Study Questions for Chapter 17: From Gene to Protein
1) Summarize the one gene – one polypeptide hypothesis.
One gene gives instruction for 1 polypeptide formation
List one exception to this hypothesis.
Genes code for RNA molecules that are never translated into a protein
2) List 3 ways in which RNA differs from DNA.
1) Sugar = Ribose
2) Bases are A, U, C, G (no T)
3) Single Stranded
3) How do the processes of transcription and translation differ in prokaryotes and eukaryotes?
Prokaryotes – both processes happen in the same place; RNA polymerases are different; transcription
terminates differently; ribosomes are slightly different (leads to medical uses – tetracycline and
streptomycin can paralyze prok ribosomes w/o impacting euk ribosomes)
4) What is meant by antisense strand of DNA?
Strand of DNA that is copied (Template of mRNA)
Is this always the same strand of a given DNA molecule? No
Is this always the same strand for a specific gene? Yes
5) How many RNA nucleotides are necessary to code for a polypeptide that is 177 amino acids long? (THINK!)
177 x 3= 531
531 + stop codon = 534
6) Summarize Nirenberg’s experiment in which he began to “crack” the genetic code of mRNA codons.
One codon in repetition (UUUUUU…)  this led to translation of polypeptide w/only phenylalanine; then
proceeded w/other poly-A, poly-G, poly C mRNA’s
7) It is now known that 61 of the 64 triplets code for an amino acid.
a) What is the purpose of the mRNA codon: AUG?
Signify “start” of translation (MET)
b) What is the purpose of the 3 triplets that do not code for an amino acid?
To end/stop translation
8a) Explain the statement: “There is redundancy in the genetic code, but no ambiguity.” Provide an example
to illustrate your explanation.
-diff. tRNA’s carry same amino acid (redundancy)
-a specific tRNA anticodon will only carry a specific amino acid
8b) What is the probable evolutionary significance of the genetic code’s universality?
A language shared by all living things must have been operating very early in the history of life – early
enough to be present in the organisms that were common ancestors of all modern orgs.
9) List two functions of the DNA region known as the promoter and one function of the DNA sequence known
as the terminator.
Promoter: 1) determines which of the 2 strands of the double helix are used as the template 2)
determine where transcription starts
Terminator: (seq. of DNA) functions as a termination signal
10) Complete the chart below summarizing the changes made to a pre-mRNA molecule in eukaryotes.
mRNA end
Description of
Function #1
Function #2
modification
5’ end
Modified (G) Cap
Protect mRNA from
“attach” here sign for
degradation
ribosomes
3’ end
Poly-A tail
Protect mRNA from
Helps mRNA leave the
30-200 (A) nucleotides
degradation
nucleus
11) What is RNA splicing? Why is this done?
RNA splicing takes out sections of mRNA that are not coding for a section of the protein; introns are
spliced out and exons are then joined together to make a continuous coding sequence
12) Introns (non-coding regions) were once thought to be “junk DNA” but now it is thought that they do have
biological and/or evolutionary importance. List 3 potential functions of introns.
1. Increase opportunity for crossing over between 2 alleles of a gene; raises probability that a
crossover will switch one version of an exon for another version on homolog. chromos.
2. Regulatory role in the cell
3. Splicing process may regulate passage of mRNA from nucleus to cytoplasm
13) Sketch and label a typical ribosome. Include its protein and RNA composition, large & small subunits,
three tRNA binding sites, and mRNA binding site.
P
mRNA binding site
14) Summarize the role in protein synthesis of each of the enzymes/proteins/structures listed in the chart
below.
Enzyme/protein/structure
RNA polymerase
Function in protein synthesis
Unwind and open DNA; hook new nucleotides into RNA chain
Aminoacyl-tRNA synthetase
Join correct amino acid to correct tRNA
Initiation factors
Proteins that bring together mRNA, initiator tRNA, small ribosomal
subunit and large ribosomal subunit
Elongation factors
Proteins that help add amino acids one by one to the 1 st amino acid
Release factors
Protein that binds directly to the stop codon in the A site; causes
addition of H2O instead of an amino acid
Ribosomal sites:
*A site
A site: holds tRNA carrying the next amino acid to be added
P site: holds tRNA carrying the growing polypeptide
E site: exit sit for empty tRNA
*P site
*E site
15) What is the purpose of polyribosomes?
When several ribosomes work on a single mRNA to make several copies of a protein
16) Describe 3 ways in which a polypeptide may be modified after translation has occurred.
1) Certain amino acids may be chemically modified (attach lipids, sugars…)
2) Enzyme excises part of chain to become active
3) 2 or more separate polypeptides joined (Hb)
17a) Ribosomes can either be “free” or “bound.” Discuss the destination of the final protein product from
each of these.
Free – function in cytosol
Bound – make protein or endomembrane system and secreted from cell (insulin)
17b) How does a “free” ribosome become a “bound” ribosome?
Sequence of 20 amino acids at or near beginning of polypeptide; recognized as it emerges from
ribosome by SRP (protein-RNA complex); brings ribosome to receptor built into ER membrane
18) Complete the following table for the functions of the various types of RNA in a eukaryotic cell.
Type of RNA
Messenger RNA (mRNA)
Function
Copy message of DNA gene and take message to ribosome
Transfer RNA (tRNA)
Translate message of nucleic acid (mRNA) into correct amino acid
order
Ribosomal RNA (rRNA)
Provide site for mRNA and tRNA to temporarily H-bond; aligning
amino acids
Small nuclear RNA (snRNA)
RNA that works with proteins as part of the spliceosome
SRP RNA
Recognizes if emerging polypeptide should go to ER; brings protein
there w/ribosome
snoRNA
Aids in processing of pre rRNA transcript for ribosome subunit
formation in the nucleolus
siRNA and miRNA
Involved in regulation of gene expression (which get expressed)
19) What is a ribozyme?
RNA molecule that functions like an enzyme and catalyzes reactions during RNA splicing
20) What is the significance of polar and nonpolar amino acids? (Not from the text… think!)
IB Things to Know:
 State 4 functions of proteins, giving a named example of each (do not include membrane proteins).
 Fibrous vs globular proteins with 2 examples of each
o Fibrous: insoluble; elongated molecules in which the secondary structure (either alpha-helices or betapleated sheets) forms the dominant structure (keratin, silk, collagen)
o Globular: highly diverse group of proteins that are soluble and form compact spheroidal molecules in
water; all have tertiary structure and some have quaternary structure (enzymes, myoglobin)