Download Plasma membrane

Chapter 03
Cell Structure
and Genetic
Control
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I. Plasma Membrane and Associated
Structures
Overview


Cells are the basic functional units of the body.
They come in a variety of shapes and sizes.
This diversity reflects their diverse functions.
 Principal parts of cells
a. Plasma membrane – selectively permeable,
gives form, and separates from the external
environment
b. Cytoplasm and organelles – fluid part of cell
and little organs that do the functions
c. Nucleus – contains DNA and directs cell
activities
Cell
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Golgi complex
Secretory
vesicle
Nuclear envelope
Centriole
Mitochondrion
Nucleolus
Lysosome
Chromatin
Plasma membrane
Nucleus
Microtubule
Granular
endoplasmic
reticulum
Agranular
endoplasmic
reticulum
Cytoplasm
(cytosol)
Ribosome
Summary of Cell Components
Structure of the Plasma Membrane

Phospholipid barrier separates the intracellular
and extracellular environments
 Hydrophobic center of the double membrane
restricts the movement of water, water-soluble
molecules, and ions.
 Many substances are selectively allowed to
pass through protein channels.
 Proteins and phospholipids are not trapped in
the membrane but constantly move laterally;
known as the fluid mosaic model.
Plasma Membrane Structure
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Extracellular side
Carbohydrate
Glycoprotein
Glycolipid
Nonpolar
end
Polar
end
Phospholipids
Proteins
Cholesterol
Intracellular side
Membrane Proteins

Integral proteins are embedded into the
membrane.
 Peripheral proteins are attached to just one
side of the membrane.
 Functions:
1) Structural support
2) Transport (channels)
3) Enzymatic control of cell processes
4) Receptors for hormones and other molecules
5) “Self” markers for the immune system
Other components of the membrane

Carbohydrates – attached to lipids (glycolipids)
and to proteins (glycoproteins); serve as
antigens and interactions with regulatory
molecules

Cholesterol – gives flexibility to the membrane
Phagocytosis (cell eating)





Bulk transport of large extracellular substances into the
cell
Some cells, like neutrophils and macrophages, can
perform amoeboid movement by extending pseudopods
to pull the cell forward.
 Relies on the bonding of proteins called integrins with
extracellular proteins
Pseudopods engulf bacteria, dead cells, or other
organic materials and then fuse together to form a food
vacuole.
The food vacuole fuses with a lysosome, and the
material is digested.
Very important for body defense, inflammation, and
apoptosis
Phagocytosis
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Pseudopod
Pseudopods
forming food
vacuole
(a)
(b)
(both): © Kwang Jeon/Visuals Unlimited, Inc.
Endocytosis

Another process for bringing large materials into
the cell
 The plasma membrane furrows inward rather
than extending outward. A small part of the
membrane surrounding the substance pinches
off and is brought in as a vesicle.
a. Pinocytosis (nonspecific)
b. Receptor-mediated endocytosis (specific)
1) Has receptor proteins in the membrane that
will bind to the substance to be brought in
2) Ex – cholesterol, AIDS virus, hepatitis virus
Endocytosis
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Extracellular
Plasma
membrane
(pit forming)
Membrane
pouching
inward
Cytoplasm
(1)
(2)
Extracellular
Cytoplasm
Vesicle
within
cell
Vesicle
(3)
(4)
(all): From M.M. Perry and A.B. Gilbert. Journal of Cell Science
39: 257–272, 1979. Reprinted by permission Company of Biologists, Ltd.
Exocytosis


Process of moving large cellular products
(proteins) out of the cell.
The Golgi apparatus packages proteins into
vesicles that fuse to the plasma membrane, and
the contents spill out of the cell.
Cilia & Flagella

Cilia - tiny, hairlike structures composed of
microtubules that project from the plasma
membrane
a. Primary cilium – most cells have this
nonmotile cilium with “9+0” structure; may
have a sensory function in some cells
b. Motile cilia beat in unison to move
substances through hollow organs.
1) Found in respiratory tract and uterine tubes
2) Have a “9+2” arrangement
Cilia
c. Base of each cilium has a pair of centrioles that
are perpendicular to each other and are parts of
a structure called the centrosome;
 One centriole is parallel to the cilium and
called the basal body
Cilia
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Cilia
(a)
10 µm
(b)
a: © Science Photo Library RF/Getty Images; b: © Biophoto Associates/Photo Researchers, Inc.
0.15 µm
Flagellum



