Lecture 3b.
Anatomy of the cell – organelles,
cell division, and the central
dogma
Anatomy of the Cell
Organelles
Question

A red blood cell is placed in a solution and a
few minutes later, the cell shrinks (crenates).
What was the tonicity of the solution?
Question

Celery begins to wilt after it has been in the
fridge for a while. You can actually restore
some of its crispness by submersing it in a
solution. What kind of solution would you
use?
Cellular level


Cytology: structure and function of cells
Cell biology
Cell Theory




Cells are smallest unit of life
All cells come from previously existing cells
through cell division
Cells perform all physiological functions
Maintains homeostasis at the level of the cell
which impacts tissues, organs, and systems
Numbers and diversity


Body contains trillions of cells
Also has thousands of different types of cells
(there are hundreds of different types of
neurons alone)
Two general classes of cells
1.
2.
Somatic cells (diploid, 2n) – two copies of
each chromosome
Sex cells (haploid, n) – one copy of each
chromosome
Organelles


Internal cell structures that perform specific
cellular functions
Cytoplasmic organelles:

Membranous


Nonmembranous


Mitochondria, peroxisomes, lysosomes, endoplasmic
reticulum, and Golgi apparatus
Cytoskeleton, centrioles, and ribosomes
Exterior: Plasma membrane
Cytoplasm


Cytoplasm – material between plasma
membrane and the nucleus
Cytosol – largely water with dissolved protein,
salts, sugars, and other solutes
Cytoplasm = cytosol + organelles
Anatomy of a Cell
Nonmembranous:
Cytoskeleton



The “skeleton” of the
cell
Dynamic, elaborate
series of protein rods
running through the
cytosol
Consists of:




Microfilaments
Intermediate filaments
Microtubules
Muscle cells contain
thick filaments
Microfilaments



The smallest in diameter (7nm), most fragile
Made of the protein actin and located in the
periphery of the cell
Attached to the cytoplasmic side of the plasma
membrane


Braces and strengthens
the cell surface
Involved in cell
movement and shape
changes
Intermediate filaments





Intermediate in size (7-11nm)
Tough, insoluble protein fibers with high tensile
strength
The most durable of the cytoskeletal fibers
Several varieties exist (e.g. keratin)
Functions


Provides shape of the cell
Resist pulling forces
Thick filaments



Only found in muscle cells
Made of protein myosin
Interact with actin microfilaments to cause
contraction
Microtubules




Largest of the fibers (25nm)
Dynamic, hollow tubes made of the spherical protein
tubulin
Microtubular array of the cell is near the nucleus
Functions:




Determine the overall shape of the cell and distribution of
organelles
Help move structures in the cell (like highways)
Form the spindle apparatus
Form centrioles and cilia
Microtubules
Figure 3.24c
Centrioles and Cilia are made
of microtubules

Centrioles



Organize mitotic spindle
during mitosis
Form the bases of cilia
and flagella
Cilia



Used to propel material
in one direction across
cell surfaces
Whip-like, motile cellular
extensions on exposed
surfaces of certain cells
Move substances
Motor Molecules



Protein complexes that function in motility
Powered by ATP
Attach to receptors on organelles
Cilia
Figure 3.27b
Cilia
Figure 3.27c
Ribosomes are the organelles
for protein synthesis



Consists of two subunits
made of rRNA and protein
Site of protein synthesis
Two kinds of ribosomes
found in cells


Free ribosomes
synthesize soluble proteins
Fixed ribosomes
synthesize proteins to be
incorporated into
membranes
Membranous organelles
Endoplasmic Reticulum



Interconnected network
of tubes and parallel
membranes enclosing
cisternae
Continuous with the
nuclear membrane
Four major functions





Synthesis
Storage
Transport
Detoxification
Two types of ER


Smooth ER
Rough ER
Rough ER




“Rough” because xternal surface studded
with fixed ribosomes
Synthesizes secreted and integral membrane
proteins and may chemically modify them
Responsible for the synthesis of
phospholipids for cell membranes
Also important for shipment of proteins to the
Golgi apparatus
Endoplasmic Reticulum (ER)
Figure 3.18a, c
Smooth ER



