Click here to
Cell Biology
The World of Cells
Nucleus
The Nucleus The largest and most clearly visible of the cell's constituents is the nucleus. Within the nucleus, DNA molecules, the cell's genetic machinery, are stored, repaired, transcribed and eventually replicated.

Around the outside of the nucleus is an envelope consisting of two layers of membrane. Scattered over the outer surface of this nuclear envelope is a series of circular openings known as nuclear pores, and between the two membranes is a small space.

Within the nuclear envelope is the nucleoplasm, and lying within the nucleoplasm are molecules of chromatin.

Chromatin consists of DNA in association with at least two kinds of proteins, basic proteins, known as histones, and other nonhistone proteins.

For much of the life of a cell the chromatin lies as a beaded tangle of fibers throughout the volume of the nucleus. Small combinations of histones form the "beads", and the "string" is the DNA molecule.

Chromosomes

as DNA is complexed with more and more histone (and other proteins) it becomes tightly packaged. Just before cell division, these compact bodies of DNA and histone can be stained with colored dyes, making them visible under the light microscope.
At a certain time in the cell's life cycle, all the DNA molecules, and the coded sets of information they carry, are duplicated. Once the cell is committed to division these duplicated DNA molecules are gathered together and packaged with more histones into tight, compact structures which can easily be seen under the light microscope.

Because these compact structures of DNA and protein can be stained with colored dyes, they are called chromosomes, which originally meant 'colored bodies'.

Today, however, the word chromosome refers to a length of DNA in a cell's nucleus that is individually replicated, stores information and has a microtubular attachment site called a centromere.

Nucleolus

One or more nucleoli are usually present within each nucleuls. Dark staining, they are specialized regions of the chromatin in and on which are synthesized the type of RNA that eventually becomes part of the ribosome.
Nondividing cells often have a dark structure within their nuclei called a nucleolus. Strands of chromatin that hold the codes for ribosomal RNA are associated with this nucleolar region, and the information carried by these regions of DNA is enzymatically copied into RNA molecules. Small packages of protein and this RNA are then combined and exported from the nucleus into the cytoplasm to become ribosomes.
Transcription

DNA molecules are freed of their histone, and the two strands unwind. A large enzyme complex (RNA Polymerase) binds to one of the strands. As it moves long the exposed DNA chain it copies the information into a molecule of mRNA.
Although the nuclear DNA contains all of the information or codes necessary to make proteins, protein synthesis does not occur in the nucleus. Instead, an enzyme called RNA polymerase attaches to one end of a specific length of DNA and moves along the molecule making an RNA copy. This process is called transcription because DNA information is being transcribed or "converted" into RNA information.

The RNA copies move out of the nucleus and associate with ribosomes in the cytoplasm, where the processing of assembling the final protein actually takes place. These RNA molecules act as messengers, therefore, ferrying the information from the DNA 'store' to the protein assembly points on the ribosomes. For this reason they are called messenger RNA (mRNA).

Figure legend: Nucleus. Often the largest and most clearly seen of cellular organelles, the nucleus houses the cell's important genetic apparatus. During much of the life cycle of the cell, DNA is loosely distributed throughout the nucleoplasm (the liquid within the nucleus). Complexed with histones and other proteins, this forms the chromatin. During cell division, the DNA and histones form stainable, clearly visible chromosomes. The nuclear envelope has a double membrane pierced at intervals by elaborate pores. Continuous with the nuclear envelope is the endoplasmic reticulum.

BIOdotEDU
© 2001, Professor John Blamire