Compare and contrast asexual and sexual reproduction

including methods of sexual reproduction and asexual reproduction (budding, fragmentation, regeneration, binary fission, vegetative reproduction)

 

Asexual Reproduction

 

In many simple organisms, reproduction is not a very complicated thing. It generally involves only one organism. The resulting offspring often have the exact same genetic information as the parent. This type of reproduction in which one parent is involved in the production of an identical offspring is called asexual reproduction.

 

The main ways that asexual reproduction can take place are:

 

Binary Fission: A situation in which the parent cell splits in half producing two identical cells. Binary fission is the form of asexual reproduction used by most prokaryotes (bacteria) and protists to reproduce. This process results in the reproduction of a living cell by division into two equal or near-equal parts.

 

Fragmentation is another way to reproduce asexually. The parent breaks into different fragments, which eventually form new individuals. This process is exemplified by certain flatworms known as planarians. This type of reproduction would also occur in molds, yeast, and mushrooms, all of which are part of the Fungi family of organisms. These organisms produce tiny filaments called Hyphae. When a piece of hyphae breaks off and grows into a new individual, this is called fragmentation.

 

In regeneration, when an animal that is capable of regeneration loses a body part, it can grow a replacement part. If the lost body part contains enough genetic information from the parent, it can regenerate into an entirely new organism. Echinoderms are examples of animals that use regeneration.

 

Budding: When conditions are favorable. yeast cells can reproduce through budding. Once a copy of the genetic material is made, a bud begins to form outside the body of the yeast cell. It continues to grow larger until, eventually, it breaks away to form a new individual cell.

 

Vegetative reproduction is form of duplication using only mitosis also called vegetative propagation or vegetative multiplication. It is a process by which new plant "individuals" arise or are obtained without production of seeds or spores. Example, a new plant grows out of the root or a shoot from an existing plant. Vegetative reproduction produces only genetically identical offspring since all divisions are by mitosis. Offspring are called clones meaning that each is an exact copy of the original organism. A plant that persists in a location through vegetative reproduction of individuals over a long period of time constitutes a clonal colony. This method of reproduction is rapid and effective allowing the spread of an organism. It is both a natural process in many plant species and one utilized or encouraged by horticulturists to obtain quantities of economically valuable plants.

 

Advantages of Asexual Reproduction

 

Asexual reproduction can be very advantageous to certain animals. For instance, animals that remain in one particular place and are unable to look for mates would need to reproduce asexually. Another advantage of asexual reproduction is that numerous offspring can be produced without "costing" the parent a great amount of energy or time. Environments that are stable and experience very little change are the best places for organisms that reproduce asexually. The cloned offspring are more likely to succeed in the same stable areas as their parents.

 

For photos of different species undergoing asexual reproduction go to: http://biology.about.com/library/weekly/aa090700a.htm

 

Sexual Reproduction

 

In sexual reproduction, two individuals produce offspring that have genetic characteristics from both parents. Sexual reproduction introduces new gene combinations in a population. New gene combinations for the species are the advantage of sexual reproduction. The disadvantage is that sexual reproduction requires organisms to find a mate (consuming time and energy, and increasing predation risk) and it is more complicated.

 

Gametes - In animals, sexual reproduction encompasses the fusion of two distinct gametes to form a zygote. Gametes are produced by meiosis. The gametes are haploid (containing only one set of chromosomes) while the zygote is diploid (containing two sets of chromosomes). In most cases, the male gamete, called the spermatozoan, is relatively motile and usually has a flagellum. On the other hand, the female gamete, called the ovum, is nonmotile and relatively large in comparison to the male gamete.

 

Haploid and diploid are terms referring to the number of sets of chromosomes in a cell. Gregor Mendel determined his peas had two sets of alleles, one from each parent. Diploid organisms are those with two (di) sets. Human beings (except for their gametes), most animals and many plants are diploid. We abbreviate diploid as 2n. Ploidy is a term referring to the number of sets of chromosomes. Haploid organisms/cells have only one set of chromosomes, abbreviated as n. Chromosomes that carry the same genes are termed homologous chromosomes. The alleles on homologous chromosomes may differ, as in the case of heterozygous individuals. Organisms (normally) receive one set of homologous chromosomes from each parent. There are 44 autosomes and 2 sex chromosomes in the human genome, for a total of 46.  (2n=46)    (n=23)

 

Meiosis is a special type of nuclear division which segregates one copy of each homologous chromosome into each new "gamete". Mitosis maintains the cell's original ploidy level (for example, one diploid 2n cell producing two diploid 2n cells; one haploid n cell producing two haploid n cells; etc.). Meiosis, on the other hand, reduces the number of sets of chromosomes by half, so that when gametic recombination (fertilization) occurs the ploidy of the parents will be reestablished.

 

For an interactive mitosis meiosis diagram Click on Activity @

http://w3.dwm.ks.edu.tw/bio/activelearner/09/ch9c1.html