Bio E2 Ch.9
The pufferfish, Fugu rubripes, has a genome that is one-tenth the size of mammalian genomes. Which of the following statements is not a possible reason for this size difference? (a) Intron sequences in Fugu are shorter than those in mammals. (b) Fugu lacks the repetitive DNA found in mammals. (c) The Fugu genome seems to have lost sequences faster than it has gained sequences over evolutionary time. (d) Fugu has lost many genes that are part of gene families.
(d) Fugu has lost many genes that are part of gene families.
Which of the following statements is false? (a) The human genome is more similar to the orangutan genome than it is to the mouse genome. (b) A comparison of genomes shows that 90% of the human genome shares regions of conserved synteny with the mouse genome. (c) Primates, dogs, mice, and chickens all have about the same number of genes. (d) Genes that code for ribosomal RNA share significant similarity in all eukaryotes but are much more difficult to recognize in archaea.
(d) Genes that code for ribosomal RNA share significant similarity in all eukaryotes but are much more difficult to recognize in archaea.
Figure Q9-22 shows the evolutionary history of the globin gene family members. Given this information, which of the following statements is true? (a) The ancestral globin gene arose 500 million years ago. (b) The α-globin gene is more closely related to the ε-globin gene than to the δ-globin gene. (c) The nucleotide sequences of the two γ-globins will be most similar because they are the closest together on the chromosome. (d) The fetal β-globins arose from a gene duplication that occurred 200 million years ago, which gave rise to a β-globin expressed in the fetus and a β-globin expressed in the adult.
(d) The fetal β-globins arose from a gene duplication that occurred 200 million years ago, which gave rise to a β-globin expressed in the fetus and a β-globin expressed in the adult.
You discover that the underlying cause of a disease is a protein that is now less stable than the non-disease-causing version of the protein. This change is most likely to be due to ________. (a) a mutation within a gene. (b) a mutation within the regulatory DNA of a gene. (c) gene duplication. (d) horizontal gene transfer.
(a) a mutation within a gene.
The nucleotide sequences between individuals differ by 0.1%, yet the human genome is made up of about 3 × 109 nucleotide pairs. Which of the following statements is false? (a) In most human cells, the homologous autosomes differ from each other by 0.1%. (b) All changes between human individuals are single-nucleotide polymorphisms. (c) Any two individuals (other than identical twins) will generally have more than 3 million genetic differences in their genomes. (d) Much of the variation between human individuals was present 100,000 years ago, when the human population was small.
(b) All changes between human individuals are single-nucleotide polymorphisms.
Which of the following statements about retroviruses is false? (a) Retroviruses are packaged with a few molecules of reverse transcriptase in each virus particle. (b) Retroviruses use the host-genome integrase enzyme to create the provirus. (c) The production of viral RNAs can occur long after the initial infection of the host cell by the retrovirus. (d) Viral RNAs are translated by host-cell ribosomes to produce the proteins required for the production of viral particles
(b) Retroviruses use the host-genome integrase enzyme to create the provirus.
The average size of a protein in a human cell is about 430 amino acids, yet the average gene in the human genome is 27,000 nucleotide pairs long. Explain.
In the human genome, the exons are relatively short, whereas the introns can be quite large. A protein 430 amino acid residues long will need fewer than 1300 nucleotides to code for it. Furthermore, many genes in the human genome undergo alternative splicing, so some of those 27,000 nucleotides may be coding for alternative exons that are not used every time the gene is transcribed.
Which of the following statements is false? (a) A mutation that arises in a mother's somatic cell often causes a disease in her daughter. (b) All mutations in an asexually reproducing single-celled organism are passed on to progeny. (c) In an evolutionary sense, somatic cells exist only to help propagate germline cells. (d) A mutation is passed on to offspring only if it is present in the germ line.
(a) A mutation that arises in a mother's somatic cell often causes a disease in her daughter.
Which of the following generalities about genomes is true? (a) All vertebrate genomes contain roughly the same number of genes. (b) All unicellular organisms contain roughly the same number of genes. (c) The larger an organism, the more genes it has. (d) The more types of cell an organism has, the more genes it has
(a) All vertebrate genomes contain roughly the same number of genes.
