Cell Cycle Phases
Cell division is how organisms grow, repair damaged tissues, and reproduce. The cell cycle describes the complete sequence a cell goes through as it grows and then divides into two genetically identical daughter cells. It has two major parts: interphase (where the cell spends most of its time) and the mitotic phase (where the cell actually divides).
Interphase and Preparation for Cell Division
Interphase is the longest part of the cell cycle. The cell isn't just sitting around during interphase; it's actively growing, copying its DNA, and preparing for division. Interphase has three distinct sub-phases:
- G1 phase (first gap phase): The cell increases in size and synthesizes proteins and organelles. This is the cell's main growth period.
- S phase (synthesis phase): The cell replicates all of its DNA. By the end of S phase, every chromosome has been duplicated into two identical sister chromatids joined at the centromere.
- G2 phase (second gap phase): The cell continues to grow and produces the specific proteins (like tubulin for spindle fibers) it'll need for mitosis. The cell also checks that DNA replication was completed correctly.
Mitosis and Cytokinesis Stages
Mitosis is the division of the nucleus, producing two nuclei that are genetically identical to each other and to the original cell. It involves separating the duplicated chromosomes so each new nucleus gets a complete set.
Cytokinesis is the division of the cytoplasm into two separate daughter cells. It overlaps with the final stages of mitosis but differs between cell types:
- In animal cells, a cleavage furrow pinches the cell membrane inward until the cell splits in two.
- In plant cells, a cell plate forms along the middle of the cell from vesicles, building a new cell wall between the two daughter cells. Plant cells can't pinch inward because of their rigid cell wall.
Mitotic Phases
Mitosis proceeds through four phases, often remembered with the mnemonic PMAT: Prophase, Metaphase, Anaphase, Telophase.
Early Mitotic Phases
Prophase:
- Chromatin condenses into tightly coiled, visible chromosomes.
- The nuclear envelope begins to break down.
- Centrioles migrate to opposite poles of the cell and begin organizing spindle fibers.
- The nucleolus disappears.
Each chromosome at this point consists of two sister chromatids joined at the centromere, since DNA was already copied during S phase.
Metaphase:
- Spindle fibers attach to the centromere of each chromosome via protein complexes called kinetochores.
- Chromosomes are pulled and pushed until they align along the metaphase plate, an imaginary plane at the cell's equator.
This alignment is critical because it ensures each daughter cell will receive one copy of every chromosome.
Later Mitotic Phases
Anaphase:
- The centromeres split, and sister chromatids separate from each other.
- Spindle fibers shorten, pulling the now-individual chromosomes toward opposite poles of the cell.
- The cell elongates as polar microtubules push the poles apart.
Anaphase is typically the shortest phase of mitosis.
Telophase:
- Chromosomes arrive at opposite poles and begin to decondense back into chromatin.
- A nuclear envelope re-forms around each set of chromosomes.
- Nucleoli reappear in each new nucleus.
- Spindle fibers break down, and cytokinesis completes the division into two daughter cells.
Chromosomal Structures
Chromatin and Chromosomes
Chromatin is the complex of DNA wrapped around histone proteins. During interphase, chromatin exists in a loose, spread-out form so the cell can access genes for transcription. When the cell enters prophase, chromatin coils and condenses into compact chromosomes. This condensation is necessary so the long DNA molecules don't tangle or break during separation.
After DNA replication in S phase, each chromosome consists of two identical copies (sister chromatids) joined together, giving the familiar X-shaped appearance.
Centromeres and Chromatids
- Centromere: The constricted region where two sister chromatids are held together. It also serves as the attachment point for spindle fibers during mitosis. Without centromeres, spindle fibers would have no way to grab and move chromosomes.
- Sister chromatids: Two identical copies of a single chromosome, produced during S phase of interphase. They remain connected at the centromere until anaphase, when they separate and each becomes its own independent chromosome in the daughter cells.
Mitotic Apparatus
Spindle Fibers
Spindle fibers are structures made of microtubules (protein polymers of tubulin) that carry out the physical work of separating chromosomes. There are three types worth knowing:
- Kinetochore microtubules attach to the centromeres of chromosomes and pull sister chromatids apart during anaphase.
- Polar microtubules extend from opposite poles and overlap in the middle, pushing the poles apart to elongate the cell.
- Astral microtubules radiate outward from the centrosomes and help anchor the spindle apparatus in position.
The metaphase plate is the imaginary line at the cell's equator where chromosomes align during metaphase. This positioning ensures that when sister chromatids separate, each daughter cell receives an equal and complete set of chromosomes.
Centrioles
Centrioles are cylindrical structures composed of microtubules arranged in a characteristic 9+0 pattern (nine triplets of microtubules with no central pair). They play an organizational role in building the spindle during cell division.
Centrosomes are the main microtubule-organizing centers of the cell. Each centrosome contains a pair of centrioles oriented at right angles to each other. During S phase, the centrosome duplicates along with the DNA. In prophase, the two centrosomes migrate to opposite poles of the cell, where they organize the spindle fibers that will separate the chromosomes.
Note: Plant cells lack centrioles but still form functional spindles using other microtubule-organizing structures, so centrioles are helpful but not strictly required for mitosis.