Tutorial: Mitosis
Tutorial: Mitosis


Copyright © Steve Kuensting, 2004, All Rights Reserved.
This web tutorial may not be distributed by any means
without the expressed permission of the author!




Introduction

One of the fundamental and truly unique characteristics of all cells is their ability to reproduce. All organisms are able to reproduce only because their cells that make them up are capable of this feature. The process of cell reproduction is called CELL DIVISION. The process of cell reproduction is a DIVISION process of the cell. All cells of a multicellular organisms arise from divisions of the original fertilized egg cell, and all cells on earth arise by division of a preexisting cell.

Mitosis introduction



Reasons for Cell Division

Cells will divide for a few reasons. For a unicellular organism, cell division is a means for reproduction. When the cell divides, one original cell has made two cells. In multicellular organisms cells divide to replace dead or damaged cells, as well as to increase the total cell number in growth.
Prokaryotic vs. Eukaryotic Reasons for Cell Division
ProkaryotesEukaryotes
ReproductionGrowth
Maintenance and Repair



Daughter Cells

Inside of a cell the nucleus contains the genetic instructions for all activities and all characteristics of the cell. These DNA instructions must be carefully passed on to daughter cells when a cell divides or the daughter cells will not survive. (The offspring cells of cell division are referred to as "daughter cells".)

Nucleus



Chromatin

Inside of a nondividing cell's nucleus the DNA is loosely scattered about. This arrangement of DNA is called CHROMATIN. Chromatin is found in all nondividing cells. Under a light microscope the chromatin appears as "dots" and dark areas inside the nucleus. Chromatin is easily visible in a human cheek cell under a light microscope.

Chromatin



Chromosomes

As a cell begins cell division, the chromatin begins to condense or package itself more neatly. It first coils around special proteins called HISTONES to organize itself into a smaller space, much like fishing line on the spool of a fishing reel.

Histones



Chromosomes

The histone-coiled DNA then folds over itself many times to further pack itself into an even smaller space. These folds then fold over themselves again to further compact in size.

DNA folding



Chromosomes

The DNA then coils itself by wrapping into loops which form a giant coil or spring-like structure called a supercoil. This further compacts and organizes the DNA for cell division.

DNA coiling



Chromosomes

The supercoiled DNA exits as thick spring-like strands called CHROMOSOMES. Chromosomes are only present in a cell during cell division, chromatin is always present in a nondividing cell.

Chromosome formation



Chromatids vs Chromosomes

A chromosome consists of two identical supercoils of DNA called CHROMATIDS. A chromatid is essentially one-half of a chromosome or one giant supercoiled piece of DNA. The two chromatids of a chromosome are held together by a structure called a CENTROMERE.

Chromatids



Chromatin vs Chromosomes

DNA structure cannot be seen with a light microscope. Chromatin appears as almost indistinguishable black dots under a high power light microscope. But, chromosomes are clearly visible and highly distinguishable with a light microscope under high power.

Chromatin vs chromosomes



Chromosome Numbers

Different organisms have different numbers of chromosomes in their cells. Some different types of organisms are listed below next to the normal chromosome number found in their cells. Notice that the chromosome number has nothing to do with the intelligence or complexity of a species.

Chromosome Counts in Different Species
SpeciesChromosome #SpeciesChromosome #
fruit flies8chimpanzee48
pea plant14amoeba50
crayfish> 100human46
mallard80mosquito6
rat42horse64
toad22redwood22
wheat42earthworm36
chicken78house cat32



Even Chromosome Numbers

Note that all of the chromosome counts were even in number. This is because all sexually reproducing organisms have chromosomes in pairs. In other words, for every chromosome in a cell, another chromosome looks just like it. In human cells, there are 23 pairs of chromosomes, not 46 different looking chromosomes.


Chromosome numbers



Homologous Pairs

The chromosomes of a similar pair have the same size and shape. These similar-looking chromosomes are called HOMOLOGOUS PAIRS. Each chromosome of a homologous pair is called a HOMOLOG. Even under a microscope, any two homologs of a pair cannot be distinguished from one another. But, the homologs of one homologous pair are very different from the homologs of other pairs.

Homologous pairs



Diploid

Most sexually reproducing organisms carry both homologs of every possible homologous pair that their species has. Because of this, they are said to be DIPLOID. If a cell or organism is diploid it means that the cell or every cell of the organism has homologous pairs of chromosomes. The notation "2N" is used to refer to the diploid state. Humans are diploid. The phrase "Diploid number" (abbreviated 2N #) refers to the total number of chromosomes of a diploid cell. For a human, that number is 46. Some different species are listed below.

Diploid Chromosome Counts
in Different Species
Species2N #
mosquito6
horse64
chimpanzee48
cat32
brown bat44
alligator32
bullfrog26



Haploid

Any cell that has only one chromosome of each homologous pair is referred to as HAPLOID. The term "1N" refers to the haploid state. Sperm and egg cells are haploid, most organisms are not. The phrase "Haploid Number" (abbreviated "1N #) refers to the total number of chromosomes found in a haploid cell. For a human, the haploid number is 23. Other organisms are listed below. Haploid cells never contain homologous pairs of chromosomes.