Which of the following statements about gene families is false? (a) Because gene duplication can occur when crossover events occur, genes are always duplicated onto homologous chromosomes. (b) Not all duplicated genes will become functional members of gene families. (c) Whole-genome duplication can contribute to the formation of gene families. (d) Duplicated genes can diverge in both their regulatory regions and their coding regions.
(a) Because gene duplication can occur when crossover events occur, genes are always duplicated onto homologous chromosomes.
Which of the following statements about homologous genes is true? (a) For protein-coding genes, homologous genes will show more similarity in their amino acid sequences than in their nucleotide sequences. (b) Fewer than 1% of human genes have homologs in the nematode and the fruit fly. (c) Most homologous genes arose by gene duplication. (d) A gene in humans that has homologs in plants and prokaryotes will show the same level of similarity in nucleotide sequence when the human and prokaryotic sequences are compared as when the human and chimpanzee sequences are compared.
(a) For protein-coding genes, homologous genes will show more similarity in their amino acid sequences than in their nucleotide sequences.
Mobile genetic elements are sometimes called "jumping genes," because they move from place to place throughout the genome. The exact mechanism by which they achieve this mobility depends on the genes contained within the mobile element. Which of the following mobile genetic elements carries both a transposase gene and a reverse transcriptase gene? (a) L1 (b) B1 (c) Alu (d) Tn3
(a) L1
Which of the following statements about what we have learned by comparing the modern-day human genome to other genomes is true? (a) Modern humans whose ancestors come from Europe or Asia share up to 4 percent of their genome with Neanderthals. (b) Accelerated changes, which were found when comparing the human genome to other mammalian genomes, were not found when comparing the modern-day human genome to the Neanderthal genome. (c) The human genome is far more gene-dense than the yeast genome. (d) In syntenic regions of the human and mouse genomes, both gene order and the placements of more than 95% of the mobile genetic elements are conserved.
(a) Modern humans whose ancestors come from Europe or Asia share up to 4 percent of their genome with Neanderthals.
Which of the following statements is true? (a) The intron structure of most genes is conserved among vertebrates. (b) The more nucleotides there are in an organism's genome, the more genes there will be in its genome. (c) Because the fly Drosophila melanogaster and humans diverged from a common ancestor so long ago, a gene in the fly will show more similarity to another gene from the same species than it will to a human gene. (d) An organism from the same Order as another will be more likely to have a genome of the same size than will a more evolutionarily diverged animal.
(a) The intron structure of most genes is conserved among vertebrates.
Which of the following processes is not thought to contribute to diversity in the genome of human individuals? (a) exon shuffling (b) single-nucleotide polymorphisms (c) CA repeats (d) duplication and deletion of large blocks of sequence
(a) exon shuffling
Which of the following regions of the genome is the least likely to be conserved over evolutionary time? (a) the upstream regulatory region of a gene that encodes the region conferring tissue specificity (b) the upstream regulatory region of a gene that binds to RNA polymerase (c) the portion of the genome that codes for proteins (d) the portion of the genome that codes for RNAs that are not translated into protein
(a) the upstream regulatory region of a gene that encodes the region conferring tissue specificity
Two individuals are represented in Figure Q9-11; individual 1 is one of the parents of individual 2. The asterisk indicates the occurrence of a single mutation. What is the chance that individual 2 will inherit the mutation in individual 1? (a) 100% (b) 50% (c) 1 in 100,000 (d) none
(b) 50%
Given the evolutionary relationship between higher primates shown in Figure Q928, which of the following statements is false? (a) The last common ancestor of humans, chimpanzees, gorillas, and orangutans lived about 14 million years ago. (b) Chimpanzees are more closely related to gorillas than to humans. (c) Humans and chimpanzees diverged about 6 million years ago. (d) Orangutans are the most divergent of the four species shown in Figure Q928.
(b) Chimpanzees are more closely related to gorillas than to humans.
Which of the following changes is least likely to arise from a point mutation in a regulatory region of a gene? (a) a mutation that changes the time in an organism's life during which a protein is expressed (b) a mutation that eliminates the production of a protein in a specific cell type (c) a mutation that changes the subcellular localization of a protein (d) a mutation that increases the level of protein production in a cell
(c) a mutation that changes the subcellular localization of a protein
Which of the following functions do you not expect to find in the set of genes found in all organisms on Earth? (a) DNA replication (b) DNA repair (c) protein production (d) RNA splicing
(d) RNA splicing
Which of the following statements about the human genome is false? (a) About 50% of the human genome is made up of mobile genetic elements. (b) More of the human genome comprises intron sequences than exon sequences. (c) About 1.5% of the human genome codes for exons. (d) Only the exons are conserved between the genomes of humans and other mammals.
