31. Adenoviruses are frequently used in gene delivery experiments. Their average genome size is
36,000 base pairs. Of their total DNA content, upto how much of adenovirus genome
can be replaced by foreign DNA?
- 5,000 base pairs
- 15,000 base pairs
- 20,000 base pairs
- 30,000 base pairs
Adenoviruses are key vectors in gene therapy due to their capacity to carry foreign DNA. The correct answer is 20,000 base pairs, as this aligns with established limits for stable replacement in common first-generation vectors.
Why Adenoviruses in Gene Delivery?
Adenoviruses have a linear double-stranded DNA genome averaging 36,000 base pairs, making them ideal for gene delivery experiments.
Researchers delete non-essential regions like E1 and E3 to insert foreign DNA while maintaining viral packaging and infectivity.
The capsid stably packages genomes near wild-type size (up to ~105% or ~38 kb), but replacements beyond optimal limits cause instability or rearrangements.Correct Answer: 20,000 Base Pairs
Up to 20,000 base pairs of the adenovirus genome can be replaced by foreign DNA in standard E1/E3-deleted vectors.
This yields a cloning capacity of about 7-8 kb additional DNA beyond the minimal 28-30 kb viral backbone, totaling ~36-38 kb for stability.
Such vectors support gene therapy without major rearrangements, as confirmed in exam contexts and vector design studies.Option Breakdown
Option Explanation Correct? 5,000 base pairs Too low; only allows tiny inserts (~2-5 kb max), insufficient for most transgenes like fluorescent proteins or shRNA cassettes. No 15,000 base pairs Underestimates capacity; E1/E3 deletions free ~8 kb, but optimized vectors handle up to ~20 kb replacement with stuffer DNA if needed. No 20,000 base pairs Matches practical limit for first-generation vectors; accommodates large transgenes while keeping total genome ~36 kb for packaging efficiency. Yes 30,000 base pairs Overstates replacement; reduces backbone to ~6 kb (unstable, <75% wild-type), leading to rearrangements—only theoretical for helper-dependent vectors with stuffers. No Stability Factors
Vector genome size critically affects virion stability: below ~33 kb or above ~38 kb risks heat instability or deletions.
E1/E3-deleted vectors (backbone ~28 kb) thus support ~8 kb inserts reliably, equating to ~20 kb total replacement when counting deleted regions.
Helper-dependent adenoviruses (hdAd) push limits to ~30 kb total with non-coding fillers but aren’t standard for basic experiments.
