Question

QUESTION 18

Briefly describe why E. coli wants to express different amounts of the lac operon genes in relation to the presence/absence of glucose and lactose and the molecular mechanism by which it does so for each of the four conditions with respect to glucose and lactose.

QUESTION 19

A rare deletion on the long arm of chromosome 15 (15q11-13) produces one of two types of genetic disorders in humans.  These are Prader-Willi syndrome (obesity, hypotonia, hypogonadism) and Angelman syndrome (epilepsy, tremors, perpetual smiling).  Three imprinted genes are present in the deleted region.  These are snrpn and ndn (lack of which is associated with Prader-Willi) and ube3A (lack of which is associated with Angelman).  snrpn and ndn are paternally imprinted for expression and ube3A is maternally imprinted.   Charlie and Pat are siblings both with Angelman syndrome and Charlie has one (of two) child with Prader-Willi Syndrome while Pat has a child with Angelman syndrome.  What are the sexes of Charlie and Pat?  Explain your answer.  Can Charlie and Pat have two normal parents or does one of them have Prader-Willi or Angelman syndrome.  Explain your answer and if one of them has a disorder tell me which parent had which disorder.

Answer 1

Q18. The lac operon codes for genes that helps E. coli to metabolize lactose as an alternate carbon and energy source when glucose is absent and lactose is present in the growth media. The activity of the lac operon is under the direct control of the glucose concentration inside the cell. Making proteins (enzymes) required for this process is energy expensive and the bacteria will be at a fitness disadvantage if these genes are transcribed and translated unnecessarily, i.e.. when glucose that is readily available for uptake and utilization is present in the growth medium. So it is essential that the transcription of these genes are regulated strictly. There are two factors, namely the lac repressor and catabolite activator protein (CAP) which act as lactose and glucose sensors respectively to regulate the lac operon.

The lac repressor normally blocks the transcription of the operon, but it stops being a repressor in the presence of lactose. On a similar note, CAP senses the glucose levels in the cell and activates the transcription of the operon when glucose levels are low, which is signaled by rising levels of cyclic AMP (cAMP). There can be four distinct conditions with respect to the levels of glucose and lactose in the cell and the operon is regulated accordingly.

Condition 1 - Glucose is present and lactose is absent.

In this condition, no transcription of the lac operon occurs as the lac repressor remains bound to the operator region of the lac operon and prevent transcription. Furthermore, CAP is inactive as the cell has glucose at its disposal and hence cAMP levels will be low (cAMP levels rise only when the cell is starved of energy).

Condition 2 - Glucose is present and lactose is present.

When both glucose and lactose are present, though the lac repressor is no longer bound to the operator site, the cell isn't started due to the availability of glucose and hence CAP will not be bound to the CAP binding site. However, very low levels of 'leaky' transcription can occur in this condition.

Condition 3 - Glucose is absent and lactose is absent.

No transcription of the lac operon occurs in this condition as although CAP can now bind due to rising cAMP levels, the lac repressor also remains bound to the operator region, preventing the binding of RNA polymerase and thereby transcription of the operon.

Condition 4 - Glucose is absent and lactose is present.

In this condition, the lac operon is active and is strongly transcribed. This is because CAP is bound to the CAP site due to glucose starvation and the lac repressor can no longer be bound to the operator region as is it binds to allolactose, an isomer of lactose. This enables the RNA polymerase to bind to the promoter, thus enabling high levels of transcription.

Q19. Maternal imprinting occurs when an allele of a particular gene inherited from the mother is transcriptionally silent and paternal imprinting occurs when the allele inherited from the father is transcriptionally silenced. Uniparental disomy (UPD) occurs when a person inherits two copies of a chromosome, from one parent and no copy from the other parent. Both Prader-Willi syndrome and Angelman syndrome can be caused by UPD. We are told that of the three genes in the deleted region,  snrpn and ndn (lack of which is associated with Prader-Willi syndrome) are paternally imprinted while that of ube3A (lack of which is associated with Angelman syndrome) is maternally imprinted.

We are told that Charlie and Pat are siblings both with Angleman syndrome, which means neither of them have a functional copy of ube3A. One of Charlie's two children has Prader-Willi syndrome (no functional copies of snrpn and ndn) while Pats child has Anglemans syndrome. We are asked to assess the sexes of Charlie and Pat.

Now using this information, we can see that Pat is a male and Charlie is a female. Let us see why this is the case. Since this is a rare deletion, it is unlikely that their spouses carry the same mutation. Pat's child has Anglemans syndrome. Now we know that the condition is resultant from the loss of ube3A, which is maternally silenced. Even if the mother had a functional allele of ube3A, since that copy is silenced, the child has inherited no functional copy of it, leading to Anglemans syndrome. Likewise, we are told that one of Charlie's two children have Prader-Willi syndrome (no functional copies of snrpn and ndn). Both these genes are paternally silenced, meaning that the copies they inherit from their father will be silent. If the child has to have Prader-Willi, it then naturally follows that Charlie is a female.

Can Charlie and Pat have two normal parents or does one of them have Prader-Willi or Angelman syndrome? Since we know that both of them have Angelman syndrome, we know that neither of them inherited a functional copy of ube3A. Since ube3A is maternally imprinted, it is more likely that their father had Angelmman syndrome.

I hope this helps :)