Figure 7.11 Model for the action of RNA polymerases during
transcription. The RNA polymerase holoenzyme attaches to
DNA at promoter sites as a consequence of the action of numerous promoter-binding transcription factors. The duplex is
by the ribosome. A specific sequence context surrounding the initiator
AUG aids ribosomal discrimination. This context is A/GCCA/GCCAUGA/G
in most mRNAs (where A/G means either nucleotide at that position).
7.5.3 Eukaryotic initiatio
or green) is hybridized in greater amount to a given DNA sequence.
Computer-generated values for color intensity then allow for accurate assessment of expression level of thousands of genes simultaneously.
Collection of gene-specific
placenta, essentially able to regenerate a complete new embryo and full
8.3.5 Current and potential stem cell uses
and points of controversy
Embryonic stem (ES) cells from mice were first successfully grown
in the laboratory i
Cell Division and Growth
Flow cytometry and fluorescence-activated cell sorting (FACS).
Individual cells in droplets are passed through a laser beam, exciting fluorescence emission. Photodetectors are used to analyze the emission and
light scatter, an
Cell Division and Growth
Cell cycle checkpoints. The cell cycle contains numerous checkpoints to maintain normal cell cycle progress and ensure production of two healthy daughter cells. For
details, see Sec. 8.2.3.
cells are exposed to the
This enzyme is much less abundant than pol I; however, its activity is
nearly 100 times that of pol I.
There have been five distinct eukaryotic DNA polymerases identified: a, b, g, d, and e. The identity of the individual polymerase
Figure 7.10 Initiation of transcription. Shown are the functions of transcription factors (TFIIATFIIH, and others) that bind at the promoter (TATA
box) and direct the RNA polymerase II to sites of active transcription.
Tissue stem cells (traditional view)
The historical view of a stem cell, particularly for a tissue, is that it
resides within a tissue as a partially differentiated cell. For example,
the hematopoietic (blood-forming) stem cell in
CLINICAL BOX 7.1
Chemotherapy Targeting Replication
The class of compounds that have been used the longest in chemotherapy
as anticancer drugs are the alkylating agents. The major alkylating agents
are derived from nitrogenous mustards t
being limited to forming more cells from their tissue of origin. However,
numerous publications have been overturning this dogma and showing
that such postnatal tissue stem cells have a much greater flexibility than
previously thought. S
also be perceived as damaged DNA, activating the DNA damage checkpoint
mechanism and stopping replication and cell growth.
Cell cycle breakdown
Another aspect of cellular senescence involves the cell cycle regulatory
proteins. A se
2!-5!-oligoadenylate synthetase, an enzyme that generates the compound
pppA(2!p5!A)n (where n refers to multiple 2! to 5! linked adenosines),
which in turn activates a preexisting ribonuclease, RNase L. RNase L
degrades all classes o
Posttranscriptional RNA processing. Shown are the
sequences at the ends of exons and within the introns that direct
accurate and efficient splicing. (Reproduced with permission from
Garland Science, Taylor and Francis Group. Molecular Bi
Model for the looping of the lagging strand through the
replisome. This allows for simultaneous replication of DNA on
both the leading and lagging strands. For details, see Sec. 7.3.1.
(Reproduced with permission from Garland Science, Ta
CLINICAL BOX 8.1
Separating Chromosomes Incorrectly: Nondisjunction
Proper attachment of spindle fibers to chromosome kinetochores is necessary for later separation of chromosomes equally between the two daughter cells. If spindle fibers
Activity of DNA ligase during DNA replication at a nick in the DNA
backbone. DNA ligase uses the energy of ATP hydrolysis to generate the phosphodiester bond at nicks in the DNA strand. The 5! end of the nick is activated
first by at
The taxanes are another class of plant-derived compounds that act via
interference with microtubule function. These compounds are isolated from
the Pacific yew tree, Taxus brevifolia, and include paclitaxel (Taxol) and docetaxel (Tax
working with cells, it is often useful to be able to accumulate an entire
population of cells at one point in the cell cycle to study the characteristics of that phase or the progression of cells through the cycle.
Isolation of a populat
7.5.4 Specific steps in translational
Initiation of translation requires four specific steps:
1. A ribosome must dissociate into its 40S and 60S subunits.
2. A ternary complex termed the preinitiation complex is formed consist
Figure 7.15 Activation and attachment of amino acids to tRNAs
during translation. (Reproduced with permission from Garland
Science, Taylor and Francis Group. Molecular Biology of the Cell
by Alberts, B. et al., 4th ed., 2002.)
Cell Division and Growth
The cell cycle. The four phases of the
eukaryotic cell cycle are G1, S, G2, (collectively termed
interphase), and M (mitosis). DNA synthesis occurs
during S phase, and cell division occurs at the end of
M phase. G0 is consider
which binds and hydrolyzes ATP and exhibits RNA helicase activity.
Unwinding of mRNA secondary structure is necessary to allow access
of the ribosomal subunits. The third protein, eIF-4G, aids in binding of
the mRNA to the 43S preini
Action of polyadenylate polymerase complex.
The enzyme complex adds the poly(A) tail (A)n to mRNAs
during posttranscriptional RNA processing. (Reproduced
with permission from Garland Science, Taylor and Francis
Group. Molecular Biolo
Incorporation of deoxynucleoside triphosphates
(dNTPs) during DNA replication. Each newly incorporated
nucleotide in DNA is attached to the 3!-OH of the preceding
nucleotide in the elongating DNA strand. Upon incorporation, the terminal
Growth of a human being from a single-celled zygote into the complex
system of approximately 100 trillion cells of an adult requires an obvious increase in cell number for growth of the organism. But this multiplication of cells must tak
present throughout a cell cycle. However, they must be complexed with
a cyclin for their activity. The cyclins are regulatory and targeting proteins for the cdks. When a particular cyclin binds to a cdk, it activates
the cdk and also tar
In mature sperm nuclei, the DNA is highly compact. Lying at the
head of the sperm cell is the acrosomal vesicle, which is a modified
secretory vesicle (see Fig. 9.2). The acrosome is loaded with enzymes
that digest complex carbohydrates a
Model of a replication fork. The model shows the direction of newly synthesized DNA with associated major activities of
replication. For details, see Sec. 7.3.1. (Reproduced with permission from Garland Science, Taylor and Francis Gr