Chapter 15 Practice Problems Flashcards
a cell with a mixture of different mtDNAs generates a daughter cell with only one kind
cytoplasmic segregation
cell with mtDNAs or cpDNAs with different genotypes
heteroplasmic
transmission of genes through maternal gamete only
maternal inheritance
transmission of genes through either a maternal gamete or a paternal gamete -not both
uniparental inheritance
having gametes of a similar size
isogamous
a specific fraction of wild type organellar DNAs is required for a wild type phenotype
threshold effect
The human nuclear genome encodes tRNAs with 32 different anticodons. The mitochondrial genome encodes only 22 different tRNAs that are sufficient to translate all mitochondrial mRNAs. The differences in the nuclear and mitochondrial genetic codes are not great enough to explain the differences in the numbers of tRNAs needed in each case. How can the difference be explained?
Fewer tRNAs are needed to translate mitochondrial mRNAs than to translate mRNAs transcribed from nuclear genes because the wobble rules are different in each case. The wobble bases of mitochondrial tRNAs can generally recognize more different bases than the wobble bases of the cytoplasmic tRNAs that help translate the transcripts of nuclear genes.
The human mitochondrial genome includes no genes for tRNA synthetics.
a. how are mitochondrial tRNAs charged with amino acids?
b. given your answer to part a ,explain how AUA specify met in mitochondria but Ile in the nucleus
a. The tRNA synthetases for making charged mitochondrial tRNAs are encoded by genes in the human nuclear genome.
b. The mitochondrial methionyl-tRNA synthetase functions only in mitochondria, where it recognizes mitochondrial tRNAMet whose anticodon can bind AUA.
Is each of these statements true of chloroplast or mitochondrial genomes, both or neither?
a. contain tRNA genes
b. encode proteins that participate in electron transport pathways
c. all genes necessary for function of the organelle are present
d. vary greatly in size from organism to organism
a. both
b. both
c. neither
d. only mtDNA
Suppose you are characterizing the DNA of a diploid plant species that had never been analyzed previously. You purify all the DNA that can be isolated from a seedling, and subject this DNA to high throughput sequencing involving millions of reads of random DNA fragments
a. if you obtained an average 100 reads of a given single copy nuclear DNA sequence, about how many reads would you obtain for mtDNA? for cpDNA?
b. beyond the number of reads, what other criteria would allow you to conclude whether a particular read was nuclear DNA, mtDNA, or cpDNA?
a. you would get 5000 × 100 = 500,000 reads of a mtDNA sequence and 500 × 100 = 50,000 reads of a cpDNA sequence
b. (i) The organellar DNAs of a previously uncharacterized plant species should display similarity with the organellar DNAs of other plant species. (ii) The organization of organellar and nuclear genes differ.
If the reads of mtDNA do not assemble into a circle and instead are seen only as linear contigs, it may be difficult to differentiate them from nuclear DNA.
which of the following characteristics of chloroplasts and/or mitochondria make them seem more similar to bacterial cells than to eukaryotic cells?
a. translocation in sensitive to chloromphenicol and erythromycin
b. alternate codons are used in mitochondria genes
c. introns are present in organelle genes
d. DNA in organelles is not arranged in nucleosomes
(a) and (d) are characteristics of chloroplasts and mitochondria that are similar to the characteristics of bacteria but dissimilar to those of eukaryotic cells.
The Saccharomyces cerevisiae nuclear gene ARG8 encodes an enzyme that catalyzes a key step in biosynthesis of the amino acid arginine. This protein is normally synthesized on cytoplasmic ribosomes, but then is transported into mitochondria, where the enzyme conducts its functions. In 1996 T.D. Fox and his colleagues constructed a strain of yeast in which a gene encoding the Arg8 protein was itself moved into mitochondrial ribosomes.
a. How could these investigators move the ARG8 gene from the nucleus into the mitochondria, while permitting the synthesis of active enzyme? In what ways would the investigators need to alter the ARG8 gene to allow it to function in the mitochondria instead of in the nucleus?
b. why might these researchers have wished to move the ARG8 gene into mitochondria in the first place?
a. introns would have to be removed, some of the codons in the nuclear gene would have to be changed because the genetic code is somewhat different in the nucleus than in mitochondria, the open reading frame of the altered nuclear gene would have to be placed under the control of a promoter, a translational start site, and a transcriptional termination site that work in mitochondria. the researchers introduced the cloned gene into yeast mitochondria using microprojectile bombardment (the biolistic gun).
b. Such a yeast strain allows researchers to select for function of the mitochondrial genetic system in mutants that are unable to respire, find arginine auxotrophs that could no longer make arginine because there was a DNA change that obliterated the function of the promoter. By sequencing such mutations in mtDNA, researchers could figure out a lot about the function of regulatory elements in the mitochondrial genome.
why are severe mitochondrial or chloroplast gene mutations usually found in heteroplastic cells instead of homoplasmic cells?
If the mutation is very debilitating to the cell, either because of the loss of energy metabolism in the case of mitochondria or of photosynthetic capability in the case of chloroplasts, a cell that is homoplasmic for the mutant genome will die
what characteristics in a human pedigree suggest a mitochondrial location for a mutation affecting the trait?
maternal inheritance; differing levels of expression of the mutant phenotype in different progeny
how could researchers have determined that the Rhesus monkeys mito and tracker were devoid of the mitochondrial DNA from their nuclear donor mother?
Scientists could use a PCR assay that distinguishes SNPs in the mtDNAs from the nuclear donor and the cytoplasm donor oocyte. The researchers would perform these tests on cells from different tissues of Mito and Tracker.
