1. The diagram below shows the arrangement of chromatin (thick black strands) around histones on a chromosome in the nucleus of a eukaryotic cell.

Transcription

1 1. The diagram below shows the arrangement of chromatin (thick black strands) around histones on a chromosome in the nucleus of a eukaryotic cell. A signal received by the cell causes the attachment of acetyl groups ( COCH 3 ) to the histones, and the chromatin-histone complex changes, as shown below. Page 1 of 7

2 Which statement best explains how this change will affect cellular activity? (A) Histone acetylation will reduce gene expression, because the acetyl groups prevent RNA polymerase from accessing the coding regions. This answer suggests the student may understand that RNA polymerase is involved in transcribing DNA into mrna, but does not understand that histone acetylation increases gene expression by exposing DNA in order for transcription to take place, whereas methylation reduces gene expression by restricting access of transcription factors to DNA. (B) Histone acetylation will reduce gene expression, because the acetyl groups will bind to chromatin and prevent proteins from initiating transcription. This answer suggests the student may understand that acetylation of histones plays a role in gene expression, but does not understand that histone acetylation does not repress genes, and that, instead, it allows for gene transcription. (C) Histone acetylation will increase gene expression, because the chromatin is no longer tightly bound to the histones and can be transcribed. This answer suggests the student understands that histone acetylation allows for gene expression because it uncoils DNA from the histone and allows genes to be exposed to transcription factors that initiate protein synthesis. (D) Histone acetylation will increase gene expression, because the acetyl-histone complexes can bind to promoter regions on the chromatin and activate the transcription of genes. This answer suggests the student may understand that gene expression can be initiated when compounds bind to a promoter, but does not understand that acetylated histone complexes do not regulate genes by binding to promoter regions and that acetylated histones instead act by separating from chromatin to expose genes for transcription. Page 2 of 7

3 2. A methyl group (CH 3 ) can be attached to a cytosine base on DNA, as shown in the diagram below. When a methyl group is attached to a base, enhancers and activator proteins are unable to access the base. A certain gene is identified that contains many methyl groups, compared to other regions of DNA. Which statement best explains the significance of methylation to this gene? (A) The gene will probably not be transcribed into RNA, because transcription factors will not be able to bind to the gene to initiate transcription. This answer suggests the student understands that DNA methylation blocks gene expression by preventing the binding of transcription factors to DNA. (B) The gene will probably be transcribed into RNA, but not translated into a polypeptide, because enhancers will be unable to convert the nucleic acid code into amino acids. This answer suggests the student may understand that DNA methylation blocks gene expression, but does not understand that this process prevents transcription because transcription factors cannot bind to DNA to initiate transcription, and that DNA methylation is not involved in the translation of RNA into an amino acid sequence. (C) The gene will probably not be transcribed into mrna, because the mrna will have defective cytosine nucleotides that will be unable to bind with complementary DNA nucleotides. Page 3 of 7

4 This answer suggests the student may understand that DNA methylation blocks gene expression, but does not understand that this process prevents transcription and the formation of mrna because DNA methylation prevents transcription factors from binding to DNA and does not affect RNA. (D) The gene will probably be transcribed and translated into an amino acid sequence, but the process will continue for a long time and produce excess polypeptides, because the mrna cannot be properly degraded. This answer suggests the student may understand that methylation of certain regulatory genes (ribonuclease) may slow the breakdown of mrna, which would result in greater production of polypeptides, but does not understand that the methylation of this gene would prevent expression because transcription factors would not be able to bind to the DNA to initiate transcription. 3. Fur coloration in Himalayan rabbits is determined by genetic and environmental factors. The pictures below show three genetically identical rabbits reared at three different temperatures. Which statement best explains why fur coloration in Himalayan rabbits is different at different temperatures? (A) Fur coloration is different because cold temperatures cause mutations in genes governing fur color in certain parts of rabbits. Page 4 of 7

5 This answer suggests the student may understand that some environmental factors can cause genetic mutations, but does not understand that cold temperatures are not associated with an increased risk of genetic mutation and that the differential expression of genes in Himalayan rabbits is caused by a difference in rearing temperature, not mutations. (B) Fur coloration is different because warm temperatures denature dark-pigmented proteins, which results in rabbits with only white fur. This answer suggests the student may understand that high temperatures can denature proteins, but does not understand that this outcome is unlikely because the temperature range affecting Himalayan rabbits is lower than the temperature needed to denature most biological enzymes, and the temperature is instead affecting gene expression and resulting in different phenotypes. (C) Fur coloration is different because different temperatures cause DNA to arrange into different sequences, and this alters the genes that express fur color in rabbits. This answer suggests the student may understand that environmental factors such as temperature may change gene expression, but does not understand that this explanation is unlikely because temperature may affect gene expression, but would not affect DNA arrangement. (D) Fur coloration is different because different temperatures cause differential expression of certain genes, which produces different fur colors in rabbits. This answer suggests the student understands that environmental factors such as temperature may affect gene expression and that this may result in different phenotypes. Page 5 of 7

6 4. The diagram below shows a hormone initiating a change in a cell. Based on the process illustrated in the diagram, which statement best explains how the hormone initiates a response in the cell? (A) The hormone initiates a response in the cell by binding to a protein receptor on RNA polymerase, allowing the enzyme to transcribe a gene. This answer suggests the student may understand that gene expression involves initiating the transcription of DNA by RNA polymerase, but does not understand that the hormone shown in the diagram does not directly initiate this because the hormone is binding to a protein receptor, not to RNA polymerase. The protein/hormone complex then binds to a regulatory region on the gene (DNA), which activates the gene for transcription. (B) The hormone initiates a response in the cell by binding to a protein receptor, initiating a series of kinase reactions that result in transcription of a gene in the nucleus. This answer suggests the student may understand that many hormones initiate gene expression via protein kinase reactions, but does not understand that this is not the pathway illustrated in the diagram because there is no indication that protein kinases are active in the process; rather, the protein/hormone complex shown in the diagram is binding to a regulatory region on the gene (DNA), which activates the gene for transcription. Page 6 of 7

7 (C) The hormone initiates a response in the cell by binding to a protein receptor, allowing the protein/hormone complex to bind to the gene and initiate transcription. This answer suggests the student understands that the hormone shown in the diagram controls gene expression by binding to a receptor protein and allowing the protein to activate a gene for transcription. (D) The hormone initiates a response in the cell by binding to a protein receptor on a ribosome, initiating transcription and translation of a gene. This answer suggests the student may understand that hormones and ribosomes are both involved in gene expression, but does not understand that a hormone binding to a receptor on a ribosome is not the correct description of the pathway shown in the diagram, because regulatory proteins interact with hormones to regulate gene expression, and ribosomes are involved with translation of the genetic code into polypeptides. Page 7 of 7