Unit 4- Cell Communication and Cell Cycle

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What is Cell to Cell communication and why is it important?

It’s critical to the function and survival of cells, responsible for growth and development of multicellular organisms. 3 types: Direct Contact, Local Signaling/Local Receptors, Long Distance Signaling

Direct Contact

Communication through Cell junctions. Signaling Substances and other material dissolved in cytoplasm can pass freely between cells. Animal Cells: Gap Junctions Plant Cells: Plasmodesmata

Examples of Direct Contact

Immune Cells: Antigen presenting Cells (APCs) communicate to t cells thru direct contact

Local regulators/Local Signaling

Local regulators: a secreting cell releases chemical messages (local regulators/ligands) that travel a short distance through the extracellular fluid. The chemical messages will cause response in a target cell. 2 types: Paracrine signaling and Synaptic signaling

Paracrine Signaling and Synaptic signaling

Paracrine Signaling: secretor cells release local regulators (ie growth factors) via exocytosis to an adjacent cell)

Synaptic signaling: Occurs in animal nervous systems. Neurons secrete neurotransmitters that diffuse across synaptic cleft (Gap between nerve cell and target cell) (depending on length of neuron can be classified as long distance)

Long Distance Signaling

Animals and plants use hormones for long distance signaling. Plants release hormones that travel in the plant vascular tissue (xylem/phloem) or thru the air to reach target tissues.

Animals use endocrine signaling. Specialized cells release hormones into circulatory system where they reach target cells. Ex: Insulin is released by pancreas, then it circulates and binds to target cells.

Three stages of cell to cell messages

Reception (ligand binds to receptors), Transduction (signal is converted), Response (A cell’s process is altered)

Reception

The detection and receiving of a ligand by receptor in the target cell.

Receptor: macromolecule that binds to a signal molecule(ligand). receptors have an area that interacts with ligand and an area that transmits a signal to another protein. That area is v specific

Two types of receptors

Plasma Membrane receptors: Most common type of receptor, binds to ligands that can’t pass thru PM (polar, large). Ex: G protein coupled receptors, Ligand gated ion channels

Intracellular Receptors: Found in cytoplasm or nucleus of target cell. Binds to ligands that can pass thru PM( nonpolar, small). Ex: Steroid,thyroid hormones, gasses like nitric oxide.

Transduction

Conversion of extracellular signal to intracellular signal that can trigger response. Requires sequence of changes in molecule series called signal transduction pathway

Signal Transduction Pathway and second messengers.

Pathway regulates protein activity thru: Step 1: phosphorylation by Enzyme protein kinase, this relays signal inside cell. Step 2: Dephosphorylation by Enzyme protein phosphatase, this shuts off pathways.

Transduction amplifies signal. Second messengers: small, non protein molecules & Ions help relay/amplify signal. Cyclic AMP (cAMP) is common 2nd messenger

Response

Final molecule in signaling pathway converts signal to response that will alter cellular process. Ex: protein that cal alter membrane permeability, Enzyme that changes metabolic process, or protein that turns genes on/off

Signal Transduction Pathways

Influence how a cell responds to its environment, can result in changes in gene expression and cell function, can alter phenotypes and cause cell death. Interruption to any part of the pathway will result in change to transduction of signal

Two main types of cell membrane receptors

GPCR- G protein coupled receptors

Ion channels

GPCRs How do they work

Largest category of cell surface receptors, animal nervous systems.

Process: Ligand binds to GPCR (gpcr is just receptor!!!), The G protein is then able to bind to the GPCR and phosphorylate GDP to GTP. The activated g protein binds to the enzyme and activates it → amplifies signal and leads to cellular response

Ligand Gated ion channels

Located in PM, Important in nervous system. Receptors act as gate, when ligand activates receptor ions can move through facilitated Diffusion. The ions move along the concentration gradient.

What is Homeostasis?

The state of relatively stable internal conditions. Set points are values for different functions that the body wants to maintain (ex body temp at 98.6 deg) 

How does the body maintain Homeostasis?

The body monitors it’s condition at all times to ensure function. Organisms detect and respond to stimuli: maintain homeostasis thru feedback loops

What are the 2 types of feedback loops? Terminology

2 types: Positive and negative

Stimulus: variable that causes response

Receptor: organ that detects stimuli and sends to brain

Effector: Muscle/gland that responds

Response: meant to inc/Dec effect of stimulus

Negative Feedback

Reduces the effect of a stimulus: Ex: Sweating, Shivering, Blood sugar, breath rate

Positive Feedback

Where the response increases the effect of a stimulus. Blood Clotting, Fruit ripening, Giving Birth

When are cells unable to maintain homeostasis? What happens when it isn’t maintained

Genetic Factors

Drug/Alcohol abuse

Extreme environmental conditions (freezing to death, ect)

When homeostasis is not maintained, cells & tissues develop diseases like diabetes or cancer

What is the cell cycle and why is it important

Cell division is necessary for multicellular organism’s growth , cell reproduction and repair. The cell cycle is the process of the cell from formation to division

Structure of DNA before division/during division

DNA associates with and wraps around proteins known as histones to form nucleosomes.

