Question

Mutations are errors in DNA that :

• A. Always harmful
• B. Increase tumor growth
• C. Occur spontaneously at a low rate
• D. Only occur on the X chromosome

Hint:

Mutations are rare, random, and spontaneous events. While some cause diseases, many are completely silent (neutral), and a small fraction provide the genetic diversity required for natural selection and evolution.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
A mutation is any permanent, heritable change in the nucleotide sequence of an organism's genome (DNA or RNA).
Mutations are the ultimate source of genetic variation and drive evolutionary processes.

Step 3: Detailed Explanation:
Let us evaluate each characteristic statement regarding mutations:
- Statement (C) is correct: Under natural conditions, mutations occur spontaneously at a very low baseline rate due to transient chemical instability of purine/pyrimidine bases, replication errors by DNA polymerase, or natural background radiation.
Cells have evolved highly efficient DNA repair mechanisms to correct these spontaneous replication errors, keeping the mutation rate extremely low (typically $10^{-6}$ to $10^{-9}$ per base pair per generation).
- Let us evaluate why other options are incorrect:
- Option (A): Many mutations are neutral (silent mutations that do not alter the amino acid sequence or occur in non-coding regions), and some can be beneficial, providing adaptive advantages (evolutionary drivers). Thus, they are not *always* harmful.
- Option (B): While some somatic mutations in oncogenes or tumor suppressor genes can cause unregulated cell division and tumors, the vast majority of mutations do not affect tumor growth.
- Option (D): Mutations can occur randomly on any chromosome (autosomes or sex chromosomes) and in organelle DNA (mitochondria, chloroplasts).
Therefore, mutations occur spontaneously at a low rate.

Step 4: Final Answer:
Mutations are genomic errors that occur spontaneously at a low rate.