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GATE 2023 BT Q.45: solution and answer

GATE 2023 · BTQ.45 · 2 marks · MSQ

Which of the following vector(s) is(are) used to clone a DNA fragment of size 220 kb?

  1. (A)

    Bacterial artificial chromosome

  2. (B)

    Yeast artificial chromosome

  3. (C)

    Cosmids

  4. (D)

    pUC19 plasmid

See the question in the paper
GATE 2023 BT question 45Tap to open full size

Answer and solution

ANSWER

The vectors capable of cloning a 220 kb DNA fragment are the Bacterial Artificial Chromosome (BAC) and Yeast Artificial Chromosome (YAC) — options A and B.

Official answer: A, BSource: Official answer key, GATE 2023 BT (IIT Kanpur)

  1. Recall typical insert-capacity ranges for cloning vectors: plasmids (e.g. pUC19) — up to about 10 kb; cosmids — about 30-45 kb, limited by the lambda phage head packaging constraint (total packaged DNA of 37-52 kb including the cos sites); BACs — about 100-300 kb, based on the single-copy F-plasmid replicon; YACs — about 100 kb up to 1-2 Mb, a linear vector with yeast telomeres and a centromere.

  2. Option (D) pUC19 plasmid: capacity of only a few kb, far too small to carry a 220 kb insert — reject.

  3. Option (C) Cosmids: capacity capped at ~30-45 kb by the phage-packaging limit; 220 kb greatly exceeds this — reject.

  4. Option (A) Bacterial artificial chromosome (BAC): F-plasmid based, routinely carries 100-300 kb inserts; 220 kb fits comfortably — accept.

  5. Option (B) Yeast artificial chromosome (YAC): linear vector with telomeres and centromere, carries inserts from ~100 kb to several Mb; 220 kb fits easily — accept.

  6. Therefore the vectors capable of cloning a 220 kb fragment are the BAC and the YAC — options A and B.

KEY IDEA

Cloning vector choice depends on insert-capacity range: plasmids (e.g. pUC19) hold only a few kb, cosmids are capped by lambda phage packaging at ~30-45 kb, while BACs (F-plasmid based, 100-300 kb) and YACs (linear yeast chromosome with telomeres/centromere, 100 kb to several Mb) are the vectors built for large (>100 kb) inserts such as 220 kb.

WATCH OUT FOR

Confusing cosmids (packaging-limited by the lambda phage head to ~30-45 kb total) with BACs, or assuming any "artificial chromosome" vector name alone (without checking insert-capacity range) is enough — some students pick cosmids because they associate "large insert" loosely with cosmids rather than knowing the phage-packaging ceiling.

From the GATE 2023 BT paper. Every question of this paper · Download the full paper · All 10 Enzymes & vectors questions, with notes

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