Molecular Genetics Review Questions for AP Biology (page 2)

By — McGraw-Hill Professional
Updated on Oct 24, 2011

Answers and Explanations

  1. D—During translation, the P site holds the tRNA carrying the growing protein, while the A site holds the tRNA carrying the next amino acid. When translation begins, the first codon bound is the AUG codon, and in the P site is the tRNA with the methionine. The next codon in the sequence determines which tRNA binds next, and the appropriate tRNA molecule sits in the A site of the ribosome. A peptide bond forms between the amino acid on the A site tRNA and the amino acid on the P site tRNA. The amino acid from the P site then moves to the A site, allowing the tRNA in the P site to leave the ribosome. Next the ribosome moves along the mRNA in such a way that the A site is now the P site and the next tRNA comes into the A site carrying the next amino acid. Answer choices A, B, C, and E are all true.
  2. D—The process of transcription leads to the production of RNA. RNA is not immediately ready to leave the nucleus after it is produced. It must first be processed, during which a 3' poly- A tail and a 5' cap are added and the introns are spliced from the RNA molecule. After this process, the RNA is free to leave the nucleus and lead the production of proteins.
  3. C—AGG codes for the amino acid arginine. AGU codes for the amino acid serine. UGA and UAG are stop codons, which signal the end of the translation process. AUG is the start codon, which also codes for methionine.
  4. B—Guanine does not pair with thymine in DNA or RNA. Watson and Crick discovered that adenine pairs with thymine (A=T) held together by two hydrogen bonds and guanine pairs with cytosine (C≡G) held together by three hydrogen bonds. One way that RNA differs from DNA is that it contains uracil instead of thymine. But in RNA, guanine still pairs with cytosine and adenine instead pairs with uracil. Watson and Crick also discovered that for the structure of DNA they discovered to be true, a purine must always be paired with a pyrimidine. Adenine and guanine are the purines, and thymine and cytosine are the pyrimidines.
  5. C—Prions are the culprit for mad cow disease. Viroids are tiny viruses that infect plants. Plasmids are small circles of DNA in bacteria that are separate from the main chromosome. They are self-replicating and are vital to the process of genetic engineering. A provirus is that which is formed during the lysogenic cycle of a virus when it falls dormant and incorporates its DNA into the host DNA. A retrovirus is an RNA virus that carries an enzyme called reverse transcriptase. A classic example of a retrovirus is HIV.
  6. B—Translation begins when the mRNA attaches to the small ribosomal subunit. The first codon for this process is always AUG. This attracts a tRNA molecule carrying methionine to attach to the AUG codon. When this occurs, the large subunit of the ribosome, containing the A site and the P site, binds to the complex. The elongation of the protein is ready to begin after the complex has been properly constructed. Answers A, C, D, and E are all in the incorrect order.
  7. A—Episomes are not involved in gene expression regulation. Episomes are plasmids that can be incorporated into a bacterial chromosome. Repressors are regulatory proteins involved in gene regulation. They work by preventing transcription by binding to the promoter region. Operons are a promoter-operator pair that controls a group of genes, such as the lac operon. Methylation is involved in gene regulation. Barr bodies, discussed in Chapter 10, are found to contain a very high level of methylated DNA. Methyl groups have been associated with inactive DNA that does not undergo transcription. Hormones can affect transcription by acting directly on the transcription machinery in the nucleus of cells.
  8. E—The mRNA produced after transcription must be modified before it can leave the nucleus and lead the translation of proteins in the ribosomes. Introns are cut out of the mRNA, and the remaining exons are ligated back together to produce the mRNA ready to be translated into a protein. Also, the 5' end is given a guanine cap, which serves to protect the RNA and also helps the mRNA attach to the ribosome. The 3' end is given the poly-A tail, which may help ease the movement from the nucleus to the cytoplasm. Methylation does not occur during posttranscriptional modification—it is a means of gene expression control.
  9. C—tRNA does not have a short lifespan. Each tRNA molecule is released and re cycled to bring more amino acids to the ribosomes to aid in translation. It is like a taxicab constantly picking up new passengers to deliver from place to place. Answer choices A, B, D, and E are all true.
  10. C—Gel electrophoresis separates DNA fragments on the basis of size—the smaller you are, the farther you go. Because C went the farthest in this gel, this must be the smallest of the five selected DNA pieces. Of the five labeled, piece A must be the largest because it moved the least.
  11. C—Person 4 should contact a lawyer. The DNA from the crime scene seems to match the DNA fingerprint from person 4. Electrophoresis is a very useful tool in forensics and can very accurately match DNA found at crime scenes with potential suspects.


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