computer algorithm . . . with all processes digitally controlled. It has also been recently discovered that the replication protocol has higher priority than the protein-manufacturing protocol. The replisome runs along the same path as the RNA polymerases (for protein transcription), but causes any polymerase to abort its task so that replication can be done reliably.

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