Within ribosomes, the genetic instructions contained in mRNA are translated into the amino acid sequences of polypeptides. Ribosomes are complex structures, each consisting of more than 50 different proteins and RNA molecules (Table 10.4). A functional ribosome consists of two subunits, a large ribosomal subunit and a small ribosomal subunit, each of which consists of one or more RNA molecules and a number of proteins. The sizes of the ribosomes and their RNA components are given in Svedberg (S) units (a measure of how rapidly an object sediments in a centrifugal field).
| Cell type | Ribosome size | Subunit | rRNA component | Proteins |
|---|---|---|---|---|
| Bacterial | 70S | Large (50S) | 23S (2900 nucleotides), 5S (120 nucleotides) | 31 |
| Small (30S) | 16S (1500 nucleotides) | 21 | ||
| Eukaryotic | 80S | Large (60S) | 28S (4700 nucleotides), 5.8S (160 nucleotides), 5S (120 nucleotides) | 49 |
| Small (40S) | 18S (1900 nucleotides) | 33 |
Ribosomal RNA is processed after transcription in both bacterial and eukaryotic cells. A precursor RNA molecule is methylated in several places, then cleaved and trimmed to produce the mature rRNAs that make up the ribosome. In eukaryotes, small nucleolar RNAs (snoRNAs) help to cleave and modify rRNAs and assemble them into a mature ribosome.
A ribosome is a complex structure consisting of several rRNA molecules and many proteins. Each functional ribosome consists of a large and a small subunit. Ribosomal RNAs in both bacterial and eukaryotic cells are modified after transcription.