7.2: Cell Cycle and Cell Division
Prokaryotic vs Eukaryotic Cell Division
Cell division is the process by which one cell (the parent) divides to form two new cells (the daughter cells). This fundamental biological process ensures growth, repair, and reproduction.
In prokaryotes, division is simpler due to their simpler cell structure:
Single circular chromosome located in the nucleoid region.
No nucleus or membrane-bound organelles.
Division is typically through a process called binary fission, a form of asexual reproduction, resulting in two genetically identical daughter cells.
In eukaryotes, division is more complex, reflecting their intricate cellular organization:
Multiple linear chromosomes organized within a nucleus.
Many membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus).
All chromosomes must be precisely duplicated, and then segregated equally into daughter nuclei. Organelles also need to be adequately partitioned during cytokinesis.
Overall, the cell cycle is a repeating sequence of growth, DNA synthesis (replication), and cell division, ensuring the accurate transmission of genetic information from one generation of cells to the next.
The Cell Cycle: Key Concepts
Cell division is one crucial stage of a cell’s life; many cells (including cancer cells) undergo division continually, while others divide only under specific conditions or not at all.
The cell cycle in prokaryotes is simple: grow \rightarrow DNA replication \rightarrow cell division (binary fission, asexual reproduction).
In eukaryotes, the cell cycle is a tightly regulated process comprising two main phases: Interphase (a period of growth and DNA replication) and the Mitotic (M) phase (a period of nuclear and cytoplasmic division).
The M phase includes both mitosis (the division of the nucleus and its contents, primarily the chromosomes) and cytokinesis (the division of the cytoplasm, leading to two separate daughter cells).
Interphase, the longest phase of the cell cycle, includes three distinct sub-phases: G1, S, and G2. During interphase, the cell grows, carries out its normal metabolic functions, duplicates its organelles, and precisely prepares its DNA for division.
Eukaryotic Cell Cycle: Phases
Interphase (G1, S, G2): the cell is actively carrying out its functions and preparing for division, but is not actually dividing.
G1 (Growth Phase 1)
The cell typically spends the majority of its life in G1, performing its specialized functions according to its cell type.
Characterized by rapid growth and high biosynthetic/metabolic activity, synthesizing new proteins, organelles, and other cellular components.
Synthesis of amino acids and proteins required for normal cell functions, including enzymes for DNA synthesis later in the S phase.
Restriction Point (R point): A critical checkpoint late in G1. Once a cell passes this point, it is committed to proceeding with the cell cycle and typically completes S, G2, and M phases. Cells can exit the cell cycle from G1 and enter a quiescent state called G0.
S (Synthesis Phase)
This is the phase where DNA replication occurs. Each chromosome is duplicated, resulting in two identical sister chromatids joined at the centromere.
Synthesis of histone proteins, which are essential for packaging the newly synthesized DNA into chromatin.
The amount of DNA in the cell effectively doubles from 2n to 4n (where n is the haploid number of chromosomes, or more accurately, the DNA content goes from 2C to 4C if C is the DNA content of a haploid cell).
G2 (Growth Phase 2)
The cell continues to grow and synthesizes proteins and organelles in preparation for mitosis.
The cell checks the duplicated chromosomes for errors or damage and makes any necessary repairs.
Accumulation of energy stores and materials required for the upcoming division (M phase).
Formation of microtubules that will be used to construct the mitotic spindle.
M (Mitotic) Phase: The period of nuclear and cytoplasmic division.
Mitosis: The process of nuclear division, ensuring that each daughter nucleus receives a complete and identical set of chromosomes. Mitosis is divided into several sub-phases:
Prophase: Chromatin condenses into visible chromosomes (each consisting of two sister chromatids). The nuclear envelope begins to break down, and the mitotic spindle starts to form from centrosomes.
Prometaphase: The nuclear envelope completely dissociates. Kinetochore microtubules attach to the kinetochores of sister chromatids, and non-kinetochore microtubules overlap at the cell's equator.
Metaphase: Chromosomes align at the metaphase plate (equatorial plane), equidistant from the two spindle poles. A checkpoint ensures all kinetochores are properly attached to microtubules.
Anaphase: Sister chromatids separate (now considered individual chromosomes) and are pulled toward opposite poles of the cell by the shortening of kinetochore microtubules. The cell elongates.
Telophase: Chromosomes arrive at the poles and begin to decondense. New nuclear envelopes form around the two sets of chromosomes. The mitotic spindle disassembles.
Cytokinesis: The division of the cytoplasm, which usually overlaps with late anaphase and telophase.
In animal cells, a contractile ring of actin and myosin filaments forms a cleavage furrow, pinching the cell into two.
In plant cells, a cell plate forms in the middle of the cell, growing outwards to form a new cell wall that divides the two daughter cells.
Cell Cycle Checkpoints
The cell cycle is regulated by checkpoints that monitor the cell's status and ensure that critical events occur accurately and in the correct order. These include:
G1 Checkpoint (Restriction Point): Ensures the cell is ready to divide, assessing cell size, nutrients, growth factors, and DNA integrity. If conditions are unfavorable, the cell can enter G0 or undergo apoptosis.
G2 Checkpoint: Verifies that DNA replication is complete and that the DNA is not damaged before entering mitosis.
M Checkpoint (Spindle Assembly Checkpoint): Occurs during metaphase, ensuring that all sister chromatids are correctly attached to the spindle microtubules before anaphase begins, preventing aneuploidy (abnormal chromosome number).
Cancer is fundamentally a disease of uncontrolled cell division. The cell cycle checkpoints discussed in the notes (G1, G2, and M checkpoints) are critical regulatory points that ensure cell division proceeds accurately. When these checkpoints fail due to mutations in the genes that encode checkpoint proteins, cells can bypass the normal controls and divide uncontrollably. For example, if the G1 checkpoint fails to detect DNA damage, the cell will proceed to replicate damaged DNA, leading to further mutations. A failure in the M checkpoint could lead to incorrect chromosome segregation, resulting in daughter cells with an abnormal number of chromosomes (aneuploidy). This uncontrolled proliferation and genomic instability are hallmarks of cancer, allowing cancerous cells to grow, invade, and spread.