Cellular Structure, Proteins, and Metabolism
65
Within the lumen of the endoplasmic reticulum, enzymes
remove the signal sequence from most proteins, so this por-
tion is not present in the fi nal protein. In addition, carbohy-
drate groups are sometimes linked to various side chains in
the proteins.
Following these modifi
cations, portions of the reticulum
membrane bud off, forming vesicles that contain the newly
synthesized proteins. These vesicles migrate to the Golgi appa-
ratus (see Figure 3–25) and fuse with the Golgi membranes.
Within the Golgi apparatus, the protein may undergo
further modifi cations. For example, additional carbohydrate
groups—important as recognition sites within the cell—may
be added.
While in the Golgi apparatus, the many different pro-
teins that have been funneled into this organelle are sorted
out according to their fi nal destinations. This sorting involves
the binding of regions of a particular protein to specifi c pro-
teins in the Golgi membrane that are destined to form vesicles
targeted to a particular destination.
Following modifi cation and sorting, the proteins are
packaged into vesicles that bud off the surface of the Golgi
membrane. Some of the vesicles travel to the plasma mem-
brane, where they fuse with the membrane and release their
contents to the extracellular fl uid, a process known as exocy-
tosis. Other vesicles may dock and fuse with lysosome mem-
branes, delivering digestive enzymes to the interior of this
organelle. Specifi c docking proteins on the surface of the
membrane where the vesicle fi nally fuses recognize the spe-
cifi c proteins on the surface of the vesicle.
In contrast to this entire story, if a protein does not have
a signal sequence, synthesis continues on a free ribosome until
the completed protein is released into the cytosol. These pro-
teins are not secreted but are destined to function within the
cell. Many remain in the cytosol, where they function, for
example, as enzymes in various metabolic pathways. Others
are targeted to particular cell organelles. For example, ribo-
somal proteins are directed to the nucleus, where they com-
bine with rRNA before returning to the cytosol as part of the
ribosomal subunits. The specifi c location of a protein is deter-
mined by binding sites on the protein that bind to specifi c
sites at the protein’s destination. For example, in the case of
the ribosomal proteins, they bind to sites on the nuclear pores
that control access to the nucleus.
SECTION B SUMMARY
Genetic Code
I. Genetic information is coded in the nucleotide sequences
of DNA molecules. A single gene contains either (a) the
information that, via mRNA, determines the amino acid
sequence in a specifi c protein, or (b) the information for
forming rRNA, tRNA, or small nuclear RNAs, which assist in
protein assembly.
II. Genetic information is transferred from DNA to mRNA
in the nucleus (transcription); then mRNA passes to the
cytoplasm, where its information is used to synthesize protein
(translation).
III. The “words” in the DNA genetic code consist of a sequence
of three nucleotide bases that specify a single amino acid. The
sequence of three-letter codes along a gene determines the
sequence of amino acids in a protein. More than one triplet can
specify a given amino acid.
Protein Synthesis
I. Table 3–2 summarizes the steps leading from DNA to protein
synthesis.
II. Transcription involves forming a primary RNA transcript by
base-pairing with the template strand of DNA containing a
single gene. Transcription also involves the removal of intron-
derived segments by spliceosomes to form mRNA, which
moves to the cytoplasm.
III. Translation of mRNA occurs on the ribosomes in the cytoplasm
when the anticodons in tRNAs, linked to single amino acids,
base-pair with the corresponding codons in mRNA.
IV. Protein transcription factors activate or repress the
transcription of specifi c genes by binding to regions of DNA
that interact with the promoter region of a gene.
V. Mutagens alter DNA molecules, resulting in the addition or
deletion of nucleotides or segments of DNA. The result is an
altered DNA sequence known as a mutation. A mutation may
(1) cause no noticeable change in cell function, (2) modify
cell function but still be compatible with cell growth and
replication, or (3) lead to the death of the cell.
Protein Degradation
I. The concentration of a particular protein in a cell depends on:
(1) the rate of the corresponding gene’s transcription, (2) the
rate of initiating protein assembly on a ribosome, (3) the rate
at which mRNA is degraded, (4) the rate of protein digestion
by enzymes associated with proteasomes, and (5) the rate of
secretion, if any, of the protein from the cell.
Protein Secretion
I. Targeting of a protein for secretion depends on the signal
sequence of amino acids that fi rst emerge from a ribosome
during protein synthesis.
SECTION B KEY TERMS
anticodon
59
codon
58
exon
58
gene
55
genome
55
histone
55
initiation factor
59
intron
58
messenger RNA (mRNA)
57
mutagen
62
mutation
62
natural selection
63
nucleosome
55
preinitiation complex
62
primary RNA transcript
58
promoter
57
proteasome
63
ribosomal RNA (rRNA)
59
RNA polymerase
57
signal sequence
63
spliceosome
58
“stop” signal
56
template strand
57
transcription
56
transcription factor
61
transfer RNA (tRNA)
59
translation
56
ubiquitin
63
SECTION B REVIEW QUESTIONS
1. Describe how the genetic code in DNA specifi es the amino
acid sequence in a protein.
2. List the four nucleotides found in mRNA.
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