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Cell Biology
The World of Cells
Eukaryotic Cells
Eukaryotic Cells Eukaryotic cells exist in a wide variety of forms, ranging from single celled creatures, such as protozoa and algae, to highly specialized cells found in animals and plants. Regardless of their roles, however, all eukaryotic cells have certain features in common.
Eukaryotic membranes are highly modified. In addition to the protein and phospholipid layer that acts as a selective barrier, single celled animals, for example, have special proteins embedded in their plasma (cell) membrane that interact with molecules outside and allow the cell to react to changing external circumstances. The plasma membrane also serves as a 'sensing organ'.

Membranes enclosing organelles often have additional roles, such as anchoring vital proteins in precise relationships to one another, and acting as internal barriers. Stretching thoughout the cytoplasm, extensive internal membrane systems process metabolic products and act as sites of protein synthesis.

Unlike prokaryotes, eukaryotic cells compartmentalize various metabolic processes inside membrane-bound organelles. For example, the breakdown of certain food molecules to provide energy takes place in the mitochondrion, and photosynthesis takes place in a chloroplast. Compartmentalization of metabolic processes makes eukaryotic cells very efficient and allows them to increase in size.
Within the nucleus, are the molecules of DNA, the macromolecules that carry all the genetic and hereditary information of the cell. Linear strands of DNA are entwinded with histone and other proteins to form chromosomes. Normally invisible, these structures can be stained with dyes, identified, and counted during the division process. Every type of eukaryotic organism has its own unique collection of chromosomes.

The fact that nuclei are present in all eukaryotic cells was recognized as early as 1833 by Robert Brown, and it is from the name "true nucleus" that we get the word "eu-karyote".

Mitosis Every time a eukaryotic cell divides to produce two new daughter cells, all the DNA molecules of the parent cell are faithfully copied and combined with histones to form compact packages. In the process called mitosis, one complete set of chromosomes is transmitted to each new daughter cell. This process ensures that each offspring is genetically identical to the parent cell.
Meiosis Most (but not all) eukaryotic cells contain two sets of their genetic information. Such cells are called diploids. During the process of sexual reproduction, specialized diploid cells undergo a form of nuclear division known as meiosis by which the total informational content is halved. The new cells have only one set of the total information found in the original cell and are said to be haploid.

Generally, haploid cells go on to form sperm or eggs. After fertilization the information carried by one sperm and one egg is pooled to produce a new diploid cell. A fertilized cell can then grow and divide by mitosis reforming a new organism.

Although haploid and diploid cells can both divide by mitosis, only diploid cells, or those with even greater chromosome complements, can divide by meiosis.

Figure legend: Eukaryotic cells are very diverse in shape, form and function. Some internal and external features, however, are common to all. These include a plasma (cell) membrane, a nucleus, mitochondria, internal membrane bound organelles and a cytoskeleton.

© 2001, Professor John Blamire