Human Physiology, 14th edition (2016)

72 Chapter 3

3.5 DNA Synthesis and Cell Division

When a cell is going to divide, each strand of the DNA

within its nucleus acts as a template for the formation of a

new complementary strand. Organs grow and repair them-

selves through a type of cell division known as mitosis. The

two daughter cells produced by mitosis both contain the

same genetic information as the parent cell. Gametes con-

tain only half the number of chromosomes as their parent

cell and are formed by a type of cell division called meiosis.

selected proteins invaginates to form a vesicle that is digested

within lysosomes. Ubiquination is even believed to tag organ-

elles such as mitochondria for selective destruction by lyso-

somes in the process of autophagy (section 3.2).

Figure 3.23 The conversion of proinsulin into
insulin. The long polypeptide chain called proinsulin is
converted into the active hormone insulin by enzymatic removal
of a length of amino acids (shown in green). The insulin molecule
produced in this way consists of two polypeptide chains (red
circles) joined by disulfide bonds.

S

S

AsnCys

Tyr

Tyr

Asn

Glu

Leu

Gln

Leu

Glu

Gly

Cys

Ser

ThrCysCys

Ser

Gly

Glu

Gly

Gly

Gly

Gly

Gly

Glu

Gly

Glu

Glu

Gly

S

S

Arg

Lys

Gln

Gln

Leu

Ser

Leu

LeuAla

Pro

Gln

Leu

Ser

Ala

Pro

Leu

Gln

Val

Val

Gln

Leu

Asp

Ala

Arg

Arg

Thr

Ly

Tyr

Thr

Pro

PhePhe

Gly

Gly

Arg

Glu

Cys

Val

Leu

Tyr

Leu

Ala

Glu

Val

Leu

His

Ser

CysLeu

HisGlnAsn

Val

Phe

Val

lle

lle

S

S

LEARNING OUTCOMES

After studying this section, you should be able to:

9. Explain the semiconservative replication of DNA in
   DNA synthesis.


10. Describe the cell cycle and identify some factors that
    affect it, and explain the significance of apoptosis.


11. Identify the phases of mitosis and meiosis, and
    distinguish between them.

| CHECKPOINTS

7a. Explain how mRNA, rRNA, and tRNA function during

the process of protein synthesis.

7b. Describe the granular endoplasmic reticulum, and

explain how the processing of secretory proteins

differs from the processing of proteins that remain

within the cell.

8. Describe posttranslational changes and other
   functions of the Golgi complex and the roles of
   ubiquitin and the proteasome.

proteins to be destroyed are first tagged by binding to mole-

cules of ubiquitin (Latin for “everywhere”), a short polypep-

tide composed of 76 amino acids. Ubiquitin bonds to one or

more lysine amino acids in the targeted cell protein, in a com-

plex process that requires many enzymes and is subject to regu-

lation. This tagging with ubiquitin is required for the proteins to

be degraded by the proteasome, a large protease enzyme com-

plex. Degradation of ubiquitin-tagged proteins within protea-

somes eliminates defective proteins (for example, incorrectly

folded proteins produced in the endoplasmic reticulum) and

promotes cell regulation. For example, the stepwise progression

through the cell cycle requires the stepwise degradation of par-

ticular cyclin proteins.

The ubiquitin-proteasome system is the major route by

which regulatory proteins in the cytoplasm are degraded. In

addition, tagging with ubiquitin helps remove selected plasma

membrane proteins (receptor proteins, for example; chapter 6,

section 6.5). In that process, the membrane containing the

Genetic information is required for the life of the cell and for the

cell to be able to perform its functions in the body. Each cell obtains

this genetic information from its parent cell through the process of

DNA replication and cell division. DNA is the only type of mole-

cule in the body capable of replicating itself, and mechanisms exist

within the dividing cell to ensure that the duplicate copies of DNA

will be properly distributed to the daughter cells.

DNA Replication

When a cell is going to divide, each DNA molecule rep-

licates itself, and each of the identical DNA copies thus