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Sabtu, 24 Januari 2009

The Nucleus

The presence of a nucleus and other membranous orhanella characterizes eukaryotic celld. The nucleus houses the genetic material, DNA, which id complexed with an array of acidic and basic protein into thin fibers. During nondivisional phase of the cell cycle, these fibers are uncoiled and dispersed into chromatin.
Duriing mitosis and meiosis chromatin fibers coil and condense into structure called chromosomes. Also present in the nucleud id the nucleolus, an amorphous component where ribosomal RNA is synthesized and where the initial stages of ribosomal assembly occur. The areas of DNA encoding rRNA are collectively referred to as the nucleolus control region (NOR).
The lack of the nuclear envelope and membranaeous organelles is characteristic of prokaryotes. In bacteria such as a E. coli, the genetic material is present as a long, circular DNA molecule that is compacted into an area referred as the nucleotid area. Part of the DNA may be attached to the cell membrane, but in general the nucleotid cinstitutes a larhe area trougout the cell. Although the DNA is compacted, it does not undergo the extensive coiling characteristic of the stages of mitosis were, in eukaryotes, chromosome become visible. Nor is the DNA in these organism associated as extensively with proteins as is eukaryotic DNA. Prokaryotic cells do not have a distinct nucleolus, but do contain genes that specify rRNA molecules.

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