Test: Genetic Code & Translation (December 17) - NEET MCQ

Since there are 64 triplet codons and only 20 amino acids, the incorporation of some amino acids must be influenced by more than one codon. Only tryptophan (UGG) and methionine (AUG) are specified by single codons. All other amino acids are specified by two (e.g., phenylalanine UUU, UUC) to six (e.g., arginine CGU, CGC, CGA, CGG, AGA, AGG) codons. The latter are called degenerate or redundant codons. In degenerate codons, generally, the first two nitrogen bases are similar while the third one is different. As the third nitrogen base has no effect on coding, the same is called wobble position.

During translation, a (RNA) is specifically linked to an amino acid, this process takes place under the direction of an enzyme, aminoacyl tRNA synthetase that is extremely specific, i.e., recognises only one amino acid. (RNA) complexed with amino acid is called as charged tRNA. The process is referred to a charging or aminoacylation of tRNA.

Most of the gene. mutations involve a change in only a single nucleotide or nitrogen base of the cistron. These gene mutations are called point mutations e.g. sickle cell anaemia in which polypeptide chain coding for haemoglobin contains valine instead of glutamic acid due to substitution of T by A in second position of triplet codon.

 codon is a sequence of three nucleotides on an mRNA strand that encodes a specific amino acid. 
Sixty-four different three nucleotide combinations (codons) can be made using the four nucleotides in mRNA (43 = 64 combinations).
Following are the codons that represent Valine and proline respectively.
Valine: GUA, GUC, GUG, GUU
proline:  CCT, CCC, CCA, CCG

In tRNA, there is a TWC loop which contains pseudouridine and ribothymidine. The loop is the site for attaching to ribosomes. Another loop, DHU loop contains dihydrouridine. It is a binding site for aminoacyl synthetase enzyme. tRNA molecules have unpaired (single-stranded) CCAOH sequence at the 3 end. This is called amino acid binding site because the amino acid becomes covalently attached to adenylic acid or A of CCA sequence during polypeptide synthesis. Anticodon loop is made up of three nitrogen bases for recognizing and attaching to the codon of mRNA.

Aminoacyl tRNA synthetase is an enzyme that attaches the appropriate amino acid onto its tRNA. This step  is called as the charging of the tRNA with appropriate amino acid to the anti-codon arm. The mRNA is hence attached to the tRNA and a small sub unit of the ribosome is assembled primarily which is the cellular factory that is responsible for the translation of mRNA to proteins.For initiation the ribosome binds to the mRNA at the start codon that is recognized by the initiator tRNA. The larger sub-unit of the ribosome has two sites for subsequent amino acids to bind and thus be close enough to each other to form a peptide bond. Hence, acting as a catalyst for the peptide bond formation. This linking is the process of the peptide bond formation between two successive amino acids and the formed polypeptide chains with multiple peptide bonds exits from the ribosome. 

mRNA has some additional sequences that are not translated and are referred as untranslated regions (UTRs). The UTRs are present at both the 5'-end (before the start codon) and the 3'-end (after the stop codon). They are required for an efficient translation process.

RNA is a five-carbon sugar which is attached with a phosphate group and a nitrogenous base. In RNA all nitrogenous bases are the same as DNA but instead of pyrimidine thymine; uracil is attached to the sugar. A DNA molecule has coding and non-coding regions. The coding regions are known as the exons and the non-coding regions are known as introns. RNA is transcribed from DNA. There are various types of RNA present in an organisms’ body that have different functions. The RNA strand that is transcribed only from the coding region of the DNA is known as messenger RNA (mRNA). Its primary function is to make proteins. Different types of RNA are formed from non-coding parts of the DNA. One example is transfer RNA (tRNA). Its main function is to help mRNA in translation. tRNA is a small molecule in comparison to mRNA. Since mRNA is formed from the coding regions of the DNA, it has a more elaborate 3D structure than tRNA. This is due to extensive base-pairing.

Since there are 64 triplet codons and only 20 amino acids, genetic code is non-ambiguous. Non-ambiguous code means that there is no ambiguity about a particular code. One codon specifies only one amino acid and not any other except GUG which normally codes for valine but in certain conditions it also codes for N-formyl methionine as initiation codon.

DNA Replication: DNA replication is the process by which a cell makes an identical copy of its DNA. It occurs during the cell cycle to ensure that each daughter cell receives a complete set of genetic instructions. DNA replication is a highly accurate process in which the DNA double helix is unwound, and each strand serves as a template for the synthesis of a complementary strand. 

Transcription: Transcription is the process by which a segment of DNA is used as a template to synthesize a complementary RNA molecule. It is the first step in the central dogma of molecular biology, which describes how genetic information is used to produce proteins. In transcription, an enzyme called RNA polymerase reads the DNA template and assembles a single-stranded RNA molecule, known as messenger RNA (mRNA), by matching RNA nucleotides to the DNA template. 

Translation: Translation is the process in which the information contained in an mRNA molecule is used to synthesize a protein. This process occurs at ribosomes, cellular structures that serve as the protein-manufacturing factories. During translation, the mRNA is read in sets of three nucleotides called codons. Each codon corresponds to a specific amino acid, the building blocks of proteins.