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rRNA: Sequence, Function & Synthesis

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Instructor: Shannon Compton

Shannon teaches Microbiology and has a Master's and a PhD in Biomedical Science. She also researches cancer and neurodegenerative diseases.

Ribosomal RNA (rRNA) is responsible for translation, or protein synthesis, in the cell. Explore what rRNA is, how it is synthesized, and its function, sequence, and importance in a cell. Updated: 09/09/2021

What Is rRNA?

To begin, ribonucleic acids (RNA) are a group of large molecules that have many different roles. They are especially important in the coding, decoding, regulation, and expression of our genes. As the name implies, ribosomal RNA (rRNA) is part of the ribosome. A ribosome is the protein builder of the cell. The rest of a ribosome is made up of proteins. In addition, there are two rRNA in each ribosome, one in the large subunit and one in the small subunit.

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  • 0:01 What is rRNA?
  • 0:35 Function
  • 1:30 Sequence
  • 2:01 Synthesis
  • 2:46 Importance
  • 4:01 Lesson Summary
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Translation is the name of the process our cells use to make proteins. Ribosomes are the organelles that can take a copy of our genetic material and translate it into a protein. The original genetic material is DNA, and it is stored in the cell's nucleus. When a specific protein is needed, our cells make a copy of the DNA. This copy is also RNA, but it is called messenger RNA, or mRNA. Thus, ribosomes translate mRNA into protein.

To make a protein, ribosomes link together amino acids. The mRNA has the specific order in which the amino acids must be joined, and each protein has a different order of amino acids. The small subunit of rRNA can read the order of amino acids. Linking amino acids together is the function of the rRNA in the large subunit of the ribosome.


rRNA Subnits
table of components of rRNA subunits

The sequence of rRNA is highly complex and derived from multiple genes. The table summarizes the components of the large and small subunit rRNA. Sedimentation rate (S) of the subunit is affected by the size of each subunit. As you can see, the large subunit of rRNA is made of two smaller pieces, while the small subunit is only one piece. The structure of the small subunit of eukaryotic rRNA has been discovered.


All RNA molecules are created by transcription, or making a copy of DNA. The protein that makes a copy of DNA is RNA polymerase. This protein is an enzyme that can read DNA and make an RNA copy. Transcription starts when the RNA polymerase enzyme binds DNA at a specific location called a promoter. Promoters are found 'upstream' of a gene. RNA polymerase reads the DNA in only one direction and continues reading and creating RNA until it reaches the end of the gene. The end of the gene has a special region called a stop codon. This region literally signals the RNA polymerase enzyme to stop copying the DNA.


Ribosomal RNA characteristics are important in medicine. Both prokaryotes (single-celled organisms, like bacteria) and eukaryotes (organisms whose cells have a nucleus and organelles, like the cells in our bodies) have ribosomes with rRNA. Prokaryotes and eukaryotes differ in the size and sequence of the rRNA in each subunit. Modern medicine exploits this difference using antibiotics. Therefore, rRNA is the target of several antibiotics, including chloramphenicol, erythromycin, and streptomycin.

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