Ch 17 and 18 Flashcards
In a research article about alkaptonuria published in 1902, Garrod suggested that humans inherit two “characters” (alleles) for a particular enzyme and that both parents must contribute a faulty version for the offspring to have the disorder. Today, would this disorder be called dominant or recessive?
Recessive
What polypeptide product would you expect from a poly-G mRNA that is 30 nucleotides long?
A polypeptide made up of 10 Gly (glycine) amino acids
The template strand of a gene contains the sequence 3’-TTCAGTCGT-5’. Draw the nontemplate sequence and the mRNA sequence, indicating 5’ and 3’ ends of each. Compare the two sequences.
Template sequence (from problem): 3’-TTCAGTCGT-5’ Nontemplate sequence: 5’-AAGTCAGCA-3’ mRNA sequence: 5’-AAGUCAGCA-3’ The nontemplate strand and mRNA nucleotide sequences are the same except that there is T in the nontemplate strand of DNA wherever there is U in the mRNA.
Imagine that the nontemplate sequence in question 3 was transcribed instead of the template sequence. Draw the mRNA sequence and translate it using Figure 17.5. (Be sure to pay attention to the 5’ and 3’ ends.) Predict how well the protein synthesized from the nontemplate strand would function, if at all.
“Template Sequence” (from nontemplate sequence in problem, written 3’ to 5’): 3’-ACGACTGAA-5’ mRNA sequence: 5’-UGCUGACUU-3’ Translated: Cys-STOP-Leu (Remember that the mRNA is antiparrallel to the DNA strand.) A protein translated from the nontemplate sequence would have a completely different amino acid sequence and would most likely be nonfunctional. (It would also be shorter because of the stop signal shown in the mRNA sequence above- and possibly others earlier in the mRNA sequence.)
Compare DNA polymerase and RNA polymerase in terms of how they function, the requirement for a template and primer, the direction of synthesis, and the type nucleotides used.
Both assemble nucleic acid chains from monomer nucleotides whose order is determined by complimentary base pairing to a template strand. Both synthesize in the 5’to3’ direction,antiparallel to the template. DNA polymerase requires a primer, but RNA polymerase can start a nucleotide start a nucleotide from scratch. DNA polymerase uses nucleotides with the sugar deoxyribose and the base T, whereas RNA polymerase uses nucleotides with the sugar ribose and the base U.
What is a promoter, and is it located at the upstream or downstream end of a transcription unit?
The promoter is the region of DNA to which RNA polymerase binds to begin transcription, and it is at the upstream end of the gene (transcription unit).
What enables RNA polymerase to start transcribing a gene at the right place on the DNA in a bacterial cell? In a eukaryotic cell?
In a bacterial cell, RNA polymerase recognizes the gene’s promoter and binds to it. In a eukaryotic cell, transcription factors mediate the binding of RNA polymerase to the promoter. In both cases, sequences in the promoter bind precisely to the RNA polymerase, so the enzyme is in the right location and orientation.
Suppose X-rays caused a sequence change in the TATA box of a particular gene’s promoter. How would that affect transcription of the gene?
The transcription factor that recognizes the TATA sequence would be unable to bind, so the RNA polymerase could not bind and the transcription of that gene probably would not occur.
How can human cells make 75,000-100,00 different proteins, given that there are about 20,000 human genes?
Due to alternative splicing of exons, each gene can result in multiple different mRNAs and can thus direct synthesis of multiple different proteins.
How is RNA splicing similar to editing a video? What would introns correspond to in this analogy?
In editing a video, segments are cut out and discarded (like introns) and the remaining segments are joined together (like exons) so that the regions of joining (“splicing”) are not noticeable.
What would be the effect of treating cells with an agent that removed the cap from mRNAs?
Once the mRNA has exited the nucleus, the cap prevents it from being degraded by hydrolytic enzymes and facilitates its attachment to ribosomes. If the caps were removed from all mRNAs, the cell would no longer be able to synthesize any proteins and would probably die.
What two processes ensure that the correct amino aid is added to a growing polypeptide chain.?
First, each aminoacyl-tRNA synthetase specifically recognizes a single amino acid and attaches it only to an appropriate tRNA. Second, a tRNA charged with its specific amino acid binds only to an mRNA codon for that amino acid
Discuss the ways in which rRNA structure likely contributes to ribosomal function.
The structure and function of the ribosome seem to depend more on the rRNAs than on the ribosomal proteins. Because it is single-stranded, an RNA molecule can hydrogen bond with itself and with other RNA molecules. RNA molecules make up the interface between the two ribosomal subunits, so presumably RNA-RNA binding helps hold the ribosome together. The binding site for mRNA in the ribosome includes rRNA that can bind the mRNA. Also, complementary bonding within an RNA molecule allows it to assume a particular 3-dimensional shape and , along with the RNA’s functional groups, presumably enables rRNA to catalyze peptide bond formation during translation.
Describe how a polypeptide to be secreted is transported to the endomembrane system.
A signal peptide on the leading of the polypeptide being synthesized is recognized by signal-recognition particle that brings the ribosome to the ER membrane. There the ribosome attaches and continues to synthesize the polypeptide, depositing it in the ER lumen.
Draw a tRNA with the anticodon 3’-CGU-5’. What two different codons could it bind to” Draw each codon on an mRNA, labeling all 5’ and 3’ ends. Add the amino acid carried by this tRNA.
Because of the wobble, the tRNA could bind to either 5’-GCA-3’ or 5’-GCG-3’, both of which code for alanine (Ala). Alanine would be attached to the tRNA
