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Chapter 3
Cell Structure and Genetic Control
3-1
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Chapter 3 Outline
Plasma
Membrane
Cytoplasm & Its Organelles
Gene Expression
DNA Replication
Cell Cycle
3-2
Plasma Membrane
3-3
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Cell
Is
basic unit of structure & function in body
Is highly organized molecular factory
Has 3 main components: plasma membrane,
cytoplasm & organelles
Fig 3.1
3-4
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Plasma Membrane
Surrounds
& gives cell form, is selectively permeable
Formed by a double layer of phospholipids
Which restricts passage of polar compounds
Fig 3.2
3-5
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Plasma Membrane continued
Proteins
customize membranes
Provide structural support
Serve as transporters, enzymes, receptors & identity
markers
Fig 3.2
3-6
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Plasma Membrane continued
Carbohydrates
in form of glycoproteins & glycolipids
are part of outer surface
Impart negative charge to surface
Fig 3.2
3-7
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Bulk Transport
Is
way cells move large molecules & particles across
plasma membrane
Some cells use phagocytosis to take in particulate
matter
E.g. white blood cells & macrophages
Fig 3.3
3-8
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Bulk Transport
Some
cells use endocytosis to take in large
compounds
Membrane invaginates to take in a vesicle of
extracellular substance
Pinocytosis is non-specific intake
3-9
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Bulk Transport
Receptor-mediated
endocytosis uses receptors to take
in specific compounds
Including some viruses
1. plasma
membrane
pit forming
3. vesicle
forming
2. membrane
pouching
inward
4.vesicle
inside cell
Fig 3.4
3-10
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Bulk Transport
Cells
use exocytosis to export products into the
extracellular fluid
Via secretory vesicles
3-11
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Surface Specializations
 Some
epithelial cells have cilia projecting from surface
 Hair-like structures that beat in unison
 E.g. cilia lining respiratory & reproductive tracts
Fig 3.5
3-12
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Surface Specializations
Some
epithelial cells have microvilli on surface to
increase surface area for absorption (Fig 3.6)
Fingerlike structures to expand surface area
3-13
Cytoplasm & Its Organelles
3-14
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Cytoplasm & Cytoskeleton
Cytoplasm
is the jellylike matrix within a cell
Consists of fluidlike cytosol plus organelles
3-15
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Cytoplasm & Cytoskeleton
 Cytoskeleton
is a latticework of
microfilaments & microtubules
filling cytoplasm
 Gives cell its shape & structure
 Forms tracks upon which things
are transported around cell
Fig 3.7
Fig 3.8
3-15
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Organelles
Are
cytoplasmic structures that perform specialized
functions for cells
Fig 3.1
3-16
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Lysosomes
Are
vesicle-like organelles containing digestive
enzymes & matter being digested
Involved in recycling cell components
Involved in programmed cell death
3-17
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Peroxisomes
Are
vesicle-like organelles containing oxidative
enzymes
Involved in detoxification in liver
3-18
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Mitochondria
Are
energy-producing organelles
Believed to have originated from symbiotic bacteria
Fig 3.10
3-19
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Ribosomes
Are
protein factories
Where cell's proteins are synthesized
Composed of 2 rRNA subunits
Fig 3.11
3-20
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Endoplasmic Reticulum (ER)
A system
of membranes specialized for synthesis or
degradation of molecules
Rough ER contains ribosomes for protein synthesis
Smooth ER contains enzymes for steroid synthesis
& inactivation
Fig 3.12
Smooth ER
Rough ER
3-21
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Golgi Complex
Is
a stack of flattened sacs
Vesicles enter from ER, contents are modified, & leave
other side
Lysosomes & secretory vesicles are formed in Golgi
Fig 3.13
3-22
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Nucleus
 Contains
cell's DNA
 Enclosed by a double membrane nuclear envelope
 Outer membrane is continuous with ER
Fig 3.15
3-23
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Nucleus
 Nuclear
pore complexes fuse inner & outer membranes
together
 Small molecules can diffuse through pore
 Proteins, RNA must be actively transported
