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Fundamentals of Cell Biology
Chapter 7: The Nucleus and DNA
Replication
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Chapter foci:
Examine the anatomy of the nucleus
Introduction to the protein complexes necessary for
building, replicating, and maintaining the structureof DNA
Discussion of mitosis and the importance of the
cytoskeleton in regulating mitosis
Chapter Summary: The Big Picture (1)
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Chapter Summary: The Big Picture (2)
Section topics:
The nucleus contains and protects most of a
eukaryotic cells DNA
DNA replication is a complex, tightly-regulatedprocess
Mitosis separates replicated chromosomes
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The nucleus contains and protects most of
a eukaryotic cells DNA
Key Concepts:
The nucleus is a highly-specialized organelle
committed primarily to protecting, copying, and
transcribing DNA.
The interior of the nucleus is highly
compartmentalized.
DNA copying, plus transcribing and splicing of RNA,are accomplished by large, highly-specialized
molecular complexes.
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The nuclear envelope is a double
membrane structure
the nuclear envelope encloses the
nucleoplasm
outer membrane of the nuclear
envelope is continuous with the ER
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Nuclear pore complexes regulate
molecular traffic into and out of the nucleus
layers of rings stacked
on top of one another
that span the nuclear
membranes, linked tofilamentous protein
fibrils to form a basket
structure
structure undergoescomplex conformational
changes when it
transports material into
and out of the nucleus
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The interior of the nucleus is highly
organized and contains subcompartments
nucleolus contains DNA that
encodes ribosomal RNAs
nucleoli are sites of high
transcriptional activity forrRNA genes
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The nuclear matrix helps to organize
chromosomes
chromosomes are
compartmentalized into
regions called chromosome
territories nuclear matrix helps control
the shape of chromosomes
and regulate
heterochromatin andeuchromatin
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DNA replication occurs at sites called
replication factories
DNA replication factories form large complexes in the
nucleus devoted to copying DNA with 100%
accuracy and no breaks
Replisome is the smallest functional unit in thefactories and are responsible for copying one
segment of DNA
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RNA polymerase complexes and
spliceosomes are distinct structures within
the nucleus RNA polymerase complexes are responsible for
transcribing the DNA sequence in genes into
mRNA, rRNA, tRNA, and other RNAs
Spliceosomes are responsible for splicing the
newly synthesized RNAs into their mature form
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DNA replication is a complex, tightly-
regulated process Key Concepts (1):
DNA replication in all organisms is performed by a
small number of highly-conserved proteins.
Both prokaryotes and eukaryotes express differentforms of DNA polymerase, the enzyme responsible
for synthesizing DNA.
DNA replication begins at specific sites called origins
of replication. During replication, double-stranded DNA is unwound
and dissociated into single strands that serve as
templates for synthesis of complementary DNA
strands.
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Key Concepts (2):
DNA synthesis occurs only in the 5-to-3direction.
DNA polymerases must bind a double-stranded portion of
a DNA molecule to begin synthesis. Most often, the double
strand consists of the template DNA strand and a short,
complementary RNA primer.
DNA ligase connects individual pieces of newly
synthesized DNA to form a complete strand.
The enzyme telomerase adds extra DNA to the ends of
chromosomes to protect them from degradation.
DNA replication is a complex, tightly-
regulated process
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DNA polymerases are enzymes that
replicate DNA
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DNA polymerases
DNA polymerases add
deoxyribonucleotides to
the 3end of DNA strand
DNA polymerasesproofread their work
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DNA replication is semi-discontinuous
DNA replication begins
at sites on
chromosomes called
origins of replication During replication,
specialized proteins
unwind and separate
the two strands to forma replication fork
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The replication complex
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DNA replication is semi-discontinuous
DNA replication requiresan RNA primer
leading/lagging strand
Okazaki fragments DNA ligases join
fragments of single-
stranded DNA
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Replication of DNA at the end of
chromosomes requires additional steps
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Cells have two main DNA repair
mechanisms
Excision repair systems
Mismatch repair
Recombination repair
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Key Concepts: The function of mitosis is to safely separate
replicated chromosomes into two daughter cells.
Mitosis is divided into five phases, based largely onmorphological changes in the location and
arrangement of chromosomes.
The microtubule cytoskeleton, including microtubule
motor proteins, is essential for proper segregation ofchromosomes.
The actin cytoskeletonis required for the actual
division of one cell into two daughter cells following
mitosis.
Mitosis Separates Replicated
Chromosomes
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Mitosis is divided into stages
1879 - Walther Flemming
described the motion of what
he saw under microscope
asthreads
(Greek, mitos)
moving in an actively
dividing cell
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Prophase prepares the cell for division
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Motors contribute to the formation of
the mature spindle in prophase
Dynein motor proteins
Kinesin-related motor proteins
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Chromosomes attach to the mitotic
spindle during prometaphase
Kinetochores attach chromosomes to the mitoticspindle
A i l f th h t th
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Arrival of the chromosomes at the
spindle equator signals the beginning
of metaphase Metaphase plate = spindle equator
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Separation of chromatids at the metaphase
plate occurs during anaphase
The onset of anaphase requires dissolving the
connections between sister chromatids
APC
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Separation of chromatids at the metaphase
plate occurs during anaphase
Anaphase is subdivided into two phases:
anaphase A
anaphase B
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Telophase - Cytokinesis
The structural rearrangements that occur inprophase begin to reverse during telophase
Cytokinesis completes mitosis by partitioning the
cytoplasm to form two new daughter cells
Fragmentation of non-nuclear organelles
ensures their equal distribution in the daughter
cells