Unit 4: Cell Communication Flashcards
ligand
A molecule (e.g., hormone, neurotransmitter) that binds to a receptor to initiate a response. Signaling molecules could be proteins, small peptides, amino acids, nucleotides, steroids, fatty acid derivatives, nitric oxide, carbon monoxide. The receptor binding with the ligand often causes a conformational change in the receptor, activating downstream pathways.
Receptor
A protein on the cell membrane or within the cell that detects and binds the ligand
Signaling cells
Cells that produce ligands
Target Cells
Cells that receive the signal
Cytoplasmic receptor
Inside the cell, small or nonpolar molecules (e.g., steroids)., Ligand-receptor complex directly influences DNA transcription.
Membrane bound receptor
Embedded in the plasma membrane, Large or polar molecules (e.g., peptides), binding triggers intracellular signaling cascades
Plasmodesmata
Plants construct channels between cells called plasmodesmata that allow ligands to move directly from one cell to another throughout the plant structure. This is juxtacrine signaling.
Autocrine
A cell signals to itself, releasing signals that bind to its own receptors.
Juxtacrine
Requires direct contact between cells. Signal molecules are membrane-bound.
Paracrine
Signals act on nearby cells.
Endocrine
Signals travel through the bloodstream to distant target cells.
Quorum Sensing
A process where bacteria communicate using chemical signals to regulate gene expression based on population density. Paracrine, as the signaling molecules act on nearby bacterial cells. Bacteria release small chemical signals into the environment. As the population of bacteria grows, the concentration of signal molecules increases. Once the concentration of autoinducers reaches a threshold, bacteria detect them via receptors and alter their gene expression collectively. This allows bacteria to coordinate group behaviors like bioluminescence
Step 1: Reception
- Ligand binds to the receptor
- Most receptor proteins are in the cell membrane but some are inside the cell
- Hydrophilic (polar) ligands bind to plasma membrane receptors
- Small or hydrophobic (nonpolar) ligands can pass through the membrane and attach to intracellular receptors (ex. steroid hormones like testosterone)
G-Protein Coupled Receptors
G proteins bind the energy-rich GTP (very similar to ATP- source of energy). GPCR systems are extremely widespread and diverse in their functions. Signaling molecule binds to the receptor. G protein changes conformation an GDP is physically replaced by GTP. G protein diffuses laterally to activate an enzyme. Transduction starts when the enzyme is activated until GTP is hydrolyzed and G protein detaches
Ion Channel Receptors
An ion channel receptor acts as a gate that opens and closes when the receptor changes shape, When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptor. Ex: acetylcholine receptor, a Na+ channel in skeletal muscle cells → muscle contraction
Protein Kinase Receptor
Membrane receptors that transfer phosphate groups from ATP to another protein. Can trigger multiple signal transduction pathways at once. Ex: insulin, cell cycle
Step 2: Transduction
- A series of chemical reactions that link ligand binding to the target cell response. It relays signals.
- The goal is to ensure that the target cells have the appropriate response to a ligand binding
- Transduction can have multiple pathways
- Can amplify a signal (make it larger) by activating multiple copies of the next component in the pathway
- Provide more opportunities for coordination and regulation
- At each step in a pathway, the signal is transduced into a different form, commonly a conformational change in a protein
Secondary Messengers
Secondary messengers are small molecules/ions that relay and amplify signals received by receptors to proteins.
Calcium
Calcium is a common secondary messenger. Ca2+ can function as a second messenger because its concentration in the cytosol is normally much lower than the concentration outside the cell; a small change in the number of calcium ions represents a relatively large percentage change in calcium concentration
Protein kinase
Enzymes that use ATP to add phosphate groups (phosphorylate) intracellular proteins. This leads to structural changes that activate or inactive a protein.
