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 beta-galactosidase.

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

Sclerenchyma

Cell Types in Plants

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

Apical meristem

Plant Growth

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 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)

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

• 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

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.