Summary
Highlights
The cell cycle is a series of phases a cell undergoes to replicate itself, turning one cell into two identical cells. This process is crucial for cell replication and controlling cell growth. This video will cover interphase and mitosis, using models to visualize the changes within the cell.
A cell is the basic unit of all living things, typically characterized by a cell membrane (a phospholipid bilayer), a nucleus housing genetic material (chromatin), and cytoplasm. The goal of the cell cycle is to duplicate the DNA and create an identical cell.
The G1 phase is the first stage where the cell prepares for replication. It involves increasing the number of organelles and synthesizing proteins and enzymes necessary for DNA replication. Most cells, depending on their type, spend a significant amount of time in this phase. This phase also includes repair of thymine dimers in the DNA.
There are three types of cells based on their proliferative capacity: labile (proliferative) cells, which constantly replicate (e.g., skin, GI tract, hematopoietic stem cells); stable cells, which replicate when stimulated (e.g., liver, kidney tubules, alveolar cells); and permanent (amitotic) cells, which do not replicate (e.g., neurons, skeletal muscle, cardiac muscle).
The S phase is where DNA replication occurs. The cell takes its genetic material, opens it up to form replication bubbles, and synthesizes new DNA using a semi-conservative model. DNA polymerases are highly efficient at this process, with few mistakes. This phase is usually about six hours long and involves a G1-S phase checkpoint to ensure DNA integrity.
In the G2 phase, the cell focuses on growth by increasing cytoplasm and other cellular components to ensure an equal distribution when the cell divides. This prepares the cell for mitosis and helps ensure the two daughter cells are of equal size and content. This phase is approximately two hours long.
Interphase collectively includes the G1, S, and G2 phases, in that order. Before transitioning from G1 to S, there's a G1-S checkpoint to regulate the cell cycle progression.
Mitosis, or the M phase, consists of four main parts: prophase, metaphase, anaphase, and telophase. In prophase, the loose chromatin condenses into visible chromosomes. The nuclear envelope begins to dissolve, and microtubule organizing centers (centrioles/centrosomes) appear, which will form the spindle fibers.
A chromosome is made of chromatin (DNA and histone proteins). After DNA replication, a chromosome consists of two identical halves called sister chromatids, joined at a centromere. The number of centromeres determines the number of chromosomes.
In metaphase, the nuclear envelope is fully dissolved. The microtubule organizing centers move to opposite poles of the cell, and polar microtubules attach to the kinetochores (protein structures on the centromeres) of the chromosomes. These chromosomes then align perfectly along the metaphase plate (the cell's midline).
Anaphase involves the separation of sister chromatids. The cohesin protein holding them together is cleaved, and motor proteins (dynein and kinesin) move the individual chromatids (now considered chromosomes) towards opposite poles of the cell along the microtubules. This ensures each pole receives an equal set of 46 chromosomes.
In telophase, the cell begins to reform the nuclear envelope around the separated chromosomes at each pole. The condensed chromatin starts to decondense. Simultaneously, actin and myosin proteins form a contractile ring, creating a cleavage furrow that indents the cell membrane. This process, called cytokinesis, eventually pinches the cell into two, equally distributing cytoplasm and organelles. The result is two daughter cells, each with 46 chromosomes (2n), an equal amount of cytoplasm, and organelles.
Visual aids demonstrate the characteristics of each mitotic phase: loose chromatin in interphase, breaking down nuclear envelope and centrosomes in prophase, chromosomes aligned on the metaphase plate, separated chromatids moving to poles in anaphase, and the reforming nuclear envelope/cleavage furrow in telophase.
After replication, cells can re-enter the cell cycle (labile cells), enter a quiescent G0 phase (stable cells, awaiting a strong stimulus), or remain permanently out of the cell cycle (permanent cells). Aging shortens telomeres, potentially leading to cellular senescence, an irreversible exit from the cell cycle. Crucial checkpoints exist at G1-S, G2-M, and Metaphase (M checkpoint) to ensure DNA integrity and proper chromosome alignment before progression.