RNAs that carry genetic signals from the DNA of cells are called mRNAs or messenger RNAs. This is called template RNA or monocistronic mRNA. It is a beginning and very temporary. Their molecular weight is 5-20 lakh daltons. About 5-10% of the cell is mRNA.
Structure of mRNA
(i) Guanine cap: Guanine is added to the 5 end of mRNA to form a cap. This is called a guanine cap. The guanine cap consists of 7 guanosine nucleotides. This cap ensures ribosome attachment during translation.
(ii) Non-coding region-1: The region which does not synthesize protein is called non-coding region. This region consists of 10-100 nucleotides after the cap. It is rich in adenine and uracil. Translation does not occur here.
(iii) Initiation codon: The codon which initiates protein synthesis is called initiation codon. The start codon is AUG. It binds to the amino acid methionine.
(iv) Coding region: The region that synthesizes the protein is called coding region. This region consists of 1500 nucleotides. Translation occurs in this region.
(v) Termination codon: The codon which terminates protein synthesis is called termination codon. The termination codons are UAA, UAG and UGA. If any one of these codons is present, protein synthesis stops.
(vi) Non-coding region-2 : This region consists of 50-150 nucleotides. No translation here.
(vii) Poly A tail: Adenine is added to the 3 end of mRNA to form poly A tail. It is composed of 200-250 adenines.
Function: mRNA carries the protein-making signal from the nucleus to the cytoplasm and forms the chain of amino acids.