Animal Cell Structure and Organelles

1. Nucleus

control center of cell that contains genetic material and regulates cellular activities.

• Nuclear Pore: Channels that regulate the exchange of materials (RNA, proteins) between the nucleus and cytoplasm.

• Nucleolus: Site of ribosomal RNA (rRNA) synthesis and ribosome assembly.

• Nuclear Envelope/Membrane: Double membrane that encloses the nucleus, separating it from the cytoplasm.

• Nucleoplasm: Viscous fluid within the nucleus, containing chromatin and nucleolus.

2. Cell Membrane

• Semi-permeable barrier that controls the movement of substances in and out of the cell.

• Composed of a phospholipid bilayer with embedded proteins.

3. Lysosome

• Membrane-bound organelles containing digestive enzymes.

• Responsible for breaking down waste materials and cellular debris.

4. Rough Endoplasmic Reticulum (RER)

• Studded with ribosomes, involved in protein synthesis and processing.

• Transports proteins to the Golgi apparatus.

5. Smooth Endoplasmic Reticulum (SER)

• Lacks ribosomes, involved in lipid synthesis, detoxification, and calcium ion storage.

6. Ribosome

• Molecular machines that synthesize proteins by translating mRNA.

• Can be free in the cytoplasm or bound to the RER.

7. Mitochondria

• Powerhouse of the cell, responsible for ATP production through cellular respiration.

• Contains its own DNA and double membrane.

8. Golgi Apparatus/Body

• Stacked membrane-bound structures that modify, sort, and package proteins and lipids for secretion or delivery to other organelles.

9. Cytoplasm

• Gel-like substance filling the cell, containing organelles, cytoskeleton, and various molecules.

• Site of many metabolic processes.

10. Centriole

• Cylindrical structures involved in cell division, forming spindle fibers that separate chromosomes.

11. Centrosome

• Region near the nucleus that organizes microtubules and is critical for cell division.

12. Secretory Vesicle

• Membrane-bound vesicles that transport materials (e.g., hormones, neurotransmitters) to the cell membrane for secretion.

13. Peroxisome

• Organelles that contain enzymes for oxidative reactions, breaking down fatty acids and detoxifying harmful substances, such as hydrogen peroxide, into water and oxygen.

Phases of Cell Cycle

The cell cycle consists of a series of phases that a cell goes through to divide and replicate. It is divided into interphase (G1, S, G2) and the mitotic phase (M phase). Some cells may enter a resting state (G0).

1. G1 Phase (Gap 1)

• Structure: The cell is in a growth phase, with a normal diploid number of chromosomes.

• Process:

  • Cell grows in size.

  • Synthesizes proteins and organelles.

  • Prepares for DNA replication.

  • Checks for DNA damage and nutrient availability.

  • Duration: Varies; can be hours to years.

2. S Phase (Synthesis)

• Structure: Chromosomes are duplicated; each chromosome now consists of two sister chromatids.

• Process:

  • DNA replication occurs.

  • Centromeres hold sister chromatids together.

  • Histones and other proteins are synthesized for chromatin structure.

  • Ensures that each daughter cell receives an identical set of chromosomes.

3. G2 Phase (Gap 2)

• Structure: The cell has two complete sets of chromosomes, each consisting of sister chromatids.

• Process:

  • Further cell growth and preparation for mitosis.

  • Synthesis of proteins required for mitosis (e.g., tubulin for spindle formation).

  • Checks for DNA replication errors and repairs any damage.

  • Organelles are duplicated.

4. M Phase (Mitosis)

• Structure: Chromosomes are condensed and visible under a microscope.

• Process:

  • Mitosis is divided into several stages: prophase, metaphase, anaphase, and telophase.

    • Prophase: Chromosomes condense; nuclear envelope breaks down.

    • Metaphase: Chromosomes align at the cell equator.

    • Anaphase: Sister chromatids are pulled apart to opposite poles.

    • Telophase: Nuclear envelope reforms; chromosomes de-condense.

  • Cytokinesis occurs, dividing the cytoplasm and resulting in two daughter cells.

5. G0 Phase (Resting Phase)

• Structure: Cells are metabolically active but not actively dividing.

• Process:

  • Cells may exit the cell cycle and enter the G0 phase, where they remain in a quiescent state

Stages of Mitosis

Mitosis is the process of cell division that results in two genetically identical daughter cells. It consists of several stages:

1. Interphase

• Structure: Chromatin (uncondensed DNA) is present; the nucleus is intact.

