RNA – Types and Structure

By | July 23, 2021
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The nucleic acids are one of the important macromolecules present in the nucleus of all eukaryotes. They are also present in all bacteria and viruses, but in non-eukaryotes, nucleic acids are not enclosed in the nuclear membrane. Some cell organelles such as mitochondria and chloroplasts (present in plant cells) also have their own nucleic acid molecules apart from the ones present in nucleus. They are responsible for storing all the information required for synthesis of all the proteins for cells growth and division. Nucleic acids also also act as genetic material for inheritance (the process by which genetic information is passed on from parent to child) and is passed from one generation to the next. Nucleic acids can be compared to a biological storage device that allows genetic information of life to be passed to successive generations.

The two nucleic acids present in the cells are Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA acts a genetic material in all living things ranging from bacteria to humans. In fact, all cells from plants and animals possess both DNA and RNA (with some rare exceptions) but it the DNA which acts as a genetic material. Viruses particles contains either DNA or RNA but rarely both. So some viruses has RNA molecules adorning the role of genetic material.

Structure of RNA:

Similar to DNA, Ribonucleic acid (RNA) is also made of polymers of nucleotides but it has few differences:

  1. DNA is double stranded and RNA is mostly single stranded
  2. RNA is made up of ribose sugar instead of deoxyribose as seen in the case of DNA.
  3. RNA contains Uracil nitrogenous base as compared to Thymine in DNA.
Comparison of RNA and DNA molecules

Functions of RNA:

The main function of RNA is to make proteins in the cell. DNA contains all the genetic information required for the synthesis of all the proteins, our body needs. However, our cells needs RNA molecules to decode the genetic information present in DNA and export the same (in the form of mRNA molecule) out of nucleus, for protein synthesis to occur in the cytoplasm. For this multi step process of protein synthesis, cells also need the help of other RNA molecules such as rRNA and tRNA.

Types of RNA: If DNA is the biological storage device for genetic information, then 3 different RNA molecules can be termed as readers of this stored information for protein synthesis. mRNA, rRNA and tRNA play vital role in decoding the encrypted message in various DNA segments termed as genes.

Messenger RNA(mRNA): mRNA is formed by a process called as Transcription, the first step of protein synthesis which happens in the nucleus. In transcription, a DNA sequence or Gene is decoded, or transcribed, into a RNA molecule of variable lengths, termed as mRNA. This length of mRNA depends on the length of the genes that is transcribed. A enzyme called as RNA polymerase is crucial for the process of transcription. The three adjacent nitrogenous bases on the mRNA molecule, constitutes a codon. Each codon specifies a particular amino acid. Amino acids are building of proteins and chain of amino acids attached to each, depending on the code of mRNA forms a polypeptide or protein. So, mRNA’s are copies of gene segments in the language of RNA (uracil in place of thymine). These transcribed mRNA’s are then transported out of nucleus into cytoplasm. In the cytoplasm, mRNA molecules are attached by ribosomes, which are the cellular factories that helps in the production of proteins based on the mRNA code.  In eukaryotes, the transcribed RNA molecule must undergo some modifications to become a mature messenger RNA (mRNA).

messenger RNA transcript

tRNA: It is a small RNA molecule with a highly folded structure and is responsible of bringing correct amino acids to form a polypeptide. This process of bringing amino acids based on the mRNA codons and decoding the mRNA sequence and forming a protein is called Translation. Each tRNA molecule has two important regions required for translation. At one end, tRNA has a trinucleotide region called the anticodon reion and on the other side, carries a specific amino acid that the mRNA codon encodes for. During translation, a tRNA molecule recognizes the codon present on mRNA and forms base pairs with its complementary anticodon sequence. It is the base pairing that ensures that the correct amino acid (present on other side of anticodon region of tRNA) is inserted into the growing polypeptide. Many tRNA’s can bind to mRNA transcript at the same time and once the amino acids forms a peptide bond among themselves, they break off from the tRNA molecule. This way many amino acids joined by means of a peptide bond, eventually forms a long polypeptide chain to form a functional protein. As the name “translation” suggests, in this process – the nucleotide sequence of the mRNA sequence is translated into a different “language” of amino acids to form protein.

mRNA and tRNA are freely moving molecules in the cytoplasm. Once mRNA is loaded onto a ribosome, a signal is sent to tRNA. After receiving a correct signal, tRNA looks for amino acid units in the cytoplasm and bring them to the ribosome loaded with mRNA to build the protein.

rRNA: The ribosome organelle is made up of rRNA and hence it got it’s name. The rRNA ensures proper alignment of mRNA to be translated on to the ribosomes. This allows tRNA to match up its anticodon with the mRNA codon.

Types of RNA involved in protein synthesis

There are other small RNA molecules which do code for any proteins such as micro RNA (miRNA) , piRNA. tRNA and rRNA also belongs to class of non coding RNA’s.

Micro RNA (miRNA): These small RNAs were first discovered in Caenorhabditis elegans by Victor Ambros’ laboratory in 1993 and later Gary Ravkun identified the first miRNA target gene. miRNAs belongs to the class of small RNA molecules found in the genomes of plants and animals. These are highly conserved small RNAs and are 22 nucleotides long. They play a important role in regulation of gene expression by binding to the 3′-untranslated regions (3′-UTR) of specific mRNAs.  The human gnome encode for more than 1900 miRNAs. In humans, they are found freely floating in blood and cerebrospinal fluid and hence serve as biomarkers in many diseases.

Image Source: Wikimedia commons – RNA and DNA comparison.

Smartservier.com – Types of RNA and Protein synthesis