Asexual reproduction

Reproduction is the process of making new living organisms. When new organisms are produced by just one parent, the process is almost always asexual, which means (without sex). There are many forms of reproduction by just one parent.

The simplest type of asexual reproduction is fission. In fission, one parent cell splits in half, producing two new cells. The new cells are identical to each other and to the original parent cell. Binary fission is the primary means of reproduction in prokaryotes, such as blue- green algae and bacteria, which lack a true nucleus. First the single, circular chromosome in a prokaryote duplicates, the process called replication. Both chromosomes then attach themselves to sites on the cell membrane. As the cell grows, a new cell membrane forms between the attachment sites, and the two chromosomes are forced apart. Then the cell membrane constricts in the center, ultimately separating the cell into two identical parts.

Budding is another type of asexual reproduction. In budding, a new organism - a bud - grows from the original organism. The bud is smaller than the original organism. It has to grow before it breaks away from the parent. The cell nucleus and cytoplasm divide into two cells of unequal size. The bud, the daughter cell, pinches off to become a new individual.

Starfish can reproduce in a special asexual way called regeneration. In regeneration, an entire new organism can grow from certain pieces, or cells, of the original organism.

One more method of asexual reproduction is through the production of tiny, asexual reproductive cells called spores. Like binary fission, spore formation begins with the replication of the chromosomes. A protective wall then forms around the cell to create the spores. After a spore leaves the parent cell, the spore can remain inactive until environmental conditions are favorable for the growth of the organism. The spore will then germinate.

Some plants produce organisms that are initially attached to the parent plant and then separate to become individual plants. This process is called vegetative propagation. Plants such as strawberries send out ‘runners’, or horizontal aboveground stems. The runners root and develop into new, individual plants when they contact fertile soil. Tuber plants, such as potatoes, propagate through underground runners that develop from ‘eyes’. Some plants form tiny plants on the edges of their leaves. The new plants drop off the leaves and root in the soil.

Sexual reproduction

Most kinds of living organisms reproduce sexually. Sexual reproduction requires one sex cell from each of two parents. It results from the joining of two specialized sex cells called gametes. The sex cell from the female parent is the egg, or an ovum. Fertilization occurs when the sperm cell (male gamete) and egg cell join and form a cell called a zygote. The cell that results is the first cell of the new organism. Because both parents contribute the same number of chromosomes to the zygote, the offspring is usually not identical to either parent but may have some characteristics of each.

Embriogenesis – the development of embryo (Gk. en, in + bryein, to swell): The early developmental stage of an organism produced from a fertilized egg; a young organisms before it emerges from the seed, egg, or the body of its mother.

Parthenogenesis (Gk. parthenos, virgin): the development without fertilization. It’s typical (distinctive) for worms, arthropods, and drones. There are natural, season and artificial parthenogenesis.

Date: 2014-12-22; view: 919