Biology | Answers to Review Questions

Author Dr. George B. Johnson, Washington University
Author Dr. Peter H. Raven, Missouri Botanical Gardens & Washington University
Contributor Dr. Susan Singer, Carleton College
Contributor Dr. Jonathan Losos, Washington University

Answers to Review Questions

Chapter 15 (p. 312)

1. Ribosomal RNA (rRNA) functions in ribosomes to provide the site where a polypeptide is assembled. Transfer RNA (tRNA) transports amino acids to ribosomes to build a polypeptide. Messenger RNA (mRNA) passes from the nucleus to the cytoplasm to be a blueprint for protein synthesis, which is produced from chromosomal DNA information.

2. RNA polymerase initiates transcription and catalyzes the binding of the appropriate mRNA base to the DNA base being transcribed. RNA polymerase recognizes specific "start" and "stop" regions at the start of and end of a gene.

3. Crick and his colleagues chemically deleted a nucleotide to change the reading frame, then determined what further deletions were required to restore the proper reading frame. They found that the reading of the code remained nonsense when one or two additional deletions were made, but three deletions corrected the reading frame. An anticodon is the complementary three-nucleotide sequence on the tRNA.

4. Only one codon at a time is receptive to a tRNA anticodon on the ribosome, and tRNA anticodons are specific for both the codons to which they bind and the amino acids they carry.

5. Translation is initiated through the formation of an initiation complex. Through elongation factors and translocation, the protein grows in length as the ribosome moves down the mRNA chain. When a "stop" nonsense codon is reached, release factors terminate translation and release the new polypeptide from the ribosome.

6. Elongation factors assist in the binding of the tRNA to the appropriate mRNA codon.

7. An intron is a segment of noncoding DNA. Exons are the segments of DNA that actually code for the production of a specific protein. Both introns and exons are transcribed into mRNA, but prior to leaving the nucleus, the introns are processed out of the so-called primary transcript, leaving only exons in the stretch of mRNA leaving the nucleus.