Genetics, DNA, and Molecular Biology - GRE Subject Test: Biology
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Consider the following scenario:
Gene-Z is a transcription factor required for transcribing the genes that contribute to populating the motor cortex with the proper number of neurons. Mutant mice for Gene-Z have 50% fewer motor neurons than controls (non-mutants). You genetically engineer a transgenic DNA construct that acts as a dominant negativeto Gene-Z.
You transfect an embryonic mouse brain with this dominant negative and measure that all motor neurons get and expressed this construct. What would you expect the number of neurons in the transfected brain to be if the dominant negative works with 100% efficacy, and a normal motor cortex has 100,000 cells in the region you are examining?
Consider the following scenario:
Gene-Z is a transcription factor required for transcribing the genes that contribute to populating the motor cortex with the proper number of neurons. Mutant mice for Gene-Z have 50% fewer motor neurons than controls (non-mutants). You genetically engineer a transgenic DNA construct that acts as a dominant negativeto Gene-Z.
You transfect an embryonic mouse brain with this dominant negative and measure that all motor neurons get and expressed this construct. What would you expect the number of neurons in the transfected brain to be if the dominant negative works with 100% efficacy, and a normal motor cortex has 100,000 cells in the region you are examining?
This question requires the knowledge that a dominant negativeis expected to work in the same direction as a genetic mutant; it is a loss-of-function of the gene, by expressing a version of the gene that outcompetes the actual gene but cannot perform the proper biological process. Because we are assuming the dominant negative works perfectly, it should act just like the mutant and reduce cell number by 50%, giving us 50,000 cells.
This question requires the knowledge that a dominant negativeis expected to work in the same direction as a genetic mutant; it is a loss-of-function of the gene, by expressing a version of the gene that outcompetes the actual gene but cannot perform the proper biological process. Because we are assuming the dominant negative works perfectly, it should act just like the mutant and reduce cell number by 50%, giving us 50,000 cells.
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Which of the following statements about the function of transcription factors is not true?
Which of the following statements about the function of transcription factors is not true?
Transcription factors can be activated or deactivated by any number of processes occurring within the cell and nucleus, and this it not limited to phosphatases (which remove phosphate groups from proteins). All of the other answers accurately describe possible activity and function of transcription factors.
Transcription factors can be activated or deactivated by any number of processes occurring within the cell and nucleus, and this it not limited to phosphatases (which remove phosphate groups from proteins). All of the other answers accurately describe possible activity and function of transcription factors.
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Which of the following is true?
I. Transcription factors typically bind directly to the genes that they regulate.
II. Transcription factors can bind enhancer and promoter regions upstream of the genes they regulate.
III. In eukaryotes, transcription factors frequently associate with coactivators.
IV. Transcription factors can either upregulate or downregulate transcription of a gene.
Which of the following is true?
I. Transcription factors typically bind directly to the genes that they regulate.
II. Transcription factors can bind enhancer and promoter regions upstream of the genes they regulate.
III. In eukaryotes, transcription factors frequently associate with coactivators.
IV. Transcription factors can either upregulate or downregulate transcription of a gene.
Only the first statement in this question is false. Transcription factors typically (in fact, almost always) bind upstream of the gene to enhancer or promoter regions, and are rarely found interacting with the gene's coding region itself.
Only the first statement in this question is false. Transcription factors typically (in fact, almost always) bind upstream of the gene to enhancer or promoter regions, and are rarely found interacting with the gene's coding region itself.
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A protein that will be embedded in the plasma membrane is likely to be translated by __________.
A protein that will be embedded in the plasma membrane is likely to be translated by __________.
Most proteins that will be embedded in the plasma membrane are translated on ribosomes located in the rough endoplasmic reticulum. There are specific mechanisms and proteins that help insert the proteins into the membrane while they are being translated. Free-floating proteins are more likely to be translated in the cytosol. The nucleus and the Golgi do not have ribosomes used for translation, though the Golgi can play an important role in transporting proteins from the rough endoplasmic reticulum to the membrane.
Most proteins that will be embedded in the plasma membrane are translated on ribosomes located in the rough endoplasmic reticulum. There are specific mechanisms and proteins that help insert the proteins into the membrane while they are being translated. Free-floating proteins are more likely to be translated in the cytosol. The nucleus and the Golgi do not have ribosomes used for translation, though the Golgi can play an important role in transporting proteins from the rough endoplasmic reticulum to the membrane.
