The main enzyme responsible for DNA replication in E. coli is?

[Molecular-Basis-of-Inheritance] [class-xii ]

• DNA Polymerase I
• DNA Polymerase II
• DNA Polymerase III
• Primase
Correct Option: C  [ DNA Polymerase III ]

Remark: DNA Polymerase III is the primary enzyme responsible for the polymerization of nucleotides during DNA replication in E. coli. It adds nucleotides to the growing chain with high speed and accuracy.

The direction of polymerization by DNA-dependent DNA polymerase is?

[Molecular-Basis-of-Inheritance] [class-xii ]

• 3-prime to 5-prime
• 5-prime to 3-prime
• Both directions
• Random
Correct Option: B  [ 5-prime to 3-prime ]

Remark: DNA polymerases can only add nucleotides to the 3-prime hydroxyl group of a growing strand. Therefore, the direction of synthesis (polymerization) is always 5-prime to 3-prime.

Which sequence on DNA defines the starting point for transcription?

[Molecular-Basis-of-Inheritance] [class-xii ]

• Terminator
• Structural gene
• Promoter
• Origin of replication
Correct Option: C  [ Promoter ]

Remark: The promoter is a specific DNA sequence located upstream of the structural gene. It provides a binding site for RNA polymerase and defines where transcription of a gene begins.

The coding strand of DNA has the sequence 5-ATGC-3. What will be the sequence of the mRNA transcript?

[Molecular-Basis-of-Inheritance] [class-xii ]

• 5-TACG-3
• 5-AUGC-3
• 3-AUGC-5
• 5-UACG-3
Correct Option: B  [ 5-AUGC-3 ]

Remark: The coding strand has the same sequence as the mRNA (except T is replaced by U) because the mRNA is complementary to the template strand (which is complementary to the coding strand). So, 5-ATGC-3 becomes 5-AUGC-3 in mRNA.

In prokaryotes, transcription and translation are?

[Molecular-Basis-of-Inheritance] [class-xii ]

• Separated by the nuclear membrane
• Coupled (occur simultaneously)
• Occurring in different compartments
• Dependent on RNA processing
Correct Option: B  [ Coupled (occur simultaneously) ]

Remark: In prokaryotes, there is no nucleus to separate the genetic material from the cytoplasm. Therefore, ribosomes can attach to the mRNA and begin translation even before transcription is fully complete, coupling the two processes.

The intervening sequences in eukaryotic genes that do not code for proteins are?

[Molecular-Basis-of-Inheritance] [class-xii ]

• Exons
• Cistrons
• Introns
• Promoters
Correct Option: C  [ Introns ]

Remark: Introns are non-coding sequences found within eukaryotic genes. They are transcribed into the primary RNA transcript but are removed (spliced out) before the mRNA is translated into protein.

Which RNA polymerase transcribes tRNA in eukaryotes?

[Molecular-Basis-of-Inheritance] [class-xii ]

• RNA Polymerase I
• RNA Polymerase II
• RNA Polymerase III
• Primase
Correct Option: C  [ RNA Polymerase III ]

Remark: In eukaryotes, RNA Polymerase III is responsible for transcribing transfer RNA (tRNA), 5S rRNA, and small nuclear RNAs (snRNAs). Mnemonic: RMT (I-rRNA, II-mRNA, III-tRNA).

How many codons code for amino acids?

[Molecular-Basis-of-Inheritance] [class-xii ]

• 64
• 61
• 20
• 3
Correct Option: B  [ 61 ]

Remark: There are 64 possible codons in the genetic code. However, 3 of them are stop codons (UAA, UAG, UGA) which do not code for any amino acid. Therefore, only 61 codons code for the 20 amino acids.

Sickle cell anemia is a result of which type of mutation?

[Molecular-Basis-of-Inheritance] [class-xii ]

• Frameshift mutation
• Deletion
• Point mutation
• Duplication
Correct Option: C  [ Point mutation ]

Remark: Sickle cell anemia is caused by a point mutation (substitution) in the beta-globin gene. A single base change (GAG to GUG) results in the substitution of Glutamic acid with Valine, altering the protein structure.

Insertion or deletion of one or two bases in DNA results in?

[Molecular-Basis-of-Inheritance] [class-xii ]

• Point mutation
• Frameshift mutation
• Silent mutation
• Missense mutation
Correct Option: B  [ Frameshift mutation ]

Remark: The genetic code is read in triplets. Inserting or deleting one or two bases shifts the reading frame from that point onward, altering all subsequent amino acids. This is called a frameshift mutation.