Haploid Chromosome Counts
in Different Species
Species1N #
mosquito3
horse32
chimpanzee24
cat16
brown bat22
alligator16
bullfrog13



2N is Twice N

Note that the diploid number is always twice the haploid number. This is because the total number of chromosomes of a diploid cell has to be twice the total of a haploid cell, since the diploid cell has pairs and the haploid cell has only individuals.

2N and 1N Chromosome Counts
in Different Species
Species1N #2N #
mosquito36
horse3264
chimpanzee2448
cat1632
brown bat2244
alligator1632
bullfrog1326



Chromosome Distribution

Chromosomes are the focus of this discussion because their passage to daughter cells in cell division is critically important. When cell division occurs, the chromosomes (containing the DNA) must be correctly distributed to the daughter cells or the daughter cell have no hope for survival.

Chromosome passage to daughter cells



Mitosis vs Cytokinesis

Every cell division consists of two phases: 1) MITOSIS - division of the nucleus and the chromosomes inside, and 2) CYTOKINESIS - division of the cytoplasm and the organelles it contains. In cell division, mitosis occurs first, and cytokinesis is second.

Mitosis before cytokinesis



Cell Division Control

The factors that control cell division include nutrients, growth factors, attachment to other cells, and the cell membrane-surface area to cytoplasm-volume ratio. As the cell grows too large for diffusion to meet its nutrient needs, it may divide. Also, when damage occurs to cells in a multicellular organism, nearby cells can somehow detect this and divide to replace the damaged cells.

Mitosis control



The Cell Cycle

Before we discuss cell division we have to take a look at what goes on in the typical "life" of a cell. All cells go through definite stages where they perform certain activities to stay alive. The sequence of events that occur in a cell's life is called the CELL CYCLE. Basically, the cell cycle consists of 2 broad phases: 1) cell division - cell divides, and 2) interphase - cell growth and development after cell division.

Cell cycle



The Cell Cycle

All cells repeat these 2 basic events in their lives. They divide and then grow, divide and grow, and so on. Both cell division and interphase consist of smaller phases that involve specific activities. Cell division consists of mitosis and cytokinesis. Interphase consists of an initial growth phase (G1), followed by a DNA replication phase (S), followed by a final growth phase (G2).

Phases of the cell cycle



The Cell Cycle

In the G1 phase, the cell doubles in size and all organelles double in number. During the S phase, ALL DNA is replicated. During the G2 phase, enzymes are produced to prepare for the next cell division. During mitosis, the cell nucleus is split, and during cytokinesis, the cytoplasm and organelles are distributed to the daughter cells.

Cell cycle phases





Cell Cycling

Not all cells continually go through the cell cycle or even cycle at the same rates. Human muscle and nerve cells stopped cycling before any human was even born, remaining "frozen" in the G1 phase for life. Muscle and nerve cells never reproduce. On the other hand, human skin cells cycle constantly to replace the dead and dying cells at the skin surface. Also, the cell cycle may require 2 hours for a fungus cell, 24 hours for a human skin cell, and 48 hours for an ameba.

Cell cycling times



Phases of Mitosis

We will now focus on the events that occur in mitosis and cytokinesis of cell division. Mitosis consists of 4 stages that involve the careful dividing of the contents of the nucleus. Cytokinesis involves only 1 phase where the cytoplasm is haphazardly split into two compartments. Mitosis is very exact, cytokinesis is inexact. This is because the genetic instructions must be VERY carefully handled, whereas the exact number of organelles passing into each daughter cell is unimportant.

Mitosis events



Phases of Mitosis

The four phases of mitosis are: 1) Prophase, 2) Metaphase,
3) Anaphase, and 4) Telophase
. Each involves distinct events and activities that occur to ensure the correct division of the chromosomes of the nucleus.

Phases of mitosis



Prophase

As a nondividing cell enters cell division, it will begin prophase of mitosis to prepare its nucleus for division. Several major events occur during prophase. The chromatin coils and folds up into chromosomes, the nuclear membrane disappears, and the nucleolus disappears. In addition, centrioles become visible in animal cells and move away from one another. (Plant cells have no centrioles.)

Prophase events



Prophase

The centrioles produce rope-like structures called Spindle Fibers (microtubules) that are important to dividing the nucleus. Those fibers that stretch across the cell from centriole to centriole are called polar fibers. Other spindle fibers that connect to the centromeres of the chromosomes are called kinetochore fibers.

Prophase



Metaphase

The next stage of mitosis involves lining up the chromosomes for division. The chromosomes are moved to the center of the cell by the kinetochore fibers.

Metaphase



Anaphase

Anaphase involves the splitting of each chromosome into 2 separate chromatids by the kinetochore fibers pulling from opposite directions. The chromatids then move to opposite ends of the cell by the kinetochore fibers pulling on them.

Anaphase



Telophase

Finally, once the chromatids have reached opposite ends of the cell, telophase begins. It is essentially the opposite of prophase. Centrioles and spindle fibers disappear. Chromatin forms from chromosomes, the nuclear membrane reforms, and the nucleoli (2 of them) reappear. There are now 2 SEPARATE NUCLEI IN ONE CELL.

