AP Biology
Notes:
Jones
Unit 3 ~Cell Communication
 Cell Signaling

There are two general types of signal transmission:
1. Intercellular
2. Intracellular

Cell signal transduction pathways affects:
a. Function of an organisms as a whole
b. Overall cellular function
c. Gene expression
d. Protein activity
 Cells may communicate by direct contact; signaling molecules in cytosol pass freely through:
- Animals = gap junctions
- Plants = plasmodesmata
- &/OR –
- Animal cells = cell-cell recognition via cell receptor molecules Local vs. Long Distance
Signaling:

Local signaling (short distance).
1. Paracrine signaling: stimulate nearby target cells to divide.
2. Synaptic signaling (animal nervous system) = chemical signals (neurotransmitters)
created by one nerve cell is sent across the gap (synapse) to a second nerve cell to
stimulate target tissue
Neuron #1
Neuron #2
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
Long distance signaling
- signal travel in blood stream to target tissue.
 2 affects
So, how does it happen?
 3 Stages of Cell Signaling:
1. Reception
2. Transduction
3. Response
 Reception can cause a stimulatory or inhibitory response
Types of Reception & how they lead to Transduction then a response:
1. Receptors embedded in plasma membrane
- Large, polar, charged signal molecules cannot cross membrane
- Attachment of signal causes conformational change of the membrane protein
receptor;
- Change in shape causes cellular response
2. Intracellular receptors located in the cytoplasm &/or nuclear membrane.
- Small, non-polar signal molecules can cross membrane
- Attach to protein receptor inside the cell
Examples of Transduction
a) Single celled organisms = ST pathways influence how the cell responds to environments.
Ex. Yeast cell “sex”
Mating pheromone (signal) triggers mating; gene expression
o Via protein receptor in membrane
b) Multicellular organisms
-
Ligand gated ion channels (e.g. muscle contraction)
-
Phosphorylation cascades = after the ligand attaches, a series of enzymes (kinases)
add a phosphate group to the next protein in the cascade sequence
 Cascading benefits:
1. To amplify the message the signal is delivering.
2. Contribute to the specificity of the response.
…In regulating and coordinating the complexity of the cell’s function.
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Termination of the Signal:
- ST’s are reversible
- Dependent on concentration of signal
- When signal is no longer present, receptor molecules revert back to inactive forms,
 Understanding signaling pathways allows humans to modify and manipulate biological system
and physiology.
- Ex. understanding of human endocrine system (hormones) allowed:
1. The development of birth control methods
2. Medicines that control depression.
3. Blood pressure and metabolism.
- Other examples:
4. Ability to control ripening of fruit
5. Agricultural production (growth hormones)
6. Biofilm control

 Cell Division
All cells arise from pre-existing cells via cell division.
Cell division is a part of the Cell Cycle
 Controlled by chemical signals of a cell’s internal and external environment.
Background:
 Genetic Information levels of organization:
Genome = all of DNA
Chromosome
Gene
-
Somatic cells = diploid: 23 pairs (46) of chromosomes
Reproduce via Mitosis
-
Gametes = haploid: 23 chromosomes
Reproduce via Meiosis
Chromatin = Relaxed DNA
Chromosomes = Condensed DNA

Chromosome anatomy
- Sister chromatids (X2) ~ identical
- Centromere
- Kinetochores

Cell Division (two types)
- Mitosis = process produces to somatic daughter cells identical to original cell.
 Asexual reproduction
 One 2N cell produces two identical 2N cells.
 Purpose = growth, repair, maintenance
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-
Meiosis = produces gametes with half the genetic information as the parent cell.
 Sexual reproduction
 Once 2N cell produces (4 or 1) N cell(s).
 Purpose = making gametes with ½ genetic information.
 Cell Cycle
Stages of the Cell Cycle:
 INTERPHASE – most of cell’s life is spent here consist of (G1, S & G2).

