Ribosomal proteins can also cross-link to the sugar-phosphate backbone of rRNA with binding sites that consist of basic residues (i.e. lysine and arginine).

[7] Yeast has been the traditional model for observation of eukaryotic rRNA behavior and processes, leading to a deficit in diversification of research.

It has only been within the last decade that technical advances (specifically in the field of Cryo-EM) have allowed for preliminary investigation into ribosomal behavior in other eukaryotes.

The combined 5.8S and 28S are roughly equivalent in size and function to the prokaryotic 23S rRNA subtype, minus expansion segments (ESs) that are localized to the surface of the ribosome which were thought to occur only in eukaryotes.

However recently, the Asgard phyla, namely, Lokiarchaeota and Heimdallarchaeota, considered the closest archaeal relatives to Eukarya, were reported to possess two supersized ESs in their 23S rRNAs.

SSU and LSU rRNA sequences are widely used for study of evolutionary relationships among organisms, since they are of ancient origin,[12] are found in all known forms of life and are resistant to horizontal gene transfer.

rRNA sequences are conserved (unchanged) over time due to their crucial role in the function of the ribosome.

The structure of rRNA is able to drastically change to affect tRNA binding to the ribosome during translation of other mRNAs.

The rRNA subsequently undergoes endo- and exonucleolytic processing to remove external and internal transcribed spacers.

Upon going under more maturation steps and subsequent exit from the nucleolus into the cytoplasm, these particles combine to form the ribosomes.

[citation needed] Universally conserved secondary structural elements in rRNA among different species show that these sequences are some of the oldest discovered.

The ribosome catalyzes ester-amide exchange, transferring the C-terminus of a nascent peptide from a tRNA to the amine of an amino acid.

These processes are able to occur due to sites within the ribosome in which these molecules can bind, formed by the rRNA stem-loops.

[15] In fact, studies have shown that the peptidyl transferase center contains no proteins, and is entirely initiated by the presence of rRNA.

The exemplary species used in the table below for their respective rRNAs are the bacterium Escherichia coli (prokaryote) and human (eukaryote).

[citation needed] In contrast, eukaryotes generally have many copies of the rRNA genes organized in tandem repeats.

[29] It was previously accepted that repeat rDNA sequences were identical and served as redundancies or failsafes to account for natural replication errors and point mutations.

The DNA for the 5S subunit occurs in tandem arrays (~200–300 true 5S genes and many dispersed pseudogenes), the largest one on the chromosome 1q41-42.

Since the 2S rRNA is small and highly abundant, its presence can interfere with construction of sRNA libraries and compromise the quantification of other sRNAs.

[32] The tertiary structure of the small subunit ribosomal RNA (SSU rRNA) has been resolved by X-ray crystallography.

In the nucleolus, rRNA is synthesized by RNA polymerase I using the specialty genes (rDNA) that encode for it, which are found repeatedly throughout the genome.

These pre-rRNA molecules are separated by external and internal spacer sequences and then methylated, which is key for later assembly and folding.

As these complexes are compacted together to form a cohesive unit, interactions between rRNA and surrounding ribosomal proteins are constantly remodeled throughout assembly in order to provide stability and protect binding sites.

The modifications that occur during maturation of rRNA have been found to contribute directly to control of gene expression by providing physical regulation of translational access of tRNA and mRNA.

Once both subunits are assembled, they are individually exported into the cytoplasm to form the 80S unit and begin initiation of translation of mRNA.

The P1 promoter is specifically responsible for regulating rRNA synthesis during moderate to high bacterial growth rates.

Because the transcriptional activity of this promoter is directly proportional to the growth rate, it is primarily responsible for rRNA regulation.

At the transcriptional level, there are both positive and negative effectors of rRNA transcription that facilitate a cell's maintenance of homeostasis: Ribosomal RNA is quite stable in comparison to other common types of RNA and persists for longer periods of time in a healthy cellular environment.

Currently, only a basic understanding of how cells are able to target functionally defective ribosomes for ubiquination and degradation in eukaryotes is available.

[62] These key traits of rRNA have become especially important for gene database projects (comprehensive online resources such as SILVA[63] or SINA[64]) where alignment of ribosomal RNA sequences from across the different biologic domains greatly eases "taxonomic assignment, phylogenetic analysis and the investigation of microbial diversity.