(d) Only the exons are conserved between the genomes of humans and other mammals.
Which of the following statements about mobile genetic elements is true? (a) Mobile genetic elements can sometimes rearrange the DNA sequences of the genome in which they are embedded by accidentally excising neighboring chromosomal regions and reinserting these sequences into different places within the genome. (b) DNA-only transposons do not code for proteins but instead rely on transposases found in cells that are infected by viruses. (c) The two major families of transposable sequences found in the human genome are DNA-only transposons that move by replicative transposition. (d) During cut-and-paste transposition, the donor DNA will no longer have the mobile genetic element embedded in its sequence when transposition is complete.
(d) During cut-and-paste transposition, the donor DNA will no longer have the mobile genetic element embedded in its sequence when transposition is complete.
In humans and in chimpanzees, 99% of the Alu retrotransposons are in corresponding positions. Which of the following statements below is the most likely explanation for this similarity? (a) The Alu retrotransposon is not capable of transposition in humans. (b) Most of the Alu sequences in the chimpanzee genome underwent duplication and divergence before humans and chimpanzees diverged. (c) The Alu retrotransposons are in the most beneficial position in the genome for primates. (d) The Alu retrotransposons must also be in the same position in flies.
(b) Most of the Alu sequences in the chimpanzee genome underwent duplication and divergence before humans and chimpanzees diverged.
You are interested in finding out how the budding yeast Saccharomyces cerevisiae is so good at making bread and have collected five new related species from the wild. You sequence the genomes of all of these new species and also consult with a fungal biologist to help you construct the phylogenetic tree shown in Figure Q9-29. You find that species V, W, and X make pretty good bread whereas species Y and Z do not, suggesting that the last common ancestor of species X and S. cerevisiae may have the genes necessary for making good bread. You compare the gene sequences of species X and S. cerevisiae and find many identical coding sequences, but you also identify nucleotides that differ between the two species. Which species would be the best to examine to determine what the sequence was in the last common ancestor of species X and S. cerevisiae? (a) species V (b) species W (c) species Y (d) species Z
(c) species
Your friend works in a lab that is studying why a particular mutant strain of Drosophila grows an eye on its wing. Your friend discovers that this mutant strain of Drosophila is expressing a transcription factor incorrectly. In the mutant Drosophila, this transcription factor, which is normally expressed in the primordial eye tissue, is now misexpressed in the primordial wing tissue, thus turning on transcription of the set of genes required to produce an eye in the wing primordial tissue. If this hypothesis is true, which of the following types of genetic change would most likely lead to this situation? (a) a mutation within the transcription factor gene that leads to a premature stop codon after the third amino acid (b) a mutation within the transcription factor gene that leads to a substitution of a positively charged amino acid for a negatively charged amino acid (c) a mutation within an upstream enhancer of the gene (d) a mutation in the TATA box of the gene
(c) a mutation within an upstream enhancer of the gene
Alternative exons can arise through the duplication and divergence of existing exons. What type of mutation below would be least tolerated during the evolution of a new exon? (a) a nucleotide change of A to G (b) a deletion of three consecutive bases (c) mutation of the first nucleotide in the intron (d) a nucleotide change that alters a TT dinucleotide to AA
(c) mutation of the first nucleotide in the intron
Propose a reason to explain why highly repetitive regions of the genome are particularly susceptible to expansions and contractions in number.
Highly repetitive regions of the genome are particularly susceptible to unequal genetic exchange during homologous recombination.
The human genome has 3.2 × 109 nucleotide pairs. At its peak, the Human Genome Project was generating raw nucleotide sequences at a rate of 1000 nucleotides per second. At the rate of 1000 nucleotides per second, how long would it take to generate 3.2 × 109 nucleotides of sequence?
It would take approximately 37 days; 60 seconds/minute means 86400 seconds in a day. At peak rate, you can obtain 86,400,000 nucleotides per day.