• Process:

  • Not a part of mitosis but prepares the cell for division.

  • Consists of three phases: G1 (cell growth), S (DNA replication), and G2 (preparation for mitosis).

  • By the end, each chromosome has been duplicated, resulting in sister chromatids.

2. Prophase

• Structure: Chromosomes condense and become visible; spindle fibers begin to form; nuclear envelope starts to break down.

• Process:

  • Chromatin condenses into distinct chromosomes.

  • Each chromosome consists of two sister chromatids joined at the centromere.

  • The mitotic spindle forms from the centrosomes, which move to opposite poles.

3. Metaphase

• Structure: Chromosomes align at the metaphase plate; spindle fibers attach to centromeres.

• Process:

  • Chromosomes are lined up along the equatorial plane of the cell.

  • Spindle fibers from opposite poles attach to the centromeres of each chromosome, ensuring proper separation.

4. Anaphase

• Structure: Sister chromatids are pulled apart; spindle fibers shorten.

• Process:

  • The centromeres split, and sister chromatids are pulled toward opposite poles of the cell.

  • This ensures that each daughter cell will receive an identical set of chromosomes.

5. Telophase

• Structure: Chromosomes de-condense back into chromatin; nuclear envelopes reform around each set of chromosomes.

• Process:

  • The separated chromosomes reach the poles and begin to de-condense.

  • The nuclear envelope re-forms around each set of chromosomes, resulting in two nuclei.

6. Cytokinesis

• Structure: Division of the cytoplasm; cleavage furrow or cell plate forms.

• Process:

  • In animal cells, the cell membrane pinches inwards (cleavage furrow) to separate the two daughter cells.

  • In plant cells, a cell plate forms along the center of the cell to divide the cytoplasm and create two distinct daughter cells.

Stages of Meiosis

Meiosis is a specialized type of cell division that reduces the chromosome number by half, resulting in four haploid cells. It consists of two main stages: Meiosis I and Meiosis II.

Meiosis I

Interphase

• Definition: The preparatory phase before meiosis begins.

• Process: Chromosomes are replicated during the S phase, resulting in sister chromatids.

Prophase I

• Definition: The first stage of Meiosis I.

• Process:

  • Chromosomes condense and become visible.

  • Homologous chromosomes pair up (synapsis) forming tetrads.

  • Crossing over occurs, exchanging genetic material between homologs.

• Structures: Tetrads, chiasmata (points of crossing over).

Metaphase I

• Definition: The second stage of Meiosis I.

• Process:

  • Tetrads align at the metaphase plate.

  • Spindle fibers attach to the centromeres of homologous chromosomes.

Anaphase I

• Definition: The third stage of Meiosis I.

• Process:

  • Homologous chromosomes are pulled apart to opposite poles.

  • Sister chromatids remain attached.

Telophase I

• Definition: The fourth stage of Meiosis I.

• Process:

  • Chromosomes reach the poles and begin to de-condense.

  • Nuclear membranes may reform.

Cytokinesis I

• Definition: The final stage of Meiosis I.

• Process:

  • Cytoplasm divides, resulting in two haploid cells.

Meiosis II

Prophase II

• Definition: The first stage of Meiosis II.

• Process:

  • Chromosomes condense again if they de-condensed in Telophase I.

  • Spindle apparatus forms.

Metaphase II

• Definition: The second stage of Meiosis II.

• Process:

  • Chromosomes align at the metaphase plate.

  • Spindle fibers attach to the centromeres of sister chromatids.

Anaphase II

• Definition: The third stage of Meiosis II.

• Process:

  • Sister chromatids are pulled apart to opposite

Telophase II

• Definition: The final stage of meiosis II where the separated chromatids reach opposite poles of the cell.

• Key Features:

  • Chromatids decondense back into chromatin.

  • Nuclear envelopes reform around each set of chromosomes.

  • Each nucleus now contains a haploid set of chromosomes (n).

Cytokinesis

• Definition: The process that follows telophase II, resulting in the physical separation of the cytoplasm into two distinct cells.

• Key Features:

  • In animal cells, a cleavage furrow forms, pinching the cell membrane inward.

  • In plant cells, a cell plate forms along the center of the cell, eventually developing into a new cell wall.

  • Results in the formation of four genetically distinct haploid cells (gametes).