• 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

• Endogenous Controls in Quantitative RT-PCR and Transcriptomics:
• Actin ($eta$-actin / ACTB): Encodes a ubiquitous cytoskeletal microfilament protein required for structural cell maintenance.
• It is utilized as a standard positive internal reference (housekeeping control) in Northern blots, RT-qPCR, and transcriptome assays because its basal mRNA expression level is fairly constant and stable across different cell types, tissues, and experimental physiological conditions.
• nalysis:
• (It is used as a positive control as actin expression is fairly constant in different cell types): Correct.

Correct option: (3) It is used as a positive control as actin expression is fairly constant in different cell types.