10.2 The Cell Cycle

The cell cycle is the ordered sequence of events a cell goes through as it grows and divides into two daughter cells. It's central to how organisms grow, repair damaged tissue, and reproduce. This section covers the phases of the cell cycle, how chromosomes behave during mitosis, how the cytoplasm divides, and how checkpoints keep the whole process under control.

The Cell Cycle

Stages of interphase

Interphase is where the cell spends most of its time. It's not a resting phase; the cell is actively growing, copying its DNA, and preparing for division. Interphase has three distinct stages:

• G1 phase (Gap 1)
  • The cell increases in size and carries out normal metabolic functions
  • Organelles like mitochondria and ribosomes are produced to support growth
  • Proteins and enzymes needed for DNA replication are synthesized, including DNA polymerase
  • This is typically the longest phase of the cell cycle
• S phase (Synthesis)
  • DNA replication occurs, so the cell goes from having one copy of each chromosome to two
  • After replication, each chromosome consists of two identical sister chromatids joined at a region called the centromere
  • The centrosomes (which will later organize the mitotic spindle) also replicate during this phase
• G2 phase (Gap 2)
  • The cell continues to grow and synthesizes proteins needed for mitosis, such as tubulin, which builds spindle fibers
  • The cell builds up energy reserves (ATP) to fuel the energy-intensive process of division
  • Final error-checking of replicated DNA happens here

Chromosome behavior during mitosis

Mitosis is the division of the nucleus into two genetically identical nuclei. It happens in four continuous stages:

• Prophase
  • Chromatin condenses into tightly coiled, visible chromosomes
  • The nuclear envelope breaks down, giving the spindle access to the chromosomes
  • The mitotic spindle begins to form as microtubules extend from the centrosomes, which migrate toward opposite poles of the cell
• Metaphase
  • Chromosomes line up along the metaphase plate (the equatorial plane of the cell)
  • Spindle fibers from opposite poles attach to the centromere of each chromosome, connecting to structures called kinetochores on each sister chromatid
• Anaphase
  • Sister chromatids separate at the centromere and are pulled toward opposite poles
  • The spindle fibers shorten, actively dragging the now-individual chromosomes to each end of the cell
  • This is the stage that ensures each daughter cell gets an identical set of chromosomes
• Telophase
  • Chromosomes arrive at the poles and begin to decondense back into loose chromatin
  • A nuclear envelope re-forms around each set of chromosomes, producing two distinct nuclei
  • The spindle fibers disassemble as the cell prepares for cytokinesis

Process of cytokinesis

Cytokinesis is the physical division of the cytoplasm, and it overlaps with the end of mitosis. The mechanism differs between animal and plant cells:

In animal cells:

1. A cleavage furrow forms as the cell membrane pinches inward at the equator
2. A contractile ring made of actin and myosin filaments tightens like a drawstring
3. The membrane pinches completely shut, producing two separate daughter cells

In plant cells:

1. A cell plate forms at the center of the cell from vesicles produced by the Golgi apparatus
2. These vesicles carry cell wall materials (like cellulose) and fuse together at the midline
3. The cell plate grows outward until it joins with the existing cell wall, creating a new wall between the two daughter cells

In both cases, cytokinesis ensures each daughter cell receives a complete set of organelles and cytoplasm. The end result is two genetically identical daughter cells.

Active vs. quiescent cell states

Not every cell is actively dividing. Cells can be in one of two general states:

• Active cell cycle (G1, S, G2, and M)
  • Cells progress through growth, DNA replication, and division
  • This progression is driven by cyclins and cyclin-dependent kinases (CDKs), proteins that act as the cell cycle's internal clock
• G0 phase (quiescent state)
  • Cells exit the active cell cycle and stop dividing
  • This happens in fully differentiated cells that have taken on a specialized role (like neurons) or in cells facing unfavorable conditions (like nutrient deprivation)
  • G0 isn't always permanent. Some cells can re-enter the cell cycle when stimulated by growth factors, such as epidermal growth factor (EGF)

Regulation and Significance of the Cell Cycle

Describe the role of checkpoints in regulating the cell cycle

Checkpoints are built-in quality control mechanisms. They prevent the cell from moving to the next phase until specific conditions are met. Think of them as gates that only open when everything checks out.

• G1 checkpoint (restriction point)
  • Evaluates cell size, nutrient availability, and whether growth factor signals are present
  • This is the main decision point: the cell either commits to division (proceeds to S phase) or exits into G0
• G2 checkpoint
  • Confirms that DNA replication is complete and checks for DNA damage
  • If errors are detected, repair mechanisms are activated before the cell can enter mitosis
• M checkpoint (spindle assembly checkpoint)
  • Verifies that every chromosome is properly attached to spindle fibers from both poles
  • Prevents the separation of sister chromatids (anaphase) until all chromosomes are correctly aligned at the metaphase plate

Explain the significance of the cell cycle in the context of growth, development, and disease

• Growth and development
  • The cell cycle produces new cells during an organism's growth, from embryogenesis through adulthood
  • Tightly regulated division is essential for proper tissue and organ formation
• Tissue repair and regeneration
  • Damaged or lost cells are replaced through cell division
  • Wound healing, for example, depends on skin cells re-entering the cell cycle to close an injury
• Disease (cancer)
  • Cancer results from uncontrolled cell division caused by failures in cell cycle regulation
  • Mutations in key regulatory genes can drive cancer development. Two important examples: p53, a tumor suppressor gene that normally halts the cycle when DNA is damaged, and Ras, a proto-oncogene that normally promotes cell growth but can become permanently "on" when mutated
  • Many cancer therapies target the cell cycle directly, such as CDK inhibitors that block the proteins driving cell division

Cell Growth and Division

Cell growth (an increase in cell size and mass) occurs primarily during G1 and G2, while cell division (the splitting into two daughter cells) is the outcome of mitosis and cytokinesis together. Chromosomes are critical to this process because they ensure each daughter cell receives an accurate copy of the genetic material.

Some cells undergo cellular differentiation, specializing into distinct cell types with specific functions (muscle cells, nerve cells, blood cells). Differentiated cells often exit the cell cycle and enter G0, though some retain the ability to divide when needed.