Cell Cycle and Cell Division

Phases of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase and the mitotic (M) phase, which includes mitosis and cytokinesis.

• Interphase (about 90% of the cycle):

  • G1 phase: Cell growth and metabolic activity.

  • S phase: DNA synthesis and chromosome duplication.

  • G2 phase: Further growth and preparation for division.

• Mitotic Phase (about 10% of the cycle):

  • Mitosis: Division of the nucleus into two genetically identical nuclei.

  • Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.

Timing of Cell Cycle: A typical human cell cycle lasts about 24 hours. M phase lasts less than 1 hour, S phase about 10-12 hours, G2 phase 4-6 hours, and G1 phase is variable (often 5-6 hours).

Special Cases: Some cells, such as those in the pancreas, may enter a non-dividing state called the G0 phase.

Key Structures in Mitosis

• Centrosomes: Organize microtubules of the spindle; duplicate during interphase.

• Kinetochore: Protein structures at centromeres where spindle fibers attach.

Visual Skills: During prometaphase, each chromosome consists of two sister chromatids, each containing one DNA double helix.

Mitosis Stages

Phases of Mitosis

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets. It is divided into several stages:

• Prophase: Chromatin condenses into visible chromosomes; nucleoli disappear; mitotic spindle begins to form.

• Prometaphase: Nuclear envelope fragments; microtubules attach to kinetochores; chromosomes condense further.

• Metaphase: Chromosomes align at the metaphase plate; kinetochores attach to spindle microtubules from opposite poles.

• Anaphase: Sister chromatids separate and move to opposite poles; cell elongates.

• Telophase: Nuclear envelopes reform around daughter nuclei; chromosomes decondense; mitosis completes.

• Cytokinesis: Cytoplasm divides, forming two daughter cells; in animal cells, a cleavage furrow forms.

The Mitotic Spindle: A Closer Look

The mitotic spindle is a structure made of microtubules and proteins that segregates chromosomes during mitosis.

• Spindle microtubules elongate by adding tubulin subunits and shorten by losing them.

• In animal cells, spindle microtubules assemble at the centrosome, which duplicates during interphase and moves apart during prophase and prometaphase.

• Each chromosome has an aster, a radial array of short microtubules.

• The spindle includes centrosomes, spindle microtubules, and asters.

• Each sister chromatid has a kinetochore, which attaches to kinetochore microtubules during prometaphase.

• Chromosomes move toward spindle poles when kinetochores attach to microtubules from opposite poles, resulting in a tug-of-war that positions chromosomes at the metaphase plate.

• Chromosomes move to opposite poles via motor proteins walking along microtubules, which depolymerize at the kinetochore ends.

• Nonkinetochore microtubules elongate the cell by pushing spindle poles apart during anaphase.

• Cytokinesis begins during anaphase or telophase, and the spindle disassembles by microtubule depolymerization.

Cytokinesis: A Closer Look

Cytokinesis in Animal Cells

Cytokinesis in animal cells occurs through a process called cleavage:

• The first sign is the appearance of a cleavage furrow, a shallow groove near the old metaphase plate.

• On the cytoplasmic side, a contractile ring of actin microfilaments interacts with myosin molecules, causing the ring to contract.

• This contraction deepens the cleavage furrow until the parent cell is pinched into two separate cells, each with its own nucleus and share of cytosol, organelles, and other subcellular structures.

Cytokinesis in Plant Cells

Cytokinesis in plant cells is different due to the presence of cell walls:

• There is no cleavage furrow. Instead, during telophase, vesicles from the Golgi apparatus move to the middle of the cell and coalesce to form a cell plate.

• The cell plate grows and its membrane fuses with the plasma membrane, resulting in two daughter cells, each with its own plasma membrane.

• A new cell wall forms from the contents of the cell plate between the daughter cells.

Phases of Mitosis in a Plant Cell

• Prophase: Chromosomes condense, the nucleolus disappears, and the mitotic spindle starts to form.

• Prometaphase: Chromosomes become visible, each consisting of two sister chromatids. The nuclear envelope fragments; later in this phase, chromosomes attach to microtubules at their kinetochores.

• Metaphase: Chromosomes align at the metaphase plate, attached to microtubules at their kinetochores.

• Anaphase: Chromatids separate and move to opposite ends of the cell as kinetochore microtubules shorten.

• Telophase: Daughter nuclei form, and cytokinesis starts with the cell plate growing toward the cell perimeter.

Binary Fission in Bacteria

Overview

Binary fission is the process by which prokaryotes (bacteria and archaea) reproduce, involving the cell growing to double its size and then dividing into two cells.

• Eukaryotic vs. prokaryotic division: In eukaryotes, cell division involves mitosis, whereas in prokaryotes, it does not.

• Bacterial chromosome: Most bacterial genes are carried on a single, circular DNA molecule with associated proteins.

Chromosome Replication and Cell Division

• Replication begins at the origin of replication, producing two origins.

• One origin moves to the opposite end of the cell as replication continues.

• As the chromosome replicates, the cell elongates.

• Once replication is complete, proteins cause the plasma membrane to pinch inward, resulting in two daughter cells, each inheriting a complete genome.

Steps of Binary Fission

1. Chromosome replication begins, and one copy of the origin moves to the other end of the cell.

2. Replication continues, with one origin at each end of the cell, and the cell elongates.

3. Replication finishes, the plasma membrane pinches inward, and a new cell wall is deposited.

4. Two daughter cells result.

Proteins Involved

• Proteins resembling eukaryotic actin and tubulin play roles in bacterial chromosome movement and cell division.

• Actin-like proteins help move chromosomes, while tubulin-like proteins help pinch the plasma membrane inward.

The Evolution of Mitosis

Prokaryotes preceded eukaryotes by over a billion years, suggesting that mitosis evolved from simpler prokaryotic cell reproduction mechanisms.

• Proteins involved in bacterial binary fission are related to eukaryotic proteins used in mitosis, supporting the hypothesis of evolutionary development.

• As eukaryotes evolved with nuclear envelopes and larger genomes, the ancestral binary fission process gave rise to mitosis.

• Variations in cell division among different groups may resemble steps in the evolution of mitosis from binary fission.

• Some unicellular eukaryotes (e.g., dinoflagellates, diatoms, and some yeasts) show intermediate stages in the evolution of mitosis.

• In dinoflagellates, chromosomes attach to the nuclear envelope, which remains intact during cell division, with microtubules passing through the nucleus inside cytoplasmic tunnels.

• In diatoms and some yeasts, the nuclear envelope also remains intact, with microtubules forming a spindle within the nucleus to separate chromosomes and split the nucleus into two daughter nuclei.

• These examples are used to hypothesize about intermediate mechanisms, acknowledging that other mechanisms may have existed in now-extinct species.

Table: Comparison of Mitosis in Animal and Plant Cells