A single whip-like structure that can propel a cell
forward
Composed of microtubules with a “9+2”
arrangement
The sperm is the only cell in the human body
with a flagellum.
Microvilli

Folds in the
plasma
membrane that
increase the
surface area for
chemical
reactions and
rapid diffusion

Examples:
Digestive tracts
and kidney
tubules
Microvilli
Lumen
Junctional
complexes
II. Cytoplasm and Organelles
Cytoplasm & Cytoskeleton

Structures within a cell
 Includes organelles, a fluid called cytosol, the
cytoskeleton, and inclusions.
 Inclusions – stored chemical aggregates such
as glycogen, melanin, and triglycerides
Cytoskeleton
a. Organized system of microtubules and
microfilaments throughout the cytoplasm
b. Proteins of the cytoskeleton are mobile.
c. They organize the intracellular environment and
allow movement of muscle cells and phagocytic
cells.
d. They form the spindle apparatus that pulls
chromosomes apart in mitosis.
e. They also serve as a “railway” system for
vesicles and organelles to move along using
molecular motors of myosin, kinesins, and
dyneins
Cytoskeleton
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Plasma
membrane
Mitochondrion
Polysome
Microtubules
Endoplasmic
reticulum
Microtubule
Ribosome
Microfilaments
Nuclear
envelope
Lysosomes

Organelles filled with digestive enzymes
a. Fuse with vacuoles that contain engulfed
extracellular particles (proteins, lipids,
polysaccharides, viral proteins, bacterium, etc)
b. Primary lysosome: only contains digestive
enzymes
c. Secondary lysosome: contains the partially
digested contents of the food vacuole or wornout organelles
d. Residual body: a lysosome filled with waste,
which can be expelled through exocytosis
Lysosomes

Besides digesting bacteria, lysosomes are also
responsible for:
a. Autophagy – process of digesting worn-out or
damaged organelles and proteins in the cell
b. Apoptosis – programmed cell death. The
lysosome spills its contents, killing the cell.
Peroxisomes




Contain enzymes specific to certain oxidative
reactions
Found in most cells but most numerous in the
liver; often oxidize toxic molecules (such as
alcohol)
Enzymes used to remove hydrogen from a
molecule and transfer it to O2, forming hydrogen
peroxide
Also contain the enzyme catalase, which
converts hydrogen peroxide into water and O2
Mitochondria

Sites of energy production through aerobic cell
respiration
 Structure
a. Have an inner membrane and an outer
membrane separated by an intermembranous
space
b. Inner membrane is folded into cristae to
increase surface area for reactions
c. Central area is fluid and called the matrix
Mitochondria
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Inner mitochondrial
membrane
Outer mitochondrial
membrane
Matrix
Cristae
(a)
(b)
a: Courtesy Keith R. Porter Endowment
Mitochondria

Most cells have mitochondria, and there can be
thousands of mitochondria in a single cell.
 Mitochondria can migrate around the cell and
can make copies of themselves.
a. Have their own DNA, all derived from mother
b. Mutations of mitochondrial DNA may
contribute to aging and disease
Ribosomes





Protein factories of the cell
Messenger RNA takes genetic information to the
ribosome so a protein can be assembled.
Very small; made of 2 subunits of ribosomal
RNA and protein
Found free in the cytoplasm or associated with
the endoplasmic reticulum
Serves as enzymes called ribozymes that are
needed for protein synthesis
Ribosome
Endoplasmic Reticulum (ER)

System of membranous passageways whose
membrane are continuous with the nuclear
membrane
 Rough ER (Granular ER)
a. Has ribosomes embedded on the outer
surface
b. Functions in protein modification
 Smooth ER (Agranular ER)
a. Has many functions, depending on the cell
 Steroid hormone metabolism, detoxification,
Ca2+ storage
Endoplasmic Reticulum
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(a)
Nucleus
Tubule
Membrane
Ribosome
(b)
(c)
a: Courtesy Keith R. Porter Endowment
Golgi Complex (Apparatus)

Consists of stacks of hollow, flattened sacs;
cavities are called cisternae
a. One side receives proteins from the ER
b. These are packaged in vesicles called endosomes,
that bud off to (1) fuse with the plasma membrane
for exocytosis, (2) fuse and become integral
proteins, (3) fuse with lysosomes inside the cell