Which of the following is true of a retrovirus but not of the Alu retrotransposon? (a) It requires cellular enzymes to make copies. (b) It can be inserted into the genome. (c) It can be excised and moved to a new location in the genome. (d) It encodes its own reverse transcriptase.
(d) It encodes its own reverse transcriptase.
A finished draft of the human genome was published in ______. (a) 1965. (b) 1984. (c) 2004. (d) 2012.
(c) 2004.
The yeast genome was sequenced more than 15 years ago, yet the total number of genes continues to be refined. The sequencing of closely related yeast species was important for validating the identity of short (less than 100 nucleotides long) open reading frames (ORFs) that were otherwise difficult to predict. What is the main reason that these short ORFs are hard to find without the genomes of other yeast for comparison? (a) Short ORFs are found only in yeast. (b) The short ORFs code for RNAs. (c) Many short stretches of DNA may lack a stop codon simply by chance, making it difficult to distinguish those DNA sequences that code for proteins from those that do not. (d) Short ORFs occur mainly in gene-rich regions, making them difficult to identify by computer programs.
(c) Many short stretches of DNA may lack a stop codon simply by chance, making it difficult to distinguish those DNA sequences that code for proteins from those that do not.
HIV is a human retrovirus that integrates into the host cell's genome and will eventually replicate, produce viral proteins, and ultimately escape from the host cell. Which of the following proteins is not encoded in the HIV genome? (a) reverse transcriptase (b) envelope protein (c) RNA polymerase (d) capsid protein
(c) RNA polymerase
The evolutionary relationships between seven different species—G, H, J, K, L, M, and N—are diagrammed in Figure Q9-33. Given this information, which of the following statements is false? (a) These are all highly related species, because the sequence divergence between the most divergent species is 3%. (b) Species M is just as related to species G as it is to species J. (c) Species N is more closely related to the last common ancestor of all of these species than to any of the other species shown in the diagram. (d) Species G and H are as closely related to each other as species J and K are to each other.
(c) Species N is more closely related to the last common ancestor of all of these species than to any of the other species shown in the diagram.
Figure Q9-27 shows the nucleotide sequence from a protein-coding region of a gene in humans, chimpanzees, and gorillas and the protein sequence produced from this gene. The seventeen amino acids encoded by this DNA are numbered below. The two codons that are not conserved in all three species have been boxed. These two codons code for amino acids 3 and 15. Which of these statements is consistent with these sequence-comparison data? (a) The gorilla sequence is more similar to the chimp sequence than to the human sequence. (b) Since these sequences are so similar, this protein must also be found in invertebrates. (c) The chimp DNA sequence has likely diverged at the DNA coding for amino acid 15 from the sequence found in the last common ancestor of humans and chimps. (d) The last common ancestor of chimps and gorillas most likely used AAA to code for amino acid number 3.
(c) The chimp DNA sequence has likely diverged at the DNA coding for amino acid 15 from the sequence found in the last common ancestor of humans and chimps.
What is the most likely explanation of why the overall mutation rates in bacteria and in humans are roughly similar? (a) Cell division needs to be fast. (b) Most mutations are silent. (c) There is a narrow range of mutation rates that offers an optimal balance between keeping the genome stable and generating sufficient diversity in a population. (d) It benefits a multicellular organism to have some variability among its cells.
(c) There is a narrow range of mutation rates that offers an optimal balance between keeping the genome stable and generating sufficient diversity in a population.
Figure Q9-16 shows an experiment used to determine the spontaneous mutation rate in E. coli. If the spontaneous mutation rate in E. coli is 1 mistake in every 109 nucleotides copied, about how many colonies would you expect to see on the plates lacking histidine if you were to assay 1011 cells from the culture for their ability to form colonies? (a) 1 (b) 2 (c) 10 (d) 100
(d) 100
You are studying a gene that has four exons and can undergo alternative splicing. Exon 1 has two alternatives, exon 2 has five alternatives, exon 3 has three alternatives, and exon 4 has four alternatives. If all possible splicing combinations were used, how many different splice isoforms could be produced for this gene? (a) 22 (b) 30 (c) 60 (d) 120
(d) 2 × 5 × 3 × 4 = 120.
Viral genomes _________. (a) can be made of DNA. (b) can be made of RNA. (c) can be either double-stranded or single-stranded. (d) All answers above are true.
