rRNA: What Is It?

Protein synthesis is an important process in all eukaryotic cells, as the protein forms structural components of each cell and is essential for life. Protein is often called the building block of cells. Three main forms of RNA exist — messenger RNA, transfer RNA and ribosomal RNA. The DNA controls all of the cell's activities and it is synthesized when the cell needs more protein. Small bits of DNA are changed into RNA through the process of protein synthesis.

When a cell is following its genetic instructions, it copies a portion of the DNA as a gene to change it to an RNA nucleotide. RNA differs from DNA in two distinct ways. The nucleotides in RNA are made of the sugar ribose and are called ribonucleotides. DNA has deoxyribose as its sugar content. RNA has the same bases as DNA of adenine, guanine and cytosine, but it has the base or uracil instead of the thymine that is in DNA. The structure of DNA and RNA are vastly different, as DNA is a double stranded helix and RNA is single-stranded. RNA chains can fold into a wide variety of many shapes in the same manner that a polypeptide chain folds up to form a protein's final shape.

There are three main types of RNA that are produced as molecules in the nucleus of human and animal cells. RNA is also located in the cytoplasm of a cell. A cell's cytoplasm is all of the contents outside of the nucleus that are enclosed by the individual cell membrane. The three main types of RNA are messenger RNA, transfer RNA and ribosomal RNA, or rRNA. Each of the three types of RNA has a distinct role in protein synthesis of transcription, decoding and translation of the genetic code that begins with DNA.

Transcription is the first step of protein synthesis in which the messenger RNA plays a very important role. Messenger RNA is unstable and doesn't live long in a cell to ensure that proteins are only made when they are needed for growth or repairing cells. Transcription is when the genetic information within the DNA of a cell is changed into a message in the form of RNA. Proteins of transcription factors unwind the DNA strand to enable the enzyme RNA polymerase to transcribe a single strand of DNA. DNA is made from four nucleotide bases of adenine, guanine, cytosine and thymine. They are combined in pairs of adenine plus guanine and cytosine plus thymine. When the RNA transcribes the DNA into a messenger RNA molecule, the adenine pairs with uracil and cytosine pairs with guanine. At the end of the transcription process, messenger RNA is transported out of the nucleus and into the cytoplasm.

Next is the translation process, during which transfer RNA plays an important role in protein synthesis. Transfer RNA is the smallest type of RNA and is usually about 70 to 90 nucleotides long in length. It translates the message within the nucleotide sequences of the messenger RNA into sequences of amino acids. Amino acids link together with other amino acids to form proteins, which are needed for all cell functions. Proteins are formed from a set of 20 amino acids. Transfer RNA is in the same shape as a cloverleaf with three hairpin loops in it. Transfer RNA has an amino acid attachment site on one end of it and a section in the middle loop that is called the anticodon site. The anticodon site recognizes the codons on the messenger RNA. A codon has three continuous nucleotide bases that create an amino acid and signal the end of the translation process. Transfer RNA and the ribosomes read the messenger RNA codons to produce a polypeptide chain, which undergoes several changes before it can become a fully functioning protein.

Ribosomal RNA (or rRNA) has a specific function. Ribosomes are made of ribosomal proteins and ribosomal RNA. Ribosomal RNA makes up about 60 percent of the ribosome's mass. They usually are composed of a large subunit and a small subunit. The subunits are synthesized in the nucleus by the nucleolus. Ribosomes are unique in nature, as they contain a binding site for the messenger RNA and two binding sites for transfer RNA in the RNA location in the large ribosomal subunit. A small ribosomal subunit attaches to a messenger RNA molecule and simultaneously an initiator transfer RNA molecule recognizes and binds to a certain codon sequence on the same ribosomal RNA molecule during translation. Next, the rRNA function includes a large ribosomal subunit joins the newly formed complex then both ribosomal subunits travel along the messenger RNA molecule as they translate the codons in the entire polypeptide chain as they pass over them. Ribosomal RNA creates the peptide bonds between amino acids in the polypeptide chain. When a termination codon is reached on the messenger RNA molecule, the translation process will end and the polypeptide chain will be released from the transfer RNA molecule at which time the ribosome splits back into the large and small subunits as they were at the beginning of the translation phase.

