Unit 4: Cell Communication, Cell Cycle, and Regulation

Unit 4 Parts 1+ 2

ligands

chemical signals; used for cells to communicate with one another to relay information necessary for their processes

can be hydrophobic or hydrophilic

autocrine signaling

a cell signaling itself to generate a response

“self signaling”

only type of signaling that is self

steps of autocrine signaling

cell secretes a ligand

• ligand then binds to a receptor on the same cell, triggering a response within the cell

what is an example of autocrine signaling?

a cancer cell

• releases its own growth hormones (ligands) that stimulate the cancer cell to grow and divide

juxtacrine signaling

“juxtaposition” - two opposites

signaling that depends on direct contact between the cell that is sending the ligand and the cell that is receiving and responding to it via a surface receptor

what are examples of juxtacrine signaling?

plasmodesmata in plants

• the ligand travels between channels that connect adjacent cells

antigen-presenting cells in the human immune system

• signal helper T cells through direct cell-to-cell contact

paracrine signaling

cell secretes a ligand that travels a short distance, eliciting an effect on cells in the nearby area

“local regulators” —> only affect cells in the immediate vicinity of the cell that is sending the signals

what is an example of paracrine signaling?

neurotransmitters

• travel the short distance across a synapse to communicate with nearby cells

endocrine signaling

some ligands travel a long distance between the sending and receiving cells

• hormones

hormones

ligands (chemical signals) that travel long distances

what are examples of endocrine signaling?

insulin
* hormone that is produced and released by the pancreas, travels through the circulatory system to trigger responses in cells all over the body

the control of blood glucose levels by insulin and glucagon

• travel long distances in the bloodstream

signal transduction

the process that determines how a cell responds internally to a signal in its environment

• is important for gene expression, cell growth and division, and the release of hormones

begins with a ligand

• the ligand interacts with specific target cells, which respond to the ligands presence

• the binding of the ligand to the cell membrane receptor then triggers a series of chemical reactions inside the cell

hydrophilic ligands

ligands that are not resistant of water

• cannot cross the phospholipid bilayer of the cell membrane and enter the cell

• interact with receptors located on the cell membrane (cell membrane receptors)

hydrophobic ligands

ligands that are resistant to water

• may enter the cell by sliding between the phospholipids of the cell membrane

• bind to intracellular receptors in the cytosol of the cell

• once bound to the intracellular receptor, the ligand can then cross the nuclear membrane and bind to DNA in the nucleus, changing the expression of genes

The Three Major Steps

1. Reception

2. Transduction

3. Response

“Rookies Take Regents”

Reception

ligand binds to a specific receptor on or in the target cell

• receptor may be located on the cell membrane or in the cytosol of the target cell

Receptors contain ligand-specific binding domains (will not bind otherwise)

Upon binding, the receptor undergoes a conformational(shape) change, which triggers the next step in the process on the inside of the cell

examples of receptors

• G-protein-coupled receptors

• receptor tyrosine kinases

Transduction

the series of chemical reactions (triggered by the binding of the ligand to its receptor) that helps the cell choose the appropriate response

• often the most complicated part of signal transduction

possible components of transduction

• signal amplification

• kinases

• phosphatases

• enzymes —> produce secondary messengers

  • ex: adenylyl cyclase —> produces the secondary messenger cyclic AMP (cAMP) from ATP

signal amplification

signaling cascades, a series of chemical reactions in which one molecule activates multiple molecules, amplifying the cell’s response to a signal

kinases

transfer phosphate groups to other molecules (which activate those molecules)

phosphatases

remove phosphate groups from other molecules(which inactivate those molecules)

Response

the final step of signal transduction; the ultimate result generated by the ligand

examples:

• the activation of genes by steroid hormones

• opening of ligand-gated ion channels

• the initiation of cell processes(apoptosis)

signal transduction pathway

the series of chemical reactions that mediate the sensing and processing of stimuli

• disruptions have a profound effect on cells

may be disrupted when molecules in the environment interfere with a ligand’s ability to bind to its receptor (ex: the cholera toxin and the G-protein-coupled receptors)

a mutation in a gene that is coding for a receptor protein could result in…

…a change in shape of the receptor such that it would no longer bind to its specific ligand

• without a functional receptor for the ligand, the cell with the mutated receptor protein would no longer be able to respond to the ligand

examples of disorders caused by mutations in receptor proteins

• androgen insensitivity syndrome (AIS)

• nephrogenic diabetes insipidus( NDI)

mutations in the gene for adenylyl cyclase…

…can interfere with a cell’s ability to produce the secondary messenger cAMP, disrupting all steps in the signal transduction process that are dependent on that secondary messenger

• any disruption to any step in the process affects not only that step but also the subsequent steps in the process

feedback mechanisms

important because they help living organisms respond to changes in the environment while maintaining the internal environment of the cell

negative feedback

“returns system to homeostasis”

• returns a system to its original condition and helps maintain homeostasis

ex: person is too hot —> cell signaling processes trigger skin cells to release sweat —> body is cooled down and returns to normal body temperature

positive feedback

“increases the deviation from homeostasis”

magnification of cell process until end result is achieved

• example: hormone oxytocin stimulates contractions of the uterine muscles in labor contractions during childbirth

