10. From DNA to Protein: Gene Expression. Chapter 10 From DNA to Protein: Gene Expression. Key Concepts 10.1 Genetics Shows That Genes Code for Proteins 10.2 DNA Expression Begins with Its Transcription to RNA
How do antibiotics such as tetracycline target bacterial protein synthesis?
Genes to proteins
Clarify your understanding about the events of gene expression in a typical eukaryotic cell by diagramming the following steps, showing where in the cell they take place, and describing what happens during them:
1. What is the role of mRNAs? If a cell has 25,000 genes, how many mRNAs can it make?
2. What is the role of rRNAs? Would you expect the genes that code for rRNAs to be conserved over the course of evolution? Why?
3. What is the role of tRNAs? How many of them does a cell need if its metabolism depends on 20 amino acids?
DNA transcription in eukaryotes
a. yields an exactly equivalent DNA strand.
b. yields a single-stranded RNA molecule that may be processed to yield mRNA.
c. yields a double-stranded tRNA molecule.
d. takes place in the cytoplasm.
e. None of the above
Promoters, terminators, start and stop codons, and splice sites
Working individually, diagram a eukaryotic gene consisting of three exons and two introns. Your DNA molecule should be double-stranded and you should illustrate the 5′ and 3′ ends. Be sure to show:
Which strand is used for transcription and in what direction does the transcription occur
The location of the promoter and terminator sequences
The location of the start and stop codons
The location of the four splice sites
What portions of this gene will be transcribed?
What portions of this gene will be included in the finished mRNA?
The term “exon” refers to
a. a large oil company.
b. a type of tRNA that is secreted outside a cell.
c. a region of a gene that is translated into a protein.
d. a region of a gene that is not translated into a protein.
e. None of the above
Understanding the genetic code
One of the most important discoveries of the 20th century was the breaking of the genetic code. This understanding underlies much of modern biology; it provides the key linking genetic sequences to amino acid sequences needed for proteins.
Working with a partner, construct an mRNA sequence that will yield the following polypeptide:
Can you identify any redundant sequences?
What does this mean for the evolution of the underlying DNA sequences?
The term “codon” refers to
a. a sequence of three amino acids uniquely coded by mRNA.
b. a sequence of tRNAs that can recognize a DNA sequence without requiring any intermediates.
c. a sequence of three nucleotides in an mRNA that is the unique code for a specific amino acid.
d. a sequence of three nucleotides in an mRNA that may redundantly code for a specific amino acid.
e. Both c and d
Working with a partner, outline the steps in the elongation of a polypeptide, specifying the roles of small and large subunits of ribosomes, tRNAs, codons, and mRNA. Be sure you can answer the following questions:
In what direction does elongation occur (i.e., does it run 5′-to-3′ or 3′-to-5′)?
What are the E, P, and A sites of the large subunit of the ribosome? Why do some people refer to the large ribosomal subunit as a ribozyme?
At what step is energy consumed? Does this occur before or after the tRNA recognizes its codon?
How does elongation of the polypeptide terminate?
What is a polyribosome (polysome)?
a. have an anticodon site that recognizes codons in mRNAs.
b. have a recognition site for a particular amino acid.
c. have a stem and loop structure.
d. interact with an mRNA strand and rRNAs in the form of small and large ribosomal subunits.
e. All of the above
Working with a partner, trace the two pathways typical for proteins once they have been synthesized. One of these pathways leads to incorporation of proteins into organelles within the cell; the other pathway leads to packaging and export via exocytosis.
What modifications do most proteins undergo?
What modifications might a protein destined for a site within the cell undergo after translation?
What modifications might a protein destined for secretion undergo before it is secreted?
In a secretory cell, why are ribosomes associated with the outside of the endoplasmic reticulum?
Would you expect secreted proteins to undergo any additional modifications once they are outside the cell? Why or why not?
a. refers to changes in nuclear DNA after a protein has been translated from mRNA.
b. rarely occurs in eukaryotic protein synthesis.
c. is unnecessary for secretion of most proteins.
d. may involve modifications in the protein’s sequence or the addition of sugars or phosphate groups.
e. All of the above