Module 7/Chapter 13

True or false is the genetic code degenerate?

True

A degenerate genetic code means

multiple different codons can specify the same amino acid

A DNA sequence produces a mutant protein in which several amino acids in the middle of the protein differ from the normal protein. What kind of mutation could have

occurred?

An addition and deletion mutation

True or false : for eukaryotic transcription Transcription initiation occurs when RNA polymerase binds to a complex of transcription factors at the TATA box.

true

True or false: for eukaryotic transcription A polycistronic mRNA may be transcribed if the gene products are used in the same pathway or needed at the same time.

False

Eukaryotic promoter regions contain a TATA box and a CAAT box. True or false

True

True or false for eukaryotic transcription The transcripts produced contain both exons and introns.

True

Fill in the blanks:

Information contained in the base sequences of

DNA

is either

replicated

and passed to offspring through a variety of reproductive processes or

transcribed

into a variety of RNAs. Certain RNAs

(

tRNA

)

carry amino acids to the site of translation where proteins are assembled. Other RNAs

(

mRNA

and

rRNA

)

provide a mechanism for ordering the sequence of amino acids during translation.

.In 1964, Nirenberg and Leder used the triplet binding assay to determine specific codon assignments. A complex of which of the following components was trapped in the nitrocellulose

filter?

charged tRNA, RNA triplet, and ribosome

The finding that virtually all organisms use the same genetic code provides the basis for declaring that the code is universal. Identify the two exceptions to such universality.

Mycoplasma capricolum

mitochondrial DNA

RNA synthesis from a DNA template is called

_______.

transcription

Which bacterial RNA polymerase subunits is found in the holoenzyme, but not the core

enzyme?

σ (sigma)

In a particular bacterial mutant, a specific gene is not producing a mRNA or the corresponding protein. Researchers sequence the gene and determine that there are no mutations in the coding portion of the gene. Choose the structures that could be mutated to explain these observations.

-35 sequence (TTGACA) Pribnow box (TATAAT)

Eukaryotic transcription is different than bacterial transcription because eukaryotic transcription

________.

requires the uncoiling of the chromatin fiber

What are four ways that mRNA can be modified?

adding a modified guanine to the 5′ end

RNA editing

splicing of introns

Adding a polyadenylation tail to the 3′ end

If one compares the base sequences of related genes from different species, one is likely to find that corresponding ________ are usually conserved, but the sequences of ________ are much less well

conserved.

exons; introns

What is

heterogeneous nuclear RNA

(hnRNA)?

pre-mRNA

A researcher is looking at a cellular sample using an electron microscope. They observe that while mRNA is being transcribed from the DNA that ribosomes are binding to the mRNA to make proteins. Is this sample from a bacterial or a eukaryotic

cell?

This sample is from a bacterial cell since in bacteria the ribosomes can bind to the mRNA as it's being synthesized and initiate translation.

Assuming the genetic code is a triplet, what effect would the addition or loss of two nucleotides have on the reading

frame?

It will change the reading frame.

What effect would the addition or loss of three, six, or nine nucleotides have on the reading frame?

It will have no effect on the reading frame, but will result in the addition or loss of amino acids in the resulting protein.

In the triplet binding technique, radioactivity remains on the filter when the amino acid corresponding to the codon is labeled.Explain the rationale for this

technique.

The basis of the technique is that if a trinucleotide contains bases (a codon) that are complementary to the anticodon of a charged

tRNAtRNA

,

a relatively large complex is formed that contains the ribosome, the

tRNAtRNA

,

and the trinucleotide. This complex is trapped in the filter, whereas the components by themselves are not trapped. If the amino acid on a charged, trapped

tRNAtRNA

is radioactive, then the filter becomes radioactive.

Can you hypothesize why a synthetic

RNARNA

composed

of a mixture of poly

Upper UU

+ poly

Upper AA

would not stimulate incorporation of

Superscript 14 Baseline Upper C14C

-phenylalanine

into protein? (Refer to the table

below.)

Artificial mRNAmRNA	Radioactivity (counts/min)
None	44
Poly Upper UU	39,800
Poly Upper AA	50
Poly Upper CC	38
Source : After M. Nirenberg and J. H. Matthaei (1961).

Because poly

is

complementary

to poly

,

double-stranded

structures will be formed.In order for an

to serve as a messenger

,

it must be

single-stranded

,

thereby exposing the bases for interaction with ribosomal subunits and

s.

Shown here is a hypothetical viral

mRNAmRNA

sequence.

5 Superscript prime Baseline minus AUGCCAAGGGAUGCAACUUGCUGA minus 3 Superscript prime5′−AUGCCAAGGGAUGCAACUUGCUGA−3′Assuming that the sequence could arise from overlapping genes, how many different polypeptide sequences can be

produced?

Two

One form of posttranscriptional modification of most eukaryotic

pre-

mRNAmRNA

s

is the addition of a

poly-

Upper AA

sequence at the

3 Superscript prime3′

end. The absence of a

poly-

Upper AA

sequence leads to rapid degradation of the transcript.

Poly-

Upper AA

sequences of various lengths are also added to many bacterial

RNARNA

transcripts where, instead of promoting stability, they enhance degradation. In both cases,

RNARNA

secondary structures, stabilizing proteins, or degrading enzymes interact with poly-A sequences.Considering the activities of

RNARNA

s,

what might be the general functions of

3 Superscript prime3′

-polyadenylation?

regulation of

mRNAmRNA

longevityregulation of

mRNAmRNA

transport to the cytoplasmregulation of

mRNAmRNA

association with cellular components, like proteins, lipids, or nucleic acids

An early proposal by George Gamow in 1954 regarding the genetic code considered the possibility that DNA served directly as the template for polypeptide synthesis.In eukaryotes, what difficulties would such a system

pose?

Protein synthesis occurs primarily in the cytoplasm, far from the location of DNA and the encoded information.

An early proposal by George Gamow in 1954 regarding the genetic code considered the possibility that DNA served directly as the template for polypeptide synthesis.What observations and theoretical considerations argue against such a

proposal?

Some of the basic amino acids can associate directly with DNA, but the acidic amino acids cannot do so. An adaptor_ system is needed for DNA to direct amino acid assembly.

What is a disadvantage of having the same

DNADNA

sequence code for two protein

products?

Mutation would affect more than one protein.