G0 = after a split = cell arrest
-
G1 (1st gap) = first growth phase; organelles duplicate
 G1 check point
-
S (synthesis) = Synthesis phase ~ DNA replication
-
G2 (2nd gap) = second growth phase ~ more growth and prepare for cell division
 G2 check point

MITOSIS or MEIOSIS = division of chromosomes (PMAT)
 M check point (meta/ana)

CYTOKINESIS = division of cytoplasm
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In Prokaryotes:
 Cell division = Binary Fission
Evolutionary relationships:
- The DNA sequences & protein structures that control Binary fission in prokaryotes
are the same in mitosis for eukaryotic cells.
 Control of the Cell Cycle
Molecules of the Cell Cycle Control System:
1.
Protein Kinases – activate/inactivate proteins via phosphorylation
- constant concentration in an inactive form
- Cdk = cyclin-dependent kinases
2.

Cyclins
- Fluctuating concentrations
- Controls protein kinase activation
External control of Cell Cycle:
 Growth factors (GF) = stimulate other cells to divide.
• Triggers signal transduction pathway that allows a cell to pass G1
 Environment
Ex. ~ When the associated cyclin accumulates during G2…
• Cdk = MPF (M-phase promoting factor/maturation promoting factor) triggers cell to pass
G2 checkpoint into the M phase.
 Cell density & size
• Density-dependent inhibition = crowded cells stop dividing.
• Due to not enough supply of GF and nutrients to supply numbers of cells for division.
And…
•
•
If a cell gets too big, not enough nutrients/molecules/ ions can cross plasma
membrane to run cell efficiently (SA:V).
Recalls cell from G0….
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Internal control of Cell Cycle:
• During M phase checkpoint…
All chromosomes must be attached at kinetochores before separation of sister chromatids

 Cancer

Cells keep dividing and can invade other tissues.
- Do not respond to depletion of GF
- Do not respond to density dependency
- Don’t stop at normal check points
 Transformation = normal cells turn into cancer cells.
 Results in a tumor
• benign
• malignant

Metastasis – physical spreading of tumor
 Treatment:
Chemotherapy = drug given through IV (blood) that interferes with the cell cycle.
Radiation = localized radiation waves damage cancer cell DNA (more than normal cells)
 DNA & Replication

Structure of DNA
-
P
pentose
P
+
phosphate
or
+
nitrogenous base
nucleotide
basic unit = nucleotide
-
joined in long strands:
• phosphate attached to 5'C of one sugar is linked to 3'C of next sugar in chain
- so strand has 3' end and 5' end
• two strands joined by H-bonds between N-bases
• strands are joined "antiparallel"
5' end
5' carbon
P
3' carbon
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P
3' end
- the 3' end of one strand is matched
with the 5' end of the other strand
•
•
these are complimentary pairs
strands form a double helix
Replication (S-Phase)
DNA information must be duplicated for
- transmission to new offspring (meiosis)
- formation of new cells during growth (mitosis)
replication is semi-conservative: each strand copies a new partner
- Meselson—Stahl experiment with 15N DNA [Figures 15.7 & 15.8]
The Process of Replication
• requires at least 14 enzymes
• the strands are split apart at specific points
• they are unwound exposing the N-bases
• each strand serves as template for new partner
- a template is like a pattern in reverse (a hand is a template for a glove)
• polymerases: responsible for synthesis of new DNA
• nucleotides joined to matching partners on the split strands
• phosphate bonds are made between the new nucleotides
• replication forks: active sites of replication
• synthesis proceeds in the 3' to 5' direction of template (which is 5’ to 3’ direction on
new strand)
- continuous on one strand (leading strand)
- but must proceed stepwise on the other (lagging strand):
– Okazaki fragments grow into each other
– connected by ligases
• proofreading: addition of nucleotides checked for accuracy
• repair: mismatched bases pairs can be replaced after replication
some details:
• origin of replication: special sites where replication begins
• helicases: unwind DNA helix
• primases: adds short RNA primer forms on each strand
- new DNA is added to RNA primer
- RNA primer is removed and replaced by DNA
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