Every Cell Comes From a Cell That Divided Before ItA whole quaking aspen forest can turn colour at the same moment because every tree is a genetic clone of one original.Those identical trees exist because new cells are made only by the division of pre-existing cells.No cell ever appears from nothing, so every cell you have was copied from an earlier one.NoteNew cells arise only by the division of cells that already exist.This single rule links growth, repair, and reproduction to one unbroken chain of cells.Cell Division: One Cell Splits Into TwoCell division is the process where a parent cell divides to produce two daughter cells.The parent cell does not survive alongside its offspring, because it becomes the daughter cells.This makes cell division essential for three things in living organisms.Growth: it increases the number of cells in an organism.Repair: it replaces old or damaged cells.Reproduction: it makes new individuals in single-celled organisms.Common MistakeDo not picture cell division like an animal parent that stays around after the young are born.The parent cell stops existing as a separate cell once it has divided.Cell Theory: Life Is One Unbroken Chain of CellsA key part of cell theory is that all cells arise from pre-existing cells.In a multicellular organism this chain traces back to the zygote, the single cell made when a sperm and egg fuse.The zygote divides again and again to build every cell in the body.Following this chain backwards connects your cells to the earliest cells on Earth, which is the continuity of life.AnalogyThink of a lit candle that can only be lit from another already-burning flame.A cell is the same, it can only come from a living cell that came before it.Before Dividing, a Cell Copies Its DNAFor the chain of life to hold, each daughter cell must receive a complete set of genetic instructions.So a cell copies its DNA before it divides, giving two identical copies of every chromosome.The detailed steps of copying DNA and separating the copies are covered in the sub-topics that follow this one.TipFor D2.1.1, remember the big idea rather than the mechanism.The examinable point here is that new cells only ever come from existing cells.Test yourself on the core idea of this sub-topic before moving on.Active recallState where new cells come from, according to cell theory.Define the terms parent cell and daughter cell.Name the single cell that all cells in a multicellular organism trace back to.Explain what is meant by the continuity of life.Give one reason why a cell must copy its DNA before it divides.
