Concept 6 AP BIO

In mammals, the dark color of skin, hair, and eyes is due to a pigment called melanin. Melanin is produced by specialized skin cells called melanocytes. The melanin is then transferred to other skin cells called keratinocytes.

The amount of melanin produced is dependent on the amount of the enzymes TYR , TRP2 , and TRP1 present inside melanocytes. The peptide hormone α– melanocyte stimulating hormone ( α–MSH ) activates a signal transduction pathway leading to the activation of MITF . MITF is a transcription factor that increases the expression of the TYR , TRP2 , and TRP1 genes (Figure 2).

• The TYR , TRP2 , and TRP1 genes are located next to each other on a single chromosome and are organized into an operon.

• The TYR , TRP2 , and TRP1 genes may be located on different chromosomes but are activated by the same transcription factor.

• The TYR , TRP2 , and TRP1 genes are identical genes since they are activated by the same transcription factor.

• The TYR , TRP2 , and TRP1 genes may be located on different chromosomes but with identical operator sequences.

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Some mammals increase melanin production in response to ultraviolet (UV) radiation. The UV radiation causes damage to DNA in keratinocytes, which activates the p53 protein. p53 increases the expression of the POMC gene. The POMC protein is then cleaved to produce α–MSH . The keratinocytes secrete α–MSH , which signals nearby melanocytes. The increased melanin absorbs UV radiation, reducing further DNA damage.

Which of the following best explains a process occurring between point 1 and point 2 in Figure 3 ?

• α–MSH is produced.

• The TYR gene is transcribed.

• Polypeptides are removed from a protein.

• A poly‑A tail is added to RNA .

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NGSS.HS-LS1-1

Scientists conducted a transformation experiment using E. coli bacteria and the pTru plasmid. Samples of the pTru plasmid (lane A ) and the chromosomal DNA from two different E. coli strains that the scientists attempted to transform (lane B and lane C ) were compared using gel electrophoresis. The results are shown in Figure 1.

Which of the following statements best explains the experimental results observed in Figure 1 ?

• E. coli in both lanes B and C have been successfully transformed and contain additional genetic information.

• E. coli in lane B have been successfully transformed and contain additional genetic information.

• E. coli in lane C have been successfully transformed and contain additional genetic information.

• Which E. coli have been transformed cannot be determined from this gel.

Which of the following best explains how the pattern of DNA arrangement in chromosomes could be used, in most cases, to determine if an organism was a prokaryote or a eukaryote?

• 
• 
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An evolutionary biologist hypothesizes that two morphologically similar plant species are not closely related. To test the hypothesis, the biologist collects DNA samples from each of the two plant species and then uses restriction enzymes to cut the DNA samples into fragments, which are then subjected to gel electrophoresis. The results are shown in Figure 1.

Given the results shown in Figure 1, which of the following correctly describes a relationship between the two species?

• Species B is the ancestor of species A because it has fewer bands.

• Species A is more complex than species B because it has more bands.

• Species B has more short fragments of DNA than species A does.

• Species A has more short fragments of DNA than species B does.

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Labeled nucleotides were supplied to a cell culture before the cells began DNA replication. A simplified representation of the process for a short segment of DNA is shown in Figure 1. Labeled DNA bases are indicated with an asterisk (*).

Which of the following best helps explain how the process represented in Figure 1 produces DNA

DNA molecules that are hybrids of the original and the newly synthesized strands? (think about the semi-conservative nature of DNA Replication)

• Each template strand is broken down into nucleotides, which are then used to synthesize both strands of a new DNA

  DNA molecule.

• Each template strand is broken into multiple fragments, which are randomly assembled into two different DNA

  DNA molecules.

• Each newly synthesized strand is associated with another newly synthesized strand to form a new DNA

  DNA molecule.

• Each newly synthesized strand remains associated with its template strand to form two copies of the original DNA

  DNA molecule.

The trp operon in E. coli is an example of a repressible operon that consists of genes coding for enzymes used to synthesize tryptophan. When tryptophan levels are high, the operon is turned off and these genes are not transcribed. However, it is also known that tryptophan does not bind directly to the operator DNA sequence. A regulatory gene called trpR has also been discovered although it is not part of the trp operon. The proposed model of how tryptophan acts as a corepressor is shown in Figure 1.

Which of the following evidence best supports a claim that tryptophan functions as a corepressor?

• Normal expression of trpR causes the trp operon to be transcribed regardless of tryptophan levels

• When the operator sequence is mutated, the trp operon is not transcribed.

• The trpR gene codes for a repressor protein that has a DNA binding domain.

• When trpR is mutated, the trp operon is transcribed regardless of tryptophan levels.

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A simplified model of a DNA replication fork is represented in Figure 1. The protein labeled Enzyme 1 carries out a specific role in the DNA replication process.

Which of the following statements best explains the role of Enzyme 1 in the DNA replication process?

• Enzyme 1 is a DNA ligase that joins together the DNA fragments at a replication fork to form continuous strands.

• Enzyme 1 is a DNA primase that catalyzes the synthesis of RNA primers on the lagging strand of a replication fork.

• Enzyme 1 is a DNA polymerase that synthesizes new DNA by using the leading and lagging strands of a replication fork as templates.

• Enzyme 1 is a topoisomerase that relieves tension in the overwound DNA in front of a replication fork.

All cells must transcribe rRNA in order to construct a functioning ribosome. Scientists have isolated and identified rRNA genes that contribute to ribosomal structure for both prokaryotes and eukaryotes. Figure 1 compares the transcription and processing of prokaryotic and eukaryotic rRNA .

Which of the following statements provides the best explanation of the processes illustrated in Figure 1 ?

• Introns are removed from the pre- rRNA , and the mature rRNA molecules are joined and then translated to produce the protein portion of the ribosome.

• Introns are removed from the pre- rRNA , and each mature rRNA molecule is translated to produce the proteins that make up the ribosomal subunits.

• Sections of the pre- rRNA are removed, and the mature rRNA molecules are available to combine with proteins to form the ribosomal subunits.

• Sections of the pre- rRNA are removed, and the mature rRNA molecules are available to bring different amino acids to the ribosome.

Cystic fibrosis ( CF ) is a progressive genetic disease that causes persistent lung infections and affects the ability to breathe. CF is inherited in an autosomal recessive manner, caused by the presence of mutations in both copies of the gene for the cystic fibrosis transmembrane conductance regulator ( CFTR ) protein. Partial nucleotide sequences and the corresponding amino acid sequences for an unaffected individual and an affected individual are modeled in Figure 1.

Based on the information in Figure 1, which type of mutation explains the nature of the change in DNA that resulted in cystic fibrosis in the affected individual?

• Substitution, because the amino acid tryptophan is replaced with glycine.

• Insertion, because an extra guanine is present, which changes the reading frame.

• Deletion, because a thymine is missing, which changes the reading frame.

• Duplication, because the amino acid leucine occurs twice, which changes the reading frame.

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