Ribosomal RNA – rRNA

Ribosomal RNA – rRNA – Ribosomal RNA, as the name suggests, is found in the ribosomes. It comprises about 80% of the total RNA of the cell. The base sequence of rRNA is complementary to that of the region of DNA where it is synthesized. In prokaryotes (e.g. bacteria) the rRNA molecule is formed on apart of the DNA strand called the ribosomal DNA.
In eukaryotes ribosomes are formed on the nucleolus. The nucleolar organiser contains ribosomal DNA which transcribes 4SS RNA. This then gives rise to 28S RNA through a series of intermediate steps. Ribosomal RNA is formed from only a small section of the DNA molecule, and hence there is no definite base relationship between rRNA and DNA as a whole.
In E. coli only 03.2% of the chromosome codes for rRNA. The synthesis of rRNA begins at gastrulation and increases as embryonic development proceeds. The base ratios of rRNA are very similar in ribosomes from many different organisms. This suggests a general structural similarity. Depending upon ionic strength, temperature and pH the RNA molecule may be a short compact rod; a compact coil or an extended strand.
Ribosomal RNA consists of a single strand twisted upon itself in some regions. It has helical regions connected by intervening single strand regions. The helical regions may show presence or absence of positive interaction. In the helical region most of the base pairs are complementary, and are joined by hydrogen bonds. In the unfolded single strand regions the bases have no complements.
Hence rRNA does not show purine-pyrimidine equality. The rRNA strands unfold upon heating and refold upon cooling. Ribosomal RNA is stable for at least two generations. The ribosome consists of proteins and RNA. The 70S ribosome of prokaryotes consists of a 30S subunit and a 50S subunit. The 30S subunit contains 16S rRNA, while the 50S subunit contains 23S and 5S rRNA.
The 80S eukaryote ribosome consists of a 40S and a 60S subunit. In vertebrates the 40S subunit contains 18S rRNA, while the 60S subunit contains 28-29S, 5.8S and 5S rRNA. In plants and invertebrates the 40S subunit contains 16-18S RNA, while the 60S subunit contains 25S and 58 and 5.8S rRNA.
Mitochondria of vertebrates and insects contain 54S-618 ribosomes (which have been called 55S ribosomes) each with a small 30S subunit and a large 40S subunit. The small subunit contains 128-138 rRNA while the large subunit contains 16S-17S rRNA and 58 rRNA. It will thus be seen that there are three types of ribosomal RNA on the basis of sedimentation and molecular weight.
Two of these classes are high molecular weight RNAs, while the third is a low molecular weight RNA. The three classes are: (I) high molecular weight rRNA with molecular weight of over a million, e.g. 21s-29s RNA, (2) high molecular weight rRNA with molecular weight below a million e. g. 12-8-188 rRNA, (3) low molecular weight rRNA e. g. 58 rRNA.