Test: Transcription, Genetic Code & Types of RNA - NEET MCQ

• In some eukaryotes, genes consist of coding nucleotide sequences, which are separated from each other by blocks of non-coding sequences. The coding sequences here are called exons and non-coding sequences are introns.
• The primary transcript from a typical eukaryotic gene contains introns as well as exons. The introns are removed from this primary transcript by a process called RNA splicing. 

Fig: RNA Splicing 

Hence, the correct option is D.

Polypeptide synthesis is signalled by two initiation codons - AUG (methionine) and GUG (valine). Polypeptide chain termination is signalled by three termination codons UAA (ochre), UAG (amber) and UGA (opal). They do not specify any amino acid and are hence called non-sense codons. Anticodon loop is present on tRNA.

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.

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.

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.

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.

Promoter gene acts as an initiation signal which functions as recognition center for RNA polyrmerase provided the operator gene is switched on. RNA polymerase is bound to the promoter gene. When the operator gene is functional, the polymerase moves over it and reaches the structural genes to perform transcription.

Griffith performed the transformation experiment on S.pneumoniae to prove that the genetic material was inheritable. Hershey and Chase performed the transduction experiment which is the infection of a bacteria by a virus called bacteriophage to prove that DNA and not proteins were the genetic material. Meselson and Stahl the experiment on E.coli to establish that replication was semi-conservative (half of parent and half synthesized). Jacob and Monad elucidated the Lac operon a transcription-ally regulated system. 

The accessibility of promoter regions of prokaryotic DNA is regulated by interaction of proteins with sequences called as operators. Repressor molecules that repress gene expression bind to the operator sits and as a result the promoter cannot initiate gene transcription. Promoter is the site for the binding of the RNA polymerase to initiate the process of transcription. It refers to a conserved sequence that is specific for the binding of the RNA polymerase that can bring the expression genes for various functions. Regulator gene is the gene of an operon that codes for a repressor molecule that can bind to the operator sequence and stop gene expression. Structural gene codes for proteins or enzymes that are essential for the cell growth and survival 
So, the correct option is 'A - (ii), B - (i), C - (iv), D - (iii)'.

The lac operon consists of three structural genes (lac Z, lac Y, lac A). Lac Z codes for 13-galactosidase which hydrolyses lactose to glucose and galactose. LacY codes for lac permease, a membrane-bound protein constituent of the lactose transport system which increase permeability of the cell to 13-galactosides. Lac A codes of thiogalactoside transacetylase, an enzyme of uncertain metabolic function. The appearance of ~3-galactosidase is coordinated with the production of permease and transacetylase. Operator gene is a gene which directly controls the synthesis of mRNA over the structural genes. Repressor is regulator protein synthesised all the time (constitutively) by the regulator igene. Repressor is meant for blocking the operator gene so that the structural genes are unable to form mRNAs. After coming in contact with inducer (e.g., lactose) the repressor undergoes conformational change in such a, way that it is unable to combine with operator. Thus, formation of inducer-repressor complex switches on the lac operon.

Regulation of gene expression can be exerted at four levels:
(i) transcriptional level during formation of primary transcript,
(ii) processing like splicing, terminal additions or modifications,
(iii) transport of mRNAs from nucleus to the cytoplasm and
(iv) translational level.

• Transcription is when RNA is made from DNA.
• During transcription, RNA polymerase makes a copy of a gene from the DNA to mRNA as needed. During transcription, the DNA site at which RNA polymerase binds is called the promoter.
• Transcription begins with binding of sigma factor proteins to the RNA polymerase holoenzyme, thus, allowing it to bind to promoter DNA. 
• RNA polymerase creates a transcription bubble, which separates the two strands of the DNA helix and initiates the transcription. 

Transcription in progress: RNA polymerase slides along the DNA and makes an RNA copy (Genes are the only parts of DNA that get copied into RNA):

Hence, the correct option is C.

• The genetic code by which DNA stores the genetic information consists of "codons" of three nucleotides. These codon translates to form protein sequences.
• A codon is defined by the initial nucleotide from which translation starts known as start codon.

Fig: Initiation Codon

Hence, the correct option is D.

The genetic code is degenerate: Some amino acids are encoded by more than one codon, in as much as there are 64 possible base triplets and only 20 amino acids. In fact, 61 of the 64 possible triplets specify particular amino acids and 3 triplets (called stop codons) designate the termination of translation.

Thus, for most amino acids, there is more than one codeword.

Fig: Codons synthesizing amino acids

Hence, the correct option is C.

GGG code is present only 1 time and the amino acid produced by GGG is glycine.

Hence, the correct option is B.

Transcription is the first step of gene expression. In this process, a particular segment of DNA is copied into mRNA. Thus. it controls gene expression.

• tRNA has an anticodon loop that has bases complementary to the code, it also has an amino acid acceptor end by which it binds to amino acids.
• The site lies at the 3' end opposite the anticodon in tRNAs are specific for each amino acid.

Fig: tRNA binding site 
Hence, the correct option is C.
NCERT Reference: Page no. 114 of topic “6.6.2 tRNA– the Adapter Molecule” of chapter 6.

• A part of one chain of DNA molecule known as the sense strand which codes a polypeptide is called cistron.
• Each cistron consists of many codons, and a codon comprises three consecutive nitrogenous bases.

Fig: DNA transcription unit

Hence, the correct option is D.
NCERT Reference: Page no. 109 of topic “6.5 TRANSCRIPTION” of chapter 6.

Three base triplets form a codon that codes for an amino acid. Thus AUG codes for methionine and GUG codes for Valine. AUG and GUG are both initiating codons for translation.

Hence, the correct option is B.
NCERT Reference: Page no. 112 of topic “6.6 GENETIC CODE” of chapter 6.

The anticodon loop is crucial for the accurate translation of mRNA into protein, as it ensures that the correct amino acid is added to the growing polypeptide chain.