The process of DNA to RNA and the product of proteins can happen at an amazingly fast speed. The RNA is almost immediately released when it separates from the DNA strand. In this manner, many RNA copies can be made from the exact same gene in a short amount of time. The synthesis of additional RNA molecules can be started before the first RNA is completed so that it can produce RNA quickly. When the RNA molecules are following each other closely, they can each move about 20 nucleotides per second in humans and animals. Over 1,000 transcriptions can occur in an hour from a single gene.

Ribosomal RNA depletion is the most abundant component in RNA, as it comprises the majority of over 80 to 90 percent of the total of the RNA in a cell. Ribosomal RNA depletion is when the rRNA is partially removed from an entire sample of RNA so as to better study the RNA sequencing reaction to focus on the other two parts of an RNA sample in the transcription.

There are three more additional types of RNA that can be produced in cells. Small nuclear RNA's function in a variety of processes of the nucleus such as splicing the pre-messenger RNA's. Small nucleolar RNA processes and chemically modifies the ribisomal RNA. Other types of RNA that are non-coding units serve to function in cellular processes such as telomere synthesis, inactivating the X chromosome and transporting proteins to the endoplasmic reticulum for good cell health.

An RNA virus has a core of the genetic material that is obtained from the DNA of a cell. It usually has a protective capsid of protein and a lipid envelope for even farther protection. An RNA virus attaches to a host cell, penetrates it, reproduces the genetic material and creates the protective capsid then emerges from the cell. RNA viruses store the genetic material of RNA and not DNA.

All healthy cells store genetic material in the DNA. The RNA is only used when DNA is replicated to form RNA and synthesize proteins needed by a healthy cell to live. DNA is much more stable than RNA so DNA makes very few mistakes when cells are dividing, however the instability of RNA and its replication can make many mistakes and it can even interact with itself to multiply a virus. RNA can make up to one mistake over 10,000 nucleotides each and every time it is copied. It is also much less able to correct genetic mistakes than DNA. When an immune system learns to recognize a virus, it forms antibodies to fight off the virus. Viruses can mutate so the immune system can't recognize it and then it can multiply. This allows RNA viruses to spread much more quickly than DNA viruses.

A virus that survives can reproduce itself in new cells through the RNA sequence and result in thousands of cells that it reproduces containing the virus. RNA viruses evolve faster than any actual living organism. High rates of mutation of RNA virus infected cells don't threaten the survival of the virus.

Two types of RNA viruses exist. They may be single stranded or sense stranded or paired as antisense strands. The double stranded antisence RNA viruses have to first change and translate themselves into single stranded sense RNA. This allows the host cell to be in a form that the ribosomes can read. Influenza A virus keeps the needed enzymes close to the nucleic acid core of the virus. When it changes from an antisense to a sense RNA, it can then be read by the ribosomes in the cell to build viral proteins and replicate.

Some RNA viruses store their information in a sense strand so it can be read directly by the cell's ribosomes and it functions like a normal messenger RNA. In this case, the ribosomes synthesize the RNA transcript and create an antisense viral cell so it can use it as a template to synthesize more viral RNA's along with the necessary proteins for the cells to live. One of the most deadly viruses of this type is Hepatitis C.

Retrovirus examples are HIV and AIDS. They store their genetic material in the form of RNA but they use the reverse transcription enzyme to turn their RNA into DNA in the infected cell. This allows many copies to be made in the host cells so the virus can infect a large amount of cells quickly.

Coronaviruses are RNA viruses as well. They primarily infect the upper respiratory and gastrointestinal tract in humans. SARS-CoV is a serious virus that infects the upper respiratory tract as well as the lower respiratory tract and it also includes gastrointestinal distress. Coronaviruses are a significant percentage of all of the common colds. Rhinoviruses are the leading cause of the common cold. Conronaviruses can lead to pneumonia also.

SARS is severe acute respiratory syndrome and it contains RNA genes that mutate very slowly. SARS is transmitted by respiratory droplets in the air from sneezing or coughing to infect others.

Norovirus infections became famous for appearing on cruise ships and being called Norwalk-like viruses. These cause gastroenteritis and it is spread from one person to another by fecal-oral route. If an infected person is working in a kitchen, they can contaminate the food by having the virus on their hands and not wearing gloves.