AP
®
BIOLOGY
2009 SCORING COMMENTARY
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Question 4
Overview
The question addressed the “central dogma” of biology, the flow of information from DNA to RNA to
protein. In the first part of the question, students had to explain the role of five specified components—
RNA polymerase, spliceosomes, codons, ribosomes, and tRNA—involved in transcription and translation
in eukaryotic cells. The second part of the question asked students to describe two specific eukaryotic
mechanisms of protein regulation, including protein synthesis and activity. In the third part of the
question, students had to explain how the central dogma does not apply to some viruses, selecting a type
of virus or a specific virus and explaining how it deviates from the central dogma.
Sample: 4A
Score: 10
The response earned 4 points in part (a). Explanations of the roles of the following each earned a point:
“RNA polymerase is an enzyme that attaches to a DNA sequence and begins transcribing it to
mRNA.”
“[I]t undergoes RNA splicing by the spliceosomes. These enzymes cut out the intron.”
“Ribosomes are where proteins are made.”
“When tRNA attaches, it brings with it an amino acid.”
The maximum of 4 points were earned in part (b). Acetylation and methylation are the mechanisms of protein
regulation named. These mechanisms each earned a point for a total of 2 points. Each mechanism is also
discussed: “Histone acetylation brings acetyl groups that are positively charged and cause the H1 histones to
not bind to each other as tightly. This loose packaging of DNA allows the RNA polymerase better access to
the DNA to transcribe for that protein,” and “DNA methylation attracts methyl groups that induce tighter
packaging of the DNA. This DNA is less likely to be transcribed and have its proteins synthesized.” These
clear explanations of protein regulation mechanisms earned 2 more points. The discussion of operon
regulation that follows earned no points since this is not eukaryotic regulation.
A point was earned in part (c) by selection of the “retrovirus (like HIV)” as an example of a virus that deviates
from the central dogma. A deviation point was also earned by explaining that “instead of creating proteins
from DNA, it uses its host cell to create DNA from the virus’s own RNA.”
Sample: 4B
Score: 8
The response earned 4 points in part (a). Explanations of the roles of the following each earned a point:
“Spliceosomes cut and paste together the interpretable sections of RNA, leaving out the intron
sections.”
“[E]ach codon codes for a specific amino acid.”
“RNA is read and translated into a polypeptide in the ribosome.”
“[E]ach amino acid is carried by tRNA, where it is matched to the proper codon.”
Two mechanism of protein regulation are identified in part (b): “feedback inhibition and allosteric regulation.”
This earned 2 points: 1 point for each mechanism. However, only 1 point was earned for the correct
discussion of the allosteric regulation mechanism: “multiple activation sights [sic] in an enzyme need to be
filled before it can operate, yeilding [sic] very specific conditions that the product is formed under.”