Proteins are modified within the cisternae based
on the type of protein
Retrograde transport – extracellular proteins
brought in by endocytosis and then through the
Golgi complex to the ER
Golgi Complex (Apparatus)
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Rough
endoplasmic
reticulum
Protein
Plasma membrane
Cisternae
Nucleus
Secretion
Secretory
storage
Ribosomes
Cytoplasm
Golgi complex
(a)
Lysosome
(b)
a: Courtesy of Mark S. Ladinsky and Kathryn D. Howell, University of Colorado
III. Cell Nucleus and Gene
Expression
Cell Nucleus

The nucleus is enclosed by a double membrane
nuclear envelope:
a. Outer membrane continuous with ER
b. Inner membrane often fused to outer by
nuclear pore complexes, which allow
movement of small molecules and RNA
into/out of the nucleus
 Contains genetic materials stored in form of
chromatin, which are highly coiled DNA
Cell Nucleus
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Inner and outer
nuclear membranes
Nucleus
Nucleus
Chromatin
Pore
Nucleolus
Inner
membrane
Outer
membrane
Ribosome
Pore
complex
DNA and Genes




The nucleus contains DNA. A gene is a length
of DNA that codes for a specific protein.
Transcription – the gene on the DNA is copied
as messenger RNA (mRNA), which then leaves
the nucleus to go to the rough ER
Translation – the mRNA is processed at the
ribosomes to assemble the proper amino acid
sequence to form a protein
These two steps can be called Genetic
expression.
Nucleolus


The nucleus also has one or more darker
regions not surrounded by a membrane; these
are called nucleolus.
The nucleolus contain DNA that codes for the
production of ribosomal RNA (rRNA).
Genome and Proteome

The genome is all the genes in a particular
individual or all the genes of a particular species.
a. Researchers believe humans have ~25,000
different genes.
 The proteome is all the proteins that are
produced from the genome.
a. More than 100,000 proteins are produced in
the human body.
Genome and Proteome

How can a gene code for more than one protein?
a. mRNA is altered after transcription by cutting and
splicing different ways
b. Proteins are made of many polypeptide chains
that can associate in different combinations
c. Post-translational modification by:
1) Glycosylation
2) Methylation
3) Phosphorylation
4) Cleavage of larger into smaller units
Chromatin

DNA in the nucleus is packaged with proteins
called histones to form chromatin.
a. Histones are positively charged and will interact
with negatively charged DNA to cause spooling
b. Two turns of DNA around 8 histone proteins
create structure called nucleosomes


Euchromatin: active in transcription, looser;
chemical changes in histones (such as
acetylation) allow molecules access to the DNA
during gene expression.
Heterochromatin: inactive regions, highly
condensed; much of the DNA is inactive.
Chromatin
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Chromosome
Region of
euchromatin
with activated
genes
Nucleosome
DNA
Chromatin and Gene Expression
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Condensed chromatin,
where nucleosomes
are compacted
Acetylation
Acetylation of chromatin
produces a more open
structure
Transcription
factor
Transcription factors
attach to chromatin,
activate genes
(producing RNA)
DNA region to be transcribed
Deacetylation
Deacetylation causes
compaction of chromation,
silencing genetic transcription
RNA Synthesis

Transcription (DNA-directed RNA synthesis)
 Occurs inside the nucleus
 Involves:
1) Start and stop regions at the beginning and
end of the gene
2) Promoters, areas of DNA that are not part
of the gene but tell enzymes involved
where to begin
3) Transcription factors that bind to the
promoter to begin transcription
RNA Synthesis

RNA polymerase, breaks the hydrogen bonds
between the base pairs of DNA and assembles
the appropriate RNA nucleotide


RNA nucleotides pair up to the DNA template
Assembly is complementary.
C-G-A-U- CG-C-T-A-G-T-C-G-G



RNA has uracil instead of thymine.
Forms precursor messenger RNA that detaches
from the DNA template
Only 1 freed DNA strand is transcribed
Types of RNA
a. Precursor messenger RNA (pre-mRNA) – made
directly by transcription
b. Messenger RNA (mRNA) – modified pre-mRNA;
contains the code to make a specific protein
c. Transfer RNA (tRNA) – carries amino acids to
mRNA for translation
d. Ribosomal RNA (rRNA) – along with protein,
forms ribosomes; site of translation; acts as an
enzyme
Pre-mRNA modification

Precursor messenger RNA is altered in the
nucleus before it leaves as messenger RNA
(mRNA).
Introns – portions of the Pre-mRNA that do not
actually code for proteins. These must be
spliced out.



