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U N I T 1
Cell and Tissue Function
at a time. After the first part of the mRNA is read by the
first ribosome, it moves onto a second and a third. As
a result, ribosomes that are actively involved in protein
synthesis are often found in clusters called
polyribosomes
.
The process of translation is not over when the genetic
code has been used to create the sequence of amino acids
that constitute a protein. To be useful to a cell, this new
polypeptide chain must be folded up into its unique three-
dimensional conformation. The folding of many proteins
is made more efficient by special classes of proteins called
molecular chaperones
. Typically, the function of a chap-
erone is to assist a newly synthesized polypeptide chain
to attain a functional conformation as a new protein and
then to assist the protein’s arrival at the site in the cell
where the protein carries out its function. Molecular chap-
erones also assist in preventing the misfolding of existing
proteins. Disruption of chaperoning mechanisms causes
intracellular molecules to become denatured and insolu-
ble. These denatured proteins tend to stick to one another,
precipitate, and form inclusion bodies. The development
of inclusion bodies is a common pathologic process in
Parkinson, Alzheimer, and Huntington diseases.
A newly synthesized polypeptide chain may also
need to combine with one or more polypeptide chains
from the same or an adjacent chromosome, bind small
cofactors for its activity, or undergo appropriate enzyme
modification. During the posttranslation process, two
or more peptide chains may combine to form a single
product. For example, two
α
-globin chains and two
β
-globin chains combine to form the hemoglobin protein
in red blood cells (see Chapter 13). The protein prod-
ucts may also be modified chemically by the addition of
various types of functional groups. For example, fatty
acids may be added, providing hydrophobic regions
for attachment to cell membranes. Other modifications
may involve cleavage of the protein, either to remove a
specific amino acid sequence or to split the molecule into
smaller chains. As an example, the two chains that make
up the circulating active insulin molecule, one contain-
ing 21 and the other 30 amino acids, were originally
part of an 82-amino-acid proinsulin molecule.
Regulation of Gene Expression
Only about 2% of the genome encodes instructions
for synthesis of proteins. Although researchers are still
learning about the functions of the other 98% of DNA,
it is now believed to consist of sequences that code for
the production of certain types of regulatory RNA,
including RNA involved in the splicing process dis-
cussed earlier. Some of the remaining genes are involved
in regulation of other genes; whereas others may play a
role in regulating various nuclear or cytoplasmic func-
tions. The degree to which a gene or particular group of
genes is active is called
gene expression
. A phenomenon
termed
induction
is an important process by which gene
expression is increased.
Gene repression
is a process by
which a regulatory gene acts to reduce or prevent gene
expression. Activator and repressor sites commonly
monitor levels of the synthesized product and regulate
gene transcription through a negative feedback mecha-
nism. Whenever product levels decrease, gene transcrip-
tion is increased, and when product levels increase, gene
transcription is repressed.
Although control of gene expression can occur in
multiple steps, many regulatory events occur at the
transcription level. As noted earlier, the initiation and
regulation of transcription require the collaboration
of a battery of proteins collectively termed
transcrip-
tion factors
. After binding to their own specific DNA
region, transcription factors can function to increase or
decrease transcriptional activity of the genes. The role
of transcription factors in gene expression explains why
neurons and liver cells that have the same DNA in their
nuclei nevertheless have completely different structures
and functions. Moreover, some transcription factors
activate genes only at specific stages of development.
For example, the PAX family of transcription factors is
involved in the development of such embryonic tissues
as the eye and portions of the nervous system.
SUMMARY CONCEPTS
■■
Genes are the fundamental unit of information
storage in the cell.They code for the assembly
of the proteins made by the cell and therefore
control inheritance and day-to-day cell function.
■■
Although every cell in the body contains the same
basic genetic information, cells from our body
express their genetic information differently, and
can therefore differ vastly in their appearance,
structure, and function.
Introns
Messenger RNA with introns
Messenger RNA with introns removed
Exon splicing in cell A
Exon splicing in cell B
EXON 3 13
EXON 1 11 EXON 2 12
EXON 4 14 EXON 5
FIGURE 5-5.
Ribonucleic acid (RNA) processing. In different
cells, an RNA strand may eventually produce different proteins
depending on the sequencing of exons during gene splicing.
This variation allows a gene to code for more than one protein.
(Courtesy of EdwardW. Carroll.)
