Molecular Basis of Inheritance
Read the following passage carefully and answer the questions given below:
In the process of translation, genetic information is translated from the language of nucleic acid polymers to the language of amino acid polymers. There is no complementarity between amino acids and nucleotides, nor can this be thought of chemically. This led to the proposal of a genetic code that would direct the sequence of amino acids during protein synthesis. In eukaryotes, monocistronic structural genes are found, in which interrupted coding sequences are present. Coding sequences, or expressed sequences, are called exons. Exons are interrupted by unexpressed sequences called introns.
The length of DNA usually depends on the number of nucleotides present in it. It is a characteristic feature of an organism. For example, a bacteriophage called φ × 174 has 5386 nucleotides, bacteriophage lambda has 48502 base pairs (bp), and Escherichia coli has 4.6 × 106 base pairs (bp). There are three major types of RNA: mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA). In eukaryotes, RNA is needed for the synthesis of proteins in ribosomes. DNA obtained from sources such as blood, blood spots, hair, skin, bone, semen, etc. has one or more polymorphisms, which are very useful in forensic applications.
Identify the correct set of stop codons.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → UAA, UAG and UGA
The genetic code has 64 codons. 61 of them code for amino acids. The other three, UAA, UAG and UGA, do not code for any amino acid. They are stop (terminator) codons, and they end translation. AUG has a double job: it codes for methionine and also acts as the start (initiator) codon.
(1) UAA, UUU and UGA — Incorrect. UUU codes for the amino acid phenylalanine, so it is not a stop codon.
(2) UAA, UAG and UGA — Correct. These are the three stop codons.
(3) UAG, AUG and UGA — Incorrect. AUG is the start codon, not a stop codon.
(4) UAA, AUG and UAC — Incorrect. AUG is the start codon, and UAC codes for tyrosine.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Genetic Code – “… (i) The codon is triplet. 61 codons code for amino acids and 3 codons do not code for any amino acids, hence they function as stop codons.”
Genetic Code – “… (vi) UAA, UAG, UGA are stop terminator codons.”