Fig
3.15
3-24
Gene Expression
3-25
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Gene Expression
Genes
are lengths of DNA that code for synthesis of
RNA
mRNA carries info for how to make a protein
Is transported out of nucleus to ribosomes where
proteins are made
3-26
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Gene Expression continued
Takes
place in 2 stages:
Transcription occurs when DNA sequence in a gene
is turned into a mRNA sequence
Translation occurs when mRNA sequence is used to
make a protein
3-27
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Gene Expression continued
Each
nucleus contains 1 or more dark areas called
nucleoli (Fig 3.14)
These contain genes actively making rRNA
3-28
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Genome & Proteome
Genome
refers to all genes in an individual or in a
species
Proteome refers to all proteins produced by a
genome
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Chromatin
 Is
made of DNA & its associated proteins (=histones)
 Histones are positively charged & form spools around which
negatively charged DNA strands wrap
 Each spool & its DNA is called a nucleosome
Fig 3.16
3-29
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Chromatin continued
Euchromatin
is the part of chromosomes active in
transcription
Light in color
Heterochromatin is highly condensed region where
genes are permanently inactivated
Darker in color
3-30
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Chromatin continued
Fig 3.17
3-31
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RNA Synthesis
One
gene codes for one polypeptide chain
Each gene is several thousand nucleotide pairs long
3-32
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RNA Synthesis continued
For
transcription RNA polymerase binds to a "start"
sequence on DNA & unzips strands
Nearby are promoter regions which regulate levels
of transcription
Transcription factors must bind to promoter to
initiate transcription
3-33
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RNA Synthesis continued
 Only
1 strand of DNA
contains the gene & is
transcribed
 Its bases pair with
complementary RNA
bases to make mRNA
 G pairs with C
 A pairs with U
 RNA polymerase
detaches when hits a
"stop" sequence
Fig 3.18
3-34
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RNA Synthesis continued
Transcription
produces 4 types of RNA:
pre-mRNA - altered in nucleus to form mRNA
mRNA - contains the code for synthesis of a protein
tRNA (transfer RNA) - decodes the info contained in
mRNA
rRNA - forms part of ribosomes
3-35
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RNA Synthesis continued
 Pre-mRNA is
much larger
than mRNA
 Contains non-coding
regions called introns
 Coding regions are called
exons
 In nucleus, introns are
removed & ends of exons
spliced together to
produce final mRNA
Fig 3.19
3-36
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Protein Synthesis
Occurs
1 amino acid at-a-time according to sequence
of base triplets in mRNA
In cytoplasm mRNA attaches to ribosomes forming a
polysome where translation occurs
Fig 3.20
3-37
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Protein Synthesis continued
 Ribosomes
read 3 mRNA bases (= a triplet) at-a-time
 Each triplet is a codon which specifies an amino acid
 Ribosomes translate codons into an amino acid sequence
that becomes a polypeptide chain
3-38
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Protein Synthesis continued
Fig 3.21
3-39
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Protein Synthesis continued
 Translation
of codons is
achieved by tRNA &
enzymes
 tRNA contains 3 loops,
one of which contains an
anticodon
 Which is
complementary to a
specific mRNA codon
 tRNA carries the
amino acid specified
by its anticodon
Fig 3.22
3-40
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Protein Synthesis continued
 In
a ribosome, anticodons of tRNA bind to mRNA codons
 Amino acids on adjacent tRNAs are brought together & linked
enzymatically by peptide bonds
 Polypeptide forms; at end detaches from ribosome
Fig3.23
3-41
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Functions of ER
 Protein
to be secreted is made in ribosomes of rough ER
 Amino acids in leader sequence of newly-made proteins
are attracted to ER membrane
 Causing new protein to enter cisternae of ER
Where leader sequence removed, protein modified
Fig 3.24
3-42
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Functions of Golgi
Secretory
proteins leave ER in vesicles & go to Golgi
In the Golgi complex carbohydrates are added to
make glycoproteins
Vesicles leave Golgi for lysosomes or exocytosis
3-43
DNA Replication
3-44
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DNA Replication
When
