New cells are created from a process called cell division. The new cells are produced when a cell, called the "mother cell" divides into new cells 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.
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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.
Cell Formation and Making a New Membrane
New lipids and phospholipids are synthesized and added to the cell membrane so that there will be enough membrane to enclose both daughter cells during cell formation. Phospholipids are made from fatty acids and glycerol phosphate inside the endoplasmic reticulum (ER), which is an organelle inside the cell. The new lipids are transported via vesicles that fuse with the plasma membrane.
New Cells Are Created From 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 once cell division has occurred.
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.
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 in a process called cytokinesis. 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.
Binary fission is a type of asexual reproduction/cell formation method where new cells are created from a single original cell. This is commonly used by prokaryotes and small unicellular organisms like protists.
As with "regular" mitosis, binary fission involves the duplication of genetic material and the separation of an original parent cell into two genetically identical daughter cells. It's quite similar to mitosis. However, because these cells are significantly simpler than eukaryotic cells, the division process is much simpler.
Organisms that reproduce via binary fission do not undergo meiosis since that process only occurs for organisms that reproduce sexually.