Mitosis

Mitosis refers to the exact copying of cells, specifically the division of the nucleus. It is crucial for growth, repair (like regenerating a lizard's tail or forming a scab), and asexual reproduction. Every cell in a multicellular organism starts from a single cell and contains the exact same DNA, maintained through mitosis.

Haploid cells (n) contain one copy of each chromosome (e.g., sperm and egg cells in humans have n=23 chromosomes). Diploid cells (2n) contain two copies of each chromosome (e.g., human somatic cells have 2n=46 chromosomes), one from each parent. These chromosomes are linear stretches of DNA.

The cell cycle describes how one cell divides into two. It begins with interphase, divided into G1, S, and G2 phases. G1 (Gap 1) is where the cell grows. S (Synthesis) is when DNA is copied through replication. G2 (Gap 2) involves further growth. Most of a cell's life (e.g., nerve cells, muscle cells) is spent in interphase, or G0 if they don't divide.

A duplicated chromosome has a characteristic 'X' shape, consisting of two identical sister chromatids joined at a centromere. Each chromatid is a copy of the DNA, created during the S phase. Chromosomes also have a short arm and a long arm. DNA is highly condensed and wrapped around histone proteins to form this compact structure from loose chromatin.

Mitosis is a continuous process, not jerky phases, remembered as IPMAT: Interphase, Prophase, Metaphase, Anaphase, and Telophase. These phases describe the stages of nuclear division.

In prophase, chromatin condenses into visible chromosomes, the nucleolus disappears, and centrosomes move to opposite sides, forming spindle fibers. In prometaphase, the nuclear envelope breaks down, and spindle microtubules attach to kinetochores (protein structures) on the centromeres of chromosomes.

Metaphase is characterized by chromosomes aligning along the metaphase plate (the cell's equator), pulled by spindle fibers. In anaphase, sister chromatids separate and are pulled apart by the spindle fibers towards opposite poles of the cell, ensuring each new cell gets an identical set of genetic material.

Telophase marks the 'end' of nuclear division, where new nuclear envelopes form around the separated chromosomes at each pole. Chromosomes decondense back into chromatin, and nucleoli reappear. Cytokinesis is the final step, involving the division of the cytoplasm and its organelles, resulting in two distinct daughter cells. In plant cells, a cell plate forms to divide the cell, whereas animal cells form a cleavage furrow.

The cell cycle and mitosis are typically well-regulated. However, uncontrolled cell growth, or uncontrolled mitosis, leads to cancer. This can be caused by mutations that suppress genes responsible for controlling cell division. These rapidly reproducing cells form tumors and can spread throughout the body (metastasis). The video illustrates the link between cigarette consumption and lung cancer as an example of carcinogens causing such mutations.