Cell physio 2

Lecturer Introduction

  • Instructor: Dr. R. Ahangari
  • Affiliation: University of Central Florida, Orlando
  • Recommended Texts:
    • Human Physiology by S.I. Fox
    • Human Anatomy by Marieb & Mallat

The Nucleus

  • Definition: The nucleus is the control center of the cell and contains genetic materials (DNA).
  • Function: Directs the cell’s activities by providing instructions for protein synthesis.
  • Structure:
    1. Nuclear Envelope:
    • Surrounds the nucleus.
    • Contains pores and is continuous with the endoplasmic reticulum.
    1. Chromatin and Chromosomes:
    • Chromatin: Granular thread-like material composed of DNA and histone proteins.
    • Chromosomes: Structures that contain coiled chromatin.
    1. Nucleoli:
    • A dark staining body within the nucleus.
    • Contains parts of chromosomes and acts as the ribosome-producing machine (contains genes coding for rRNA).

Deoxyribonucleic Acid (DNA)

  • Structure:
    • DNA molecule in chromatin is a double helix made up of nucleotide molecules.
    • Nucleotides consist of:
      • Sugar.
      • Phosphate.
      • One of four bases:
      • Thymine (T)
      • Adenine (A)
      • Cytosine (C)
      • Guanine (G)
    • Nucleotides bind to hold the DNA helix together like a ladder.
    • DNA helix wraps around clusters of histone proteins, regulating gene expression and transcription.
    • Each cluster of DNA and histones is termed a nucleosome.
  • Chromosomes:
    • Contains a single, very long molecule of DNA.
    • A typical human cell contains 46 chromosomes.
    • During cell division, the chromatin is coiled, making the chromosomes appear as thick rods.

The Cell Life Cycle

  • Definition: A series of changes a cell experiences from formation until reproduction.
  • Two Major Periods:
    1. Interphase:
    • Cell grows and carries out its usual activities.
    1. Cell Division (Mitosis):
    • The cell divides into two cells.
  • Importance: Essential for growth and repair of the body.

Interphase

  • Definition: The non-dividing phase of the cell cycle.
  • Function:
    • Cells maintain life-sustaining activities and prepare for cell division.
  • Subphases:
    1. G1 Phase (Gap 1):
    • Cells are active, growing vigorously, and centrioles start replicating.
    1. S Phase (Synthetic):
    • DNA replicates for the two future daughter cells to have identical genetic material.
    1. G2 Phase (Gap 2):
    • Synthesis of enzymes needed for cell division.
    • Completion of centriole replication and preparation for division.

Cell Division (Mitosis)

  • Definition: The process by which a cell divides to produce two daughter cells.
  • Stages:
    1. Prophase:
    • Formation of asters (microtubule arrays) from the centrosome.
    • Chromatin coils into chromosomes (two identical chromatids per chromosome).
    • Nucleoli disappear, centriole pairs separate, and the nuclear envelope fragments.
    • Microtubules align to form mitotic spindles, pushing centrioles apart.
    1. Metaphase:
    • Chromosomes align at the middle of the cell, forming a metaphase plate.
    • Separase, an enzyme, cleaves cohesin to separate the chromatids.
    1. Anaphase:
    • V-shaped chromatids are pulled apart by kinetochore spindles.
    • Polar spindles elongate the cell.
    1. Telophase:
    • Reversal of prophase events.
    • Chromosomes uncoil and resume extension.
    • Nuclear envelope reforms, and nucleoli reappear.

Cytokinesis

  • Definition: The process of division in which the cell cytoplasm divides, resulting in two separate daughter cells.

Meiosis

  • Definition: A specialized process of cell division for the production of gametes.
  • Characteristics: Involves two divisions resulting in four gametes, each with half the number of chromosomes (23 single chromosomes, or 1N).
  • Meiosis I:
    • Events:
    1. Synapsis: Pairing of 46 homologous duplicated chromosomes.
    2. Crossing Over: Large segments of DNA are exchanged between homologous chromosomes.
    3. Alignment: Homologous duplicated chromosomes align at the metaphase plate.
    4. Disjunction: Homologous chromosomes separate; centromeres do not split.
    5. Cell Division: Two secondary gametocytes (23 duplicated chromosomes, 2N) are formed.
  • Meiosis II:
    • Events:
    1. Synapsis: Absent
    2. Crossing Over: Absent
    3. Alignment: 23 duplicated chromosomes align at the metaphase plate.
    4. Disjunction: 23 duplicated chromosomes separate into single chromosomes (23, 1N).
    5. Cell Division: Four gametes (23 single chromosomes, 1N) are produced.

Clinical Considerations

  • Aneuploidy: Abnormal number of chromosomes, can manifest as trisomy (extra chromosome) or monosomy (missing chromosome).
    • Examples:
    • Down Syndrome: Trisomy 21
    • Klinefelter Syndrome: XXY
    • Turner Syndrome: XO

Gametes

  • Composition: Contain 23 single chromosomes (22 autosomes and 1 sex chromosome) and 1N amount of DNA.
  • Haploid Definition: Cells containing 23 single chromosomes are referred to as haploid.
  • Sex Chromosomes:
    • Female gametes contain only the X sex chromosome.
    • Male gametes can carry either X or Y sex chromosome, determining the genetic sex of the individual.

Cellular Diversity

  • Variety: There are approximately 200 different cell types in the human body, each with distinct shapes and functions.

Aging

  • Definition: Aging is complex and involves various factors, including cell damage.
  • Mechanisms of Aging:
    1. Damage due to free radicals from normal metabolism or environmental factors (radiation, pollutants).
    2. Mitochondrial Theory: Decreased energy production by free radical-damaged mitochondria leads to cellular weakening and aging.
    3. Vitamins C and E act as antioxidants, preventing excess free radicals.
    4. Caloric intake restriction lowers metabolic rate, slowing aging process.
    5. Genetic Theories of Aging: Aging may be programmed into our genes, known as senescence.

Apoptosis (Programmed Cell Death)

  • Definition: A method of cell removal from tissues in an orderly process for normal maintenance or during development.
  • Morphological Features:
    1. Chromatin condensation, nuclear breakdown, plasma membrane fragmentation.
    2. The cell shrinks and forms membrane-enclosed fragments called apoptotic bodies.
  • Signal Mechanisms:
    • Induction of apoptosis via cytokines like tumor necrosis factor (TNF); activation of caspases degrades proteins leading to apoptosis characteristics.
  • Clinical Implications:
    • Excess apoptosis can lead to nerve cell loss in diseases like Alzheimer’s and stroke.
    • Insufficient apoptosis may be linked to cancers and autoimmune diseases.

Cancer

  • Definition: A mass of cells that divide and multiply abnormally, referred to as a neoplasm.
  • Classification of Neoplasms:
    • Benign Neoplasm: Localized, often encapsulated, grows slowly, seldom fatal.
    • Malignant Neoplasm (Cancer): Not encapsulated, grows rapidly, contains immature cells, capable of invading other tissues (metastasis) through lymphatics or blood.

Oncogenes

  • Definition: Result from mutations of proto-oncogenes, regulatory genes that normally control cell proliferation and development.
  • Formation: Caused by genetic accidents or viral infections leading to oncogene formation.
  • Function: Oncogenes dominate normal proto-oncogenes, causing deregulation of cell division, leading to cancerous conditions.
  • Examples of Cancer Caused by Oncogenes:
    • Bladder cancer
    • Acute myelogenous leukemia