Why is it called smooth?
Responsible for the synthesis and storage of
lipids and carbohydrates
Detoxification of drugs and toxins
Very large and developed
in liver cells. Why?
Stucture  function
Smooth ER

Catalyzes the following reactions in various
organs of the body




In the liver – lipid and cholesterol metabolism,
breakdown of glycogen and, along with the
kidneys, detoxification of drugs
In the testes – synthesis of steroid-based
hormones
In the intestinal cells – absorption, synthesis, and
transport of fats
In skeletal and cardiac muscle – storage and
release of calcium
Golgi Apparatus


Typically contains 5-6
Stacked and flattened
membranous sacs called
cisternae
Functions in modification,
concentration, and
packaging of proteins:



Modifies and packages
secreted proteins
Packages special enzymes
Also renews the cell
membrane (adds lipids)
Golgi Apparatus



Transport vessels from the
ER fuse with the cis face of
the Golgi apparatus
Proteins then pass through
the Golgi apparatus to the
trans face
Secretory vesicles leave the
trans face of the Golgi stack
and move to designated
parts of the cell
Figure 3.20a
Pathways of the Golgi Apparatus
Cisterna
Rough ER
Proteins in cisterna
Phagosome
Membrane
Vesicle
Lysosomes containing acid
hydrolase enzymes
Vesicle incorporated
into plasma membrane
Pathway 3
Coatomer
coat
Golgi
apparatus
Pathway 2
Secretory vesicles
Pathway 1
Plasma membrane
Proteins
Secretion by exocytosis
Extracellular fluid
Figure 3.21
Lysosomes





Spherical membranous bags containing digestive
enzymes
Arise by budding off of Golgi
Digest ingested bacteria, viruses, and toxins
Degrades and recyles nonfunctional organelles
Specializations:


Breakdown glycogen and release thyroid hormones
Secretory lysosomes are found in white blood cells,
immune cells, and melanocytes
Like a “disposal” for large objects from inside and
outside the cell
Lysosome Functions
Endomembrane System
 System


Produce, store, and export biological molecules
Degrade potentially harmful substances
 System

of organelles that function to:
includes:
Nuclear envelope, smooth and rough ER,
lysosomes, vesicles, Golgi apparatus, and the
plasma membrane
Endomembrane System
Figure 3.23
Lysosomal storage diseases

Tay-Sachs
Lack an enzyme (protein) called
hexosaminidase A (hex A) necessary for
breaking down certain fatty substances in
brain cells called gangliosides (see AM
p.29)
Peroxisomes





Membranous sacs containing enzymes that detoxify
harmful or toxic substances (which cells have a lot?)
Breaks down fatty acids and some organic
compounds
Produced by the division of existing peroxisomes
Convert free radicals to hydrogen peroxide (H2O2)
Catalase enzyme coverts H2O2 to H2O and O2
Like a smaller-scale recycler for molecule-sized things
within the cell (cf. lysosome).
Mitochondria


“Powerhouse” of the
cell…makes most of
the cell’s ATP via
aerobic cellular
respiration
Double membrane
structure with shelf-like
cristae
Nucleus - structure





Consists of a double
membrane
Contains nuclear
envelope, nucleoli, and
chromatin
Holds the DNA in the
form of chromosomes
Also has RNA and
enzymes
Has pores for entry/exit
Nucleus - Functions



Functions as the gene-containing control
center of the cell
Contains the genetic library with blueprints for
nearly all cellular proteins
Regulates gene expression: dictates the
kinds and amounts of proteins to be
synthesized
Nucleus - Contents
Nuclear Envelope
 Selectively permeable
double membrane barrier
containing pores
 Outer membrane is
continuous with the rough
ER and is studded with
ribosomes
 Inner membrane is lined
with the nuclear lamina,
which maintains the shape
of the nucleus
 Pores regulates transport of
large molecules into and out
of the nucleus
Nucleolus
 Dark-staining spherical
body (or bodies) within the
nucleus
 Site of ribosome production
Nucleus –
Chromatin



Threadlike strands
of DNA and
histones
Arranged in
fundamental units
called nucleosomes
Form condensed,
barlike bodies of
chromosomes when
the nucleus starts to
divide
Figure 3.29
SUMMARY


Diversity of human cells
Structures and functions of membranous and
nonmembranous organelles
Anatomy of the Cell
Cell Life Cycle - Mitosis
Cell Cycle

