Download Test: Gene Regulation Free Response Questions It is known that

Test: Gene Regulation
Free Response Questions
1. It is known that very little of the human genome actually codes for proteins.
a. Briefly explain how microRNAs, also called miRNA, regulate gene expression.
miRNA: a single stranded micro RNA forms a complex with proteins. The complex
binds to mRNA with complementary base sequences and the mRNA is either
degraded or translation is blocked. If the homology between miRNA and the mRNA
strand is adequate, then the mRNA will be degraded. If the two strands are not
complementary then the mRNA will be block so translation is inhibited.
b. Do siRNAs regulate gene expression by affecting translation or transcription?
Explain.
siRNA affects translation by degrading mRNA or blocking translation.
Experiments show that siRNA can also silence transcription by changing the
chromatin structure.
2. a. What type of evidence established that Bicoid protein is a morphogen that
determines the anterior end of a fruit fly.
Bicoid mRNA in Drosophila melanogaster was shown to be localized at one end of
the unfertilized egg; later in development bicoid protein occurred in a gradient that
was most concentrated in the anterior cells of the embryo. Also, injection of bicoid
mRNA into various regions of early embryos caused anterior structures to form at
those sites.
b. Summarize the flow of genetic information during replication of a retrovirus.
Indicate the enzymes that catalyze this flow.
A retrovirus is a RNA virus. The virus injects its mRNA along with reverse
transcriptase into host cells. The reverse transcriptase is use to make DNA from
viral mRNA. Once a double stranded DNA is made, it embeds itself into the host
genome and or uses host RNA polymerase to transcribe viral proteins and
assembles new viruses, which can infect other cells and continue its life cycle.
3. a. List three genetic changes that can convert a proto-oncogene into an oncogene.
Proto-oncogenes are genes that code for proteins that are responsible for normal
cell growth. Mutations in proto-oncogenes result in i. more copies of the genes being present than normal – Amplification
ii.
Translocation or transposition may bring the gene under the control of a more
active promoter
iii. Point mutation may occur in control elements, increasing gene expression or
may occur in the gene that creates a more active or resilient protein.
b. List three possible functions of tumor-suppressor proteins.
i.
ii.
iii.
iv.
Tumor suppressor proteins may function in repair of damaged DNA
The proteins may control cell to cell adhesion
The p53 protein can activate genes that halt the cell cycle by binding to CDKs
The proteins could signal pathways that inhibit or halt the cell cycle by
binding to miRNAs
v. The proteins can activate suicide genes – leading the cell to apoptosis
4. Bacteria often respond to environmental change by regulating transcription.
a. Describe an operon model for gene regulation.
In bacteria, the genes for different enzymes of a single metabolic pathway may
be grouped together into one transcription unit, served by a single promoter.
Thus these genes are coordinately controlled. An operon typically includes:
Operator – DNA segment near or within a promoter that controls the access of
RNA polymerase to the genes
Promoter – which binds to the RNA polymerase allowing transcription of all the
genes in the operon.
Repressor – binds to operator and turns the operator off preventing
transcription
Inducer – inactivates a repressor protein
b. How does the lac operon control lactose metabolism in E.coli.
The lac operon, an inducible operon in E.coli, contains a cluster of 3 genes – lacZ,
lacY and lac A, along with a promoter and an operator. Lac I codes for a repressor
protein which binds to the operator, switching off all the genes in the operon. When
lactose is present, an isomer, allolactose binds to the repressor and inactivates it.
This allows RNA polymerase to bind to the promoter and allows transcription of the
genes.
Positive control of the operon is when an activator CAP binds to cAMP and
stimulates transcription with efficiency.
5. Eukaryotic cells in multicellular organisms regulate gene expression to produce
specialized cells. Describe gene regulation at the following stages in eukaryotic cells:
a. Chromosomal level
Differences between cells with same genome are a result of differential gene
expression.
DNA packing and location – genes with highly condensed heterochromatin are
not expressed
Histone acetylation – attachment of acetyl group to histone tails opens
chromatin for transcription
DNA methylation – addition of methyl groups to DNA leads to condensation of
the chromatin and reduces transcription.
Epigenetic inheritance - no change in DNA chromatin modification
b. At transcriptional Initiation.
A typical eukaryotic gene consists of a promoter to which a transcription initiation
complex attaches, including RNA polymerase II. A sequence of introns interspersed
among coding exons are transcribed.
Control elements are non-coding regions that regulate transcription by serving as
binding site for transcription factors.
General transcription factors include the TATA box.
Distal elements include enhancers that may be upstream or downstream of the gene
or could be within an intron.
A protein mediated bend bring activators bound to enhancers in contact with
mediator proteins and general transcription factors at the promoter.