In this task, you are given a question and an answer, you would be asked to create the sentence based on the Question-Answer provided. It should be contained within the Question-Answer provided.

[EX Q]: Question: Mitosis actually occurs in how many phases? Answer: four
[EX A]: Mitosis actually occurs in four phases.

[EX Q]: Question: What are intervertebral discs made of? Answer: fibrocartilage
[EX A]: Intervertebral discs are made of fibrocartilage.

[EX Q]: Question: What happens to structural genes in the presence of tryptophan? Answer: they are not transcribed
[EX A]:
They are not transcribed happens to structural genes in the presence of tryptophan.