O6

O6. Many organisms reproduce by asexual means.

 

Student Outcome: O6.1

Understand that, in eukaryotes, asexual reproduction involves mitosis.

The Stages of Mitosis (warning: long document)

Mitosis is a continuum. Humans have decided to separate the process into stages for the convenience of our discussion. Some humans even separate the stages into sub-stages and intermediate stages.

Properly, mitosis refers to what happens to the chromosomes in the nucleus.

Cytoplasmic division occurs during the accompanying cytokinesis.

The Spindle Complex

Since chromosomes are being distributed into new nuclei, a critical component of the process of mitosis is how the chromosomes are moved. Movement of chromosomes involves sets of microtubules, known as the spindle apparatus.

Microtubules of the spindle complex extend from each pole of the cell and overlap each other at the equator of the cell. Poisons that affect microtubule function block cell division. Spindle formation is one of the events of prophase.

 

Prophase

Chromosome Condensation

• Duplicated chromosomes start to condense and tightly coil to become visible as threadlike structures as prophase starts. Chromosomes continue to condense and become thicker as prophase progresses.
• The nucleolus region (an aggregation of chromosome bits and concentrated RNA and protein) of the nucleus will disassociate.
• The duplicated chromosomes are firmly attached at their centromeres throughout the condensation and coiling.

Nuclear membrane

• The nuclear membrane degrades in later prophase into small vesicles, which can be used to synthesis new nuclear membrane material in the new cells.

Microtubule Organization

• Microtubules initiate spindle formation and determine the poles of the cell. The spindle apparatus will extend from the poles of the cell to the center of the cell surrounding the nuclear region and to the opposite pole of the cell.
• Some microtubules from each pole of the cell attach to each duplicated chromosome's kinetochores located in the centromere region.
• Other microtubules overlap each other from the poles through the equator region of the cell.
• In many cells, clusters of microtubules form around the centriole pairs, which replicated during interphase. Microtubules move centriole pairs to the respective poles of the dividing cell. These regions are sometimes called the asters. Centrioles are not essential to mitosis. Cells that lack centrioles still form the spindle complex. It's just a way to ensure that the new cells will have a pair of centrioles in their cytoplasm.

Note: some researchers choose to call the events that include the degradation of the nuclear membrane and the attachment of the spindles to chromosomes prometaphase.

 

Metaphase

• The spindle apparatus moves the chromosomes to the equator of the cell, aligning the centromeres of each duplicated chromosome along the equator.
• Chromosomes are moved by a combination of pulling and pushing of spindle microtubules.
• The length of the spindle microtubules is regulated by the kinetochores to facilitate the alignment of centromeres at the equator.
• The ultimate alignment of chromosomes along the equator plane of the cell is metaphase, and the chromosomes are often called the metaphase plate.

 

Anaphase

• Centromeres of each duplicated chromosome separate to start anaphase. You can't actually see this; the separating chromosomes are the first visual sign of anaphase.
• Kinetochore motor proteins pull their chromosomes along the spindle microtubules from the equator to the poles of the cells. The microtubules disintegrate behind the moving chromosomes, becoming shorter.
• The overlapping polar microtubules lengthen moving the poles of the cell further apart, and, in animal cells elongating the cell.
• Since sister chromatids are identical, each of the two clusters of chromosomes being pulled to the two poles of the cell has one copy of each original chromosome. As the chromosomes are pulled toward the poles, they begin to lengthen out.

 

Telophase

• The spindle microtubules disperse and the spindle apparatus disappears.
• Chromosomes stretch back out and become indistinct as chromatin.
• Membrane vesicles form new nuclear membranes around each group of chromosomes (at the two poles).
• Each new nucleus has chromosomes identical to the original cell and the same number of chromosomes as the original cell.

Cytokinesis: Separation of the Cytoplasmic Contents

Mitosis describes events of chromosomes and nuclei. Most cells accompany mitosis with cytokinesis, the separation of the cytoplasm of the original cell into two new cells. This is not always the case. Some organisms (including many fungi and algae) are "multinucleate", they just have one cell body with many nuclei. Some animal tissues are also multinucleate.

Cytokinesis coincides with the events of telophase or occurs immediately after, so that at the completion of mitosis, the original cell is separated into two cells, each with a nucleus and DNA identical to that of the original cell.

 

Source: http://scidiv.bcc.ctc.edu/

 

Here is video of mitosis in a real cell.

Go here if it doesn't work.

 

Here is another video of mitosis using phase contrast microscope.

 

Here is a animated version of mitosis.

Here is another animation by Biology in Motion. Go here.

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Student Outcome: O6.2

Explain why the offspring of asexual reproduction are genetically identical to their parent. In asexual reproduction genetic variation occurs only through mutation.

 

Non-Sexual (Asexual) Reproduction

Mitosis is used for all asexual reproduction or propagation. This is especially common in plants, fungi and protists. Animals less commonly reproduce asexually. There are many claims for the world's largest organism based on the ability to make more. Asexual reproduction produces offspring genetically identical to the original parent, as would be expected of any mitosis.

 

Source: as above

 

Although this video is about binary fission, it is also an example of asexual reproduction