Introns may regulate the expression of the area
that do code for protein
Exons – portions of Pre-mRNA that contain
codes for proteins.
Pre-mRNA modification


Alternative splicing allows one Pre-mRNA to be
used to make multiple proteins.
Exons are joined together by spliceosomes and
snRNPs (snurps) which are small nuclear
ribonucleoproteins, to form mRNA
Messenger RNA Synthesis
and Splicing
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A T
C G
T A
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DNA
DNA (gene)
G C
A
C
T
U
C G
RNA
Introns
A
C G
G C
A
T
U
Transcription
Pre-mRNA
U A
Exon Intron
G C
Exon
Intron Exon
U
A
C G
C
U
T
C G
A
T
G
C
C G
Exons spliced together
G C
A
mRNA
RNA Interference

RNA molecules that don’t code for proteins may
prevent some mRNA molecules from being
translated.
 Two types:
 siRNA (short interfering)
 Formed from double-stranded RNA and cut
by Dicer enzyme into short segments
 miRNA (micro interfering)
 Formed from introns of Pre-mRNA and cut
by Dicer enzyme into short segments
RNA interference

One of the strands of siRNA and miRNA enter a
protein particles called RNA-induced silencing
complex (RISC)
 Can bind to mRNA where it is complementary
 Prevents tRNA from bringing amino acids to
mRNA, so translation is prevented – the gene
is silenced
 RNA interference may help in cancer treatments
as well as other genetic conditions
IV. Protein Synthesis and Secretion
Protein Synthesis




Translation
mRNA attaches to a string of ribosomes to form
a polyribosome.
Codon - a group of three bases on mRNA that
provide a code for an amino acid
The order of the codons gives the order of amino
acids in a polypeptide
Polyribosome
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Ribosomes
Newly synthesized protein
mRNA
© E. Kiselva-D. Fawcett/Visuals Unlimited, Inc.
Triplets,Codons, Amino Acids
Amino Acids
Protein Synthesis
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T
G
A
C
C
G
C
DNA
double
helix
G
C
A
T
T
C
A
T
G
C
G
G
C
C
A
G
G
G
C
G
G
C
Translation
Transcription
DNA coding strand
T
A
C
C
C
G
A
G
G
T
A
G
C
C
G
C
G
T
C
G
T
A
U
G
G
G
C
U
C
C
A
U
C
G
G
C
G
C
A
G
C
A
Messenger RNA
Codon 1
Codon 2
Codon 3
Codon 4
Codon 5
Codon 6
Codon 7
Methionine
Glycine
Serine
Isoleucine
Glycine
Alanine
Alanine
Protein
Transfer RNA (tRNA)

A single strand of RNA
bent into a cloverleaf
shape

One end has the anticodon,
which is three nucleotides
that will be complementary to
the proper codon.
The other end has the
appropriate amino acid
bonded by aminoacyl-tRNA
synthetase enzyme

A
C
C
Amino acidaccepting end
Loop 3
Loop 1
Loop 2
UUA
Anticodon
(a)
Loop 3
Amino acidaccepting end
Loop 1
Loop 2
Anticodon
Formation of a Polypeptide
1.
2.
3.
4.
5.
6.
The mRNA moves through the ribosome, with the proper
tRNA attaching at each codon.
Amino acids attached to the tRNAs form peptide bonds
to each other and disassociate from the tRNA.
tRNAs disassociate from the mRNA as they lose their
amino acids.
This continues until a stop codon is encountered and the
whole complex disassociates.
Interactions between amino acids of the growing
polypeptide chain cause bends and folds into secondary
and tertiary structures
Chaperone proteins aid the formation of correct
interactions and folds
Translation
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Codons
mRNA
3
Anticodons
C
I
E
1
Codons
U
A
C
G
C
G
A
U
U
A
C
G
Next amino acid
6
tRNA
I
H
D
G
tRNA
H
F
G
G
E
F
5
Next amino acid
D
E
2
4
C
B
C
3
A
5
D
2
1
Growing
polypeptide
chain
A
4
B
3
2
1
Ribosome
Functions of the ER and Golgi Complex

