Human Physiology, 14th edition (2016)

Cell Structure and Genetic Control 71

including gene activation and inactivation. Other proteins

modify the activity of particular enzymes, and so help to regu-

late the cell. Examples of such regulatory proteins include the

cyclins, which help control the cell cycle (see fig. 3.25 ).

Because proteins have so many important functions, the

processes of genetic transcription and translation have to be

physiologically regulated. Hormones and other chemical

signals can turn specific genes on or off, regulating protein

synthesis. However, for critically important proteins, tighter

control is required. Regulatory proteins are rapidly degraded

(hydrolyzed, or digested), quickly ending their effects so that

other proteins can produce new actions. This affords a much

tighter control of specific regulatory proteins than would be

possible if they persisted longer and only their synthesis was

regulated.

Protease enzymes (those that digest proteins) located in

the lysosomes digest many types of cellular proteins. In recent

years, however, scientists learned that critical regulatory pro-

teins are also degraded outside of lysosomes in a process that

requires cellular energy (ATP). In this process, the regulatory

Figure 3.21 The translation of messenger RNA (mRNA). (1) The anticodon of an aminoacyl-tRNA bonds with a codon on

the mRNA, so that the specific amino acid it carries can form a peptide bond with the last amino acid of a growing polypeptide. (2) The

tRNA that brought the next-to-last amino acid dissociates from the mRNA, so that the growing polypeptide is attached to only the last

tRNA. (3) Another tRNA carrying another amino acid will bond to the next codon in the mRNA, so that this amino acid will be at the new

growing end of the polypeptide.

Codons

Codons

mRNA

A

B

C

D

E

F

G

H

I

tRNA

4

5

tRNA

1

2

3

Next amino acid

Growing

polypeptide

chain

Ribosome

Anticodons

tRNA

tRNA

G

G

UA

AU

CG

CG

UA

C

C

D

E

Next amino acid

2

3

4

5

6

tRNA

tRNA

tRNA

A B

C

D

E

F

G

H

I

1

1

2

3

Figure 3.22 How secretory proteins enter the

endoplasmic reticulum. A protein destined for secretion

begins with a leader sequence that enables it to be inserted into

the cisterna (cavity) of the endoplasmic reticulum. Once it has

been inserted, the leader sequence is removed and carbohydrate

is added to the protein.

Ribosome

mRNA

Carbohydrate

Cisterna of

endoplasmic reticulum

Protein

Granular endoplasmic

reticulum

Cytoplasm

Leader

sequence

removed

Leader

sequence

Free ribosome

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