Module 5 Lesson 5 - 2

Asexual Reproduction

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Recall from Lesson 1 that two basic methods of cell reproduction exist. Mitosis produces two new daughter cells from a parent cell quickly but with little variation, and meiosis produces daughter cells with a reduced number of chromosomes that must fertilize or be fertilized by other cells to produce varied offspring.

If mitotic division leads to a new organism, it is asexual reproduction. Examples of asexual reproduction are binary fission, budding, fragmentation, and parthenogenesis. If meiosis, followed by fertilization, leads to new offspring, it is sexual reproduction.

Binary Fission

Sometimes reproduction can be very simple. Consider bacteria. They do not have chromosomes, and they hold their entire DNA in a single loop. When they wish to divide, they duplicate their DNA loop, attach the ends of each to their cell wall, and grow a new wall between the loops. This process is called binary fission and can be completed in as little as 20 minutes.

This leads quickly to large populations of bacteria. As a result, each organism is identical, and the whole population could be susceptible to a toxin or a change in the environment. This toxin or environmental change could lead to the extinction of the population because no individual would have the genetic variation for resistance.

Budding

Another quick method common to small aquatic organisms is budding. In budding, the parent organism begins growing a new organism from its body through mitosis. The new organism, or the 'bud', separates and becomes a new individual. In a similar way, many plants grow long horizontal stems that reach new areas and then grow new full plants on the ends of those stems. An example is the strawberry plants that might be in your garden. The horizontal stems usually are called runners.

Fragmentation

An artificial way to reproduce that is very similar to budding is to take a cutting of a plant and encourage it to root. This kind of reproduction is called fragmentation. In this way, a clone is made of the parent plant because the new cells will grow through mitosis.

Although asexual reproduction is rare in animals, some unique cases are interesting. You may be familiar with sea stars and their ability to reproduce through fragmentation. Flat worms can do it, too. Even more strange, when a flat worm is cut only partway down the middle, it will re-grow each half of the head that is missing and then share the tail!

 

Parthenogenesis

Other animals can produce unfertilized eggs that grow into whole organisms. This is called parthenogenesis. A queen bee does this to produce more sterile male drones. Female Komodo dragons can produce offspring this way when no males are present! When males are present, they can reproduce in the regular sexual way.

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One advantage of asexual reproduction is the speed at which more offspring are produced. To understand how different the resulting numbers of offspring can be in sexual reproduction and asexual reproduction, watch this video by Dr. Brett Finlay from Howard Hughes Medical Institute on Bacterial Growth  

Dr. Finlay discusses how bacteria can grow rapidly to incredible numbers, and he explains factors that limit this explosive growth.