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.
Which of the following facts about exons is not correct?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → They are not seen in processed DNA.
In eukaryotes, the coding part of a gene is split. The coding or expressed sequences are called exons. They are interrupted by non-coding sequences called introns. All of these are present in the DNA of the gene. When the gene is copied into RNA, the introns are cut out and the exons are joined together (splicing). So exons are the sequences that appear in the mature or processed RNA.
(1) They are also known as expressed sequences. — A correct fact about exons. Exons are the coding, expressed sequences.
(2) They are seen in mature or processed RNA. — A correct fact about exons. Only exons remain after splicing.
(3) They are interrupted by introns. — A correct fact about exons. In the gene, introns lie between the exons.
(4) They are not seen in processed DNA. — Not a correct fact about exons. Exons are part of the DNA of the gene. Processing (splicing) happens to the RNA, and it removes the introns, not the exons.
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:
Transcription Unit and the Gene – “… In eukaryotes, the monocistronic structural genes have interrupted coding sequences – the genes in eukaryotes are split. The coding sequences or expressed sequences are defined as exons. Exons are said to be those sequence that appear in mature or processed RNA. The exons are interrupted by introns.”