Interphase


Growth (G1),
synthesis (S),
growth (G2)
Mitotic phase

Mitosis and
cytokinesis
Figure 3.30
Cell Cycle

Most of a cell’s life is spent in a nondividing
state (interphase)


Cell is preparing to divide or performing its normal
cell functions
During interphase the DNA, is referred to as
chromatin
Interphase

G0 : cells that cease dividing (often permanently)


G1 (gap 1): metabolic activity and vigorous growth



perform specialized cell functions only
organelle duplication, protein synthesis
S (synthesis): DNA replication
G2 (gap 2) : final preparation for division

finishes protein synthesis and centrioles replicate
G0
Cells that are no longer dividing are said to be
in G0.
Q: What cells can you think of that are in G0?
Chromosomes

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
Humans have 23 pairs; one of each pair comes from
Mom, the other comes from Dad
The two chromosomes of each pair are called
homologous chromosomes
Each Chromosome is a long molecule of DNA.
Each contains thousands of genes arranged in a
single file.
Each gene is a segment of DNA
Each gene represents blueprints for a protein
Cell Division - Mitosis



Necessary for growth and maintenance of
organisms
Responsible for humans developing from a
single cell to 75 trillion cells
Mitosis divides duplicated DNA into 2
identical sets of chromosomes:


DNA coils tightly into chromatids
chromatids connect at a centromere
Mitosis

The phases of mitosis are:





Prophase
Metaphase
Anaphase
Telophase
Cytokinesis


Cleavage furrow formed in late anaphase by
contractile ring
Cytoplasm is pinched into two parts just after
mitosis ends
Mitosis - Overview
Prophase



Chromatin condenses into chromosomes
Nucleoli disappear
Centriole pairs separate and the mitotic
spindle is formed
Prophase
Figure 3.32.3
Metaphase

Chromosomes with
sister chromatids
cluster at the middle of
the cell with their
centromeres aligned at
the exact center, or
equator, of the cell
called the metaphase
plate
Anaphase



Centromeres of the
chromosomes split
Motor proteins in
kinetochores pull one of
each sister chromatids
toward poles
At this point, each
chromatid is now called
a chromosome
Telophase and
Cytokinesis




New sets of
chromosomes unwind
into chromatin
New nuclear membrane
is formed from the
rough ER
Nucleoli reappear
Cytokinesis completes
cell division

Cleavage furrow may be
visable as early as late
anaphase
Control of Cell Division




Surface-to-volume ratio of cells
Chemical signals such as growth factors and
hormones
Contact inhibition
Cyclins and cyclin-dependent kinases (Cdks)
complexes
Regulation of cell division

Mitotic Rate and Energy




slower mitotic rate means longer cell life
cell division requires energy (ATP)
Muscle cells, neurons rarely divide
Exposed cells (skin and digestive tract) live
only days or hours
Nucleus Controls Cell
Structure and Function

Direct control through synthesis of:



structural proteins
secretions (environmental response)
Indirect control over metabolism through
enzymes
Anatomy of the Cell
Central Dogma
Topics – Central Dogma


DNA and DNA replication
Protein synthesis



RNA
Transcription (RNA synthesis)
Translation (protein synthesis)
Deoxyribonucleic acid (DNA) function




Contains genes which are functional units
of heredity
Each gene contains the instuctions for how
to make one or more proteins
Exists in the nucleus as chromatin, when
cell prepares to divide the DNA is
replicated and coiled to form a
chromosome (two chromatids)
Always found in the nucleus
DNA structure




The DNA molecule resembles a spiral ladder
called a double Helix (It is double stranded)
Contains alternating sugar/phosphate
backbone attached by covalent bonds and
Nitrogen containing bases (A, T, C, and G)
Monomers of DNA are called nucleotides.
Strands run anti-parallel and have orientation
DNA bases

1.
2.
3.
4.
There are four different
nitrogenous bases;
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)


DNA bases are
complementary:
A-------T
T-------A
G-------C
C-------G
Held together by
hydrogen bonds
Complementarity
means given one
strand, you can always
predict the other
KEY CONCEPT




The nucleus contains chromosomes
Chromosomes contain DNA
DNA stores genetic instructions for proteins
Proteins determine cell structure and function
The Central Dogma

