2. Ribonucleic Acid (rna)

Location. In a prokaryotic cell, whole of the RNA is found in the cytoplasm because there is no nucleus. In a eukaryotic cell, bulk of RNA occurs in the cytoplasm, and a small amount in the nucleus. It is mainly confined to the ribosomes in the cytoplasm and to the nucleolus in the nucleus.

Quantity. The amount of RNA in a cell van: with nutritional states, types of tissue, etc.

Composition. RNA molecule is a single chain of ribonucleotide, or ribotide, units, ribonucleotide unit further consists of 3 different molecules: phosphate, ribose sugar and nitrogenous base. The nitrogenous base may be 3 purine (adenine or guanine) or a pyrimidine (cvtosine or uracil). Thus, there are four types of ribonucleotides in RNA, namely, adenine-riboss-

phosphate, or adenosine monophosphate | (AMP); guanine-ribose-phosphate, or guanosine monophosphate (GMP); cytosine-ribose-phosphate, or cytidine monophosphate (CMP); and uracil-ribose-phosphate, or uridine ! monophosphate (UMP). In an RNA chain, the phosphate group at 5' position of sugar molecule in one ribonucleotide is joined to the >ugar-molecule of the next ribonucleotide at 3' position by phosphodiester bond. This forms a 'backbone" of alternating sugar-phosphate- | „gar molecules. The nitrogenous base I molecules are joined to the sugar molecules at carbon 1' positions of this "backbone" by : gfycosidic bonds and project on its one side (Fig. 1 20.14). Purines and pyrimidines do not occur in equal amounts in the RNA.

Variety. The four nitrogenous bases (A, U, C, G) found in RNA may occur in any sequence, jmd there may be any number of sequences in a I

iecule. This gives an infinite variety to RNA molecule (Fig. 20.15).

Types. There are three major types of | RNA in every cell: messenger RNA, or mRNA;

jsomal RNA, or rRNA; and transfer RNA, % or tRNA. The three types of RNAs are synthesized from different regions of the DNA template. Only one DNA strand, called the sense strand, is apparently selected for transcription. I The basis for selection is not known. In aryotic cells, the RNAs are formed in the nucleus and pass into the cytoplasm througl nuclear pores. In prokaryotic cells, RNA is releasee by the nucleoid directly into the cytoplasm afte synthesis. All the three types of RNAs an iscribed in both prokaryotes and eukaryotes a; joined to form a polypeptide, hence its name. It is also called informational RNA, or template RNA.

It forms about 5% of the total RNA of a cell. Its molecule is the longest of all the RNA types. Its length is related to the size of the polypeptide to be synthesized with its information. There is a specific mRNA for each polypeptide. Because of the varia­tion in size in mRNA population in a cell, the mRNA is often called heterogeneous nuclear RNA, or hn RNA. One mRNA molecule normally specifies a single polypeptide chain and is said to be monocistronic. It occurs in the eukaryotes. In some case, mostly prokaryotes, one mRNA can specify more than one polypeptide, and is described as polycistronic.

An mRNA has at its 5' end a methylated cap, a small noncoding region and an initiation codon and carries at its 3' end a termination codon, a small noncoding region and a poly A tail (Fig. 20.17).

(c) Transfer RNA. It has many varieties. Each variety carries a specific amino acid to the mes­senger RNA on the ribosomes to form a polypep­tide. The tRNA forms the remaining 15% of the total RNA of a cell. Its molecule is the smallest of all the RNA types, and is folded to assume the form of a clover leaf. It contains some nucleotides havingunusual bases. A tRNA molecule has four regions: (/) carrier end (3' end) where amino acid joins for incorporation into a polypeptide, it carries CCA-OH group ; (ii) recognition end which is opposite to the carrier end and bears an anticodon that bindi to a matching codon of mRNA; (Hi) enzyme site or. one side for the binding of an enzyme ; and (fv) ribosome site on the other side for attachment to a ribosome (Fig. 20.18).

Other RNAs. There are 3 other forms of RNA from their functional point of view : genetic c: genomic, small nuclear RNA or snRNA and smaE cytoplasmic RNA or scRNA.

1 Genetic (Genomic) RNA. It occurs in cer­tain viruses known as riboviruses. It may be single-stranded (tobacco mosaic virus or TMV) or double-stranded (rheovirus). Genomic RNA ma] form a DNA copy of it which then transcribes RNA copies of its kind (human immunodeficiency virui or HIV), or synthesize new genomic RNA on itself (TMV).

1. Small Nuclear RNAs (snRNAs). These stag in the nucleus and help in processing RNAs.

2. Small Cytoplasmic RNAs (scRNAs). Thea:

come out into the cytoplasm and join prote:r

molecules to form signal recognition particles. The latter help in the synthesis of secretory proteins.

Functions. RNA plays a multiple role in cells.

(i) It brings about protein synthesis in the cell. All the three types of RNAs play a role in this process.

(ii) RNA is the genetic material of certain viruses.

(Hi) The rRNA is a component of ribosomes, the site of protein synthesis.

(iv) Some RNA is associated with chromatin fibres during interphase. It initiates replication of DNA.

(v) RNA primer is essential for starting replication of DNA.

(W) Folding of prokaryotic DNA into a nucleoid needs the help of RNA.

(yii) Certains RNAs act as enzymes, e.g., ribonuclease.