Phosphorylation Cascade
In this process, a series of protein kinases add a phosphate to the next one in line, activating it. This leads to amplification of the signal. Phosphatase enzymes then remove the phosphates
Step 3: Response
There can be many possible responses to a cell signal:
- Activate or inactivate intracellular proteins
- Trigger multiple receptors and different responses
- Example of responses: turn transcription on/off or regulate activity of proteins in cytoplasm
- What can happen if the protein synthesis is altered? Cell behavior is altered - protein production can be altered
what can stop the response from happening?
- no more ligands
- ligand stops binding
- reception happens but something goes wrong in transduction (like an interfering molecule) and response doesn’t happen
How are steroid and protein hormones different?
steroid hormones are hydrophobic while protein hormones are hydrophilic
Second messengers tend to be water-soluble and small. This accounts for their ability to __________
diffuse rapidly through the cytoplasm and amplify the signal efficiently
Homeostasis
Steady state or internal balance. Cells maintain a relatively constant internal environment even when the external environment changes significantly. The internal conditions’ typical state is the set point. Fluctuations in that condition above or below the set point serve as the stimulus. A receptor or sensor detects the stimulus and triggers a response that returns the condition to the set point
Positive feedback
Do more of something. Amplifying a condition that is advantageous for the organism
Vaccines
- a weak or dead form of the germ is introduced
- immune system develops antibodies
- antibodies fight off germ if it invades again
Nerve Signaling
- One way to maintain homeostasis is by nervous system communication
- Its paracrine signalling
- The sending cell sends a message through the axon to the receiving cell. The receiving cell receives it in the synapse (synaptic cleft) where the receptors will receive the neurotransmitters.
- Message travels from dendrites down axon as an action potential. This opens Ca2+ channels. Neurotransmitter vesicles fuse with membrane and release neurotransmitter to synapse → diffusion. Neurotransmitter binds with protein receptor on dendrites of second neuron. Ligand-gated ion channels open and message continues through second neuron.
- Neurotransmitter in synapse degraded or reabsorbed
Negative feedback
Do less of something. Stopping a condition that is detrimental or limiting a condition to specified levels
Immune Signaling
- Another way in which organisms maintain homeostasis is by detecting foreign cells and particles like pathogens and cancer cells.
- Once the pathogen is detected and identified, other systems in the organism’s body can attack the invader, thus keeping the organism healthier.
- Cells of the human immune system are finely tuned to recognize and respond quickly to disease-causing organisms.
- When a cell is infected by a virus, they will send out interferons (signaling proteins) to local cells to help slow down viral spread
- Interferon does different things for different cells:
- Alert your immune system so it can go after the virus or cancer
- Help your immune system recognize the virus or cancer
- Tell immune cells to attack
- Stop virus and cancer cells from growing and dividing
- Help healthy cells fight infection - Antigens and Antibodies: Shape-specific recognition between antigen (surface proteins on foreign pathogens) and antibodies (proteins produced by B cells of immune system). Production of antibodies speeds immune response after primary response
Different neurotransmitters
- Acetylcholine: transmit signal to skeletal muscle
- Epinephrine (adrenaline) & norepinephrine: fight-or-flight response
- Dopamine: widespread in brain, affects sleep, mood, attention & learning, lack of dopamine in brain associated with Parkinson’s disease, excessive dopamine linked to schizophrenia
- Serotonin: widespread in brain, affects sleep, mood, attention & learning
Phases of cell division
Interphase (G1, S, and G2), M-Phase (prophase, metaphase, anaphase, telophase) cytokinesis
Interphase
- Newly divided cells are given the opportunity to grow, maintain normal cell function, and prepare for division.
- Cells grow, replicate DNA, and prepare for division.