(d) All answers above are true.
Which of the following statements about the globin gene family is true? (a) The globin protein, which can carry oxygen molecules throughout an organism's body, was first seen in ancient vertebrate species about 500 million years ago. (b) The gene duplication that led to the expansion of the globin gene family led to the separation and distribution of globin on many chromosomes in mammals, such that no chromosome has more than a single functional member of the globin gene family. (c) As globin gene family members diverged over the course of evolution, all the DNA sequence variations that have accumulated between family members are within the regulatory DNA sequences that affect when and how strongly each globin gene is expressed. (d) Some of the duplicated globin genes that arose during vertebrate evolution acquired inactivating mutations and became pseudogenes in modern vertebrates.
(d) Some of the duplicated globin genes that arose during vertebrate evolution acquired inactivating mutations and became pseudogenes in modern vertebrates.
You isolate a pathogenic strain of E. coli from a patient and discover that this E. coli strain is resistant to an antibiotic. Common laboratory strains of E. coli are not resistant to this antibiotic, nor are any other previously isolated pathogenic E. coli strains. However, such resistance has been observed in other bacteria in the hospital in which the patient was treated. This newly discovered antibiotic resistance in E. coli is most likely due to _______. (a) a mutation within a gene. (b) a mutation within the regulatory DNA of a gene. (c) gene duplication. (d) horizontal gene transfer.
(d) horizontal gene transfer
Which of the following would contribute most to successful exon shuffling? (a) shorter introns (b) a haploid genome (c) exons that code for more than one protein domain (d) introns that contain regions of similarity to one another
(d) introns that contain regions of similarity to one another
Match the type of phenotypic change below with the type of genetic change most likely to cause it. Each type of genetic change may be used more than once, or may not be used at all. Phenotypic changes: 1. A protein normally localized in the nucleus is now localized in the cytoplasm. _________ 2. A protein acquires a DNA-binding domain. _________ 3. Tandem copies of a gene are found in the genome. _________ 4. A copy of a bacterial gene is now found integrated on a human chromosome. _________ 5. A protein becomes much more unstable. _________ 6. A protein normally expressed only in the liver is now expressed in blood cells. ________ Types of genetic change: A. mutation within a gene B. gene duplication C. mutation in a regulatory region D. exon shuffling E. horizontal gene transfer
1—A; 2—D; 3—B; 4—E; 5—A; 6—C
For each statement below, indicate whether it is true or false, and explain why. A. All highly conserved stretches of DNA in the genome are transcribed into RNA. B. To find functionally important regions of the genome, it is more useful to compare species whose last common ancestor lived 100 million years ago rather than 5 million years ago. C. Most mutations and genome alterations have neutral consequences. D. Proteins required for growth, metabolism, and cell division are more highly conserved than those involved in development and in response to the environment. E. Introns and transposons tend to slow the evolution of new genes.
A. False. Many highly conserved stretches of DNA are not transcribed but instead contain information critical for regulating where and when genes are expressed. B. True. Species that diverged recently have many identical stretches of DNA sequence by chance, whereas sequence similarity between species that diverged long ago is probably due to functional constraints. The sequences that are necessary to preserve the function of the gene will not be able to undergo changes and thus are more likely to be similar between species that diverged long ago. C. True. Most genomic changes do not alter the amino acid sequence of proteins or the regulatory properties of genes. Even some mutations that cause minor alterations have little effect on protein function. D. True. All organisms need to perform a similar basic set of fundamental functions, such as those for metabolism, protein synthesis, and DNA replication. Proteins involved in these functions are shared by descent, and their evolution is constrained. Different species and cells are likely to require different developmental paths and to encounter different environmental challenges, so the proteins involved in these processes will tend to be more variable. For example, bacteria do not undergo elaborate developmental programs and so lack many of the regulators of development found in eukaryotes. E. False. Introns and transposons can act as sites where recombinational crossovers occur. Transposons can also catalyze genetic rearrangements. Rearrangements occurring within these sequences are less likely to be detrimental than those occurring elsewhere in the genome. In general, only the short intron sequences required for splicing are important to intron function; alterations in sequences outside the splicing sites may have no consequences for intron function and thus will not be subject to purifying selection.
