Exploring Our Fluid Earth

All living organisms are capable of growing and producing offspring. All eukaryotic organisms—including aquatic plants and algae—grow through the process of mitosis. Mitosis is a process where one cell divides into two cells (Fig. 2.46). Chromosomes in the original cell are duplicated to ensure that the two new cells have full copies of the necessary genetic information.

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Fig. 2.46. Major events in mitosis

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Image courtesy of Mysid, Wikimedia Commons

The process of mitosis generates new cells that are genetically identical to each other. Mitosis helps organisms grow in size and repair damaged tissue. Some species of algae are capable of growing very quickly. The giant kelp Macrocystis pyrifera can grow as much as 30 centimeters (cm) in length in a single day.

Some organisms can use mitosis to reproduce asexually. The offspring of asexual reproduction are genetically identical to each other and to their parent. Most single-celled, microorganisms reproduce asexually by duplicating their genetic material and dividing in half. For example, phytoplankton reproduce primarily through asexual reproduction. Some single-celled eukaryotes, including some plants and animals, reproduce asexually in a processes called fragmentation or budding.

Sexual reproduction is the production of offspring through the combination of sex cells or gametes. Meiosis is the process of producing gametes, each of which has half of the genetic material needed to create a new organism (Fig. 2.47).

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Fig. 2.47. Major events in meiosis (see descriptions of events 1–5 in the following text). 1. Chromosomes are duplicated 2. 3. 4. 5. Second division separates each chromosome, leaving one copy of each type per cell.

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Image courtesy of Peter Coxhead, Wikimedia Commons

1. Chromosomes are duplicated. Meiosis begins in a fashion similar to mitosis with chromosome replication.
   
    
2. Matched sets of chromosomes pair together.
   
    
3. Genes are swapped between matched chromosomes. The process of crossing over, or recombination, exchanges genetic information between chromosomes in a cell. The resulting chromosomes are brand new, unique combinations of genetic information.
   
    
4. First division separates one of each chromosome pair. The parent cell divides in half as in mitosis, producing two cells with a complete amount of DNA (although they are not identical because of crossing over).
   
    
5. Second division separates each chromosome, leaving one copy of each chromosome per cell. The two new cells divide a second time to produce four new gametes. These gametes contain one-half of the genetic information needed to form a new individual.
   
    
6. Each parent provides one gamete to the process of fertilization, which results in a cell called a zygote with a full compliment of chromosomes.
   
    
7. Offspring produced through sexual reproduction are genetically distinct from both parents, since each of their gametes has a unique combination of chromosomes.
   
    

In summary, mitosis produces two identical cells, each with the full amount of DNA. Meiosis produces four genetically unique cells, each with half the amount of DNA. See Table 2.10 for a comparison of mitosis and meiosis.

Many species of algae have complex life histories and can reproduce through both sexual and asexual means. It is common for algae to have an alternation of generation, where one generation is made through mitotic cell division and the other is made from cells created through meiotic cell division.