GENETICS AND MOLECULAR BIOLOGY HOMEWORK

Question

25.You are studying a new type of cancer and you discover that there is a mutation in a gene that encodes a histone acetylase. You hypothesize that the mechanism by which this mutation causes cancer is:

A - Introducing random mutations

B - By altering the expression of a gene involved in the control a cell proliferation

C - Mutating DNA repair genes

D - By introducing a mutation in a gene involved in the control a cell proliferation

26.Often mutations in cyclin dependent kinases, CDKs, can cause cancer. This is because:

A - CDKs control apoptosis

B - CDKs control the cell cycle progression

C - CDKs affect DNA repair

D - CDKs affect chromatin remodeling

27.BAX proteins stimulate apoptosis. Mutations in BAX genes can help developing cancers but not necessarily trigger a tumor. This is because:

A - Mutations in BAX do not contribute to tumorigenesis, but BAX can prevent the formation of a tumor

B - Mutations in BAX may over-stimulate apoptosis

C - Mutations in BAX contribute to tumorigenesis only with other inhibitors of apoptosis

D - Mutations in BAX can prevent the formation of tumors

28.When lactose is present:

A - Lactose represses the activation of the genes in the lac operon by binding the operator

B - Lactose represses the activation of the genes in the lac operon by binding the repressor

C - Lactose induces the activation of the genes in the lac operon by binding the operator

D - Lactose induces the activation of the genes in the lac operon by binding the repressor

29.When both lactose and glucose are available to E.coli, which part of the lac operon regulation assures that glucose will be metabolized first?

A - The repressor binds to the operator in the absence of lactose

B - The repressor cannot bind to the operator in the presence of lactose

C - CAP binds to the promoter only in the presence of low glucose

D - RNA polymerase transcribes the lac operon to make a single RNA molecule

30.The tryptophan (trp) operon is a repressible system. This means:

A - The genes are normally repressed, but the presence of trp can activate the genes

B - The genes are normally repressed

C - The genes are normally expressed when trp is present

D - The genes are normally expressed, but the presence of trp can repress the operon

Answer 1

25 (B). Acetylation of histones alters the binding interactions between histone proteins and DNA that form nucleosomes and condense DNA into chromatin. Highly condensed DNA is inaccessible to RNA polymerase and is not wasily transcribed. If the accessibility of genes controlling cell proliferation is altered, cancer could develop.

26 (B) Cyclins are proteins that control the progression of the cell cycle. There are specific cyclins that determine transitioning from G1, S, G2 and M phases of the cell cycle. Many of these proteins are activated or inactivated by phosphorylation, so changed their phosphorylation state could effect cell proliferation and can contribute to cancer development.

27 (A) BAX proteinss (Bcl-2 associated proteins) are involved in apoptosis. They function to degrade the mitochondria and release cytochrome c, one of the key steps for programmed cell death. Loss of function of BAX would decrease the apoptic response, which could enable a cancer cell to continue to grow. However, loss of BAX cell function by itself would not cause a cell to proliferate. There would need to be mutations in other genes that wojuld cause increased cell growth.

28 (D) One of the control mechanisms of the lac operon is a constituitively expressed repressor protein that binds the promoter region of the operator. When the repressor protein is bound to the promoter, the op[erator gene is inaccessible to RNA polymerase and the lac operon is inactive. Lactose binds to the repressor protein and changes its conformation, so the repressor protein no longer binds to the promoter region. The operator is now accessible and the lac operon is activated.

29 (C) When both lactose and glucose are present, lactose will bind to the repressor protein. The repressor protein will no longer bind to the promoter region, and the lac operon will be accessible for transcription. However, transcription is also governed by CAP (catabolite activator protein). When glucose is low, cAMP levels in the cell are high, and cAMP binds to CAP. This puts CAP into its active form so it can bind to the promoter and increase transcription. Keep in mind, however, that even when CAP is inactive there can be very low levels of transcription. This is often called "leaky" transcriptional regulation.

30 (D) Repressible systems are actively transcribed under normal circumstances, but can be shut down, or repressed by a physiological response. In the case of the trp operon, there is a repressor protein that is constituiitively produced, but is inactive unless bound with tryptophan. When tryptophan levels are high, transcription is down regulated because tryptophan binds to the repressor which can then bind to the promoter region of the trp operon making it inaccessible to RNA polymerase.

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