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Which of the following is true concerning the genetic code?
Which of the following is true concerning the genetic code?
The genetic code is unambiguous, meaning that each given codon will always code for the same amino acid. An amino acid, however, can be coded for by multiple codons, making the genetic code degenerative in nature. Once a stop codon is reached during translation, the ribosome stops making the protein.
The genetic code is unambiguous, meaning that each given codon will always code for the same amino acid. An amino acid, however, can be coded for by multiple codons, making the genetic code degenerative in nature. Once a stop codon is reached during translation, the ribosome stops making the protein.
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What site in the ribosome contains the elongating polypeptide?
What site in the ribosome contains the elongating polypeptide?
A tRNA that is attached to one amino acid will enter the ribosomal complex at the A site. It will then receive the growing polypeptide chain from the previous tRNA and move into the P site. Once handing off the chain, the tRNA that no longer has an amino acid will exit the ribosome at the E site.
The peptide chain is always anchored in the P site, where peptide bond synthesis occurs.
A tRNA that is attached to one amino acid will enter the ribosomal complex at the A site. It will then receive the growing polypeptide chain from the previous tRNA and move into the P site. Once handing off the chain, the tRNA that no longer has an amino acid will exit the ribosome at the E site.
The peptide chain is always anchored in the P site, where peptide bond synthesis occurs.
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Which of the following is not an example of an epigenetic cause of trait variation?
Which of the following is not an example of an epigenetic cause of trait variation?
Epigenetics are changes to the genome that result in phenotypic variation that have nothing to do with changes in the actual DNA sequence. All listed answers occur independently of DNA sequence, except for "different exon sequences," which is the actual sequence of an exon. This referces to alternative splicing, an is not related to the modification of DNA or histones.
Epigenetics are changes to the genome that result in phenotypic variation that have nothing to do with changes in the actual DNA sequence. All listed answers occur independently of DNA sequence, except for "different exon sequences," which is the actual sequence of an exon. This referces to alternative splicing, an is not related to the modification of DNA or histones.
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In terms of epigenetics, acetylated histones usually result in which of the following?
In terms of epigenetics, acetylated histones usually result in which of the following?
The correct answer is an increase in gene expression. Histone acetylation removes positive charges on the histones, reducing the affinity of DNA for histones. Remember that DNA is negatively charged due to the phosphate groups on its backbone. DNA and histones are attracted to each other because histones are positively charged due to being rich in basic amino acid residues. Acetylation relaxes the tightly bound DNA allowing transcription factors to bind promoter regions. DNA deacetylation and methylation supress gene transcription by making DNA and histones associate more tightly together, decreasing the ability of transcription factors and/or RNA polymerase to bind the DNA. Histone modifications such as acetylation, deacetylation, and methylation do not directly affect the amount of DNA. If a histone is acetylated on a part of the DNA which codes for the genes for ribosome production, then an increase in ribosomal production and assembly could occur, but genes coding for ribosomes are greatly outnumbered by other genes, and thus, this is not the usual result of acetylating histones.
The correct answer is an increase in gene expression. Histone acetylation removes positive charges on the histones, reducing the affinity of DNA for histones. Remember that DNA is negatively charged due to the phosphate groups on its backbone. DNA and histones are attracted to each other because histones are positively charged due to being rich in basic amino acid residues. Acetylation relaxes the tightly bound DNA allowing transcription factors to bind promoter regions. DNA deacetylation and methylation supress gene transcription by making DNA and histones associate more tightly together, decreasing the ability of transcription factors and/or RNA polymerase to bind the DNA. Histone modifications such as acetylation, deacetylation, and methylation do not directly affect the amount of DNA. If a histone is acetylated on a part of the DNA which codes for the genes for ribosome production, then an increase in ribosomal production and assembly could occur, but genes coding for ribosomes are greatly outnumbered by other genes, and thus, this is not the usual result of acetylating histones.
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Which of the following is true about the effects of epigenetics on gene expression?
Which of the following is true about the effects of epigenetics on gene expression?
Methylation and acetylation of histones occurs on lysine residues, thereby decreasing or increasing gene expression, respectively. Methylation increases the affinity for histones and DNA, where acetylation decreases the affinity for histones and DNA. Gene expression is in part controlled by modification of histone proteins, rather non-histone chromosomal proteins.
