Bio SSAT Review
-Gene expression in prokaryotes and eukaryotes
-Plant physiology and biodiversity (revisited)
-Human body systems: mechanisms of function
Gene expression
• How does a prokaryote “decide” when to
express its genes?
• How does one cell in a multicellular eukaryote
express genes differently from other cells in
the same organism?
• What mechanisms do the cells of a eukaryotic
organism utilize to regulate protein synthesis?
Prokaryotes and the operon model
• The operon:
– Promoter: binding site for RNA polymerase
– Operator: binding site for regulatory protein
– Structural gene: gene to be transcribed and
translated
The lac operon
• Controls production of beta-galactosidase, an
enzyme that bacteria use to break down
lactose
• Bacteria don’t anticipate having lactose in the
environment to use as food, so it normally has
the beta-galactosidase gene “turned off”
• This is an example of an inducible operon
(normally switched off, but can be turned on
when needed)
The lac operon
The lac operon
• Repressor protein made by a separate gene
upstream attaches to operator, preventing RNA
polymerase from reaching the gene.
• When lactose (the inducer) is present, it binds
to the repressor protein, causing a
conformational change.
• The repressor protein can no longer bind to the
operator region, so the gene is now switched
on.
• Protein synthesis occurs to make betagalactosidase.
The trp operon
• Some operons are instead repressible, i.e., they
are normally turned on but can be switched off to
conserve energy/resources
• Bacteria normally need to express the genes to
make tryptophan, an essential amino acid
• The repressor protein is made in an inactive state,
i.e., it cannot bind to the operator
• Only when the co-repressor is present (in this
case, tryptophan), then it binds to the repressor
protein and activates it
• The repressor then binds to the operator and
prevents further expression (turns gene off)
The trp operon
Eukaryotic gene expression
• Summary animation
• Pre-transcriptional regulation:
– Transcription factors, enhancer region
– Euchromatin vs. heterochromatin
• Post-transcriptional:
– Pre-mRNA splicing; introns removed by
spliceosomes, leaving exons to be expressed
– Tagging mRNA for export: 5’ cap and poly-A tail
– mRNA degradation  protein degradation
Cell Types in Plants
•Parenchyma – cells used for metabolic support,
i.e. photosynthesis, water storage, etc.
•Collenchyma – cells used for support; ususally
grouped in strands to support areas of plant that
are still lengthening
•Sclerenchyma – thick, rigid cell walls; used for
support/strength in areas of plant that are no
longer growing
Cell Types in Plants
Sclerenchyma
Tissue Types
• Dermal Tissue – made up of epidermis and cuticle
(outermost layer of cells)
• Ground tissue system – functions in storage,
metabolism, and support (found between dermal
and vascular in non-woody plant parts
• Vascular tissue system – transport and support;
xylem – water, phloem – nutrients
Tissue Types
Plant Growth
• Meristems – regions of continual cell division
– Apical meristems – located in tips of stems and roots,
allow plants to grow in length
– Intercalary meristems – found in some monocots at
bases of leafs and stems (ex. Allow grass leaves to
quickly re-grow after mowing
– Lateral meristems – allow stems and roots to increase
in diameter; vascular cambium produces new xylem
and phloem
Plant Growth
Apical
meristem
Plant Growth
Primary growth – growth in length
Secondary growth – growth in diameter
Plant Roots
• Taproot – large, primary root (ex. Carrots,
some trees
• Fibrous roots – small, numerous roots; found
in many monocots, i.e. grass
Plant Roots
Root Structures
Leaf Structure
Leaf Structure
• Mesophyll – ground tissue (parenchyma cells)
rich in chloroplasts
– Palisade mesophyll - cylindrical cells containing
many chloroplasts; located just below epidermis,
site of most photosynthesis
– Spongy mesophyll – irregularly shaped cells
surrounded by air space; air space allows gases
and water to diffuse in/out of leaf
Leaf Structure
• Stomata – openings on the underside of the
leaf; allow for gas and water exchange
• Guard cells – regulate opening and closing of
stomata to control gas and water exchange
(transpiration)
Stem structure and functions
• Contain xylem and phloem to transport water
and nutrients, respectively
– Sugars, organic compounds, and hormones
transported through phloem
• Provide structure and support for leaves
• Storage of nutrients
Plant Reproduction
• Alternation of generations: a haploid
gametophyte phase alternates with a diploid
sporophyte phase during a plant’s life
Alternation of Generations
Plant Hormones
Hormones are chemical messengers secreted by one cell that
causes a response in another.
Major groups of plant hormones:
Auxins
– Promote cell elongation; contributes to overall plant growth
Cytokinins
– Promote cell division
Gibberellins
– Promote seed germination
Ethylene and Abscisic Acid
– Promote ripening and abscission (detachment of fruit, flowers or
leaves)
Plant Tropisms
Tropisms are movements toward (positive) or away
from (negative) a stimulus in the environment.
Common plant tropisms:
• Phototropism
– Growth response to light
• Gravitropism
– Response to gravity
• Thigmotropism
– Response to contact with a solid
Photoperiodism
• Plants have a wide variety of flowering
strategies involving what time of year they will
flower and, consequently, reproduce. In many
plants, flowering is dependent on the duration
of day and night; this is called
photoperiodism.
Human organ systems:
mechanisms of function
• Organ systems  organs  tissues  cells
• Specialized cell structure determines the
mechanism of function for specific organs
within a system
• Focus on:
– The neuron transmitting an action potential
– The sarcomere and muscle contraction
– The nephron and kidney function
The neuron at rest
The action potential
Steps in an action potential
1. A neuron is at rest (-70 mV).
2. A stimulus opens voltage-gated Na +
channels; Na + moves into the cell as
depolarization begins.
3. As membrane potential reaches the
threshold (-55 mV), more Na + gates open
and depolarization continues.
Steps in an action potential
4. The membrane potential reaches its peak;
Na+ gates close and K + channels open; K +
leaves the cell and repolarization begins.
5. When membrane potential reaches resting
potential, K + channels close.
6. Overshoot creates hyperpolarization; Na + /K +
pump corrects during refractory period.
7. Neuron returns to resting potential.
Action potential review
• A narrated, step-by-step animation with quiz
• Another helpful animation
Saltatory conduction speeds up nervous transmission!
Skeletal muscle organization
The synapse
The sarcomere
The sliding-filament theory
• ACh neurotransmitter binds to receptors on
muscle cell; triggers Ca2+ release
• Ca2+ enters the myofibril, binding to troponin
and exposing the actin binding site
• Myosin heads now free to bind to actin;
power stroke pulls actin over myosin,
shortening the sarcomere
• ATP hydrolysis returns the myosin head to
original position
• Narrated, step-by-step animation with quiz
Kidney structure
Nephron function
• Filtration
– Removing solutes from the
blood to tubule
• Reabsorption
– Moving solutes from
tubule back to blood
• Secretion
– Transporting solutes from
blood into tubule
One last thing…
endocrine system hints
• Remember to understand the fundamentals of
positive and negative feedback loops.