7. The only amino acid that does not need to enter the A site before entering the P site on a ribosome during the process of
translation is methionine. Methionine is coded for by AUG, the start codon, and therefore it is always the first amino acid in a newly synthesized polypeptide. Since it is the first amino acid, there will be no amino acid before it to form a peptide bond with and hence it need not enter the A site. It automatically enters the P site to form a peptide bond with the amino acid that comes in after it.
8. Before translation can be initiated, a ribosome must bind to the mRNA transcript. The two ribosome units (60S and 40S)
recognize the 5' cap on the mRNA. The intact ribosome moves along the mRNA until it encounters the start codon, AUG. Translation is initiated when methionine, the amino acid encoded by the start codon, is brought into the P site of the ribosome by its corresponding tRNA. The tRNA and mRNA interact via complementary base pairing. The methionine tRNA possesses the nucleotide sequence UAC on its base, which is complementary to AUG. UAC is the anticodon.
The polypeptide is elongated from this point on. The second codon after the start codon is found in the A site of the
ribosome. Its corresponding tRNA will bring in the appropriate amino acid to the site. At this point a peptide bond is formed between methionine (P site) and the amino acid in the A site. The ribosome now shifts over one codon. Methionine exits the P site, the second amino acid and it’s corresponding tRNA enters the P site, and the A site is vacant. The corresponding amino acid to the third codon is now brought in to the A site and a peptide bond is formed between the second amino acid and the third.
The process of elongation continues until a stop codon is reached signalling termination. Once a stop codon (UAG,
UGA, or UAA) is reached, the ribosome stalls. A protein, known as the release factor, recognizes that the ribosome has stalled and causes the ribosome to dismantle and fall off the mRNA. The newly formed polypeptide chain is also released.
9. Translation is the process of expressing to another medium. Typically, translation means to express in another language.
Translation during protein synthesis lends itself to the same concept. During protein synthesis, mRNA, which consists of ribonucleotides, is translated into protein, which consists of amino acids, using the genetic code. The process of translation has been appropriately named in protein synthesis.
Making Connections 10. Student answers will vary depending on research. Tetracycline, streptomycin, and chloramphenicol arrest bacterial growth
by blocking various steps in proteins synthesis.
(a) Tetracycline inhibits the binding of the tRNA to the small subunit of the ribosome. Therefore, the tRNA cannot
(b) Streptomycin induces mRNA misreading and inhibits the initiation of the process of translation. Streptomycin
accomplishes the inhibition by binding to the small subunit of the ribosome. If misreading occurs, then dysfunctional proteins are built.
(c) Chloramphenicol inhibits the enzyme peptidyl transferase. Peptidyl transferase is the enzyme that forms the peptide
bond between the two amino acids found in the P site and A site.
5.5 CONTROL MECHANISMS SECTION QUESTIONS (Page 258) Understanding Concepts 1. operon: a cluster of genes under the control of one promoter and one operator in prokaryotic cells; acts as a simple
operator: regulatory sequences of DNA to which a repressor protein binds corepressor: a molecule (usually the product of an operon) that binds to a repressor to activate it housekeeping gene: a gene that is switched on all the time because it is needed for life functions vital to an organism signal molecule: a molecule that activates an activator protein or represses a repressor protein 2. It is to a cell’s advantage to have some of its genes under regulation because not all proteins are required at all times, nor
are all diminished quickly. It would be a waste of the cell’s resources (energy and materials) if all genes were transcribed and translated at all times. By having some genes under regulation, the cell can manage the inventory (protein product) as it is needed.
3. ,I WKH OHYHO RI ODFWRVH LV ORZ WKHQ WKH HQ]\PHV -JDODFWRVLGDVH -galactosidase permease, and transacetylase are not
required by the cell. These enzymes are involved in the metabolism of lactose, and their expression is under the regulation of the lac operon. Under low lactose levels, RNA polymerase is blocked from transcribing the genes for the lactose metabolizing enzymes. LacI, a repressor protein, is bound to the operator, which follows the promoter. When RNA polymerase binds to the promoter, it cannot get past the LacI repressor protein, and transcription is blocked. If lactose is introduced to the system, the enzymes must be transcribed. On introduction of lactose, lactose binds to the LacI repressor, changing LacI’s shape and making it fall off the operator. Now that LacI has been removed from the operator, RNA
polymerase can proceed with transcription as normal. The three enzymes involved in the metabolism of lactose are transcribed and expressed.
4. (a) The LacI protein will be dysfunctional, if it functions at all. It will not be able to bind either to the lac operon operator
or to lactose. Therefore, transcription of the three genes of the lac operon will take place continuously, irrespective the level of lactose in the cell.
(b) If a mutation is found in the second gene of the lac operon, which is the lacY JHQH WKH HQ]\PH -galactosidase
permease will be nonfunctional. Lactose will not be able to enter the cell and will accumulate in space found between cells, assuming that initially this enzyme is expressed. Furthermore, the other genes found in the lac operon will not be expressed either. Since lactose will not be able to enter the cell, it cannot bind to the LacI protein; therefore, transcription will be blocked.
(c) If RNA polymerase has difficulty binding to the promoter, then transcription will not take place. The level of lactose
will accumulate in the cell initially to a certain point. Since the permease is now not being expressed, lactose will accumulate between cells as well.
5. If the levels of tryptophan are high, then tryptophan need not be produced by the trp operon. Tryptophan will bind to the
repressor protein, changing its conformation. The trp repressor–tryptophan complex can now bind to the operator of the trp operon, blocking transcription of the genes needed to synthesize tryptophan. If the levels of tryptophan are lowered, there is not enough tryptophan available to bind to the repressor protein. The repressor protein changes shape without tryptophan and can no longer bind to the operator. RNA polymerase is able to transcribe the tryptophan synthesizing genes.
Applying Inquiry Skills 6. (a) Molecule A is acting as a corepressor molecule. When the levels of molecule A are low, the transcription of the genes
is high, indicating that its presence does not block transcription. Conversely, when the levels of molecule A are high, gene transcription is low, indicating that it’s presence has blocked transcription in some manner. Molecule B is acting as an inducer. When the levels of molecule B are low, the transcription of genes is low also, indicating that molecule B must be present for the transcription of the genes to take place.
A’s system resembles the trp operon, while molecule B’s resembles the lac operon.
(c) Generally, when an operon controls the genes responsible for the synthesis of a protein, the protein acts as a
corepressor. When an operon controls the genes responsible for the degradation of a protein, the protein acts as an inducer.
5.6 MUTATIONS Explore an Issue Debate: Cell Phones and Brain Cancer (Page 262) Statement: The electromagnetic radiation associated with the use of cell phones does not contribute to the development of brain cancer. Student answers will vary. A list of points and counterpoint follows: Point Counterpoint
Mice used in studies do not absorb energy in the
DNA is the same molecule in humans and mice.
same manner as humans do. It is questionable when one extrapolates directly to humans. Mice in cancer studies are usually transgenic:
An Australian study conducted that found that mice
They are bred to be susceptible to cancers in the who were subjected to pulsed digital radiation for immune system. Hence, studies involving mice
two and a half hours a day doubled their incidence
may not reflect accurately the effect of cell
of cancer. The rate was much higher than the
control group of transgenic mice who were not exposed to any radiation.
Definitive studies that can conclusively illustrate
It takes many years for cancer to surface (5 to 20
the effects of cell phone usage on humans have
years); hence, it is not possible to have a definitive
yet to be executed. More than 75 studies have
study—yet the risk of brain cancer is still a
been conducted over the last five years.
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