Methylation and acetylation of histones occurs on lysine residues, thereby decreasing or increasing gene expression, respectively. Methylation increases the affinity for histones and DNA, where acetylation decreases the affinity for histones and DNA. Gene expression is in part controlled by modification of histone proteins, rather non-histone chromosomal proteins.
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Which of the following choices could activate a proto-oncogene?
Which of the following choices could activate a proto-oncogene?
Proto-oncogenes are genes that have the ability to become oncogenes (genes that cause cancer). There are many ways to activate proto-oncogenes. Gene duplication can cause an increase in the expression of a particular protein, which can lead to cancer. Exposure to mutagens can cause a mutation on a proto-oncogene, which causes it to become activated. Chromosomal translocations can relocate proto-oncogenes to areas where they are expressed more rapidly. Most proto-oncogenes are involved in cell cycle regulation. Irregular expression of these genes can allow the cell to progress through the cell cycle too rapidly, resulting in unregulated cell division and tumor formation.
Proto-oncogenes are genes that have the ability to become oncogenes (genes that cause cancer). There are many ways to activate proto-oncogenes. Gene duplication can cause an increase in the expression of a particular protein, which can lead to cancer. Exposure to mutagens can cause a mutation on a proto-oncogene, which causes it to become activated. Chromosomal translocations can relocate proto-oncogenes to areas where they are expressed more rapidly. Most proto-oncogenes are involved in cell cycle regulation. Irregular expression of these genes can allow the cell to progress through the cell cycle too rapidly, resulting in unregulated cell division and tumor formation.
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Which of the following best describes an oncogene?
Which of the following best describes an oncogene?
Oncogenes can be thought of as cancerous genes, or rather a gene that has the potential to cause cancer. They typically occur when a normal proto-oncogene undergoes a mutation. Proto-oncogenes normally code for growth and development in cells, and tightly regulate these processes. If mutated, these newly cancerous genes can stimulate unregulated growth, a symptom characteristic of cancerous cells.
Oncogenes can be thought of as cancerous genes, or rather a gene that has the potential to cause cancer. They typically occur when a normal proto-oncogene undergoes a mutation. Proto-oncogenes normally code for growth and development in cells, and tightly regulate these processes. If mutated, these newly cancerous genes can stimulate unregulated growth, a symptom characteristic of cancerous cells.
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Which of the following is not true of DNA repair?
Which of the following is not true of DNA repair?
DNA repair can, and does, occur during replication. An easy example of this is the proofreading function of several DNA polymerases. This function is carried out due to the enzymes containing an exonuclease function that allows them to excise incorrect base pairs. p53 is an incredibly important protein that is expressed heavily when DNA damage is detected. It is responsible for activating both DNA repair pathways and apoptotic pathways, preventing the cell from passing replication and cell cycle checkpoints. If the DNA damage is irreparable, the cell may undergo apoptosis.
DNA repair can, and does, occur during replication. An easy example of this is the proofreading function of several DNA polymerases. This function is carried out due to the enzymes containing an exonuclease function that allows them to excise incorrect base pairs. p53 is an incredibly important protein that is expressed heavily when DNA damage is detected. It is responsible for activating both DNA repair pathways and apoptotic pathways, preventing the cell from passing replication and cell cycle checkpoints. If the DNA damage is irreparable, the cell may undergo apoptosis.
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Upon double-stranded DNA break, the cell can search the genome for a homologous sequence to serve as a template for repairing the damaged sequence. What is this process known as?
Upon double-stranded DNA break, the cell can search the genome for a homologous sequence to serve as a template for repairing the damaged sequence. What is this process known as?
The correct answer is homology directed repair. Using flanking homologous regions upstream and downstream of the double stranded break, the cell is able to determine the precise sequence that is in the damaged region and repair that sequence.
The correct answer is homology directed repair. Using flanking homologous regions upstream and downstream of the double stranded break, the cell is able to determine the precise sequence that is in the damaged region and repair that sequence.
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Which statement correctly describes the process of DNA replication?
Which statement correctly describes the process of DNA replication?
DNA replicates in a semiconservative process. Parental strands are used as templates to synthesize daughter strands, which remain adhered to the parental template creating hybrid molecules of old and new DNA.
