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Introduction
Cytoplasm
Nucleic Acids
DNA
Proteins
More Proteins
Ribosomes
Inclusions
Membranes
Membrane Functions
Cell Wall
More Cell Wall
Flagella
Surface Structures


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Ribosomes

©2001 Timothy Paustian, University of Wisconsin-Madison

Structure

RNA

Ribosomes are a combination of protein and RNA. There are three different RNA species. All of them are single stranded Nucleic acids. For more about their structure see Nucleic Acids

Ribosomal RNA (rRNA)

rRNA is part of the ribosome structure and assists in the catalytic role of the ribosome. During rapid growth the cell has to synthesize large amounts of rRNA and there are several copies of the rRNA genes on the chromosome.

5SrRNA.jpg

Figure 1 - A wire frame model of the 5S ribosomal RNA of E. coli

Transfer RNA (tRNA)

These are adapter molecules in protein synthesis that convert the genetic code from the language of nucleic acid to that of amino acids - the building blocks of proteins.

tRNA.JPG

Figure 2 - A wire frame model of transfer RNA

Messenger RNA (mRNA)

Messenger RNA directs the incorporation of amino acids into proteins. It can be thought of as a "photocopy" of DNA that the ribosome works from.

mRNA.jpg

Figure 3 - mRNA at work in the cell. The ribosomes in this picture are in a polysome stucture.

Protein

There are 52 different proteins in ribosomes that perform all sorts of functions. These polypeptides, along with the ribosomal RNA will self-assemble into a functional unit if they are added together in the proper order.

Function

Ribosomes are the protein synthesizing factories of the cell. They translate the information in mRNA into protein sequences. Ribosomes also give the cytoplasm its granular look in the EM. Often they aggregate to form structures known as "polysomes" as shown in the mRNA figure above. Ribosomes sit down on mRNA at two sites. The A site, where the new amino acid is accepted and the P site, where the growing polypeptide is held. A detailed treatment of protein synthesis is in Chapter 7

Remember procaryotes and eucaryotes have different size ribosomes and have different proteins. Eucaryotic ribosomes may be free in the cytoplasm or bound to an internal membrane structure known as the endoplasmic reticulum. Eucaryotes also have 70S ribosomes (procaryotic) in their mitochondrion and chloroplasts. In fact it is thought that mitochondrion and chloroplasts originate from bacteria.

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