Cell Signaling

Local Signaling

• connect cytosol through plasmodesmata (plant cells) and gap junctions (animals cells)

• synaptic signaling nerves in animal cells

Long distance

• hormones (endocrine signaling)

Signaling pathways are multistep process

• This allows for control and regulation

• Multistep pathways can amplify a signal: A few molecules can produce a large cellular response

• Multistep pathways provide more opportunities for coordination and regulation of the cellular response than simpler systems do

3 Steps of Signaling Pathways

1) Reception

Ligands bind to a receptor on a target molecule

• ligands are water soluble, chemical messengers, and signaling molecules

The binding is highly specific and causes a shape change in the receptor

GPCR

G protein-coupled receptor

Ligand-Gated Ion Channels

Ligand → receptor → open gate → ions → electric signal

2) Transduction

• proteins can activate other proteins

• signal is amplified

• phosphorylation cascade

  • sequence of signaling pathway events where one enzyme phosphorylates another

  • adding phosphate groups to proteins activates them

  • protein kinases transfer phosphates from ATP to proteins

Dephosphorylation - removal of a phosphate group from an organic compound by hydrolysis. It is a reversible post-translational modification

• provide a mechanism for turning off the signal transduction pathway

• make protein kinases available for reuse, enabling the cell to respond to the signal again

Secondary messengers are small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion

• commonly cAMP and calcium ions

3) Response

The goal of signal transduction is to create a response of cellular activity in the cytoplasm or nucleus

Because of this, signaling pathways regulate the synthesis and activity of enzymes and proteins

• utilize transcription factors to transcribe genes

Feedback loops

Positive feedback - the response amplifies the stimulus and moved away from a set point

• Childbirth

  • pressure on the cervix leads to more contractions which lead to more pressure

• Apples

  • ripe apple release ethylene which ripens other apples

Negative feedback - The response maintains homeostasis and stays at a set point

• High heart rate

  • high pulse and high breathing lead to more O2 intake which dampens the stimulus

• Body Temp

  • 

Mitosis

The goal if mitosis is to create genetically identical daughter cells

it occurs in somatic cells to repair tissue and plays a role in growth

it only happens in eukaryotic cells, prokarotic cells divide by binary fission

Interphase

most time of the cell cycle is spent in interphase

G1 is a gap phase

There is a checkpoint at the end to regulate the cell cycle

cyclin starts building up

S is the phase where DNA is replicated

G2 is another gap phase

The g2 checkpoint checks that DNA is fully replicated and not damaged

cyclin accumulation is highest in G2 and binds to Cdk to make MPF which regulates the shift from g2 to M phase

M Phase

M phase includes cell division

Prophase

• The chromosomes start to condense (making them easier to pull apart later on)

• The mitotic spindle begins to form. The spindle is a structure made of microtubules, strong fibers that are part of the cell’s “skeleton.” Its job is to organize the chromosomes and move them around during mitosis. The spindle grows between the centrosomes as they move apart

• The nucleolus (or nucleoli, plural), a part of the nucleus where ribosomes are made, disappears. This is a sign that the nucleus is getting ready to break down

Late Prophase (Prometaphase)

• The chromosomes become even more condensed, so they are very compact

• The nuclear envelope breaks down, releasing the chromosomes

• The mitotic spindle grows more, and some of the microtubules start to “capture” chromosomes

Metaphase

• All the chromosomes align at the metaphase plate (not a physical structure, just a term for the plane where the chromosomes line up)

• At this stage, the two kinetochores of each chromosome should be attached to microtubules from opposite spindle poles

Spindle Checkpoint

• helps ensure that the sister chromatids will split evenly between the two daughter cells when they separate in the next step

• if a chromosome is not properly aligned or attached, the cell will halt division until the problem is fixed

• prevents nondisjunction

Anaphase

• The chromosomes of each pair are pulled towards opposite ends of the cell

• Microtubules not attached to chromosomes elongate and push apart, separating the poles and making the cell longer

Telophase and Cytokinesis

• Two new nuclei form, one for each set of chromosomes. Nuclear membranes and nucleoli reappear

• The chromosomes begin to decondense and return to their “stringy” form

Regulation

Cyclin

when cyclin has built up enough to bind to Cdk, it indicates enough time has passed for the cell to mature

cyclin and Cdk bind to make MPF (maturation promoting factor)

MPF promotes the entrance into mitosis from the G₂ phase

Other features of division and cancer cells

Quorum sensing

• bacteria use chemical signals to measure population density

• Use of chemical messengers by microbes to communicate with other nearby cells and to regulate specific pathways in response to population density (quorum sensing)

Density dependant

• stops cell division when there are already a lot of cells

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• cancer cells experience unregulated cell division and continue dividing

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Anchorage dependence

• The requirement that a cell must be attached to a substratum in order to initiate a cell division