132 ❯ STEP 4. Review the Knowledge You Need to Score High
- D—During translation, the P siteholds 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 ribo-
some 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. - 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. - 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.- B—Guanine does not pair with thymine in
DNA or RNA. Watson and Crick discovered that
adenine pairs with thymine (AT) held together
by two hydrogen bonds and guanine pairs with
cytosine (CG) 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 ade-
nine 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 gua-
nine are the purines, and thymineand cytosine are
the pyrimidines. - C—Prions are the culprit for mad cow disease.
Viroids are tiny viruses that infect plants.
Plasmidsare 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 provirusis
that which is formed during the lysogenic cycle
of a virus when it falls dormant and incorpo-
rates 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.
For questions 10 and 11, please use the following gel: 10.Which of the DNA pieces in the gel is smallest
in size?
A. A
B. B
C. C
D. D
E. E
11.If well 1 is DNA from a crime scene, which indi-
vidual should contact a lawyer?
A. Person 2
B. Person 3
C. Person 4
D. Person 5
E. Person 6
1 2 3 4 5 6A
B
D
E
C