COMPARISON OF ANIMAL CELL WITH PRECELLULAR FORMS OF LIFE

  • The complexity of the cell:

    • The cell is a highly developed organism that evolved over hundreds of millions of years after the earliest forms of life, similar to present-day viruses, first appeared on Earth.

    • The diameter of a typical cell is approximately 1000 times greater than that of the smallest viruses.

    • The functions and anatomical organization of the cell are more intricate compared to that of a virus.

VIRUS

  • Essential life-giving constituents:

    • A virus is composed of nucleic acid (either DNA or RNA) encapsulated in a protein coat.

    • The nucleic acid is identical to the basic nucleic acid constituents found in mammalian cells and can replicate itself under suitable conditions.

    • This capacity for reproduction allows a virus to propagate its lineage across generations, qualifying it as a living structure in evolutionary terms.

  • Evolution of viruses:

    • As life evolved, additional chemicals outside of nucleic acids and simple proteins became integral to the virus.

    • Specialized functions began to emerge in different regions of the virus, improving its operational efficiency.

RICKETTSIAL AND BACTERIAL STAGES

  • Development of organelles:

    • Within more complex organisms, organelles emerged as distinct physical structures composed of chemical aggregates.

    • These organelles perform functions more efficiently than dispersed chemicals within the cell's fluid matrix.

  • Nucleated cell:

    • Further evolutionary developments led to nucleated cells, where organelles, especially the nucleus, became more complex.

    • The nucleus serves as the control center for cellular activities, ensuring the precise reproduction of new cells generation after generation, with each new cell retaining almost the exact structure as its progenitor.

FUNCTIONAL SYSTEMS OF THE CELL

  • Nutrient acquisition:

    • To live, grow, and reproduce, cells must absorb nutrients and other necessary substances from their surrounding environment.

  • Mechanisms of substance transport through the cell membrane:

    • Diffusion:

    • Involves the simple movement of substances through the membrane, driven by the random motion of molecules.

    • Active Transport:

    • Involves the transport of substances across the membrane via a physical protein structure that penetrates through the membrane.

    • Endocytosis:

    • A specialized function of the cell membrane enabling the ingestion of very large particles into the cell.

    • Exocytosis:

    • The process of expelling waste or remnants of digestive vesicles through the cell membrane.

  • Types of Endocytosis:

    • Pinocytosis:

    • The ingestion of extracellular fluid and particulate substances into the cytoplasm through vesicles.

    • Phagocytosis:

    • The ingestion of large particles, such as bacteria, whole cells, or portions of degenerating tissue.

CELL DIVISION

  • Mitosis:

    • Involves somatic cells and is crucial for:

    • Maintaining cell size, growth, and repair.

    • Replacing old, worn-out cells and facilitating healing and regeneration.

    • Supporting evidence for the basic relationships among organisms.

  • Meiosis:

    • Involves gametes for sexual reproduction.

  • Historical context:

    • Mitosis was first described in 1875 by Eduard Strasburger and the term was coined by W. Flemming in 1882.

    • Mitosis is defined as the multiplication of a body cell into two daughter cells that are equal in size and genetic composition, including the same number of chromosomes as the parent cell.

    • The two main events in this process include karyokinesis (division of the nucleus) and cytokinesis (division of the cytoplasm).

MEIOSIS

  • Coined by J.B. Farmer and J.E. Moore in 1905, meiosis occurs in gonads and reduces diploid chromosomes to a haploid number (N).

  • Cell Cycle Stages:

    • Interphase (I-phase):

    • Long undividing phase that spans from the end of telophase to the beginning of the next mitotic phase.

    • The cell undergoes growth by synthesizing biological molecules (lipids, proteins, carbohydrates, nucleic acids).

    • Subdivided into:

      • G1 phase (initial growth): Gap between previous mitosis and the onset of DNA synthesis.

      • S phase: Duplication of each chromosome through DNA replication.

      • G2 phase: Between DNA synthesis and nuclear division where further growth occurs and preparation for mitosis continues.

MITOTIC PHASE

  • Describes how duplicated chromosomes are evenly distributed to daughter cells containing identical hereditary information as the parent cell.

    • Other cell components (such as organelles) are also divided, although not as precisely as the DNA.

  • Prophase:

    • Divided into early, middle, and late prophase:

    • Early Prophase:

      • The cell’s shape transitions to a rounded form, and cytoplasm becomes viscous. Centrioles become organized and move towards opposite poles of the cell.

    • Middle Prophase:

      • Long microtubules are organized, forming the mitotic spindle and spindle fibers. Chromosomes change into discernible thickened forms.

    • Late Prophase:

      • Nuclear envelope breaks, and spindle fully adopts its shape as chromosomes are released into the cytoplasm.

  • Metaphase:

    • Spindle occupies the nuclear region with chromosomes aligned at the spindle's equatorial plane, forming the metaphase plate.

  • Anaphase:

    • Sister chromatids of each chromosome separate towards opposite poles concurrently, each now referred to as individual chromosomes.

  • Telophase:

    • The nucleus is reconstructed around each group of chromosomes, which unwind and elongate. The nucleoli reform, while the spindle disappears.

  • Cytokinesis:

    • The division of cytoplasm that separates daughter nuclei formed from karyokinesis, ensuring equal distribution of organelles.

MEIOSIS I

  • Prophase I: Divided into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.

    • Leptotene:

    • Chromosomes condense and become thicker; each is double due to prior DNA replication.

    • Zygotene:

    • Synapsis occurs, pairing homologous chromosomes into bivalents. The synaptonemal complex forms between them.

    • Pachytene:

    • Recombination occurs due to crossing over among homologous chromatids; dyads and tetrads form.

    • Diplotene:

    • Homologous chromosomes partially separate and form visible tetrads, and chiasmata mark exchange sites.

    • Diakinesis:

    • Chromosomes condense further, centrioles separate to opposite cell ends, and the nuclear envelope and nucleolus disintegrate.

  • Metaphase I:

    • Tetrads align at the spindle's equator forming parallel metaphase plates.

  • Anaphase I:

    • Dyads separate, leading to disjunction and producing haploid sets of chromosomes moving to opposing spindle poles.

  • Telophase I:

    • Chromosomes at each pole unfold, the nuclear envelope and nucleolus reform, and cytokinesis occurs, yielding two daughter cells.

MEIOSIS II

  • Prophase II:

    • Centrioles move apart and spindle forms, with chromosomes visible again.

  • Metaphase II:

    • Chromosomes align at the spindle's equator, and kinetochores connect to microtubules.

  • Anaphase II:

    • Chromatids of each chromosome separate and migrate to spindle poles, resulting in haploid chromosomes at each pole.

  • Telophase II:

    • Chromosomes decondense, nuclear envelopes reform, and the spindle dissipates.

  • End result of meiosis:

    • In animals, meiosis produces mature gametes which do not undergo further division without fusion with another gamete.

    • In contrast, in plants, the resultant spores may develop into new individuals independently of fusion.

CYTOKINESIS

  • In Animal Cells:

    • Formation of a cleavage furrow using contractile proteins (actin and myosin).

    • Completion of cell division leads to two separate daughter cells.

  • In Plant Cells:

    • Vesicles form a cell plate at the equator, allowing the development of new cell walls adjacent to the equator after exocytosis.

    • Middle lamella forms from pectins between separated daughter cell membranes.