Telophase



Cytokinesis

In some cells, cytokinesis starts during telophase, in others, after telophase. Cytokinesis in plant cells is different than cytokinesis in animal cells. In animal cells, cytokinesis involves a pinching inward of the cell membrane until the cell splits in two. In plant cells a cell plate is formed between the two nuclei that eventually splits the cell.

Cytokinesis



Cytokinesis

Cytokinesis is not an exact splitting process. Quite often, one cell ends up with more cytoplasm and organelles than the other cell. This causes no harm, however, unless one cell is missing a particular organelle, which is quite rare. The mitosis and cytokinesis have thus resulted in the formation of two new daughter cells which will then enter the G1 phase of interphase and begin to grow.

Animation



Cell Division Animated: Animal Cell Mitosis

Animal cell mitosis animation
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =
Loop

Cell Division Animated: Plant Cell Mitosis

Plant cell mitosis animation
The animations are Copyright © 1989, Steve Kuensting, All Rights Reserved.
Speed = | Delay = milliseconds | Frame # =
Loop

Explanation

One of the major differences between animal and plant cell division is cytokinesis. Animal cells are flexible because they lack the cell wall so the cell can pinch inward to split the cell in half. In plant cells the cell wall prevents that, so instead a special wall -- the Cell Plate -- is built to split the cell. Then a new cell membrane and cell wall is built by each daughter cell on either side of the cell plate, after which, the cell plate dissolves away.

Plant vs animal cell division



Animal vs Plant Cell Division

Another difference between animal and plant cell division is the fact that plant cells have no centrioles. It is not known what produces the spindle fibers in the plant cell because the centrioles function in animal cell division is to produce the spindle fibers.

Plant vs animal cell division



After Mitosis

Each of the daughter cells will then enter the G1 phase and double in size and organelle number, then replicate its DNA, then enter the G2 phase to prepare for another cell division. The cell cycle usually never stops except for special cells (muscle or nerve).

After mitosis



Mitosis and Cloning

Since each chromosome contains two identical chromatids which are split from one another in cell division, mitosis ensures that all information is correctly distributed to the daughter cells. At the end of cell division each cell contains the diploid number of chromatids (in chromatin form). Each of the chromatids will replicate itself to produce full chromosomes (in chromatin form) for the next cell division. In this way, genetic instructions are never lost. Mitosis only separates identical copies of genetic instructions so that each cell receives the total set of instructions.

Replication importance



Mitosis Conserves 2N Number

In normal cell division, the diploid number is conserved, even though the daughter cells contain chromatids instead of full chromosomes. In this way, the daughter cells are clones of one another and are identical to one another. They have exactly the same instructions, one chromatid of each original chromosome. This is how every cell of a multicellular organism has the same set of instructions. Mitosis and replication in the cell cycle ensure it.

Mitosis conserves chromosome count



Quiz

  1. What is the name of the process of nuclear division in a cell?

  2. What is the name of the division of the cytoplasm of a cell?

  3. What is the name of the replication phase of the cell cycle?

  4. What structure holds two chromatids together in a chromosome?

  5. What structure produces spindle fibers in an animal cell?

  6. Are two chromatids of a chromosome identical to one another? (Type 'yes' or 'no')

  7. What proteins are found in a chromosome that help it coil and fold up in cell division?

  8. What is the first phase of mitosis?

  9. What is the name of spindle fibers that connect to the centromeres of chromosomes?

  10. During what part of cell division do chromosomes line up at the center of the cell?

  11. What structure do plant cells lack that animal cells have which is important to animal cell division?

  12. What thin barrier disappears in prophase?

  13. How many nuclei are present in a cell at the end of telophase?

  14. What is the name of the phase of cell growth and development that occur in a nondividing cell in the cell cycle?

  15. What phase of the cell cycle immediately follows mitosis?

  16. What is the diploid number of chromosomes for a human cell?

  17. During what part of mitosis are chromatids pulled to opposite poles?

  18. During what part of animal cell mitosis are centrioles pulled to opposite poles?

  19. What is the haploid number of chromosomes for a human cell?

  20. Which period of the cell cycle is the longest? (interphase or cell division)

  21. If an organism was diploid and it had 44 total chromosomes in a typical cell, what would the haploid number be?

  22. During what part of mitosis do nuclear membranes reform?

  23. What organelle disappears during cell division?

  24. What term is used to describe the identical looking pairs of chromosomes found in a cell in cell division?

  25. What symbol(s) represent the diploid state?

  26. What symbol(s) represent the haploid state?

  27. During which phase of the cell cycle does a cell prepare for cell division?

  28. What is the name of the structure formed to split a plant cell?

  29. How many chromatids are found in 30 chromosomes?

  30. Do diploid cells have homologous pairs of chromosomes?


Diagrams
1st
Diagrams - name the labeled structures/phases.
Quiz 1

2nd
Diagrams - name the labeled structures/phases.
Quiz 2
Answers