- Cells spend 90% of their time in interphase
G1 (Interphase)
- where the cell is growing
- G1 Checkpoint - it’s a crossroad for whether the cell should go through cell division. G1 checkpoint checks: Do we NEED a new cell? Is this particular cell healthy enough to divide? Are there enough resources around to divide? (ATP, proteins, water, etc)
- If the cell does NOT pass G1 Checkpoint: No cell division (G0 phase). Cell just continues normal functions until it dies. A cell can exit the G0 phase and re-enter G1. Must have specific environmental cues. Happens if body needs a new cell (one has died) or if there is an increase in nutrients
- If the cell DOES pass G1 Checkpoint: Cell Division (G1 phase continued). Cell starts to prepare for replication. Makes copies of cell organelles. Cell grows in size. Produces proteins needed for DNA replication and cell division. Moves into the 2nd part of interphase: S phase
S (Interphase)
- copies of DNA are made
- When DNA replicates, the original DNA is connected to its copy. Where they are connected is called a centromere
- During DNA replication, the chromatin condenses into sister chromatids (not immediately, but as the cell prepares for division).
- At this point, DNA is still in chromatin form, but after replication, it will form sister chromatids. It doesn’t yet take the fully condensed chromosome form, which happens later in the cell cycle (during prophase).
G2 (Interphase)
- The cytoplasmic components are doubled in preparation for division
- This is the last phase before cell division
- The cell continues to grow
- G2 checkpoint - Cell checks replicated DNA for any mistakes. If the DNA is wrong, it will try fixing it with DNA proteins . If they can’t fix, the cell will not continue onto mitosis and will kill the cell (apoptosis).
- If the cell does NOT pass G2 Checkpoint: apoptosis. The cell must go through apoptosis because it prevents the cell from dividing and creating a new cell with DNA that differs from the original host.
- If the cell DOES pass G2 Checkpoint: cell goes to mitosis
Prophase (Mitosis)
- Chromatin condenses into chromosomes (Raveled up into sister chromatids)
- Centrioles (in animal cell) move to opposite ends of the cell
- Protein fibers form across the cell
- The nucleolus disappears
- The nuclear membrane breaks down
Metaphase (Mitosis)
- Fibers align double chromosomes across the center of the cell
- Spindle fibers (attached to kinetochores) coordinate movement
M spindle checkpoint
- Spindle fibers pull back and forth on the sister chromatids to line them up perfectly
- Occurs between metaphase and anaphase
- It ensures proper chromosome segregation, making sure that all chromosomes are correctly attached to spindle fibers and aligned before the cell progresses to anaphase.
- If the cell does NOT pass M Checkpoint: apoptosis
- If the cell DOES pass M Checkpoint: cell divides
Anaphase (Mitosis)
- Fibers separate double chromosomes into single chromosomes (chromatids)
- Chromosomes separate at the centromere
- Single chromosomes (chromatids) migrate to opposite sides of the cell
Telophase (Mitosis)
- Nuclear envelope reappears and establishes two separate nuclei
- Each nucleus contains a complete genome
- Chromosomes will begin to uncoil
Cytokinesis
- Separate the cell into two daughter cells, each containing identical genome
- NOT part of mitosis
- in animal cells, cleavage furrow forms and is the indentation that forms in the cell membrane during cell division, eventually splitting the cell into two
- in plant cells, cell plate forms
Cyclins
proteins that control the progression of a cell through the cell cycle
Cyclin-dependent kinases (CDKs)
- kinases that are only active when attached to a cyclin
- Kinases are proteins that activate or inactivate other proteins by phosphorylating them
Cell Cycle Regulation (CDKs and Cyclins)
- The binding of cyclin to CDK changes the shape of CDK such that its active site is exposed (allosteric regulation)
CDK can now phosphorylate target proteins that regulate cell cycle - The concentration of CDKs does not fluctuate
- The concentration of cyclins does
- Certain cyclins are made at certain times during the cell cycle, and their concentration will rise and fall
- Cyclins are destroyed after they are no longer needed by the cell
- CDKs are only active (initiate the next step of the cell cycle) when attached to a cyclin
- The reception, transduction, and response of signals can allow for the activation, inactivation, synthesis, or inhibited synthesis of proteins associated with cell division
Growth factors
- released by some cells and stimulate surrounding cells to divide
- Ex. Platelets release platelet-derived growth factor (PDGF). Fibroblasts (connective tissue) have receptors for PDGF. When PDGF binds to the receptors, a signal transduction pathway stimulates fibroblast division
Density-dependent inhibition
crowded cells stop dividing
Anchorage dependence
to divide, cell must be attached to something
Proto-oncogenes
- normal genes that control cell growth
- Normal function - when proto-oncogenes are activated, it’s a signal for cell division to start
- Mutated - the gene is always activated, so the cell never stops growing
- This ignores the G1 checkpoint that makes sure there is room for the new cells/cell is healthy
- Once mutated, gene is called an oncogene
- Proto-oncogene mutations are dominant; only one copy of the gene needs to be mutated for the effects to be seen in cells.