Figure Q9-35 shows a hypothetical phylogenetic tree. Use this tree to answer the following questions. A. How many years ago did species M and N diverge from their last common ancestor? B. How much nucleotide divergence is there on average between species M and N? C. Are species M and N more or less closely related to each other than species P and S are? D. In looking for functionally important nucleotide sequences, is it more informative to compare the genome sequences of species M and N or those of species M and Q?
A. M and N diverged 10 million years ago. B. There is an average of 2.0% nucleotide substitution in species M compared with species N (follow the path connecting the two species, which is twice the distance between each one and their common ancestor). C. Neither more nor less. They show roughly the same degree of relatedness. The sequence divergence between species M and N is about 2.0%, the same as that between species P and S. Both pairs of species diverged 10 million years ago. D. It is more informative to compare species that are separated by a greater evolutionary distance; thus, comparing species M and Q, which diverged 20 million years ago, will be better able to identify sequences that are likely to be important for function. Closely related species share many sequences by chance, because there has been insufficient time for neutral mutations to accumulate.
Some types of gene are more highly conserved than others. For each of the following pairs of gene functions, choose the one that is more likely to be highly conserved. A. genes involved in sexual reproduction / genes involved in sugar metabolism B. DNA replication / developmental pathways C. hormone production / lipid synthesis
A. sugar metabolism B. DNA replication C. lipid synthesis These pathways or phenomena are fundamental to the growth and proliferation of all cells, including bacteria, and thus are likely to be highly conserved from species to species.
Your friend discovered a new multicellular organism living under the polar ice caps, and brought it back to the laboratory, where it seems to be growing well. Your friend is particularly interested in the proteins that allow this organism to survive in extreme cold. Because he is interested in proteins and because he has learned that most of the human genome does not code for exons, he is considering sequencing expressed sequence tags from this organism. What do you think the pitfalls of this approach might be? Explain.
Although expressed sequence tags (ESTs) can be very useful in identifying genes, the use of ESTs in this case may not work for several reasons. Two of these are: 1. ESTs are made from mRNAs, and thus represent actively transcribed genes. Your friend is studying the proteins that permit survival in extreme cold. Since the organism is no longer living in the extreme cold, the genes required for survival may no longer be expressed. 2. Because ESTs are made from mRNAs and because genes can be expressed at different levels, you will sequence the ESTs from genes that are abundantly transcribed more often than those that may be transcribed rarely.
Most variation between individual humans is in the form of __________________. __________________ may arise by recombination within introns and can create proteins with novel combinations of domains. Scientists and government regulators must be very careful when introducing herbicideresistant transgenic corn plants into the environment, because if resistant weeds arise from __________________ then the herbicides could become useless. Families of related genes can arise from a single ancestral copy by __________________ and subsequent __________________. divergence purifying selection exon shuffling single-nucleotide polymorphisms gene duplication synteny horizontal gene transfer unequal crossing-over
Most variation between individual humans is in the form of single-nucleotide polymorphisms. Exon shuffling may arise by recombination within introns and can create proteins with novel combinations of domains. Scientists and government regulators must be very careful when introducing herbicide-resistant transgenic corn plants into the environment, because if resistant weeds arise from horizontal gene transfer then the herbicides could become useless. Families of related genes can arise from a single ancestral copy by gene duplication and subsequent divergence.
It is thought that all eukaryotes have about 300 genes in common. Would you predict that these genes would be used at different times during the life cycle of multicellular animals? Explain your answer.
No, these genes are likely to be involved in basic cellular functions such as DNA replication and protein production, and in the basic functions of eukaryotic cells such as the functioning of the nucleus and the movement of items between cellular compartments. Genes involved in basic cellular functions are likely to be used all the time in an organism's life and are not likely to be activated at a specific stage of life. The genes found in all eukaryotes probably existed in the primordial eukaryotic cells.
Sexual reproduction in a multicellular organism involves specialized reproductive cells, called __________________s, which come together to form a __________________ that will divide to produce both reproductive and __________________ cells. A point mutation in the DNA is considered a __________________ mutation if it changes a nucleotide that leads to no phenotypic consequence; a point mutation is considered __________________ if it changes a nucleotide within a gene and causes the protein to be nonfunctional. cellulose intron common neutral deleterious somatic gamete unequal homologous zygote
Sexual reproduction in a multicellular organism involves specialized reproductive cells, called gametes, which come together to form a zygote that will divide to produce both reproductive and somatic cells. A point mutation in the DNA is considered a neutral mutation if it changes a nucleotide that leads to no phenotypic consequence; a point mutation is considered deleterious if it changes a nucleotide within a gene and causes the protein to be nonfunctional.
