Unit 1

• Open Reading Frame (ORF):
• In molecular genetics, an Open Reading Frame (ORF) refers to a continuous stretch of DNA or RNA nucleotides that starts with an initiation codon (typically $5'\text{-AUG-}3'$), followed by a continuous series of nucleotide triplets (codons) that specify the amino acid sequence of a polypeptide, and terminates at a stop codon ($5'\text{-UAA/UAG/UGA-}3'$).
• nalysis:
• (The nucleotides of a gene that make up the codons specifying amino acids): Correct definition.

Correct option: (2) The nucleotides of a gene that make up the codons specifying amino acids.

• Mechanisms of DNA Mutations:
• Spontaneous and induced mutations arise through replication slippage, unequal crossing over, or mutagens causing the insertion or deletion of one or a few nucleotides (indels).
• If the indel length is not a multiple of three, it causes a catastrophic frameshift mutation, completely altering the downstream amino acid reading frame.
• nalysis:
• (Insertion or deletion of one or a few nucleotides): Correct source of genetic mutation.

Correct option: (2) Insertion or deletion of one or a few nucleotides.

• Mechanisms of DNA Repair:
• Base Excision Repair (BER) & Nucleotide Excision Repair (NER): Remove damaged bases/adducts.
• Mismatch Repair (MMR): Corrects post-replicative base mispairings and insertion/deletion loops.
• Non-Homologous End-Joining (NHEJ): Directly ligates double-strand breaks without a homologous template.
• Branch Migration: A physical thermodynamic process where homologous strands exchange pairing partners during meiotic or mitotic homologous recombination (Holliday junction movement); it is NOT a DNA repair system.
• nalysis:
• (Branch migration): Correct (not a repair system).

Correct option: (4) Branch migration