• Biology
• Medicine
• Tutorials
• Dictionary
• Articles
• Sitemap

1. The Essentials of Cellular Reproduction

2. Prokaryotic and Eukaryotic Chromosomes

compared

3. The Eukaryotic Cell Cycle

4. Mitosis

5. Meiosis

6. Cytokinesis

7. Cell Division and asexual reproduction

1. The Essentials of Cellular Reproduction

• Cell division transmits a complete set of hereditary information to each daughter cell.
• Cell division transmits essential cytoplasmic materials to each daughter cell.
• The activities of a cell from one cell division to the next constitute the cell cycle.

2. Prokaryotic and Eukaryotic Chromosomes compared:

Prokaryotic cells (the bacterium Escherichia coli)

The Eukaryotic chromosome

• The eukaryotic chromosome consists of a single strand of DNA, complexed with proteins.
• Eukaryotic chromosomes often occur in pairs, containing similar genetic information.

  Here's a set of HUMAN chromosomes...
  

3. The Eukaryotic Cell Cycle

• Growth, replication of chromosomes, and most cell functions occur during interphase.
• Cells vary in the duration of interphase.
• Cell division consists of nuclear division and cytoplasmic division.

4. Mitosis - "Normal Cell Division"

Due to their increased numbers of chromosomes, organelles and complexity, eukaryote cell division is more complicated, although the same processes of replication, segregation, and cytokinesis still occur.

Mitosis is the process of forming (generally) identical daughter cells by replicating and dividing the original chromosomes, in effect making a cellular xerox. Commonly the two processes of cell division are confused. Mitosis deals only with the segregation of the chromosomes and organelles into daughter cells.

Click here to view an animated GIF of mitosis from http://www.biology.uc.edu/vgenetic/mitosis/mitosis.htm.

Eukaryotic chromosomes occur in the cell in greater numbers than prokaryotic chromosomes. The condensed replicated chromosomes have several points of interest. The kinetochore is the point where microtubules of the spindle apparatus attach. Replicated chromosomes consist of two molecules of DNA (along with their associated histone proteins) known as chromatids. The area where both chromatids are in contact with each other is known as the centromere the kinetochores are on the outer sides of the centromere. Remember that chromosomes are condensed chromatin (DNA plus histone proteins).

The above image is modified from http://www.whfreeman.com/life/update/.

During mitosis replicated chromosomes are positioned near the middle of the cytoplasm and then segregated so that each daughter cell receives a copy of the original DNA (if you start with 46 in the parent cell, you should end up with 46 chromosomes in each daughter cell). To do this cells utilize microtubules (referred to as the spindle apparatus) to "pull" chromosomes into each "cell". The microtubules have the 9+2 arrangement discussed earlier. Animal cells (except for a group of worms known as nematodes) have a centriole. Plants and most other eukaryotic organisms lack centrioles. Prokaryotes, of course, lack spindles and centrioles; the cell membrane assumes this function when it pulls the by-then replicated chromosomes apart during binary fission. Cells that contain centrioles also have a series of smaller microtubules, the aster, that extend from the centrioles to the cell membrane. The aster is thought to serve as a brace for the functioning of the spindle fibers.

The above image is modified from http://www.whfreeman.com/life/update/.

The phases of mitosis are sometimes difficult to separate. Remember that the process is a dynamic one, not the static process displayed of necessity in a textbook.

• During prophase, the chromosomes condense and the spindle forms and attaches to the chromosomes.
  
  
  
• During metaphase, the chromosomes are aligned along the equator of the cell.

  

• During anaphase, sister chromatids separate andare pulled to opposite poles of the cell.
  
• During telophase, nuclear envelopes re-form around each group of chromosomes.
  

1 cell, 2n -> 2 cells, 2n each
part of cell cycle, where cells normally divide (for somatic
cells)

crossovers are rare

no change in chromosome number (usually 2n -> 2n)

one cell division

no pairing of homologues (normally)

centromeres divide at anaphase

Conservative process: (daughter cells' genotypes
identical to parental genotype)
cells can be either diploid or haploid

5. Meiosis - Produces Gametes

1 cell, 2n -> 4 cells, 1n each

• produces sex cells (gametes)
• crossovers are quite common
• chromosome number is halved (usually 2n -> 1n)
• two cell divisions
• full synapsis of homologues at prophase I
• centromeres do NOT divide at anaphase I,

• but they DO at anaphase II
• promotes variation amongst the products of meiosis
• cell undergoing meiosis is diploid
  
  

A Comparison of Mitosis and Meiosis:

6. Cytokinesis

7. Cell Division and asexual reproduction

• Black Market Antibiotics Endangering Public Health.
  
• In an Antarctic Desert, Signs of Life in the Simplest of Forms.
  
• New Research on Soviet Anthrax Leads to Questions About Vaccine.
  
• NASA Still Dreams of Building an Outpost for People on Mars.
  

Related article:

• Cellular Reproduction-Chapter 9-An Introduction to Life on P2008-03-06 05:34:10
• Harvesting Energy from Food: Glycolysis and Cellular Respira2008-03-06 05:31:00
• Homeostatic Mechanisms and Cellular Communication - The Huma2008-01-07 11:22:15
• Protein Activity and Cellular Metabolism - The Human Physiol2008-01-07 11:22:06