1. Ch. 17 From Gene to Protein Thought Questions
Ms. Whipple – Brethren Christian High School

2. 1. What is Gene Expression?
Gene expression is the process by which DNA directs protein synthesis, includes two stages called transcription and translation.
Proteins are the link between genotype and phenotype.
Section 17.1

3. 2. Briefly explain how Beadle and Tatum’s experiment supported the one-gene to one-enzyme hypothesis?
George Beadle and Edward Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal media
Using crosses, they and their coworkers identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine
They developed a one gene–one enzyme hypothesis, which states that each gene dictates production of a specific enzyme
Section 17.1

4. Figure 17.2
EXPERIMENT
RESULTS
Classes of Neurospora crassa
Growth: Wild-type cells growing and dividing
No growth: Mutant cells cannot grow and divide
Wild type
Class I mutants
Class II mutants
Class III mutants
Minimal medium (MM)
(control)
Minimal medium
MM  ornithine
Condition
MM  citrulline
MM  arginine (control)
Can grow with or without any supplements
Can grow on ornithine, citrulline, or arginine
Can grow only on citrulline or arginine
Require arginine to grow
Summary of results
Figure 17.2 Inquiry: Do individual genes specify the enzymes that function in a biochemical pathway?
CONCLUSION
Gene (codes for enzyme)
Class I mutants (mutation in gene A)
Class II mutants (mutation in gene B)
Class III mutants (mutation in gene C)
Wild type
Precursor
Precursor
Precursor
Precursor
Gene A
Enzyme A
Enzyme A
Enzyme A
Enzyme A
Ornithine
Ornithine
Ornithine
Ornithine
Gene B
Enzyme B
Enzyme B
Enzyme B
Enzyme B
Citrulline
Citrulline
Citrulline
Citrulline
Gene C
Enzyme C
Enzyme C
Enzyme C
Enzyme C
Section 17.1
Arginine
Arginine
Arginine
Arginine

5. 3. How did this one gene to one-enzyme hypothesis develop over the years of research? How is it understood now?
Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one protein
Many proteins are composed of several polypeptides, each of which has its own gene
Therefore, Beadle and Tatum’s hypothesis is now restated as the one gene–one polypeptide hypothesis
Note that it is common to refer to gene products as proteins rather than polypeptides
Section 17.1

6. 4. When making a protein from a gene, the cell will go through Transcription and then Translation. Please describe the process of Transcription. What is the role of Messenger RNA (mRNA) in this process?
Getting from DNA to protein requires two major stages: transcription and translation.
During transcription, a DNA strand provides a template for the synthesis of a complementary RNA strand.
Just as a DNA strand provides a template for the synthesis of each new complementary strand during DNA replication, it provides a template for assembling a sequence of RNA nucleotides.
Transcription of a protein-coding gene produces a messenger RNA (mRNA) molecule.
Section 17.1

7. 5. Describe the process of Translation.
Translation is the synthesis of a polypeptide, using the information in mRNA.
During translation, there is a change of language. The nucleotides from mRNA will now be read in groups of three called codon
The sites of translation are the ribosomes, complex particles that facilitate the orderly assembly of amino acids into polypeptide chains.
Transcription and translation occur in all organisms, from all three domains of life.
Section 17.1

8. Nuclear envelope DNA Pre-mRNA TRANSCRIPTION (b) Eukaryotic cell
Figure 17.3b-1
Nuclear envelope
DNA
TRANSCRIPTION
Pre-mRNA
Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information.
(b) Eukaryotic cell
Section 17.1

9. Nuclear envelope DNA Pre-mRNA mRNA TRANSCRIPTION RNA PROCESSING
Figure 17.3b-2
Nuclear envelope
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information.
(b) Eukaryotic cell
Section 17.1

10. Nuclear envelope DNA Pre-mRNA mRNA Ribosome Polypeptide TRANSCRIPTION
Figure 17.3b-3
Nuclear envelope
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
Figure 17.3 Overview: the roles of transcription and translation in the flow of genetic information.
TRANSLATION
Ribosome
Polypeptide
(b) Eukaryotic cell
Section 17.1

11. 6. What is a Codon? Why is your genetic code “read” this way?
There are 20 amino acids, but there are only four nucleotide bases in DNA so each amino acid cannot be linked to one nucleotide.
The flow of information from gene to protein is based on a triplet code: a series of non-overlapping, three-nucleotide words
The words of a gene are transcribed into complementary non- overlapping three-nucleotide words of mRNA
These words are then translated into a chain of amino acids, forming a polypeptide
Section 17.1

12. DNA template strand 5 DNA 3 molecule Gene 1 5 3 TRANSCRIPTION
Figure 17.4
DNA template strand 5
DNA 3 A C C A A A C C G A G T
molecule T G G T T T G G C T C A
Gene 1 5 3
TRANSCRIPTION
Gene 2 U G G U U U G G C U C A
mRNA 5 3
Codon
TRANSLATION
Figure 17.4 The triplet code.
Protein
Trp
Phe
Gly
Ser
Gene 3
Amino acid
Section 17.1

13. 7. What is a Template Strand? Is it the same side for every gene?
During transcription, one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript
The template strand is always the same strand for a given gene but not the same side for all genes.
Section 17.1

14. 8. Critical Thinking: Why do you think many of the amino acids have multiple codon options for translation? (Hint: what would this protect against?)
Having multiple codon options for amino acids helps protect against detrimental mutations of the DNA code.
For example, Leucine is coded for by CUU; CUA; CUC; and CUG. If there is a mutation of the last nucleotide, the protein will remain the same.
Section 17.1

15. First mRNA base (5 end of codon) Third mRNA base (3 end of codon)
Figure 17.5
Second mRNA base U C A G
UUU
UCU
UAU
UGU U
Phe
Tyr
Cys
UUC
UCC
UAC
UGC C U
Ser
UUA
UCA
UAA Stop
UGA Stop A
Leu
UUG
UCG
UAG Stop
UGG
Trp G
CUU
CCU
CAU
CGU U
His
CUC
CCC
CAC
CGC C C
Leu
Pro
Arg
CUA
CCA
CAA
CGA A
Gln
CUG
CCG
CAG
CGG G
First mRNA base (5 end of codon)
Third mRNA base (3 end of codon)
AUU
ACU
AAU
AGU U
Asn
Ser
AUC
Ile
ACC
AAC
AGC C A
Thr
Figure 17.5 The codon table for mRNA.
AUA
ACA
AAA
AGA A
Lys
Arg
AUG
Met or start
ACG
AAG
AGG G
GUU
GCU
GAU
GGU U
Asp
GUC
GCC
GAC
GGC C G
Val
Ala
Gly
Gly
GUA
GCA
GAA
GGA A
Glu
GUG
GCG
GAG
GGG G
Section 17.1

16. 9. What is a Reading Frame? Why is this so important when it comes to sending messages?
To extract the message from the genetic code requires specifying the correct starting point.
The starting point establishes the reading frame; subsequent codons are read in groups of three nucleotides.
The cell’s protein-synthesizing machinery reads the message as a series of non-overlapping three-letter words.
Section 17.1

17. 10. What evidence from the Genetic code & Protein synthesis supports the theory of the Evolution of Species?
The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals
Genes can be transcribed and translated after being transplanted from one species to another

18. (a) Tobacco plant expressing (b) Pig expressing a jellyfish
Figure 17.6
Figure 17.6 Expression of genes from different species.
(a) Tobacco plant expressing
(b) Pig expressing a jellyfish
a firefly gene
gene
Section 17.1