Ribosomal RNA rRNA

A ribosome is a cytoplasmic nucleoprotein structure that acts as the machinery for the synthesis of proteins from the mRNA templates. On the ribosomes, the mRNA and tRNA molecules interact to translate into a specific protein molecule information transcribed from the gene. In active protein synthesis, many ribosomes are associated with an mRNA molecule in an assembly called the polysome.

The components of the mammalian ribosome, which has a molecular weight of about 4.2 X 106 and a sedimentation velocity of 80S (Svedberg units), are shown in Table 35-2. The mammalian ribosome contains two major nucleoprotein subunits—a larger one with a molecular weight of 2.8 X 106 (60S) and a smaller subunit with a molecular weight of 1.4 X 106 (40S). The 60S subunit contains a 5S ribosomal RNA (rRNA), a 5.8S rRNA, and a 28S rRNA; there are also probably more than 50 specific polypeptides. The 40S subunit is smaller and contains a single 18S rRNA and approximately 30 distinct polypeptide chains. All of the ribosomal RNA molecules except the 5S rRNA are processed from a single 45S precursor RNA molecule in the nucleolus (Chapter 37). 5S rRNA is independently transcribed. The highly methylated ribosomal RNA molecules are packaged in the nucleolus with the specific ribosomal proteins. In the cytoplasm, the ribo-somes remain quite stable and capable of many translation cycles. The functions of the ribosomal RNA molecules in the ribosomal particle are not fully understood, but they are necessary for ribosomal assembly and seem to play key roles in the binding of mRNA to

OH OH

OH OH

Y CAP

' O-

P

O

O

II P-

O-

O-

OCH3

HC CH

Figure 35-10. The cap structure attached to the 5' terminal of most eukaryotic messenger RNA molecules. A 7-methylguanosine triphosphate (black) is attached at the 5' terminal of the mRNA (shown in blue), which usually contains a 2'-0-methylpurine nucleotide. These modifications (the cap and methyl group) are added after the mRNA is transcribed from DNA.

ribosomes and its translation. Recent studies suggest that an rRNA component performs the peptidyl transferase activity and thus is an enzyme (a ribozyme).

D. Small Stable RNA_

A large number of discrete, highly conserved, and small stable RNA species are found in eukaryotic cells. The majority of these molecules are complexed with proteins to form ribonucleoproteins and are distributed in the nucleus, in the cytoplasm, or in both. They range in size from 90 to 300 nucleotides and are present in 100,000-1,000,000 copies per cell.

Small nuclear RNAs (snRNAs), a subset of these RNAs, are significantly involved in mRNA processing and gene regulation. Of the several snRNAs, U1, U2, U4, U5, and U6 are involved in intron removal and the processing of hnRNA into mRNA (Chapter 37). The U7 snRNA may be involved in production of the correct 3' ends of histone mRNA—which lacks a poly(A) tail. The U4 and U6 snRNAs may also be required for poly(A) processing.

Region of hydrogen bonding between base pairs

Region of hydrogen bonding between base pairs

TyC arm

Extra arm

Alkylated purine

Anticodon arm

Figure 35-11. Typical aminoacyl tRNA in which the amino acid (aa) is attached to the 3' CCA terminal. The anticodon, TYC, and dihydrouracil (D) arms are indicated, as are the positions of the intramolecular hydrogen bonding between these base pairs. (From Watson JD: Molecular Biology of the Gene, 3rd ed. Copyright © 1976, 1970, 1965, by W.A. Benjamin, Inc., Menlo Park, California.)

TyC arm

Extra arm

Alkylated purine

Anticodon arm

Figure 35-11. Typical aminoacyl tRNA in which the amino acid (aa) is attached to the 3' CCA terminal. The anticodon, TYC, and dihydrouracil (D) arms are indicated, as are the positions of the intramolecular hydrogen bonding between these base pairs. (From Watson JD: Molecular Biology of the Gene, 3rd ed. Copyright © 1976, 1970, 1965, by W.A. Benjamin, Inc., Menlo Park, California.)

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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