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4.5 Cell Cycle

Keywords

English Term 中文翻译 Definition & Explanation
Cell Cycle 细胞周期 A highly regulated series of events that controls the growth and reproduction of eukaryotic cells.
Interphase 分裂间期 The longest phase of the cell cycle, consisting of G1, S, and G2 phases, during which the cell grows and copies its DNA.
Mitosis 有丝分裂 The process of nuclear division in eukaryotic cells that ensures the transfer of a complete genome to two genetically identical daughter cells.
Sister Chromatids 姐妹染色单体 Two identical copies of a single replicated chromosome that are connected by a centromere.
Centromere 着丝粒 The specialized DNA sequence of a chromosome that links a pair of sister chromatids.
Cytokinesis 胞质分裂 The physical division of the cytoplasm into two daughter cells following mitosis.

1. Overview and Interphase (The Prep Work)

The cell cycle is a highly regulated series of events that alternates between preparing for division (Interphase) and actively dividing (Mitosis and Cytokinesis).

Interphase accounts for about 90% of the cell cycle. It is not a resting phase; rather, it is a period of intense metabolic activity and growth, consisting of three sequential stages:

  • \(G_1\) Phase (Gap 1): The cell is metabolically active, rapidly growing, and duplicating its organelles and cytosolic components.
  • \(S\) Phase (Synthesis): This is the most critical prep step. DNA (in the loose form of chromatin) replicates. Each chromosome is duplicated to form two identical sister chromatids connected tightly at a region called the centromere.
  • \(G_2\) Phase (Gap 2): The cell continues to grow. Protein synthesis occurs, ATP is produced in large quantities to fuel the upcoming division, and centrosomes (the structures that organize the spindle fibers) replicate.

The \(G_0\) Phase: Hitting the Brakes

Not all cells divide constantly. A cell can exit the standard cell cycle and enter a non-dividing stage called \(G_0\) phase. * Some cells (like mature human nerve and muscle cells) stay in \(G_0\) permanently and never divide again. * Other cells may just be held at this stage temporarily and can re-enter the cell cycle (going back to \(G_1\)) in response to appropriate external cues (like growth factors).

(Placeholder: A circular diagram showing the phases of the cell cycle, emphasizing that Interphase takes up the vast majority of the cycle's time.)

2. Mitosis: Distributing the Genome

Mitosis is the process that ensures the transfer of a complete genome from a parent cell to two genetically identical daughter cells. In eukaryotes, mitosis plays a crucial role in: 1. Growth of the organism. 2. Tissue repair (e.g., healing a scraped knee). 3. Asexual reproduction (in certain organisms).

Mitosis occurs in four sequential steps.

Memory Trick: PMAT

You can easily remember the order of mitosis phases using the acronym PMAT (Prophase, Metaphase, Anaphase, Telophase).

  1. Prophase (The Setup): The loose chromatin condenses into tightly coiled, visible chromosomes (sister chromatids). The mitotic spindle begins to form, and the centrosomes move to opposite poles of the cell. The nuclear envelope begins to break down.
  2. Metaphase (The Alignment): Spindle fibers attach to the centromeres and align the chromosomes in a single file line along the equator (the metaphase plate) of the cell.
  3. Anaphase (The Separation): The paired sister chromatids are abruptly pulled apart. The spindle fibers pull the separated chromatids (now considered individual chromosomes) toward opposite poles of the cell.
  4. Telophase (The Cleanup): The chromosomes reach the poles and begin to uncoil back into chromatin. The mitotic spindle breaks down, and a new nuclear envelope develops around each set of chromosomes, forming two distinct nuclei.
(Placeholder: A sequence of cell illustrations showing chromosomes condensing, lining up, splitting, and forming two new nuclei.)

3. Cytokinesis: Splitting the Cell

Mitosis divides the nucleus, but Cytokinesis divides the rest of the cell (the cytoplasm). This process overlaps with the end of telophase and results in two separate, identical daughter cells.

Because plant cells have rigid cell walls, cytokinesis looks different in animals and plants: * In Animal Cells: A contractile ring of microfilaments pinches the cell inward, forming a cleavage furrow that acts like a drawstring, eventually pinching the cell in two. * In Plant Cells: Vesicles derived from the Golgi apparatus move to the center of the cell and fuse to form a cell plate. This plate gradually expands outward until it fuses with the surrounding plasma membrane, creating a new cell wall between the daughter cells.

(Placeholder: Diagram highlighting the physical difference between the pinching cleavage furrow of animal cells and the building of a cell plate in plant cells.)

Quiz

Source: Campbell Biology Practice Test - Chapter 12 (The Cell Cycle)