Molecular Genetics ❮ 133
Rapid Review
Briefly review the following terms:
DNA:contains A and G (purines), C and T (pyrimidines), arranged in a double helix of
two strands held together by hydrogen bonds (A with T, and C with G).
RNA:contains A and G (purines), C and U (pyrimidines), single stranded. There are three
types: mRNA (blueprints for proteins), tRNA (brings acids to ribosomes), and rRNA
(make up ribosomes).
DNA replication:occurs during S-phase, semi-conservative, built in 5′to 3′direction.
Helicase unzips the double strand, DNA polymerase comes in and adds on the nucleotides.
Proofreading enzymes minimize errors of process.
Frameshift mutation:deletion or addition of nucleotides (not a multiple of 3); shifts read-
ing frame.
Missense mutation:substitution of wrong nucleotide into DNA (e.g., sickle cell anemia);
still produces a protein.
- 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 sub-
unit 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.
- A—Episomes are not involved in gene expression
regulation.Episomesare plasmids that can be incor-
porated into a bacterial chromosome. Repressors
are regulatory proteins involved in gene regula-
tion. They work by preventing transcription by
binding to the promoter region. Operonsare 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 tran-
scription by acting directly on the transcription
machinery in the nucleus of cells.
- E—The mRNA produced after transcription
must be modified before it can leave the nucleus
and lead the translation of proteins in the ribo-
somes. Introns are cut out of the mRNA, and the
remaining exons are ligated back together to pro-
duce 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 posttran-
scriptional modification—it is a means of gene
expression control.
- C—tRNA does not have a short lifespan. Each
tRNA molecule is released and recycled 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.
- C—Gel electrophoresis separates DNA frag-
ments 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.
- 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 accu-
rately match DNA found at crime scenes with
potential suspects.
http://www.ebook3000.com
