A student wants to construct a plasmid 'BacLight', that when transformed into a bacterium will produce light IF AND ONLY IF the sugar lactose is present in the medium. For this, she uses the following DNA sequences: promoter of repressor ( $\mathrm{P}_{\mathrm{i}}$ ), promoter of $\operatorname{lac} Z\left(\mathrm{P}_{\mathrm{z}}\right)$, operator $(\mathrm{O})$ and repressor gene lacI from the lactose operon; and a structural gene X , which encodes a protein that produces light. The different DNA elements (II - V) and a vector (I) were digested with restriction enzymes and ligated to produce the plasmid. The DNAs used along with restriction enzyme sites at their ends are given below. Slash (/) in the sequence represents the point at which the restriction enzyme cuts the DNA.
$\begin{aligned} \text { I }: & 5^{\prime}-(\mathrm{G} / \mathrm{GATCC})-\text { Vector }-(\mathrm{T} / \mathrm{CTAGA})-3^{\prime} \\ \mathrm{II}: & 5^{\prime}-(\mathrm{T} / \mathrm{CTAGA})-\mathrm{P}_{\mathrm{i}}-l a c I-(\mathrm{G} / \mathrm{GATCC})-3^{\prime} \\ \mathrm{III}: & 5^{\prime}-(\mathrm{T} / \mathrm{CTAGA})-\mathrm{P}_{\mathrm{z}}-(\mathrm{G} / \mathrm{CTAGC})-3^{\prime} \\ \mathrm{IV}: & 5^{\prime}-(\mathrm{G} / \mathrm{CTAGC})-\mathrm{O}-(\mathrm{A} / \mathrm{CTAGT})-3^{\prime} \\ \mathrm{V}: & 5^{\prime}-(\mathrm{A} / \mathrm{CTAGT})-\mathrm{Gene} \mathrm{X}-(\mathrm{T} / \mathrm{CTAGA})-3^{\prime}\end{aligned}$

Choose the correct sequence of DNAs that can possibly be ligated to produce the plasmid 'BacLight'.

• A

  Vector $-\mathrm{P}_{\mathrm{i}}-\operatorname{lac} I-\mathrm{P}_2-\mathrm{O}-$ Gene $\mathrm{X}-$ Vector
• Correct answer
  B

  Vector $-\mathrm{P}_{\mathrm{z}}-\mathrm{O}-$ Gene $\mathrm{X}-\mathrm{P}_{\mathrm{i}}$ - lacI-Vector
• C

  Vector $-\mathrm{P}_{\mathrm{z}}-$ Gene $\mathrm{X}-\mathrm{O}-\mathrm{P}_{\mathrm{i}}-$ lac $I$-Vector
• D

  Vector $-\mathrm{P}_{\mathrm{z}}-\mathrm{O}-\mathrm{P}_{\mathrm{i}}-$ lacI -Gene $\mathrm{X}-$ Vector

Hence, the correct answer is option: 'B'.