DNA  RNA  Protein
DNA Replication
2. Translation
1. Transcription
DNA Replication





Copies ALL the DNA in a cell in order to
distribute it into two daughter cells during cell
division
Occurs only during “S Phase” of mitosis
Requires the enzyme DNA Polymerase
Splits the two original DNA strands and builds
new complementary DNA strands to make
two complete and identical sets of the genetic
material
DNA NEVER LEAVES THE NUCLEUS
DNA Replication
DNA Replication: Product
DNA Template
DNA Complementary
A------------------------T
T------------------------A
G------------------------C
C------------------------G
A-------------------------T
T-------------------------A
Protein Synthesis: from gene
to protein




DNA serves as master blueprint for protein
synthesis
Genes are segments of DNA carrying
instructions for a polypeptide chain
Triplets of nucleotide bases form the genetic
library
Each triplet specifies coding for an amino
acid
DNA instructions become
proteins in two steps
1. Gene transcribed into mRNA
2. mRNA translated into protein
Protein synthesis requires:




several enzymes
ribosomes
3 types of RNA
Ribonucleic acid (RNA)

Unlike DNA:





Single stranded
Bases are A, C, G, U (instead of T)
Has ribose sugar instead of deoxyribose
Like DNA
 Contains alternating sugar/phosphate backbone
attached by covalent bonds
3 types



mRNA - messenger (translated into protein)
rRNA - ribosomal (makes up most of ribosomes)
tRNA - transfer (helps in translation from mRNA to protein)
2. Transcription

The process of making a single strand of
RNA from the DNA code in a gene




In transcription a complementary RNA strand is
made from the DNA template strand
Only a short portion of the DNA is “copied” into
RNA – that portion is called a gene
Requires the enzyme RNA Polymerase to
build the RNA strand
Finished product called mRNA leaves the
nucleus to be translated into protein in the
cytoplasm
Overview of Transcription
Transcription: Product
DNA
RNA Strand
A------------------U
T------------------A
G------------------C
C------------------G
A------------------U
T------------------A
3. Translation
AKA: Protein Synthesis - the mRNA strand
is “read” by the ribosomes and a strand of
amino acids is made.
 Secreted and integral proteins are made on
the rough ER, those that will stay in the
cytoplasm are made on free ribosomes.
the language of nucleic acids (mRNA) is
“translated” into the language of amino acids
(protein)

How is the language translated?
The Genetic Code
 RNA stores genetic information in sets of
three nucleotides called codons.
 Each codon specifies a particular amino acid
(3 nucleic acid bases = 1 amino acid)
 There are 64 codons and only 20 amino
acids
 An adapter molecule allows mRNA codons to
be read and the proper amino acids to be put
into the growing protein
The “genetic code”
Overview
of Translation
amino acid
anticodon
codon
tRNA
Translation






mRNA moves into the cytoplasm through a nuclear
pore and is bound by a ribosome (free or fixed)
Adapter molecule tRNA delivers amino acids to
ribosome
tRNA is like the translator
Each tRNA has an anticodon that matches and
binds to the codon on the mRNA
1 mRNA codon translates to 1 amino acid
Enzymes in the ribosome join amino acids with
peptide bonds
Resulting protein has specific sequence of amino
acids (Why important?)
Figure 3–13
KEY CONCEPT

Genes:



are functional units of DNA
contain instructions for 1 or more proteins
Protein synthesis requires:



several enzymes
ribosomes
3 types of RNA
Genetic Code


There are 43 = 64 codons and only 20 amino
acids
This means there are more than one codon
for each amino acid. In other words, several
codons specify for the same amino acid.
Question

Why does this redundancy exist in the
genetic code?

What is the consequence of two different
codons coding for the same amino acid?
Mutations

Mutation is a change in the nucleotide
sequence of a gene:


Causes:




can change gene function
exposure to chemicals
exposure to radiation
mistakes during DNA replication
Mutations can lead to cancer
Mutations

Changes in the DNA
May or may not cause a change in the protein
and/or a change in the function of that protein
Can be “silent” mutations

SUMMARY


Structures and functions of DNA, RNA, and
chromosomes
Central Dogma: DNA replication,
transcription, translation