Tobacco’s New Leaf ■ 187REVIEWING THE SC IENCE
● (^) Most genes contain
instructions for building
proteins. Gene expression is
the process by which genes
are transcribed into RNA and
then translated into a protein.
● (^) During transcription, which
occurs in the nucleus,
RNA polymerase binds the
promoter of a gene and
produces a messenger
RNA (mRNA) version of the
gene sequence from free
nucleotides.
● (^) Next, during RNA splicing,
introns are snipped out of the
pre-mRNA sequence, and the
remaining exons are joined.
The mRNA is transported out
of the nucleus.
● (^) During translation, which
occurs in the cytoplasm,
ribosomes convert the
sequences of bases in
an mRNA molecule to the
sequence of amino acids
in a protein, with the help
of ribosomes composed of
ribosomal RNA (rRNA) and
proteins, and associated with
transfer RNA (tRNA).
● (^) Ribosomes read the mRNA
information in sets of three
bases at a time, called
codons. There are 64
possible codons, including a
start codon (AUG) and three
possible stop codons (UAA,
UAG, and UGA).
● (^) The genetic code is
redundant because there are
64 codons and only 20 amino
acids, and universal because
nearly all organisms on Earth
use the same code.
● (^) Gene expression is regulated
at many different points in
the pathway from gene to
protein. Organisms rely on
gene regulation to respond
to signals inside the body
and to external cues in the
environment.
THE QUESTIONS
The Basics
(^1) Link each term with the correct definition.
GENE EXPRESSION 1. The production of RNA, using the
information in the DNA sequence of a
gene.
GENE REGULATION 2. The flow of information from gene to
protein.
TRANSCRIPTION 3. The control of gene expression
in response to environmental or
developmental needs.
TRANSLATION 4. The linking of amino acids in the
precise sequence dictated by an
mRNA base sequence.
(^2) For each of the following, identify the type of RNA involved
(mRNA, rRNA, or tRNA).
a. Transports the correct amino acid to the ribosome, using
the information encoded in the mRNA.
b. Is a major component of ribosomes.
c. Specifies the order of amino acids in a protein, using a
series of three-base codons, where different amino acids
are specified by particular codons.
d. Contains a three-base anticodon that pairs with a comple-
mentary codon revealed in the mRNA.
e. Assists in making the bonds that link amino acids together
to make a protein.
(^3) Select the correct terms:
The genetic code demonstrates (ambiguity / redundancy) because
some amino acids are coded by more than one codon. The lack of
(ambiguity / redundancy) in the genetic code is evidenced by the
fact that each codon codes for one, and only one, amino acid.
(^4) In the diagram of transcription shown here, fill in the blanks
with the appropriate terms: (a) gene; (b) promoter; (c) terminator;
(d) RNA polymerase; (e) mRNA.