  • the contraction of the uterine muscles triggers the production of even more oxytocin, which in turn increases the contractions of the uterine muscles

    • causes labor contractions to amplify, getting strong and stronger during childbirth

why the cell cycle is important

helps the growth, repair, and reproduction of cells in living organisms

• controlling the rate of the cell cycle ensures that these processes occur in a timely manner while also preventing the development of uncontrolled cell growth or tumors

phases of the cell cycle

“I Miss Cat”

interphase, mitosis, cytokinesis

• nondividing cells will leave the cell cycle and enter a stage called G0

“I Grabbed(1) Shrek’s Goodies(2)”

Interphase

the longest phase of the cycle

• the cell grows so that it has enough material to divide between two daughter cells; the cell also replicates its genetic material (DNA) during this phase

The three sequential stages of interphase

1. G1 - the cell grows and prepares for the replication of DNA, and some cellular organelles (such as centrioles) are replicated

2. S - DNA is replicated

• when it begins, each chromosome consists of one chromatid

• after DNA replication is completed, each chromosome has two identical chromatids held together by one centromere

• at the end of the S stage, the cell contains twice the amount of DNA it had at the end of G1, but the same number of chromosomes

3. G2 - the cell continues to grow and prepares the materials needed for mitosis, such as the proteins that will make up the spindle fibers

mitosis

the process that makes sure there is an accurate transfer of a parent cell’s complete genome to each of the two resulting daughter cells

• four stages: prophase, metaphase, anaphase, telophase

“Musa

Probably

Misses

Alastar

Too

“

prophase

the first stage of mitosis

• the nuclear membrane dissolves and the chromosomes condense and become visible

• spindle fibers begin to form, and centrosomes move to opposite poles of the cell

metaphase

the second stage of mitosis

• spindle fibers have fully attached to the centromeres of each chromosome

• chromosomes are then aligned along the “equator” of the cell in a single column

• the center of the mitotic spindle is called the metaphase plate

anaphase

the third stage of mitosis

• each chromosome splits at its centromere as opposing spindle fibers begin to shorten

• the identical chromatids are pulled toward opposite ends of the cell

• each chromatid now has its own centromere and is considered a separate chromosome

• the cell has twice the number of chromosomes that it had at the start of the cell cycle

telophase

the final stage of mitosis

• two new nuclear membranes form

• each of the two nuclei now contain the same number of chromosomes and the same genetic information as the parent cell

cytokinesis

the division of the cytoplasm along with all of its cellular contents, between the two daughter cells

• occurs after mitosis

cytokinesis in animal cells

a cleavage furrow is formed, which partitions the cytosol and its contents between the two new cells

cytokinesis in plant cells

a cell plate is built within the dividing cell, providing new cell wall material for each daughter cell

nondividing cells

some cells stop dividing either temporarily or permanently

• may stop when they reach their mature, fully differentiated state or when environmental conditions are not favorable for continued growth

exit the cell and are in G0

G0

where most cells are during the cell cycle

cells may enter at any point int he cell cycle and may reenter the cell cycle if stimulated to do so by appropriate molecular signals

checkpoints

in the Cell Cycle; regulate it

controlled by the interactions between cyclins and cyclin-dependent kinases

cyclin-dependent kinases

present at constant levels throughout the cell cycle

• add phosphate groups to other molecules, activating those molecules

• themselves are inactive until they are bound to cyclin proteins

levels of proteins vary during the cell cycle, reaching their maximum just before mitosis starts

mitosis-promoting factor

a complex formed when cyclins are bound to cyclin-dependent kinases

• triggers the start of mitosis

somatic body cells

cells that are not involved with sexual reproduction

• division of cells are regulated by density-dependent inhibition

• exhibit anchorage dependence: cells need to be attached toa surface in order to divide

proto-oncogenes

genes that propel cell division at a specific rate, much as an accelerator propels a car

• necessary for regulated and controlled cell growth

• are said to function in a dominant way

oncogenes

mutated proto-oncogenes; promote abnormally high rates of cell division

• acts similar to how an accelerator stuck in the down position would cause a car to go too fast

• can cause tumors to form when cell division occurs too quickly and too often without regard for the neighboring cells

a mutation in a single allele of a proto-oncogene can cause…

…a cell to grow out of control and can cause a tumor to form

tumor suppressor genes

code for proteins that detect mutation in cells that may cause tumors to develop

• function much like the brakes on a car, preventing cell division from occurring at an abnormally fast rate

• of a single mutation in a ____ gene allele occurs, the cell will still possess one remaining unmutated ___ suppressor allele that is functional

  • the not mutated allele will help the organism identify cells that are dividing at a rate that is too fast

• if both are mutated, however, the growth of a tumar may occur

• function in a recessive way

apoptosis

programmed cell death; sometimes necessary for the continued function of an organism

ex: when a cell acquires a mutation that could cause cancer

benign tumor

cells are abnormal, but not considered cancerous (yet)

• cells remain at only the tumor site and are unable to spread elsewhere in the body

malignant tumor

mass of cancerous cells that lose their anchorage dependency and can leave the tumor site

metastasis

when cells separate from the tumor and spread elsewhere in the body