84 Chapter 3
Summary
2. In chromatin, DNA is wound around regulatory proteins
known as histones to form particles called nucleosomes.
3. Chromatin that is active in directing RNA synthesis is
euchromatin; the highly condensed, inactive chromatin
is heterochromatin.
C. RNA is single-stranded. Four types are produced within the
nucleus: ribosomal RNA, transfer RNA, precursor messen-
ger RNA, and messenger RNA.
D. Active euchromatin directs the synthesis of RNA in a pro-
cess called transcription.
1. The enzyme RNA polymerase causes separation of the
two strands of DNA along the region of the DNA that
constitutes a gene.
2. One of the two separated strands of DNA serves as a
template for the production of RNA. This occurs by
complementary base pairing between the DNA bases
and ribonucleotide bases.
E. The human genome is now known to contain approximately
25,000 genes, while the human proteome consists of about
100,000 proteins.
1. A gene is transcribed into pre-mRNA, which is then
cut and spliced in alternative ways to produce a number
of different mRNA molecules that code for different
proteins.
2. The RNA nucleotide sequences that are spliced
together to make mRNA are called exons; the RNA
nucleotides between them that are removed are known
as introns.
3. Some RNA molecules, known as short interfering RNA
(siRNA), participate in silencing the expression of
mRNA molecules that contain base sequences that are at
least partially complementary to the siRNA.3.4 Protein Synthesis and Secretion 68
A. Messenger RNA leaves the nucleus and attaches to the
ribosomes.
B. Each transfer RNA, with a specific base triplet in its antico-
don, binds to a specific amino acid.
1. As the mRNA moves through the ribosomes,
complementary base pairing between tRNA anticodons
and mRNA codons occurs.
2. As each successive tRNA molecule binds to its
complementary codon, the amino acid it carries is added
to the end of a growing polypeptide chain.
C. Proteins destined for secretion are produced in ribosomes
located on the granular endoplasmic reticulum and enter the
cisternae of this organelle.
D. Secretory proteins move from the granular endoplasmic
reticulum to the Golgi complex.
1. The Golgi complex modifies the proteins it contains,
separates different proteins, and packages them in
vesicles.
2. Secretory vesicles from the Golgi complex fuse with
the plasma membrane and release their products by
exocytosis.3.1 Plasma Membrane and Associated
Structures 51
A. The structure of the plasma membrane is described by a
fluid-mosaic model.
- The membrane is composed predominantly of a double
layer of phospholipids. - The membrane also contains proteins, most of which
span its entire width.
B. Some cells move by extending pseudopods; cilia and flagella
protrude from the cell membrane of some specialized cells.
C. In the process of endocytosis, invaginations of the plasma
membrane allow the cells to take up molecules from the
external environment. - In phagocytosis, the cell extends pseudopods that
eventually fuse together to create a food vacuole;
pinocytosis involves the formation of a narrow furrow in
the membrane, which eventually fuses. - Receptor-mediated endocytosis requires the interaction
of a specific molecule in the extracellular environment
with a specific receptor protein in the cell membrane. - Exocytosis, the reverse of endocytosis, is a process that
allows the cell to secrete its products.
3.2 Cytoplasm and Its Organelles 56
A. Microfilaments and microtubules produce a cytoskeleton
that aids movements of organelles within a cell.
B. Lysosomes contain digestive enzymes and are responsible
for the elimination of structures and molecules within the
cell and for digestion of the contents of phagocytic food
vacuoles.
C. Mitochondria serve as the major sites for energy production
within the cell. They have an outer membrane with a smooth
contour and an inner membrane with infoldings called cristae.
D. Ribosomes are small protein factories composed of ribo-
somal RNA and protein arranged into two subunits.
E. The endoplasmic reticulum is a system of membranous
tubules in the cell.
- The granular endoplasmic reticulum is covered with
ribosomes and is involved in protein synthesis. - The agranular endoplasmic reticulum provides a site
for many enzymatic reactions and, in skeletal muscles,
serves to store Ca^2 1.
F. The Golgi complex is a series of membranous sacs that
receive products from the endoplasmic reticulum, modify
those products, and release the products within vesicles.
3.3 Cell Nucleus and Gene Expression 62
A. The cell nucleus is surrounded by a double-layered nuclear
envelope. At some points, the two layers are fused by nuclear
pore complexes that allow for the passage of molecules.
B. Genetic expression occurs in two stages: transcription (RNA
synthesis) and translation (protein synthesis).
- The DNA in the nucleus is combined with proteins to
form the threadlike material known as chromatin.