|MadSci Network: Cell Biology|
Danielle, I'll assume, since you're asking about ribosomes, that you know a little bit about them already, so I'm not going to go into details in the way they work. Just remember, ribosomes are made up of ribosomal proteins, and ribosomal RNA (rRNA). Proteins are made in the ribosome from the messenger RNA (mRNA) through transfer RNAs (tRNA). The tRNA molecules are amino-acylated (that is they have an amino acid attached to them); the tRNAs match up with the mRNA sequence, then the ribosome connects the attached amino acid to the growing protein chain. The number of mRNA molecules in all cells is variable, and usually quite transient. That's because of the fact that certain proteins need only be made for short periods of time until there's enough for the cell's needs; the mRNA is degraded as a means of ensuring too much of a given protein isn't made by the ribosomes. In response to your question concerning the number of ribosomes per cell, I have to cheat a little bit. As you might imagine, the number of ribosomes per cell in higher organisms will vary depending on the type of cell. For example, a mature red blood cell has expelled its nucleus which means it can't make messenger RNA, and so can not make new protein; in this case, the answer is zero ribosomes. On the other hand, liver cells (the primary metabolic tissue of humans) will have a large number since these cells are called upon to perform many functions that vary depending on the composition of our last meal, how much energy we need to run around, etc. In the single cellular bacterium E. coli, the distribution of various cellular components is more or less constant. I found these interesting facts in one of my old textbooks: Distribution of cellular components in E. coli (from "Genes VI" by Benjamin Lewin) Proportion Component of cell mass Copies/Cell --------------------- ---------------- --------------- Cell wall 10% Cell membrane 10% Soluble protein 40% 1,000,000 Small Molecules 3% 5,500,000 DNA 2% 1 mRNA 2% 2500 tRNA 3% 160,000 rRNA 21% Ribosomal protein 9% Complete ribosome (rRNA + Ribosomal protein) 30% 20,000 The actual number of ribosomes synthesising a particular protein is not precisely determined in either prokaryotic cells (bacteria) nor in eukaryotic cells (higher organisms), but is a matter of statistical fluctuation. An overall view of the energy devoted to protein synthesis in E. coli is evidenced by the fact that 30% of the cells mass is accounted for by ribosomes; they contain roughly 10% of the total bacterial protein and about 80% of the total mass of cellular RNA. In eukaryotic cells, their proportion of total protein is less, but their absolute number is greater (possibly as much as one million copies or more, though I couldn't find any numerical estimate) and they still contain most of the mass of RNA of the cell. The tRNA molecules outnumber the ribosomes by almost tenfold, so that the rate of protein synthesis is not limited by the availability of protein building blocks. Because of their instability, it has been difficult for researchers to estimate the number of mRNA molecules, but a reasonable guess would be 2000 to 300, in varying states of synthesis, decomposition, and stages of the cellular lifecycle. If you're interested in additional reading, you can head to your local college library and look at some of the basic biochemistry or molecular biology text books; for more advanced reading you might try the "RNA World" Suggested reading: "Genes VI" by Benjamin Lewin; Oxford University Press, New York, c1997. Call Number: QH430 .L487 1997 "Biochemistry" by C. K. Mathews and K.E. van Holde; Benjamin/Cummings Pub. Co., Redwood City, Calif., c1990. Call Number: QP514.2 .M384 1990 "The RNA world" edited by R. F. Gesteland, T. R. Cech, J. F. Atkins. Cold Spring Harbor Laboratory Press, N.Y., c1999. Call Number: QP623 .R6 1999 I hope this helps out. Best of luck in school, and stay interested in science! Regards, Dr. Jim Kranz
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