Some living things reproduce by cloning; producing offspring genetically identical to the parent. Organisms that procreate sexually “mix it up” genetically to create truly unique offspring.
Asexual vs. Sexual Reproduction
* Prokaryote Binary Fission *
Many single-celled organisms reproduce by binary fission; the splitting of a parent cell after genetic material has been duplicated. Each “daughter cell” is a clone, an exact copy of the parent.
* Eukaryotes and Asexual Reproduction *
Some multicellular organisms can also reproduce asexually, to produce clones (offspring genetically identical to parent). The hydra, a relative of jellyfish, can reproduce via budding. A miniature version of the parent grows as a bud of mitotically dividing cells. When the bud is sufficiently developed, it detaches from the parent and becomes an independent hydra.
Another example of asexual reproduction is the fragmentation of starfish. If part of an arm breaks off, the starfish not only regenerates the arm, but the broken piece of arm can grow into an entire starfish!
Starfish (a.k.a. seastars) prey on abalone, a type of mollusk. Human who harvest abalone must compete with starfish for the shellfish delicacies. At one time it was common practice for abalone fishermen to try to reduce the number of starfish by catching them, cutting them up and dumping the starfish pieces back into the sea. Ooops! The fishermen inadvertently increased the number of starfish!
* Asexual Reproduction Is Mitosis *
When these eukaryotic, multicellular organisms, like starfish and hydra, reproduce via budding or fragmentation, the reproduction is accomplished through mitosis; a type of cell division in which cells divide to produce genetically identical clone cells.
This is the same process of cellular division that allows the human body to grow and develop from a fertilized egg into an adult human. Mitosis also enables our cells to repair injuries to the body, producing new cells to replace older cells or damaged tissue. Most cells in the body are produced by mitosis.
Only gametes (sperm and eggs) are produced by meiosis, a different type of cell division that produces only sex cells.
* Sexual Reproduction Is Meiosis *
Although sexually reproducing organisms often resemble their parents more closely than they resemble less related individuals, offspring that result from sexual reproduction are not clones of their parents, nor are they identical to their siblings.
Sexual reproduction allows for multiple instances of “genetic shuffling”; including the independent assortment and crossover of meiosis and the combination of genes from two parents.
Offspring acquire genes from parents by inheriting a set of chromosomes from each parent; half of the genes from a female parent and half from a male parent. These genes, and the chromosomes in which they reside, are transmitted to the offspring through the parents’ gametes (sperm of the father and the egg of the mother). The fusion of two gametes to produce a single zygote.
* More Information *
To learn more about cell biology, see these excellent website Cells Alive
Campbell, N. A. & Reece J. B. (2005) Biology, seventh edition. Pearson Education Inc.
Campbell, N. A., Reece J. B. & Simon, E. (2004) Essential Biology with Physiology. Pearson Education Inc.