Cell Communication

• Types of Signaling

  • Local Signaling: Nearby cells communicate through local regulators (e.g., paracrine signaling) or synaptic signaling (neurons).

  • Long-Distance Signaling: Hormones travel through the bloodstream to target distant cells, often interacting with specific receptors to avoid entering non-target cells.

• Three Stages of Cell Communication

  • Reception: Detection of a signaling molecule (ligand) by a cell’s receptor.

  • Transduction: A series of steps where the signal is amplified, often through a cascade, to generate a specific response.

  • Response: The cell's reaction to the signal, such as altering gene expression or enzyme activity.

• Types of Receptors

  • G Protein-Coupled Receptors: Work with G proteins to activate enzymes that trigger cellular responses (e.g., adrenaline response).

  • Ligand-Gated Ion Channels: These open or close in response to a ligand binding, allowing ions to flow in/out of the cell.

  • Transcription Factors: Proteins that control which genes are turned on or off in response to signals.

• Phosphorylation and Dephosphorylation

  • Protein Kinases: Enzymes that add phosphate groups to proteins, activating them in processes like phosphorylation cascades.

  • Protein Phosphatases: Enzymes that remove phosphate groups, deactivating proteins.

• Second Messengers

  • cAMP (cyclic AMP): A common second messenger that quickly spreads through diffusion and is involved in adrenaline signaling.

Feedback Mechanisms

• Negative Feedback: The body reduces or stops a process in response to a stimulus.

  • Example: The pancreas releases insulin to lower blood glucose when levels are high.

• Positive Feedback: The body amplifies a response to a stimulus.

  • Example: Oxytocin promotes uterine contractions during childbirth, which in turn triggers more oxytocin release.

Endocrine System

• Endocrine Glands: Specialized organs (e.g., pituitary, thyroid, pancreas) that release hormones to regulate various functions.

• Types of Hormones

  • Peptide Hormones: Water-soluble and bind to receptors on the cell membrane.

  • Steroid Hormones: Fat-soluble, pass through cell membranes, and bind to intracellular receptors.

• Neuroendocrine Signaling: Nerve signals can stimulate the release of hormones.

  • Example: Oxytocin is released in response to a baby suckling, triggering milk secretion.

Nervous System Signal Transmission

• Neuron Structure: Signals travel from dendrite → cell body → axon.

• Action Potential: Nerve impulse created by ion exchange (Na+ in, K+ out).

• Synapse: The gap between neurons; neurotransmitters cross this gap to continue the signal.

Immune System

• Innate Immunity: General defenses against pathogens (e.g., skin, mucous barriers, inflammatory response).

• Adaptive Immunity: Specific responses to pathogens using lymphocytes (T and B cells).

  • B Cells: Produce antibodies that recognize specific antigens.

  • T Cells: Attack infected cells directly.

• Vaccination: Introduces a harmless part of a pathogen to stimulate memory cell production for faster response upon actual infection.

The Cell Cycle

The cell cycle is the life of a cell, spanning from its birth to the point it divides into two daughter cells.

Why Is the Cell Cycle Important?

• Healing: Helps repair wounds.

• Growth: Enables organisms to grow by producing new cells.

• Functionality: Produces genetically identical cells to prevent malfunctions in body systems.

Cell Division Overview

• Purpose: Distributes identical DNA to two daughter cells (except in meiosis).

• Replication: The DNA (genome) is replicated before cell division.

  • DNA is stored in chromosomes made of chromatin (long, thin strands of genetic material).

  • During division, chromatin condenses into a compact form to fit inside the cell.

• Chromosome Structure:

  • Each duplicated chromosome consists of two sister chromatids (copies of the original chromosome), joined at a centromere.

  • These chromatids separate into two new nuclei and cells.

  • Mitosis separates the nucleus, and cytokinesis separates the cytoplasm.

• Fun Fact: Organisms have a specific number of chromosomes. Any deviation can lead to genetic diseases.

Phases of the Cell Cycle

The cell cycle consists of two main phases:

1. Interphase (95% of the cycle): Includes:

   • G1 Phase (Gap 1): Growth and organelle production.

   • S Phase: DNA replication.

   • G2 Phase (Gap 2): Preparation for mitosis.

2. M Phase (Mitosis): Includes mitosis and cytokinesis.

Steps of Mitosis

1. Prophase:

   • The mitotic spindle forms (made of microtubules).

   • Centrosomes (in animal cells) move apart.

2. Prometaphase:

   • Nuclear envelope fragments.

   • Spindle microtubules attach to kinetochores on chromosomes to move them.

   • Non-kinetochore microtubules elongate the cell.

3. Metaphase:

   • Chromosomes align along the metaphase plate (midway between the spindle poles).

4. Anaphase:

   • Cohesins (proteins holding sister chromatids together) are cleaved by the enzyme separase.

   • Sister chromatids move to opposite poles using ATP.

5. Telophase:

   • Two nuclei form, and the cell begins splitting.

Cytokinesis:

• Animals: Division occurs via a cleavage furrow that pinches the cell into two.

• Plants: A cell plate forms, enlarging until it fuses with the plasma membrane.

Special Forms of Cell Division

• Binary Fission (Prokaryotes):

  • The cell grows to double its size, replicates its DNA at the origin of replication, and divides.

  • Likely an evolutionary predecessor to mitosis.

Regulation of the Cell Cycle

The timing and rate of cell division are crucial for normal growth and development.

• Unregulated Growth: Can lead to cancer.

Control System

• Cell Cycle Control System: A system of molecules that coordinates key events through chemical signals in the cytoplasm.

