New cells are produced by cell division. The two new cells produced when a cell divides are called daughter cells. When two daughter cells have the same number of chromosomes as the original cell, the process is called mitosis. Meiosis is a special type of cell division that halves the number of chromosomes to create eggs and sperm. Daughter cells can be about the same size as the original cell, or a small portion can bud off, creating a smaller daughter cell. In either case, the genetic material has to be duplicated and the contents of the cell need to be divided.
Making New Chromosomes
Chromosomes are made up of a double helix of DNA (deoxyribonucleic acid) and many proteins. Some of the proteins are structural, helping the chromosomes stay compacted in the nucleus. Other proteins regulate how genes are read and turned into RNA (ribonucleic acid) or help copy the strands of DNA so that new chromosomes can be made. Each DNA strand in the double helix is complementary to its partner, so as the DNA double helix gradually unwinds, the proteins can make new complementary strands, creating two chromosomes where there had been one.
Making a New Membrane
New lipids are synthesized and added to the cell membrane so that there will be enough membrane to enclose both daughter cells. Phospholipids are made from fatty acids and glycerol phosphate inside the endoplasmic reticulum (ER), an organelle inside the cell. The new lipids are transported via vesicles that fuse with the plasma membrane.
Making New Proteins
Cells constantly make new proteins, and many are made before cells divide. Some of the proteins need to be divided between the two daughter cells so they can continue to function. Other proteins create the mitotic spindle, which organizes and sorts the chromosomes into the daughter cells. Still other proteins make a "contractile ring" that gradually squeezes the original cell into two cells.
Making New Organelles
Cells also continually make new organelles, just like they make new proteins. While each daughter cell needs to have exactly one copy of each chromosome, the exact number of other organelles can vary. Copies of the ER and Golgi apparatus (which together synthesize most of the molecules used by the cell) and mitochondria (which make energy for the cell) are randomly divided between the two daughter cells after the chromosomes have been segregated.
Dividing the Cells
After the chromosomes have been copied and separated carefully so that each daughter cell has one copy of each chromosome, the contents of the cell are divided by the gradual contraction of a band of proteins under the cell membrane. The contractile ring gets smaller and smaller until there are two cells; it's almost like the twisting that turns a balloon into a balloon animal. Once the cells have divided, they can start growing and getting ready to divide again.