Molecular Basis of Inheritance Test - 85

The genetic code consists of 64 triplets of nulceotides. These triplets are called codons. With three exceptions, each codon encodes for one of the 20 amino acids used in the synthesis of proteins. That produces some redundancy in the code: most of the amino acids being encoded by more than one codon, which is called as degeneracy of the genetic code.

A naturally occurring coding strand composed of alternating C and U residues (CUCUCUCUCUCUCUCUC) would result in the formation of polypeptide containing alternating leu and ser residues because CUC and UCU code for leucine and serine respectively.

There are three codons that do not code for any amino acid. They are called the stop codons or terminator codons UAA, UAG and UGA. They are found at the end of the mRNA and gives a signal to terminate the translation and releases the protein. Hence,the correct answer is option C.

The stop codon is the code of the three nucleotides in the segment of the mRNA which results in the termination of the activity of the ribosome during the process of translation. The protein which is formed gets released when the ribosome reaches the stop codon. The codons UAG, UGA and UAA are known as the stop codons. There are no anticodons or tRNA for this stop codons. So, the presence of these codons results in termination of the process.

Nirenberg and Matthaei developed extracts from the bacterial cells. This made a protein even there is no intact living cells was present. Then he adds an artificial RNA Poly-U to the extract caused to prepare the protein that is entirely of phenylalanine. Thus this experiment showed that RNA controlled the production of specific types of protein and the genetic code is a triplet codon.

In the genetic code, a stop codon is a nucleotide triplet within messenger RNA that signals a termination of translation. Stop codons signal the termination of this process by binding release factors, which cause the ribosomal units to disassociate, releasing the amino acid chain. Examples are UAA, UGA and UAG.

An ochre mutation is a nonsense mutation that changes a sense codon (one specifying an amino acid) into the stop codon UAA, causing premature termination of the polypeptide chain during translation. The mutation, the codon, and the mutant are all called as ochre. 

In option A and B, the given sequence is changed from ATTGCC to ATCGCC and ATTGCA respectively by replacement of one base with other; exchange of one nitrogenous base for other base is called substitution. It changes only single codon and thereby replaces single amino acid with other; entire reading frame is not affected. In option C, the given sequence is changed from ATTGCC to ATTCCCGCC by addition of three bases and thus one amino acid but there is no effect on rest of the reading frame. In option D, the given sequence is changed from ATTGCC to ATTTGCC by addition of “T” at 4th place. The gene mutations that alter the base sequence of whole genetic frame from the point of the mutation are called frame shift mutations. Here, the reading frame of base sequence is shifted forward due to insertion of one nucleotide base which in turn changes the succeeding codon sequence from ATT- GCC to ATT- TGC- C and cause the most serious alternation among the given options. Hence, option D is correct answer.