Mitosis

Why do cells need to divide?

Cells divide for three major reasons:

  • Growth: From a single cell to a whole organism—cell division makes it possible.

  • Repair & Replacement: Lost skin cells? A broken bone? Mitosis steps in to rebuild and replace.

  • Reproduction: Life passes itself on through cell division—either asexually (like bacteria using binary fission) or sexually (via meiosis).


Inside the nucleus of a eukaryotic cell, DNA usually exists as a loose tangle called chromatin. But before a cell divides, it bundles that DNA into chromosomes, thanks to proteins called histones.

Each chromosome duplicates itself, creating sister chromatids—identical copies joined at a region called the centromere.

Humans have:

  • 46 chromosomes in body cells (diploid = 2n)

  • 23 chromosomes in sex cells (haploid = n)

Want to see chromosomes? Scientists use karyotypes, which visually arrange chromosomes by size and type. This helps identify the sex chromosomes (XX for females, XY for males) and the autosomes (all the others).


Most of a cell’s life is spent in interphase, getting ready for the big event:

  1. G1 Phase: Growth and protein production

  2. S Phase: DNA replication

  3. G2 Phase: Final prep and error check

  4. G0 Phase: A pause—some cells stay here and don’t divide again

After interphase comes the mitotic phase, where the action happens:

  • Mitosis: Nucleus divides

  • Cytokinesis: Cytoplasm divides

Cyclins are the proteins that act as the cell's timekeepers, making sure things don’t move forward unless everything checks out.Mitosis produces two identical daughter cells with the same number of chromosomes as the parent. Here’s how:

  1. Interphase: DNA is loose (chromatin), preparing for division.

  2. Prophase: Chromatin condenses into chromosomes; spindle fibers form.

  3. Metaphase: Chromosomes line up in the middle.

  4. Anaphase: Sister chromatids are pulled apart.

  5. Telophase: Two new nuclei form; chromosomes unravel.

  6. Cytokinesis: Cell splits—animals pinch, plants build a wall.

Here is a chart that I found very helpful in explaining the steps of mitosis:








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