Explain how ESTs are identified and how they aid in finding the genes within an organism's genome.
To identify expressed sequence tags, or ESTs, mRNA must first be isolated from cells. This mRNA is converted into complementary DNA (cDNA) with the use of specialized nucleic acid polymerases. The nucleotide sequence of a short region of each cDNA is then determined. Each short sequence (or EST) corresponds to a portion of a gene that was expressed in the cells from which the mRNA was isolated; each sequence can be used as a tag to identify or manipulate the gene from which it came. A collection of ESTs can be input into a computer to search for matches to the total genome sequence and can thereby identify the sequences and chromosomal locations of many genes.
Which of the following statements about pseudogenes is false? (a) Pseudogenes code for microRNAs. (b) Pseudogenes share significant nucleotide similarity with functional genes. (c) Pseudogenes are no longer expressed in the cell. (d) There are estimated to be approximately 20,000 pseudogenes in the human genome.
(a) Pseudogenes code for microRNAs.
The number of distinct protein species found in humans and other organisms can vastly exceed the number of genes. This is largely due to ______________. (a) protein degradation. (b) alternative splicing. (c) homologous genes. (d) mutation.
(b) alternative splicing.
Which of the following DNA sequences is not commonly carried on a DNA-only transposon? (a) transposase gene (b) reverse transcriptase gene (c) recognition site for transposase (d) antibiotic-resistance gene
(b) reverse transcriptase gene
For each statement below, indicate whether it is true or false. A. To meet a challenge or develop a new function, evolution essentially builds from first principles, designing from scratch, to find the best possible solution. B. Nearly every instance of DNA duplication leads to a new functional gene. C. A pseudogene is very similar to a functional gene but cannot be expressed because of mutations. D. Most genes in vertebrates are unique, and only a few genes are members of multigene families. E. Horizontal gene transfer is very rare and thus has had little influence on the genomes of bacteria.
A. False. Evolution can work only by tinkering with the tools and materials on hand, not by starting from scratch to make completely new genes or pathways. New functions arise from the ancestral functions by a process of gradual mutational change, and thus may not represent the best possible solution to a problem. B. False. Many duplications are subsequently lost or become pseudogenes, and only a few evolve into new genes. C. True. Pseudogenes look very similar to normal genes but cannot produce a fulllength protein, as a result of one or more disabling mutations. D. False. A large proportion of the genes in vertebrates (and many other species) are members of multigene families. E. False. By some estimates, 20% of the genomic DNA in some bacterial species arose by horizontal gene transfer.
For each statement below, indicate whether it is true or false, and explain why. A. The increased complexity of humans compared with flies and worms is largely due to the vastly larger number of genes in humans. B. Repeats of the CA dinucleotide are useful for crime investigations and other forensic applications. C. Most single-nucleotide polymorphisms cause no observable functional differences between individual humans. D. There is little conserved synteny between human and mouse genes. E. The differences between multicellular organisms are largely explained by the different kinds of genes carried on their chromosomes.
A. False. The number of genes differs only by about a factor of two. It is thought that the increased complexity of humans is due largely to differences in when and where the genes are expressed. Differences in the timing of splicing may also be a major contributor to the relative complexity of humans. B. True. There are CA repeats in many locations throughout the genome. Because the number of repeats at a given location varies greatly between individuals and families, it can be used as an identifying characteristic to match two samples (such as blood samples) from the same or related individuals. C. True. Nearly all single-nucleotide polymorphisms have no effect on the appearance or behavior of the individual, but a few cause important differences. D. False. Human and mouse chromosomes show extensive synteny, with blocks of chromosomal DNA exhibiting homologous genes arranged in the same order between the two species. E. False. Multicellular organisms are built from essentially the same toolbox of gene building blocks, but the parts are put together differently because of regulatory differences that dictate when and where and how much of each protein is made. Alternative splicing can also have an important role, as it can generate several proteins from a single gene in some species, yet the homologous gene in other species may produce only one protein.