Newly formed proteins destined to leave the cell
are made on the rough (granular) ER.
a. The first ~30 amino acids (leader sequence)
are hydrophobic and are attracted to the lipid
portion of the membrane of the granular ER.
b. The growing polypeptide chain enters the
cisternae of the ER.
c. The leader leader sequence is removed and
other portions may be removed or added.
Granular ER Role in Protein Synthesis
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Cytoplasm
Ribosome
Granular endoplasmic
reticulum
Free ribosome
mRNA
Leader
sequence
Leader
sequence
removed
Protein
Carbohydrate
Cisterna of
endoplasmic reticulum
Functions of the ER and Golgi Complex

Secretory proteins are next sent to the Golgi
complex.
a. Proteins may be further modified, including the
production of glycoproteins.
b. Proteins are separated according to
destination.
c. Proteins are packaged and shipped in vesicles
to their destinations.
Protein Degradation

Regulatory proteins are rapidly degraded,
making their effects short-lived. This allows for
greater control of cell functions.
a. In the lysosome, proteins are digested by
proteases.
b. Outside the lysosome, proteins to be
destroyed are bonded by a molecule called
ubiquitin, which marks them for degradation
by a proteasome (large protease complex).
c. Ubiquitin may also tag membrane proteins
and organelles for destruction
V. DNA Synthesis and Cell Division
DNA Replication
1. Before cell division, each DNA molecule must
replicate itself so that one of each copy can be
distributed to the two new cells.
2. Involves two types of enzymes:
a. Helicases break hydrogen bonds between the
DNA strands. This creates a fork in the
double-stranded molecule where nucleotides
can be added to both strands.
DNA Replication Enzymes
b. DNA polymerase attaches complementary
nucleotides to the exposed strand.
 Two new molecules are being made from the
original one, each with half old and half new
DNA.
 This is called semiconservative replication.
DNA Replication
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Region of parental DNA helix.
(Both backbones are light.)
A
T
C
G
A
C
G
G
C
A
T
Region of replication. Parental
DNA is unzipped and new
nucleotides are pairing with
those in parental strands.
A
T
G
C
A
T
A
C
A
Region of completed replication.
Each double helix is composed
of an old parental strand (light
purple) and a new daughter
strand (blue). The two DNA
molecules formed are
identical to the original
DNA helix and to one another.
G
A
T
C
C G
T
C
G
T
T
A
G
C
C
G
A
T
C
G
G
C
G
G
C
The Cell Cycle




Divided into
interphase, mitosis,
and cytokinesis
Interphase is divided
into G1, S, and G2
Mitosis is divided into
Prophase, Metaphase,
Anaphase, and
Telophase
Cytokinesis overlaps
the last parts of mitosis
Mitotic Phase
Mitosis
G2
Final growth and
activity before
mitosis
G1
Centrioles
replicate
S
DNA replication
Interphase
Interphase
a. Lots of RNA synthesis occurring
b. G1 Phase: The cell is performing the functions
characteristic of cells in that tissue.
1) Cyclin D moves the cell quickly through G1.
 Overactivity of the gene for cyclin D has been
implicated in some cancers.
2) p53 is a transcription factor that can stall a
gene at the G1/S checkpoint, repair DNA
damage or promote apoptosis
3) If a cell does not divide, it remains in a
modified G1 phase its whole life.
Interphase
c. S phase: If a cell is
going to divide, it
performs DNA replication
at this time
d. G2 Phase:
Chromosomes start to
condense; consist of two
One (duplicated) chromosome
Centromere
strands called sister
chromatids joined by a
centromere.
Chromatid
Histone
DNA
Interphase
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Chromatin
Nucleolus
Centrosomes
Interphase
• The chromosomes are in an extended form and seen as chromatin in
the electron microscope.
• The nucleus is visible.
(all): © Ed Reschke
Mitosis
1. Prophase: Chromosomes become visible,
nuclear membrane and nucleolus disappear,
centrioles move apart, spindle fibers form
Chromatid pairs
Spindle fibers
Prophase
• The chromosomes are seen to consist of two chromatids joined by a centromere.
• The centrioles move apart toward opposite poles of the cell.
• Spindle fibers are produced and extend from each centrosome.
• The nuclear membrane starts to disappear.
• The nucleolus is no longer visible.
Mitosis
2. Metaphase: Chromosomes line up in the center
of the cell and have attached to spindle fibers
Spindle
fibers
Metaphase
• The chromosomes are lined up at the equator of the cell.
• The spindle fibers from each centriole are attached to the centromeres of the chromosomes.
• The nuclear membrane has disappeared.
Mitosis
3. Anaphase: Centromeres split as the spindle
fibers shorten and pull chromatids to opposite
sides.
Anaphase
• The centromeres split, and the sister chromatids separate as each is
pulled to an opposite pole.
Mitosis
4. Telophase: Cytoplasm is divided (cytokinesis)
and cells separate, new nuclear membrane and
nucleolus appears, chromosomes lengthen
Furrowing
Nucleolus
Telophase
• The chromosomes become longer, thinner, and less distinct.
• New nuclear membranes form.
• The nucleolus reappears.
• Cell division is nearly complete.
Role of the Centrosome


