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RNA
RNA

RNA Structure

Image Credit: Darryl Leja, NHGRI (with modification by RB)

What is RNA?

RNA molecules are single stranded nucleic acids composed of nucleotides. RNA plays a major role in protein synthesis as it is involved in the transcription, decoding, and translation of the genetic code to produce proteins. RNA stands for ribonucleic acid and like DNA, RNA nucleotides contain three components:
  • A Nitrogenous Base
  • A Five-Carbon Sugar
  • A Phosphate Group
RNA nitrogenous bases include adenine (A), guanine (G), cytosine (C) and uracil (U). The five-carbon (pentose) sugar in RNA is ribose. RNA molecules are polymers of nucleotides joined to one another by covalent bonds between the phosphate of one nucleotide and the sugar of another. These linkages are called phosphodiester linkages.

Although single stranded, RNA is not always linear. It has the ability to fold into complex three dimensional shapes and form hairpin loops. When this occurs, the nitrogenous bases bind to one another. Adenine pairs with uracil (A-U) and guanine pairs with cytosine (G-C). Hairpin loops are commonly observed in RNA molecules such as messenger RNA (mRNA) and transfer RNA (tRNA).

Types of RNA

RNA molecules are produced in the nucleus of our cells and can also be found in the cytoplasm. The three primary types of RNA molecules are messenger RNA, transfer RNA and ribosomal RNA.
  • Messenger RNA (mRNA) plays an important role in the transcription of DNA. Transcription is the process in protein synthesis that involves copying the genetic information contained within DNA into an RNA message. During transcription, certain proteins called transcription factors unwind the DNA strand and allow the enzyme RNA polymerase to transcribe only a single strand of DNA. DNA contains the four nucleotide bases adenine (A), guanine (G), cytosine (C) and thymine (T) which are paired together (A-T and C-G). When RNA polymerase transcribes the DNA into a mRNA molecule, adenine pairs with uracil and cytosine pairs with guanine (A-U and C-G). At the end of transcription, mRNA is transported to the cytoplasm for the completion of protein synthesis.


  • Transfer RNA (tRNA) plays an important role in the translation portion of protein synthesis. Its job is to translate the message within the nucleotide sequences of mRNA into specific amino acid sequences. The amino acid sequences are joined together to form a protein. Transfer RNA is shaped like a clover leaf with three hairpin loops. It contains an amino acid attachment site on one end and a special section in the middle loop called the anticodon site. The anticodon recognizes a specific area on mRNA called a codon. A codon consists of three continuous nucleotide bases that code for an amino acid or signal the end of translation. Transfer RNA along with ribosomes read the mRNA codons and produce a polypeptide chain. The polypeptide chain undergoes several modifications before becoming a fully functioning protein.


  • Ribosomal RNA (rRNA) is a component of cell organelles called ribosomes. A ribosome consists of ribosomal proteins and rRNA. Ribosomes are typically composed of two subunits: a large subunit and a small subunit. Ribosomal subunits are synthesized in the nucleus by the nucleolus. Ribosomes contain a binding site for mRNA and two binding sites for tRNA located in the large ribosomal subunit. During translation, a small ribosomal subunit attaches to a mRNA molecule. At the same time, an initiator tRNA molecule recognizes and binds to a specific codon sequence on the same mRNA molecule. A large ribosomal subunit then joins the newly formed complex. Both ribosomal subunits travel along the mRNA molecule translating the codons on mRNA into a polypeptide chain as they go. Ribosomal RNA is responsible for creating the peptide bonds between the amino acids in the polypeptide chain. When a termination codon is reached on the mRNA molecule, the translation process ends. The polypeptide chain is released from the tRNA molecule and the ribosome splits back into large and small subunits.

MicroRNAs

Some RNAs, known as small regulatory RNAs, have the ability to regulate gene expression. MicroRNAs (miRNAs) are a type of regulatory RNA that can inhibit gene expression by halting translation. They do so by binding to a specific location on mRNA, preventing the molecule from being translated. MicroRNAs have also been linked to the development of some types of cancers and a particular chromosome mutation called a translocation.

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