cells divide, DNA replicates itself & identical
copies go to 2 daughter cells
3-45
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DNA Replication continued
Helicases
break hydrogen bonds to produce 2 free
strands of DNA
DNA polymerase binds to each strand & makes new
complementary copy of old strand
Using A-T, C-G pairing rules
Thus each copy is composed of 1 new strand & 1
old strand (called semiconservative replication)
Original DNA sequence is preserved
3-46
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DNA Replication continued
Fig 3.26
3-47
Cell Cycle
3-48
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Cell Cycle
Most
cells of body are in interphase--the non-dividing
stage of life cycle
3-49
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Cell Cycle continued
Interphase
is subdivided
into:
G1 - cell performs
normal physiological
roles
S - DNA is replicated
in preparation for
division
G2 - chromatin
condenses prior to
division
Fig 3.27
3-50
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Cyclins
Are
proteins that promote different phases of cell cycle
Overactivity of genes that code for cyclins is
associated with cancer
3-51
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Oncogenes
Are
genes whose mutations are associated with
cancer
Tumor suppressor genes inhibit cancer development
E.g. gene p53 inhibits cyclin activity
Mutations in p53 are associated with cancer
3-52
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Cell Death
Occurs
in 2 ways:
Necrosis occurs when pathological changes kill a
cell
Apoptosis occurs as a normal physiological
response
Also called programmed cell death
3-53
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Mitosis (M phase)
Is
phase of life cycle
when cell divides
Chromosomes are
condensed &
duplicated
Consist of 2
duplicate strands
called chromatids
Which are
connected by a
centromere
Fig 3.28
3-54
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Mitosis (M phase) continued
Consists
of 4 stages: prophase, metaphase,
anaphase, telophase
3-55
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Mitosis (M phase) continued
In
prophase chromosomes become visible distinct
structures
In metaphase chromosomes line up single file along
equator
Positioned there by spindle fibers
In anaphase centromeres split
Spindle fibers pull each chromatid to opposite poles
In telophase cytoplasm is divided (= cytokinesis),
producing 2 daughter cells
3-56
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Mitosis (M phase) continued
Fig 3.29
3-57
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Mitosis (M phase) continued
Fig 3.29
3-58
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Mitosis (M phase) continued
Fig 3.29
3-59
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Role of Centrosome
All
animal cells have a centrosome located near
nucleus in interphase
Contains 2 centrioles
centrioles
Fig 3.30
3-60
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Role of Centrosome continued
Centrosome
is duplicated in G1 if cell is going to divide
Replicates move to opposite poles by metaphase
Microtubules grow from centrosomes to form
spindle fibers
Which attach to centromeres of chromosomes
Spindle fibers pull chromosomes to opposite poles
during anaphase
3-61
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Telomeres
Are
non-coding regions of DNA at ends of
chromosomes
Each time a cell divides, a length of telomere is lost
Because DNA polymerase can’t copy the very end
of DNA strand
When telomere is used up, cell becomes senescent
Believed to represent a molecular clock for aging
That ticks down with each division
3-62
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Telomeres continued
Germinal
& cancer cells can divide indefinitely & do not
age
Have enzyme telomerase which replaces
nucleotides lost from telomere during divisions
3-63
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Meiosis
Is
type of cell division occurring in ovaries & testes to
produce gametes (ova & sperm)
Has 2 divisional sequences--DNA is replicated once &
divided twice
3-64
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Meiosis continued
In
1st division homologous chromosomes pair along
equator of cell rather than singly as in mitosis
1 member of homolog pair is pulled to each pole
This gives each daughter cell 23 different
chromosomes, consisting of 2 chromatids
3-65
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Meiosis continued
In
2nd division each daughter divides, chromosomes
split into 2 chromatids
1 goes to each new daughter cell
Each daughter contains 23 chromosomes
 Rather than 46 like mother cell
 Which is why meiosis is called reduction
division
3-66
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Meiosis continued
Fig 3.33
3-67