Cell Signaling

  1. Cell signaling is for communication and coordination of activities within an organism.

    1. Signals are divided into 3 main distance categories

      1. Direct/adjacent signaling occurs when the cells that produce and receive the signals are in physical contact with each other.

        1. Cell-cell recognition occurs when signals are received by a cell through receptor proteins on its surface.

        2. Cell junctions are openings between cells that allow signals to travel through, such as plasmodesmata in plant cells or gap junctions in animal cells.

      2. Local signaling is close by but not touching

        1. Paracrine signaling releases signals to nearby cells . . example: growth factor

        2. Synaptic signaling is only between nerve cells and the signal is a neurotransmitter

      3. Long-distance signaling travel usually through blood

        1. Endocrine (hormonal) signaling involves the release of hormones, which act as signals to communicate with distant cells in the body.

    2. Cell signaling can be broken down into 3 main phases

      1. Reception is the process of a specific receptor protein receiving a signal, which can occur either on the cell surface or inside the cell.

      2. Transduction includes all processes that occur after reception, involving relay molecules that carry out the signal transduction pathway.

      3. Response is the final outcome of the signal, fulfilling whatever function is necessary for the cell.

  2. The cell cycle is the life cycle of the cell, including cell division, which produces two genetically identical cells to repair, replace, reproduce aesexually (clone) and grow.

    1. Interphase is the longest phase of the cell cycle, taking up 90% of its duration, during which the cell grows and doubles its contents to prepare for cell division.

      1. G1 (Gap 1) is the phase where the cell grows and begins doubling specific organelles in preparation for division.

      2. S (Synthesis) is the phase where the cell continues to grow and doubles its DNA.

        1. DNA is packed with proteins to form chromatin, which coils into chromosomes. The entire set of chromosomes makes up the cell’s genome.

        2. Somatic cells are body cells that are diploid, meaning they have 2 sets of genetic information, while gametes are haploid and contain only one set of genetic information.

          1. Gametes are sex cells (egg and sperm)

        3. During the S phase, each chromosome is replicated, resulting in two sister chromatids that are tightly connected, especially at the centromere.

      3. G2 is the phase where the cell completes its growth and duplicates the centrosome in preparing for cell division.

    2. The mitotic phase makes up about 10% of the cell cycle and includes the process of cell division.

      1. Mitosis is the first phase of the M phase, during which the nucleus divides. It consists of 5 main stages:

        1. Prophase is the 1st step.

          1. Chromosomes condense and the nuclear envelope disppears.

          2. Spindle forms and pushes centrosomes apart

        2. Prometaphase is the 2nd phase.

          1. Nuclear envelope breaks down.

          2. Chromosomes further condense and get kinetochores.

          3. The spindle fibers attach to the kinetochores, engaging in a "tug-of-war" while also connecting to non-kinetochore microtubules, which help push the centrosomes further apart.

        3. Metaphase is the 3rd step.

          1. Chromosomes are aligned along the metaphase plate at the center.

          2. Centrosomes are positioned at opposite ends of the cell, with the spindle fibers from each centrosome attached to one sister chromatid of each chromosome.

        4. Anaphase is the 4th step. There are 2 times the amount of chromosomes.

          1. Sister chromatids become full chromosomes.

          2. The spindle continues pushing the centrosomes and elongates the cell.

        5. Telophase is the final phase and appears as the reverse of metaphase and prometaphase.

          1. The chromosomes decondense and nucleoli reappear.

          2. The spindle breaks down.

          3. Cytokinesis is happening simultaneously.

      2. Cytokinesis is the division of the cytoplasm resulting in 2 daughter cells.

        1. Animal cells undergo cleavage using an actin and myosin ring, which pinches the cell in half.

        2. Plant cells form a cell plate from vesicles derived from the Golgi apparatus and endoplasmic reticulum, creating a new cell wall between the daughter cells.

  3. The cell cycle control system regulates the cell cycle using checkpoints that either let the cycle move to the next phase or pause it if needed.

    1. Signals allow cells to pass checkpoints; otherwise, they enter the G0 phase.

      1. Cyclin is an internal signal that cycles between low and high concentrations. It activates CDKs, forming MPF, which helps the cell pass the G2 checkpoint.

      2. Other external signals that allow cell division or not

        1. Growth factors are local signals that stimulate cell division, such as PDGF (Platelet-Derived Growth Factor).

        2. Density-dependent inhibition is a direct signal between adjacent cells that stops cell division when there isn't enough space.

        3. Anchorage dependence is another direct signal between adjacent cells that promotes cell division only if a proper attachment is available.

    2. Transformation occurs when cells lose control of their cell cycle, dividing without signals and undergoing unlimited division, often forming a growth called a tumor.

      1. Benign tumors are considered “good” because they don’t spread, don’t harm the organ they are in, and can usually be removed with surgery.

      2. Malignant tumors are cancerous. They damage the organs they affect and often spread through metastasis, traveling via blood or lymph vessels.

        1. Malignant tumors can be treated through chemotherapy, radiation, or surgery.