The original DNA molecules is "unzipped" by helicase to create the replication fork. DNA polymerase then begins to recruit nucleotides to bind to the exposed template, building the new DNA strand along the parental strand.
DNA replicates in a semiconservative process. Parental strands are used as templates to synthesize daughter strands, which remain adhered to the parental template creating hybrid molecules of old and new DNA.
The original DNA molecules is "unzipped" by helicase to create the replication fork. DNA polymerase then begins to recruit nucleotides to bind to the exposed template, building the new DNA strand along the parental strand.
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Which of the following proteins is known for its ability to break hydrogen bonds?
Which of the following proteins is known for its ability to break hydrogen bonds?
Before replication, the DNA helix must be unwound so that the strands can be replicated by DNA polymerase. This unwinding is accomplished by DNA helicase, which interferes with the hydrogen bonds between nucleotide pairs. This intervention creates a small separation between the two strands, known as the replication fork. DNA polymerase binds to the replication fork and recruits nucleotides to build the new DNA strand.
Topoisomerase is responsible for cleaving phosphodiester bonds in order to release torsional tension in the DNA backbone. DNA ligase synthesizes phosphodiester bonds, both on the daughter strand of DNA and in the regions cleaved by topoisomerase. Primase is responsible for synthesizing RNA primers that serve to help recruit and bind DNA polymerase in the replication fork.
Before replication, the DNA helix must be unwound so that the strands can be replicated by DNA polymerase. This unwinding is accomplished by DNA helicase, which interferes with the hydrogen bonds between nucleotide pairs. This intervention creates a small separation between the two strands, known as the replication fork. DNA polymerase binds to the replication fork and recruits nucleotides to build the new DNA strand.
Topoisomerase is responsible for cleaving phosphodiester bonds in order to release torsional tension in the DNA backbone. DNA ligase synthesizes phosphodiester bonds, both on the daughter strand of DNA and in the regions cleaved by topoisomerase. Primase is responsible for synthesizing RNA primers that serve to help recruit and bind DNA polymerase in the replication fork.
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__________ is a protein that synthesizes RNA primers on __________ during DNA replication.
__________ is a protein that synthesizes RNA primers on __________ during DNA replication.
In order for DNA polymerase to begin synthesizing base pairs, an RNA primer is needed to assist the binding of DNA polymerase to the DNA template strand. This primer is synthesized by the enzyme primase. Because DNA polymerase always needs an RNA primer before it can bind, primase must synthesize RNA primers on both the leading and lagging strands.
RNA polymerase transcribes molecules of RNA from DNA sequences during transcription, and is not involved in DNA replication.
In order for DNA polymerase to begin synthesizing base pairs, an RNA primer is needed to assist the binding of DNA polymerase to the DNA template strand. This primer is synthesized by the enzyme primase. Because DNA polymerase always needs an RNA primer before it can bind, primase must synthesize RNA primers on both the leading and lagging strands.
RNA polymerase transcribes molecules of RNA from DNA sequences during transcription, and is not involved in DNA replication.
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Which of the following is not true of DNA replication?
Which of the following is not true of DNA replication?
DNA replication occurs during the S phase of the cell cycle, significantly before prophase of mitosis. During prophase chromosomes are condensed into easily segregated forms, but replication has already occurred. The S phase is the intermediate period of interphase in the cell cycle. The G2 phase follows the S phase, and is subsequently followed by the M phase (mitosis).
The short fragments synthesized on the lagging strand are known as Okazaki fragments. DNA replication does occur in the 5' to 3' direction; this is also the reason that the lagging strand must be synthesized away from the replication fork. DNA is denatured (separated) at the replication fork by an enzyme known as helicase, which breaks the hydrogen bonds between base pairs to allow DNA polymerase and other replication proteins to bind to single-strand DNA.
DNA replication occurs during the S phase of the cell cycle, significantly before prophase of mitosis. During prophase chromosomes are condensed into easily segregated forms, but replication has already occurred. The S phase is the intermediate period of interphase in the cell cycle. The G2 phase follows the S phase, and is subsequently followed by the M phase (mitosis).
The short fragments synthesized on the lagging strand are known as Okazaki fragments. DNA replication does occur in the 5' to 3' direction; this is also the reason that the lagging strand must be synthesized away from the replication fork. DNA is denatured (separated) at the replication fork by an enzyme known as helicase, which breaks the hydrogen bonds between base pairs to allow DNA polymerase and other replication proteins to bind to single-strand DNA.
