Chapter 3
Cells: The Living Units
109
3
double-membrane vesicles called
autophagosomes
. Tey are
then delivered to lysosomes for digestion of the contents to
amino acids, fatty acids, and the like, which the cell reuses.
Autophagy may have evolved as a response to cell starva-
tion and it speeds up in response to several kinds of stress, such
as low oxygen, high temperature, or lack of growth factors.
However, autophagosomes are busy continuously. Although
autophagy can lead to programmed cell death (see apoptosis,
p. 110), it makes a greater contribution to cell survival and pro-
vides a fail-safe system against complete self-destruction when
such a dire response is not necessary.
Autophagy is exactly what the doctor ordered for disposal of
large cytoplasmic structures and protein aggregates. But lyso-
somal enzymes do not have access to soluble proteins that are
misfolded, damaged, or unneeded and need to be disposed of.
Examples of unneeded proteins include some that are used only
in cell division and must be degraded at precise points in the cell
cycle, and short-lived transcription factors.
So how does the cell prevent such proteins from accumulat-
ing while stopping the cytosolic enzymes from destroying vir-
tually all soluble proteins? It seems that the cell has a different
strategy for destroying such proteins.
Check Your Understanding
29.
Codons and anticodons are both three-base sequences. How
do they differ?
30.
How do the A, P, and E ribosomal sites differ functionally
during protein synthesis?
31.
What is the role of DNA in transcription?
For answers, see Appendix H.
Degradation of Organelles
and Cytosolic Proteins
Define autophagy and indicate its major cellular function.
Describe the importance of ubiquitin-dependent
degradation of soluble proteins.
Te workings of the cytoplasm are complex and seemingly unend-
ing. Without some system to get rid of malfunctioning or obsolete
organelles, cells would soon become gummed up with debris.
Not to worry. Te process called
autophagy
(“self-eat-
ing”) sweeps up bits of cytoplasm and excess organelles into
DNA
molecule
Gene 1
Gene 2
Gene 4
DNA:
DNA base sequence
(triplets) of the gene codes
for synthesis of a particular
polypeptide chain
T
A
1
C
G
T
A G C G A
T
T
T
C C C
T
G C G A
A
A
A
C
T
2
3
4
5
6
7
8
9
Codons
Triplets
tRNA
A
U
1
G C C
A
U C G C U
A
A
A G G G A
C G C U U U
U
A
C G G
U A G C G A
U
U U C C C U G C G A
A
A
U G A
2
3
4
5
6
7
8
9
Met
Pro
Ser
Leu
Lys
Gly
Arg
Phe
Start
translation
Stop;
detach
G
Anticodon
mRNA:
Base sequence
(codons) of the transcribed
mRNA
tRNA:
Consecutive base
sequences of tRNA
anticodons recognize the
mRNA codons calling for
the amino acids they
transport
Polypeptide:
Amino acid
sequence of the
polypeptide chain
Figure 3.40
Information transfer from
DNA to RNA to polypeptide.
Information
is transferred from the DNA of the gene to
the complementary messenger RNA mole-
cule, whose codons are then “read” (trans-
lated) by transfer RNA anticodons. Notice
that the “reading” of the mRNA by tRNA
anticodons reestablishes the base (triplet)
sequence of the DNA genetic code (except
that T is replaced by U).
previous page 143 Human Anatomy and Physiology (9th ed ) 2012 read online next page 145 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off