When cell is not actively dividing: Strings of nucleosomes form chromatid

When cell is ready to divide: After DNA replication, chromatin condenses to form chromosome, densely packed to allow division.

Chromosomes

When chromosomes are replicated, each has duplicated copy. Copies form together to form sister chromatids.

Centromere: Region on each sister chromatid where they are most closely attached

Kinetochore: Proteins attached to the centromere that link each chromatid to mitotic spindle

What is a genome and what form does it take in organisms.

Genome: All genetic material in a cell. 

Prokaryotes: single Circular DNA

Eukaryotes: 1 or more linear chromosomes: Humans have 23 pairs (46 chromosomes)

Homologous Pairs

2 chromosomes that have same length, same centromere position, have genes that code for diff versions of same characteristic. (1 from mom, one from dad), They form one pair, each cell has 23 pairs, therefore 46 chromosomes

Somatic Cells

Body Cells, Diploid (2n) have 2 sets of chromosomes, one from each gamete type. Humans 2n=46, 23 from mom and 23 from dad

Gametes

Haploid, Reproductive Cells (eggs/sperm), 1 set of chromosomes n=23

Cell Cycle’s 2 main stages

Interphase (G1, S, G2), and Mitosis(inc cytokinesis)

Interphase and it’s 3 phases

Longest phase of cell cycle(90%) 

G1(first Gap phase)→ Cell grows and carries out normal functions

S (synthesis phase)→ DNA replication and chromosome duplication occurs. SAME # OF CHROMOSOMES, DOUBLE THE DNA)

G2 (second Gap phase) →Final growth and preparation for mitosis

M phase

Mitosis: Nucleus Divides

Cytokinesis: cytoplasm divides

Results in 2 identical Daughter cells

Prophase

Chromatin condenses to chromosomes, Nucleoli disappear, Duplicated chromosomes appear as sister chromatids. Mitotic spindle begins to form, Centrosomes move away from each other 

Prometaphase

Nuclear Envelope is Destroyed, Microtubules enter nuclear area and attach to kinetochores

Metaphase

Centrosomes line up at opposite poles, Chromosomes line up at metaphase plate (middle), Microtubules are attached to each kinetochore

Anaphase

Sister Chromatids split and move to opposite ends of cells due to microtubule shortening. Cell Elongates

Telophase and Cytokinesis

2 Daughter nuclei form, Nucleoli reform, Chromosomes decondense into chromatin

Cytokinesis occurs: 

Animals: Cleavage furrow occurs due to contractile ring of actin filaments

Plants: Vesicles produced by golgi travel to middle of cell and form cell plate

G1 Checkpoint

Most important checkpoint, checks for cell size, growth factors, and DNA damage. Stop/Go signals→ If smth is wrong, cells go into nondividing (quiescent ) state known as G0. If nothing is wrong, cell can proceed with the rest of the cycle

G0 Phase

Cell purgatory. Some cells stay like this forever, not dividing (muscle/ nerve cells). Cells can be called out of cell purgatory.

G2 Checkpoint

Checks for completion of DNA replication and any DNA damage. If there is DNA damage, cell will attempt to repair itself, if unsuccessful will undergo cell death (apoptosis). If all is good cell will proceed to mitosis

M (Spindle) Checkpoint

Checks for microtubule attachment to kinetochores in Metaphase. If improperly attached→ mitosis will be paused until microtubules are properly attached, if they can’t the cell will js kinda chill and not divide. If everything is good→ proceed to anaphase

Internal Control System in cells

Cyclins: Proteins that signal diff stages of cell cycle. Synthesized and Degraded at various pts throughout cycle.

CDKs: Cyclin Dependent Kinases: Conc is constant throughout cell cycle, only active when specific cyclin is present

What do Cyclins and CDK do?

Each cyclin-CDK has specific effect. When Cyclin activates CDK, it phosphorylates other proteins to trigger signal transduction pathway.

Growth Factors

Hormone released by cells that stimulate cell growth→ Signal transduction pathway is initiated (CDKs are activated which causes cell cycle progression)

Contact (density) Inhibition

Cell surface receptors that recognize contact with other cells (no space to duplicate/grow). Initiates Signal transduction pathway that halts cell cycle in G1 phase

Anchorage dependence

Cells Rely on attachment to other cells or extracellular matrix to divide.

What happens when cell cycle unchecked?

Genetic mutations cause issues w cell cycle regulation → Cells multiply unchecked → cancer cells.

Benign Tumor: Tumor that is restricted to one area and not cancerous yet

Malignant: Cancer cells can spread and proliferate. Metastasis: cancer cells detach from tumor and move around