Cells: The Living Units
Cell Growth and Reproduction
The Cell Cycle
Te cell cycle is the series of changes that a cell goes through from
the time it is formed until it divides.
Interphase is the nondividing phase of the cell cycle. Interphase
consists of G
, S, and G
subphases. During G
, the cell grows and
centriole replication begins. During the S phase, DNA replicates.
, the ﬁnal preparations for division are made. Many
checkpoints occur during interphase at which the cell gets the
go-ahead signal to go through mitosis or is prevented from
continuing to mitosis.
DNA replication occurs before cell division, ensuring that both
daughter cells have identical genes. Te DNA helix uncoils, and
each DNA nucleotide strand acts as a template for the formation
of a complementary strand. Base pairing provides the guide for
the proper positioning of nucleotides.
Te semiconservative replication of a DNA molecule produces
two DNA molecules identical to the parent molecule, each
formed of one “old” and one “new” strand.
Cell division, essential for body growth and repair, occurs during
the M phase. Cell division consists of two distinct phases: mitosis
(nuclear division) and cytokinesis (division of the cytoplasm).
Mitosis, consisting of prophase, metaphase, anaphase, and
telophase, parcels out the replicated chromosomes to two
daughter nuclei, each genetically identical to the mother nucleus.
Cytokinesis, which begins late in mitosis, divides the cytoplasmic
mass into two parts.
Cell division is stimulated by certain chemicals (including growth
factors and some hormones) and increasing cell size. Lack of
space and inhibitory chemicals deter cell division. Cyclin-Cdk
complexes regulate cell division.
A gene is deﬁned as a DNA segment that provides the instructions
to synthesize one polypeptide chain. Since the major structural
materials of the body are proteins, and all enzymes are proteins, this
amply covers the synthesis of all biological molecules.
Te base sequence of exon DNA provides the information for
protein structure. Each three-base sequence (triplet) calls for a
particular amino acid to be built into a polypeptide chain.
Te RNA molecules acting in protein synthesis are synthesized
on single strands of the DNA template. RNA nucleotides are
joined according to base-pairing rules.
Instructions for making a polypeptide chain are carried from
the DNA to the ribosomes via messenger RNA. Ribosomal RNA
forms part of the protein synthesis sites. A transfer RNA ferries
each amino acid to the ribosome and binds to a codon on the
mRNA strand specifying its amino acid.
Protein synthesis involves (a) transcription, synthesis of a
complementary mRNA, and (b) translation, “reading” of the
mRNA by tRNA and peptide bonding of the amino acids into the
polypeptide chain. Ribosomes coordinate translation.
Other Roles of DNA
Introns and other DNA sequences encode many RNA species
that may interfere with or promote the function of speciﬁc genes.
Degradation of Organelles and Cytosolic Proteins
Organelles and large protein aggregates are picked up by
autosomes and delivered to lysosomes for digestion. Tis process,
autophagy, is very important for keeping the cytoplasm free of
deteriorating organelles and other debris.
binding results in changes in protein structure or function
within the targeted cell.
Te cytoplasm, the cellular region between the nuclear and
plasma membranes, consists of the cytosol (ﬂuid cytoplasmic
environment), inclusions (nonliving nutrient stores, pigment
granules, crystals, etc.), and cytoplasmic organelles.
Te cytoplasm is the major functional area of the cell. Tese
functions are mediated by organelles.
Mitochondria, organelles limited by a double membrane, are sites
of A±P formation. Teir internal enzymes carry out the oxidative
reactions of cellular respiration.
Ribosomes, composed of two subunits containing ribosomal
RNA and proteins, are the sites of protein synthesis. Tey may be
free or attached to membranes.
Te rough endoplasmic reticulum is a ribosome-studded
membrane system. Its cisterns act as sites for protein modiﬁcation.
Its external face acts in phospholipid synthesis. Vesicles pinched oﬀ
from the ER transport the proteins to other cell sites.
Te smooth endoplasmic reticulum synthesizes lipid and
steroid molecules. It also acts in fat metabolism and in drug
detoxiﬁcation. In muscle cells, it is a calcium ion depot.
Te Golgi apparatus is a membranous system close to the nucleus
that packages protein secretions for export, packages enzymes
into lysosomes for cellular use, and modiﬁes proteins destined to
become part of cellular membranes.
Lysosomes are membranous sacs of acid hydrolases packaged by
the Golgi apparatus. Sites of intracellular digestion, they degrade
worn-out organelles and tissues that are no longer useful, and
they release ionic calcium from bone.
Peroxisomes are membranous sacs containing oxidase enzymes
that protect the cell from the destructive eﬀects of free radicals
and other toxic substances by converting them ﬁrst to hydrogen
peroxide and then water.
Te cytoskeleton includes microtubules, intermediate ﬁlaments,
and microﬁlaments. Microtubules organize the cytoskeleton
and are important in intracellular transport. Microﬁlaments are
important in cell motility or movement of cell parts. Motility
functions involve motor proteins. Intermediate ﬁlaments help
cells resist mechanical stress and connect other elements.
Centrioles organize the mitotic spindle and are the bases of cilia
Microvilli are extensions of the plasma membrane that increase
its surface area for absorption.
Te nucleus is the control center of the cell. Most cells have
a single nucleus. Without a nucleus, a cell cannot divide or
synthesize more proteins, and is destined to die.
Te nucleus is surrounded by the nuclear envelope, a double
membrane penetrated by fairly large pores.
Nucleoli are nuclear sites of ribosome subunit synthesis.
Chromatin is a complex network of slender threads containing
histone proteins and DNA. Te chromatin units are called
nucleosomes. When a cell begins to divide, the chromatin coils
and condenses, forming chromosomes.