Module 5

Module 5—Cell Division: The Processes of Mitosis and Meiosis

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Lesson 1 presented the 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 there is meiosis followed by fertilization, this leads to new offspring, and it is known as sexual reproduction.

 

Asexual Reproduction

 

 

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 divide, they duplicate their DNA loop, attach the ends of each loop 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. Binary fission can produce 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, or “runners,” on the ends of those stems. An excellent example is strawberry plants.

 

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, you 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 from page 573 up to the heading “Alternation of Generations” on page 575 of your textbook.

 

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 stranger, if a flat worm is cut only partway down the middle, it will regrow 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 male Komodo dragons present, reproduction is sexual.

 

Try This

 

One of the advantages of asexual reproduction is the speed at which more offspring are created. To study the difference in numbers of offspring produced in sexual versus asexual reproduction, do an Internet search using the search terms “generation calculator” and “Stanford.” When you load the calculator, start with a simulation of ten generations and only two offspring per generation.

 

TR 1. Why is there such a big difference between asexual and sexual reproduction?

TR 2. What happens if you increase the number of offspring per generation?

TR 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 gatekeepers 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 multicellular 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. Read from “Alternation of Generations” on page 575 to the end of page 577 of your textbook for more information.

 

 

Ferns are an example of alternation of generations. Consider the graphic here or on page 576 in your textbook. Can you tell which generation is haploid and which is diploid? Remember, haploid means only one set of chromosomes (n), so no homologues are present, while diploid means to have two sets or (2n). Note the role of mitosis, meiosis, and fertilization in alternation of generations. The process that changes an organism from haploid to diploid is fertilization. The process that reduces the organism back to haploid is meiosis.

 

Some life forms will reproduce either sexually or asexually. Bacteria are a good example of this versatility in reproduction. They may reproduce asexually by binary fission, or grow extensions into neighbouring 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. This spore can resist drying out and may lay dormant for a long time. When conditions are favourable, the spore will undergo meiosis to form haploid yeast cells that will likely colonize through budding (mitosis).

 

What are the potential advantages and disadvantages of each type of reproduction? Read pages 578 to 580 of your textbook. Create a summary table or graphic organizer for each type of reproduction and save it in your course folder for review.

 

 

Watch and Listen

 

To review mitosis, meiosis, and how they are part of various reproductive strategies, watch the video “Asexual Reproduction and Alternation of Generations: Successful Game Plans for Survival.” Compare their lists of advantages and disadvantages with the lists you have created.

 

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