bio ssat review -gene expression in prokaryotes and eukaryotes -plant physiology and biodiversity...
TRANSCRIPT
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Bio SSAT Review
-Gene expression in prokaryotes and eukaryotes-Plant physiology and biodiversity (revisited)
-Human body systems: mechanisms of function
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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?
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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
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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)
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The lac operon
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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 beta-galactosidase.
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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)
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The trp operon
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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
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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
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Sclerenchyma
Cell Types in Plants
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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
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Tissue Types
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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
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Apical meristem
Plant Growth
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Plant Growth Primary growth – growth in length
Secondary growth – growth in diameter
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Plant Roots
• Taproot – large, primary root (ex. Carrots, some trees
• Fibrous roots – small, numerous roots; found in many monocots, i.e. grass
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Plant Roots
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Root Structures
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Leaf Structure
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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
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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)
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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
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Plant Reproduction
• Alternation of generations: a haploid gametophyte phase alternates with a diploid sporophyte phase during a plant’s life
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Alternation of Generations
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Plant HormonesHormones 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)
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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
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• 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.
Photoperiodism
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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
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The neuron at rest
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The action potential
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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.
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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.
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Action potential review• A narrated, step-by-step animation with quiz• Another helpful animation
Saltatory conduction speeds up nervous transmission!
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Skeletal muscle organization
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The synapse
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The sarcomere
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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
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Kidney structure
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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
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One last thing…endocrine system hints
• Remember to understand the fundamentals of positive and negative feedback loops.
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