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Which prokaryotic polymerase is primarily responsible for filling in DNA nucleotides into the gap created by the removal of RNA primers?
Which prokaryotic polymerase is primarily responsible for filling in DNA nucleotides into the gap created by the removal of RNA primers?
DNA polymerase I replaces the RNA primer gap with DNA nucleotides. This polymerase is unique in that it has 5' 
3' exonuclease activity. This RNA primer is created by primase, it is removed and replaced with DNA by DNA polymerase I, and the remaining nick is sealed by DNA ligase. Bacterial DNA polymerase III, in contrast, is the main polymerase for bacterial elongation. The function of DNA polymerase II is not completely understood. The remaining answer choices are not involved in prokaryotic DNA replication.
DNA polymerase I replaces the RNA primer gap with DNA nucleotides. This polymerase is unique in that it has 5'
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Colorblindness is a recessive sex-linked disease that is caused by a defective gene on the X-chromosome. If a mother who is a carrier for the trait mates with a normal sighted male, what percentage of their daughters will have the disease?
Colorblindness is a recessive sex-linked disease that is caused by a defective gene on the X-chromosome. If a mother who is a carrier for the trait mates with a normal sighted male, what percentage of their daughters will have the disease?
Since colorblindness is a recessive disease, all copies of the X-chromosome must have the diseased allele in order for the person to be colorblind. Daughters have two copies of the X-chromosome: one from the mother and the other from the father. Males only have one copy of the X-chromosome (from the mother) and a Y-chromosome from the father.
Since we know that the father has normal vision, he does NOT carry the colorblind allele. Since the daughters for this couple can only potentially receive one colorblind allele (from the mother), all of their daughters will have normal vision. This means that there is a zero percent chance for colorblindness in their daughters.
The cross would look like this, taking Xb as the colorblind allele:
Parents: XXb x XY
Offspring: XX or XXb (normal daughters), XY (normal son), YXb (colorblind son)
The chance of a colorblind daughter will be zero, but the chance of a colorblind son will be 50%.
Since colorblindness is a recessive disease, all copies of the X-chromosome must have the diseased allele in order for the person to be colorblind. Daughters have two copies of the X-chromosome: one from the mother and the other from the father. Males only have one copy of the X-chromosome (from the mother) and a Y-chromosome from the father.
Since we know that the father has normal vision, he does NOT carry the colorblind allele. Since the daughters for this couple can only potentially receive one colorblind allele (from the mother), all of their daughters will have normal vision. This means that there is a zero percent chance for colorblindness in their daughters.
The cross would look like this, taking Xb as the colorblind allele:
Parents: XXb x XY
Offspring: XX or XXb (normal daughters), XY (normal son), YXb (colorblind son)
The chance of a colorblind daughter will be zero, but the chance of a colorblind son will be 50%.
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Hemophilia A is a blood clotting disorder that is usually inherited as an X-linked recessive trait. If a woman who does not have hemophilia A has a daughter with a man who does not have hemophilia A, what is the probability that the daughter will have hemophilia A if the maternal grandfather did have hemophilia A?
Hemophilia A is a blood clotting disorder that is usually inherited as an X-linked recessive trait. If a woman who does not have hemophilia A has a daughter with a man who does not have hemophilia A, what is the probability that the daughter will have hemophilia A if the maternal grandfather did have hemophilia A?
X-linked recessive inheritance dictates that expression of themutant phenotype will only occur if the individual is homozygous for the mutation on the X-chromosomes. Therefore, a female must have inherited two mutant X-chromosomes to have hemophilia A, while a male only requires one mutant X-chromosome to have the disorder. By virtue of the father not having hemophilia A, we know the daughter is inheriting at least one wild-type X-chromosome, and therefore there is zero chance she will be homozygous and have hemophilia A.
X-linked recessive inheritance dictates that expression of themutant phenotype will only occur if the individual is homozygous for the mutation on the X-chromosomes. Therefore, a female must have inherited two mutant X-chromosomes to have hemophilia A, while a male only requires one mutant X-chromosome to have the disorder. By virtue of the father not having hemophilia A, we know the daughter is inheriting at least one wild-type X-chromosome, and therefore there is zero chance she will be homozygous and have hemophilia A.
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