The genes that encode ribosomal RNA (rRNA) make up a multi-gene family. Ribosomal RNA genes are located in the nucleolus of the cell. Animal cells typically have 100-200 rRNA genes, while plants can have as many as 5,000. These genes are found in tandem arrays, which are comprised of a repeating unit. This unit is made of the rRNA genes that are transcribed as well as non-transcibed spacer sequences. The number of rRNA genes varies widely between and among species, espcially in plants because they can tolerate more variation than animals. This variation is caused by unequal crossover and mutations.
In eukaryotes, ribosomes contain four rRNA molecules: 28S, 5.8S, 5S, and 18S. The 18S, 5.8S, and 28S genes are found adjacent to each other in that order and are tandemly repeated from 100 to 1000 times. RNA polymerase I transcribes these genes to produce a pre-rRNA molecule. Ribonucleases then remove spacer sequences to make three rRNA molecules. The 5S genes are usually located somewhere else in the DNA and are transcribed by RNA polymerase III. These four rRNA types along with ribosomal proteins form a ribosome.
rRNA gene sequences
Sequences for rRNA genes can be searched through the silva database. A search for Vitis vinifera gives 30 rRNA genes, and a search for Fragaria vesca gives only 6 rRNA genes. Information about each sequence can be found through a link on the search page. For example, this is one of the rRNA genes found in "Vitis vinfera".
Types of Ribosomes
There are at least three types of ribosomes: 80S ribosomes of animal origin characterized by 29/18S rRNA; 80S ribosomes of plant cells containing 25/16S rRNA; and 70S bacterial ribosomes made up of 23/16S rRNA. A 1966 study by Stutz and Noll found that chloroplast ribosomes in plant cells are 70S rather than 80S and are indistinguishable from bacterial rRNA .
RBS for Eukaryotes
A ribosomal binding site (RBS) is a sequence on the 5' untranslated region of mRNA that binds to the ribosome and orients it in the correct direction for translation . In prokaryotes, the RBS is usually 7 nucleotides before the start codon and is called the Shine-Dalgarno sequence. In eukaryotes, the 5' guanine cap of mRNA functions as the RBS. After recognizing the 5' cap and binding to the mRNA, the ribosome moves along the mRNA until it reaches the Kozak sequence, which has a similar function as the Shine-Dalgarno sequence, and the start codon  .