Located near the nucleus of a nondividing cell.
Centrosome contains:
 Two centrioles at the center, which replicate
in interphase and move away from each other
in prophase.
 Pericentriolar materials form around the
centrioles. Spindle fibers form from this and
attach to the centromere of the replicated
sister chromatids
 Other tubules cause the formation of the
cleavage furrow for cytokinesis
Role of the Centrosome


In nondividing cells, the
centrosome migrates to
the plasma membrane
and forms the nonmotile
primary cilium.
In ciliated cells, hundreds
of centrosomes form and
become the basal bodies
of the cilia
(a)
(b)
Telomeres and Cell Division




Telomeres – protective cap on the ends of DNA
that prevents DNA degradation.
Each time DNA is replicated, a little more
telomere is lost, which will cause the cell to
eventually lose its ability to divide.
Damaged telomeres activate p53 which induces
cell cycle arrest, senescence, and apoptosis
Cells that can divide indefinitely, such as those in
bone marrow, have an enzyme called
telomerase that replicates the telomere.
Hypertrophy and Hyperplasia


Hyperplasia: growth due to an increase in the
number of cells; responsible for the growth of
most body regions
Hypertrophy: growth due to an increase in cell
size; responsible for increase in skeletal muscle
size
Cell Death
a. Necrosis: Cell dies pathologically due to
deprivation of blood supply.
b. Apoptosis: Programmed cell death is
performed by enzymes called caspases.
1) Extrinsic: “Death ligands” attach to the cell and
mark it for destruction.
2) Intrinsic: Intercellular signals trigger death due to
DNA damage, cancer, infection, or oxidative
stress.
c. “Knocked-out” mice with their p53 gene removed
are used to study cancer and apoptosis
Meiosis

Homologous Chromosomes
a. Humans have 23 pairs of chromosomes, one
set from each parent. 22 pairs are
autosomes and 1 pair are sex chromosomes
b. Each pair is called homologous
chromosomes, and they have the same
genes on them (but not identical DNA).
 Meiosis – process by which two cell division
steps produce gametes (ova and sperm); only
occurs in the gonads (ovaries and testes)
Homologous Chromosomes
Meiosis I – reduction division
1) Prophase I: Homologous chromosomes pair
up; parts are often swapped in a process
called crossing-over.
2) Metaphase I: Homologous chromosomes
randomly line up in the center of the cell;
maternal and paternal chromosomes are
shuffled.
Crossing-over
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(a) First meiotic prophase
Chromosomes pairing
(b) Crossing-over
Chromosomes crossing-over
Meiosis I
3) Anaphase I: Homologous chromosomes are
pulled apart.
4) Telophase I: Homologous chromosomes are
separated. This results in two daughter cells
with 23 chromosomes each (haploid).
a) This is reduction division since each cell now
has half as many chromosomes.
b) Necessary for sexual reproduction
c) Crossing-over and shuffling of chromosomes
in metaphase 1 result in genetic
recombination and genetic diversity
Meiosis II
1) Proceeds like mitosis with phases prophase II
through telophase II.
2) Sister chromatids line up in the center of the cell.
Centromeres are broken and pulled to opposite
poles.
3) Results in 4 cells with 23 chromosomes each.
Meiosis
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Prophase I
Tetrad
Metaphase I
Anaphase I
Telophase I
Daughter
cell
Daughter
cell
Prophase II
Metaphase II
Anaphase II
Telophase
II
Daughter cells
Daughter cells
Meiosis
Epigenetic Inheritance
1. Silenced genes are passed to daughter cells
during mitosis and meiosis without a change in
DNA base sequence
2. Mechanisms of epigenetic inheritance
a. Posttranslational modifications of histone
proteins
b. Methylation of cytosine bases in DNA that
precede guanine
3. Problems with epigenetic inheritance contribute
to cancer, fragile X syndrome, and systemic
lupus erythematosus