Tumor-suppressor genes
- Normal function - slowing cell division, repairing DNA mistakes, or apoptosis
- Mutated - Cell does not stop division if mistakes are found
When mutations occur, the cell stops being able to do its job - Tumor suppressor genes are recessive BOTH copies of the gene need to be mutated. Inherited mutation in tumor suppressor genes CAN potentially increase the risk of cancer in future generations, but is NOT a guarantee
p53 gene
- Tumor suppressor gene
- Scientists call the p53 gene the “guardian of the genome.”
- Causes cell cycle arrest to repair DNA then cell cycle can restart or it leads to apoptosis. This prevents cancer from being formed.
- Mutation: The cell cycle continues and if there is bad DNA it is not repaired and the cell cycle is not paused so cells continue to reproduce so cancer can be caused
Cancer
- Unregulated cell division - often results from mutations in proto-oncogenes and tumor suppressor genes
- Cancer cells do not respond normally to the body’s control mechanisms and do not need growth factors to grow and divide:
- They may make their own growth factor
- They may convey a growth factor’s signal without the presence of the growth factor
- They may have an abnormal cell cycle control system
proliferation
the growth of cells
DNA Helicase
unwinds the DNA to make replication fork
DNA ligase
seals the Okazaki fragments on the lagging strand by forming phosphodiester bonds, creating a continuous strand of DNA.
Topoisomerase
relieves the tension and prevents supercoiling in the DNA ahead of the replication fork by making temporary cuts in the DNA strand, allowing it to untwist and then rejoining the strand.
DNA polymerase
adds new nucleotides to the growing DNA strand in the 5’ to 3’ direction, using the original strand as a template. It also has a proofreading function to check for errors during replication. fills any remaining gaps between Okazaki fragments on the lagging strand.
RNA primase
synthesizes short RNA primers on the single-stranded DNA templates, providing a starting point for DNA polymerase to begin adding nucleotides.
RNA primase adds one primer to the leading strand because DNA synthesis is continuous in this case. DNA polymerase can extend the DNA strand continuously in the 5’ to 3’ direction, following the replication fork.
RNA primase adds multiple primers to the lagging strand because DNA synthesis is discontinuous here.
Exonuclease
Exonucleases are enzymes that remove nucleotides one at a time from the ends of DNA or RNA molecules, essential for proofreading, repair, and primer removal.
Okazaki Fragment
Short segments of DNA that are synthesized on the lagging strand during DNA replication. They are separated by RNA primers and later joined together by the enzyme DNA ligase.
purine
Adenine and guanine
pyrmadine
Cytosine, thymine, and uracil
kinetochore
a complex of proteins associated with the centromere of a chromosome during cell division, to which the microtubules of the spindle attach.
difference between steroid hormone and protein hormone
steroid hormones are hydrophobic/nonpolar and protein hormones are hydrophilic/polar
this means that steroid hormone receptors can be in the cytoplasm and protein hormone receptors are outside the cell membrane
Haploid vs Diploid
Haploid cells contain only one set of Chromosomes (n). Diploid, as the name indicates, contains two sets of chromosomes (2n). Haploid cells are formed by the process of meiosis. Diploid cells undergo mitosis.
