935 Words Essay on Ribonucleic Acid (RNA)

DNA is the storehouse of all necessary information for the maintenance and perpetuation of life. However, it does not carry out the work itself.

It has recruited its agents to transfer and translate the coded information to a readable form. The readable form here refers to the proteins, which ultimately do the necessary work on behalf of the DNA. These recruited agents are called ribonucleic acids (RNA).

There are three forms of RNA: (1) messenger (m) RNA, (2) transfer (t) RNA and (3) ribosomal (r) RNA. Like the DNA, the RNA molecules are polynucleotide chains and are necessarily single stranded. However, on occasions, the single stranded molecule forms stems and loops by forming hydrogen bonds between complementary nucleotides present far apart.

The nucleotides present in RNA (ribonucleotides) are the same as in DNA except for uridylic acid (nucleotide of uracil) substituted for thymidylic acid (nucleotide of thymine) and 13- D-ribose for 6-D-deoxyribose. In some viruses, the genetic material is RNA instead of DNA. For example, in tobacco mosaic virus (TMV) and retroviruses, the genetic material is linear and single stranded RNA, while in reoviruses, linear and double stranded.

The flow of genetic information is reversed in retroviruses, i.e. the RNA is used as a template for the synthesis of DNA. The enzyme catalyzing this conversion is known as RNA dependent DNA polymerase/ reverse transcriptase.

Messenger RNA / RNA: It is the carrier of coded genetic information present in DNA in the form of the nucleotide sequence.

This nucleotide sequence is copied into the complementary rib nucleotide sequence of an RNA known as mRNA by a process call transcription. The enzyme catalyzing the transcription process is called DNA dependent polymerase or simply RNA polymerase.

In prokaryotes, the newly transcribed mRNA called polycistronic or polygenic, as all the contiguous structural genes of a transcription unit transcribed as a single long RNA strand, which is translated into individual polypeptides. On contrary, the transcribed mRNA is monocistronic in eukaryotes.

Most eukaryotic genes interrupted by non-coding sequences called introns. The coding sequences are known as ego such genes are called split /interrupted genes. Both sequences of a split gene are transcri so that the newly transcribed RNA contains both exon and intron sequences.

This RNA is n functional and is variously known as primary transcript / precursor RNA / pre RNA. Followi transcription, the intron sequences are removed and the exon sequences joined (spliced) toge to form a mature or functional RNA the process of removing introns and joining of exons known as RNA splicing.

Transcription and splicing occur in the nucleus. The nucleus cont long and unprocessed RNA molecules, which are nonfunctional until they are processed, mass of nuclear precursor RNA (specifically precursor mRNA) is referred to as heterogenic nuclear RNA (RNA).

Transfer RNA /RNA: This RNA is also called adapter or soluble RNA. Compari of several Tran molecules has revealed a common structural plan.

All RNA molecules single stranded and relatively small having 75-85 ribonucleotide residues. In solution, all f molecules fold into their native three dimensional secondary structures due to intra-star complementary base pairing.

This structure resembles a twisted English alphabet ‘L’. When d in two dimensions, complementary base pairing gives rise to a cloverleaf structure withf arms. Each arm has a hydrogen bonded stem formed by complementary base pai all except the amino acid arm end in loops.

Longer tRNA molecules have, in addition, an arm. One aunt, the amino acid arm has two free ends, designated 5′ and 3′. There is a G resi at-the 5′ end, while the 3′ end ends in a trinucleotide, CCA.

The amino acid is linked to the residue at the 3′ end. There is a group of three ribonucleotides in the loop of the anticodon These thre ribonucleotides together constitute the anticodon, the amino acid binding signal, arm, therefore, is called the anticodon arm. The other two arms are called D arm and arm.

A polypeptide has several amino acids joined together by peptide bonds. A peptide bond is formed by a reaction between a carboxyl (COOH) group of a preceding amino acid and the amino (NH₂) group of a succeeding amino acid. During translation, the amino acids are joined by peptide bonds in a correct sequence in conformation with the ribonucleotide sequence of mRNA. Amino acids do not have a means to read the ribonucleotide sequence. Therefore, a link is established between the amino acids and the mRNA.

Transfer RNA (tRNA) functions as a link, which has a two-fold function. It reads the ribonucleotide sequence of the mRNA on the one hand and carries and presents an amino acid to the ribosome on the other, in conformation with the ribonucleotide sequence of the mRNA.’

Ribosomal RNA / RNA: Ribosomal RNAs / rRNAs form an integral part of ribosome structure. In both prokaryotic and eukaryotic cells, these are transcribed as Lar molecules that are cleaved subsequently to yield smaller rRNAs. In eukaryotic cells, the molecu is transcribed from a specific DNA template known as the nucleolus organizer, catalyzed by t enzyme, RNA polymerase I, while in Escherichia coli, the process is catalyzed by the sa~ RNA polymerase that inscribes mRNA and tRNA.

The large precursor is cleaved in the nucleol into 28S, 18S and 5.8S rRNAs in the eukaryotic cell. Another small rRNA (5S) is transcri from a different template. In E. coli, the precursor is cleaved into 23S, 16S and 5S rRN Mitochondria and chloroplasts have their own rRNAs. The 18S and 16 S rRNAs bind to sequence of mRNA for correct positioning of ribosomes during protein synthesis. Table 4 summerizes the rRNA types and their sedimentation co-efficients.