• Key Checkpoints:

  1. G1 Checkpoint: Determines if DNA synthesis can begin.

     • If a cell doesn't receive the "go" signal, it enters a non-dividing state called G0.

     • The G0 phase is the phase in the cell cycle in which the cell is neither dividing nor preparing for division; it's in a resting phase. The cell enters this phase after it's done dividing or duplicating (mitosis).

  2. G2 Checkpoint: Ensures DNA synthesis is complete + grows more.

  3. M Checkpoint: Ensures sister chromatids can separate correctly.

Regulatory Signals

• Internal Signals in Cell Cycle Regulation

  1. Cyclins and CDKs (Cyclin-Dependent Kinases):

     • Cyclins: Proteins whose levels fluctuate during the cell cycle, controlling its progression (peaks during mitosis and degrades post-mitosis).

     • CDKs: Enzymes activated by cyclins that regulate the cycle by phosphorylating target proteins.

     • Together, cyclins and CDKs act as molecular switches to transition between cell cycle phases.

  2. MPF (Maturation Promoting Factor):

     • A specific cyclin-CDK complex formed during the G2 phase.

     • Functions:

       • Triggers progression from G2 to M phase by phosphorylating proteins required for mitosis, such as those involved in chromosome condensation and spindle formation.

       • Degrades cyclins after mitosis to reset the cycle.

  3. APC (Anaphase-Promoting Complex):

     • A multi-protein complex that regulates the transition from metaphase to anaphase.

     • Functions:

       • Ensures all chromatids are properly attached to the spindle microtubules before separation.

       • Activates separase, which cleaves cohesins holding sister chromatids together.

       • Marks cyclins for degradation to ensure proper cycle progression and exit from mitosis.

• External Signals:

  • Growth Factors: Proteins that stimulate cell growth.

  • Physical Factors:

    • Density-Dependent Inhibition: Cells stop dividing when space is limited.

    • Anchorage Dependence: Cells must be attached to a surface to divide.

Cancer and Loss of Control

• Transformation: Cells gain the ability to divide indefinitely, becoming cancerous.

• Tumors:

  • Benign Tumors: Stay localized.

  • Malignant Tumors: Spread to other tissues (metastasis).

Meiosis and Sexual Life Cycle: Detailed Notes

MEIOSIS Overview

Meiosis is a specialized type of cell division that reduces the chromosome number by half, producing haploid gametes (sperm and egg cells) in sexually reproducing organisms. It ensures genetic diversity through recombination and independent assortment of chromosomes.

Importance of Meiosis in the Sexual Life Cycle

1. Genetic Variation:

   • Meiosis introduces variation through crossing over and independent assortment.

   • Variation drives evolution and adaptation.

2. Chromosome Number Maintenance:

   • Diploid organisms produce haploid gametes via meiosis, maintaining a stable chromosome number across generations.

3. Formation of Gametes:

   • In animals, meiosis produces sperm and egg cells.

   • In plants, meiosis generates spores, leading to gametophyte development.

Phases of Meiosis

Meiosis consists of two consecutive divisions: Meiosis I and Meiosis II.

Meiosis I: Reduction Division

• Reduces the chromosome number from diploid (2n) to haploid (n).

1. Prophase I:

   • Synapsis: Homologous chromosomes pair up to form tetrads (bivalents).

   • Crossing Over: Non-sister chromatids exchange genetic material at points called chiasmata.

   • Spindle Formation: Centrosomes move to poles, and spindle fibers attach to homologs.

2. Metaphase I:

   • Homologous chromosomes align on the metaphase plate.

   • Independent assortment occurs, leading to random distribution of maternal and paternal chromosomes.

3. Anaphase I:

   • Homologous chromosomes are pulled apart to opposite poles.

   • Sister chromatids remain attached at centromeres.

4. Telophase I and Cytokinesis:

   • Two haploid cells form, each with one set of chromosomes (still in their duplicated form).

Meiosis II: Equational Division

• Similar to mitosis but starts with haploid cells.

1. Prophase II:

   • Chromosomes condense, spindle apparatus forms, and nuclear envelope breaks down.

2. Metaphase II:

   • Chromosomes align on the metaphase plate.

   • Spindle fibers attach to centromeres.

3. Anaphase II:

   • Sister chromatids are separated and pulled to opposite poles.

4. Telophase II and Cytokinesis:

   • Four genetically unique haploid cells form.

Mechanisms Contributing to Genetic Variation

1. Crossing Over:

   • Occurs during Prophase I.

   • Results in recombinant chromosomes, mixing parental genes.

2. Independent Assortment:

   • Alignment of homologous chromosomes in Metaphase I is random.

   • Creates multiple combinations of chromosomes.

3. Random Fertilization:

   • Any sperm can fertilize any egg, further increasing genetic diversity.

Comparison: Mitosis vs. Meiosis

Sexual Life Cycle Stages

1. Gametic (Animal Life Cycle):

   • Dominant diploid stage.

   • Meiosis produces haploid gametes directly.

2. Zygotic (Fungi, Algae):

   • Dominant haploid stage.

   • Zygote undergoes meiosis immediately after formation.

3. Sporic (Plant Life Cycle):

   • Alternation of generations:

     • Diploid sporophyte undergoes meiosis to produce haploid spores.

     • Haploid gametophyte produces gametes through mitosis.

Errors in Meiosis

1. Non-Disjunction:

   • Failure of homologous chromosomes or sister chromatids to separate.

   • Leads to aneuploidy (e.g., Down syndrome, Turner syndrome).

2. Translocations and Deletions:

   • Errors during crossing over can result in structural chromosome abnormalities.