MCB Quiz problem 7

The following reaction involving the breakdown of sugar A occurs in a bacterial cell. The gene that codes for the enzyme is part of an operon.
In an experiment, some of the bacterial cells were cultured in three flasks with 100 mg/ml of sugar A added. In flasks 1 and 2, compounds L and M were added respectively. Nothing was added to flask 3. The following results were obtained.


Concentration of B (mg/ml)

Flask 1
Flask 2
Flask 3

Addition of…
L
M
(nothing)
At 0 minutes

0
0
0
At 40 minutes

110
2
98
At 80 minutes

201
3
100
At 120 minutes

250
5
100



a) Which of the compounds (L or M) might be a gene repressor? Explain your answer. (5 marks)


M. The reason is because the function of the repressor is to stop the DNA from getting transcripted. If the concentration of the B is very less, this means that there are very less proteins present inside the cell, and this could be due to the large amount of repressor. The reason is because large amount of repressor will slow down the transcription process. The DNA will be transcripted to produce the enzymes, and therefore the sugar will be able to break down. Lesser amount of B means that there are also not enough amount of enzymes for breaking, and this shows that the transcription process is being slowed down by the repressor, therefore the translation process is also being slowed down, therefore, the proteins being produced are also lesser, therefore, the amount of B is less.

b) Suggest with explanations what compound L might be. (5 marks)

Compound L increases the production of the B, therefore, it could be the enzyme that breaks down the sugar A.
It can also be a inductor which is able to bind with the repressor, and therefore preventing the repressor from binding onto the operator of the DNA, and therefore, it is able to go through more transcription, and therefore going through more translation, and then, it will be able to produce more enzymes that is able to break down the sugar A into B and C, and therefore, the concentration of B increases.

爱不释手（喜爱到极点而不舍得放手。形容非常喜欢。）

从前有一个读书人，他非常喜欢苏东坡的诗作， 每当抄录到苏东坡的一首诗，便反复吟咏， 爱不释手。

有一天， 他到西湖游玩，偶然间发现一块石壁上面刻有一首诗，题款人是“苏东”。 这一发现， 他高兴得像拾到宝一样，虽然题款的名字少了一个“坡”字，他肯定只是刻石的人漏刻罢了，一定是苏东坡的诗作。

他即刻拿出笔墨纸砚， 把这首诗抄录下来，然后坐在西湖边的石椅上摇头摆脑地吟咏，越吟咏越觉得有味道，越肯定它是苏东坡的作品。

有一个游人看见他这样入迷，便开口向他借来看。

“不！不行！”他生怕那游人来抢， 把手抄本捏得更紧，“我只能念给你听！”

当他把这首诗念完后，游人忽然哈哈大笑， 说道：“你虽然对苏东坡的诗作爱不释手， 但也要分清楚是不是苏东坡的作品啊！ 告诉你吧， 我叫苏东， 和苏东坡一点关系也没有，这首诗是我十年前游西湖时刻的！”