Cracking the AP Biology Exam
AN OVERVIEW OF MEIOSIS
To preserve the diploid number of chromosomes in an organism, each parent must contribute only half of its chromosomes. This is the point of meiosis. Meiosis is the production of gametes. Since sexually reproducing organisms need only haploid cells for reproduction, meiosis is limited to sex cells in special sex organs called gonads. In males, the gonads are the testes, while in females they are the ovaries. The special cells in these organs—also known as germ cells—produce haploid cells (n), which then combine to restore the diploid (2n) number during fertilization:
female gamete (n) + male gamete (n) = zygote (2n)
When it comes to genetic variation, meiosis is a big plus. Variation, in fact, is the driving force of evolution. The more variation there is in a population, the more likely it is that some members of the population will survive extreme changes in the environment. Meiosis is far more likely to produce these sorts of variations than is mitosis, and therefore confers selective advantage on sexually reproducing organisms. We’ll come back to this theme in Chapter 12.