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Nucleus

The nucleus was the first internal organelle seen in cells, but its role was a mystery for a long time. At one time it was believed the nucleus was extruded from the cell and grew into a new cell, but slowly by the time of Mendel this idea was seen to be false.

Today we know that the nucleus is one of the largest organelles in most cells and is the site of information storage and usage. Bounded by a double membrane, the nuclear envelope, the nucleus consists of an internal fluid, the nucleoplasm in which is suspended the chromatin. This chromatin consists of genetic information carried on DNA molecules protected and regulated by proteins. Normally the chromatin is a loose collection of fibers that is difficult to see, but at the time of division the threads of material condense and collect into much more compact structures that have become known as chromosomes.

During the active growth and metabolism phase of a cell, the DNA genes are exposed in the nucleus where they can be copied into RNA molecules. A special type of RNA known as ribosomal RNA is made from a collection of genes on the DNA and then processed and altered. The site within the nucleus where this takes place can sometimes be distinguished as a darker staining region called the nucleolus. After the ribosomal RNA has be synthesized and processed it is exported from the nucleus through elaborate, circular pores called the nuclear pores which penetrate the double nuclear envelope.




Science@a Distance
© 2002, Professor John Blamire