Module 5 Cell Division

Read

Recall from lesson one that there are two basic methods of cell reproduction. Mitosis, which creates two new daughter cells from a parent cell quickly but with little variation, and meiosis, which creates daughter cells with a reduced number of chromosomes that must fertilize or be fertilized by other cells to create varied offspring. If mitotic division leads to a new organism, it is generally referred to as asexual reproduction. If meiosis, followed by fertilization, leads to new offspring, it is known as sexual reproduction.

Asexual Reproduction

Sometimes reproduction can be very simple. Consider bacteria. They do not have chromosomes, and hold their entire DNA in a single loop. If 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 to large populations of bacteria, or prokaryotes, quickly. 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.

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’, then 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 excellent example you may have in your garden would be strawberry plants (we sometimes call them runners when they spread).

An artificial way to reproduce much like budding is to take a cutting of a plant and encourage it to root. This kind of reproduction is called fragmentation. In this way we are actually making a clone of the parent plant since the new cells will grow through mitosis. To review these and other asexual methods of reproduction read pp. 573-575 of your text.

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

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

Try This

One of the advantages of asexual reproduction is the speed at which you are creating more offspring. To get an idea of how different the resulting numbers of offspring can be between sexual and asexual reproduction, try out this generation calculator. When you load the calculator, start with a simulation of 10 generation and only two offspring per generation.

1. Why is there such a big difference between asexual and sexual?
2. What happens if you increase the number of offspring per generation?
3. Is there a point where sexual reproduction approaches the high numbers attained by asexual reproduction?

Sexual Reproduction and Alternation of Generations

With sexual reproduction there are always two important processes: meiosis and fertilization. These processes act as gate keepers between life as a diploid organism and life as a haploid organism. Recall that diploid cells have two sets of chromosomes present while haploid cells only have one. Haploid cells do not contain homologous chromosomes.

Animals do alternate between diploid and haploid. However their life cycle is nearly completely dominated by the diploid generation. The haploid side does not grow and produce large cell masses through mitosis.

Plants alternate between haploid and diploid generations with growth on both sides. This variance between generations is known as alternation of generations. The diploid generation is referred to as the sporophyte generation, as it produces haploid spores to start the next generation. Each haploid spore grows through mitosis into a multi cellular structure referred to as the gametophyte. It is the gametophyte that actually produces male and female gamete cells that will fertilize to form a new sporophyte generation.

Ferns are a good example of an organism that spends much time in both parts of the life cycle. Consider the graphic here, or in your text on p. 576. Can you tell which generation is haploid and which is diploid? Remember, haploid means only one set of chromosomes, so no homologues , while diploid means to have two sets or (2n). What process changes an organism from haploid to diploid? If you guessed fertilization, you were right.Which process reduces the organism back to haploid? Meiosis is correct.

Some life forms will reproduce either sexually or asexually. Bacteria can grow extensions into neighboring bacteria and exchange genetic information. This is called conjugation, and it results in a new, genetically varied daughter cell. Yeast can also reproduce many times asexually, and if conditions are harsh, they will fuse with another yeast cell and form a spore through meiosis. This spore can resist drying out and may lay dormant for a long time. When conditions are right, the spore will germinate into haploid yeast cells that will likely colonize through budding (mitosis).

Try This

What are the potential advantages and disadvantages of each type of reproduction? Read this section on pp. 578–579 of your textbook. Create a summary table for each type of reproduction and save it in your course folder for review.

Watch and Listen

Watch the following video on asexual reproduction and the alternation of generations. It will review mitosis and meiosis and go through the reproductive strategies we have introduced above. Compare their lists of advantages and disadvantages with the list you just created in the Try This.

Alterations of Generations: