Biology, Answering the Big Questions of Life/cell division
How do things grow?
We know that living things grow. A plant will get taller and produce new leaves and flowers. A baby will grow into a child and then an adult. Growing is a complex business. Growing is capable to a certain living organism.
Growth means getting larger in size, and for multi-cellular organisms this is done by making more cells.
Plants have special tissues called meristems where growth occurs. Root apical meristems grow down into the soil, and shoot apical meristems grow up. A third meristem found as a ring in the stem of some plants grows outward.
Animals, however grow by making new cells all over their body, and when an animal first develops as an embryo, almost all cells are dividing together. Later, as the animal reaches adulthood, most cells are not dividing. Some tissues, such as skin, divide constantly, while others, such as nerve cells, don't divide at all.
Single celled organisms increase their numbers by dividing and making more cells like themselves.
When cells are dividing, they grow in size for a bit and then split in two. The cells must make copies of their chromosomes (the information storing part of the cell) before they split. Bacteria only have one chromosome. Therefore they can split very simply by making a copy of the chromosome, and making sure one chromosome ends up in each cell after it divides. This process is called binary fission.
Eukaryotic cells ( cells with a nucleus containing several chromosomes) can't be so casual about cell division. In order to make two cells containing all of the information found in the original cell, they must copy each chromosome and then divide. This more complex process is called Mitosis.
There is even another form of cell division that is reserved for sex cells. When humans reproduce, the mother gives half of her chromosomes and the father gives half of his chromosomes to the offspring. If the mother and father gave all of their chromosomes to their child, it would have twice as many chromosomes as it's parents. A special kind of cell division is used to make these haploid cells containing half the number of chromosomes as the parents. This sexual cell division is called Meiosis.