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CHAPTER # 3(d)
CELLS:
THE LIVING UNITS
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Cell Cycle
• Defines changes from formation of the cell
until it reproduces
• Includes:
• Interphase
• Cell division (mitotic phase)
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Interphase
• Period from cell formation to cell division
• Nuclear material called chromatin
• Four subphases:
• G1 (gap 1)—vigorous growth and metabolism
• G0—gap phase in cells that permanently
cease dividing
• S (synthetic)—DNA replication
• G2 (gap 2)—preparation for division
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G1 checkpoint
(restriction point)
S
Growth and DNA
synthesis
G1
Growth
M
G2
Growth and final
preparations for
division
G2 checkpoint
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Figure 3.31
Interphase
Centrosomes
(each has 2
centrioles)
Nucleolus
Interphase
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Plasma
membrane
Chromatin
Nuclear
envelope
Figure 3.33
DNA Replication
• DNA helices begin unwinding from the
nucleosomes
• Helicase untwists the double helix and
exposes complementary chains
• The Y-shaped site of replication is the
replication fork
• Each nucleotide strand serves as a template
for building a new complementary strand
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DNA Replication
• DNA polymerase only works in one direction
• Continuous leading strand is synthesized
• Discontinuous lagging strand is synthesized in
segments
• DNA ligase splices together short segments of
discontinuous strand
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DNA Replication
• End result: two DNA molecules formed from
the original
• This process is called semiconservative
replication
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Chromosome
Free nucleotides
DNA polymerase
Old strand acts as a
template for synthesis
of new strand
Leading strand
Old DNA
Helicase unwinds
the double helix and
exposes the bases
Replication
fork
Adenine
Thymine
Cytosine
Guanine
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Two new strands (leading and lagging)
synthesized in opposite directions
Lagging
strand
DNA polymerase Old (template) strand
Figure 3.32
DNA Replication
PLAY
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Animation: DNA Replication
Cell Division
• Mitotic (M) phase of the cell cycle
• Essential for body growth and tissue repair
• Does not occur in most mature cells of
nervous tissue, skeletal muscle, and cardiac
muscle
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Cell Division
•
Includes two distinct events:
1. Mitosis—four stages of nuclear division:
•
Prophase
•
Metaphase
•
Anaphase
•
Telophase
2. Cytokinesis—division of cytoplasm by
cleavage furrow
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G1 checkpoint
(restriction point)
S
Growth and DNA
synthesis
G1
Growth
M
G2
Growth and final
preparations for
division
G2 checkpoint
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Figure 3.31
Cell Division
PLAY
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Animation: Mitosis
Prophase
• Chromosomes become visible, each with two
chromatids joined at a centromere
• Centrosomes separate and migrate toward
opposite poles
• Mitotic spindles and asters form
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Prophase
• Nuclear envelope fragments
• Kinetochore microtubules attach to
kinetochore of centromeres and draw them
toward the equator of the cell
• Polar microtubules assist in forcing the poles
apart
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Early Prophase
Early mitotic
spindle
Aster
Early Prophase
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Chromosome
consisting of two
sister chromatids
Centromere
Figure 3.33
Late Prophase
Polar microtubule
Spindle pole
Fragments
of nuclear
envelope
Kinetochore
Late Prophase
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Kinetochore
microtubule
Figure 3.33
Metaphase
• Centromeres of chromosomes are aligned at
the equator
• This plane midway between the poles is
called the metaphase plate
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Metaphase
Spindle
Metaphase
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Metaphase
plate
Figure 3.33
Anaphase
• Shortest phase
• Centromeres of chromosomes split
simultaneously—each chromatid now
becomes a chromosome
• Chromosomes (V shaped) are pulled toward
poles by motor proteins of kinetochores
• Polar microtubules continue forcing the poles
apart
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Anaphase
Anaphase
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Daughter
chromosomes
Figure 3.33
Telophase
• Begins when chromosome movement stops
• The two sets of chromosomes uncoil to form
chromatin
• New nuclear membrane forms around each
chromatin mass
• Nucleoli reappear
• Spindle disappears
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Cytokinesis
• Begins during late anaphase
• Ring of actin microfilaments contracts to form
a cleavage furrow
• Two daughter cells are pinched apart, each
containing a nucleus identical to the original
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Nuclear
envelope
forming
Nucleolus
forming
Contractile
ring at
cleavage
furrow
Telophase and Cytokinesis
Telophase
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Figure 3.33
Control of Cell Division
• “Go” signals:
• Critical volume of cell when area of membrane
is inadequate for exchange
• Chemicals (e.g., growth factors, hormones,
cyclins, and cyclin-dependent kinases (Cdks))
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Control of Cell Division
• “Stop” signals:
• Contact inhibition
• Growth-inhibiting factors produced by
repressor genes
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Protein Synthesis
• DNA is the master blueprint for protein
synthesis
• Gene: Segment of DNA with blueprint for one
polypeptide
• Triplets of nucleotide bases form genetic
library
• Each triplet specifies coding for an amino acid
PLAY
Animation: DNA and RNA
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Nuclear
envelope
Transcription
RNA Processing
DNA
Pre-mRNA
mRNA
Translation
Nuclear
pores
Ribosome
Polypeptide
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Figure 3.34
Roles of the Three Main Types of RNA
• Messenger RNA (mRNA)
• Carries instructions for building a polypeptide,
from gene in DNA to ribosomes in cytoplasm
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Roles of the Three Main Types of RNA
• Ribosomal RNA (rRNA)
• A structural component of ribosomes that,
along with tRNA, helps translate message
from mRNA
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Roles of the Three Main Types of RNA
• Transfer RNAs (tRNAs)
• Bind to amino acids and pair with bases of
codons of mRNA at ribosome to begin process
of protein synthesis
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