Connection for AP® Courses
As we explored the cell cycle and mitosis in a previous chapter, we learned that cells divide to grow, replace other cells, and reproduce asexually. Without mutation, or changes in the DNA, the daughter cells produced by mitosis receive a set of genetic instructions that is identical to that of the parent cell. Because changes in genes drive both the unity and diversity of life, organisms without genetic variation cannot evolve through natural selection. Evolution occurs only because organisms have developed ways to vary their genetic material. This occurs through mutations in DNA, recombination of genes during meiosis, and meiosis followed by fertilization in sexually reproducing organisms.
Sexual reproduction requires that diploid (2n) organisms produce haploid (1n) cells through meiosis and that these haploid cells fuse to form new diploid offspring. The union of these two haploid cells, one from each parent, is fertilization. Although the processes of meiosis and mitosis share similarities, their end products are different. Recall that eukaryotic DNA is contained in chromosomes, and that chromosomes occur in homologous pairs—homologues. At fertilization, the male parent contributes one member of each homologous pair to the offspring, and the female parent contributes the other. With the exception of the sex chromosomes, homologous chromosomes contain the same genes, but these genes can have different variations, called alleles. For example, you might have inherited an allele for brown eyes from your father and an allele for blue eyes from your mother. As in mitosis, homologous chromosomes are duplicated during the S-stage, or synthesis, of interphase. However, unlike mitosis, in which there is just one nuclear division, meiosis has two complete rounds of nuclear division—meiosis I and meiosis II. These result in four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell (1n). The first division, meiosis I, separates homologous chromosomes, and the second division, meiosis II, separates chromatids. Remember—during meiosis, DNA replicates ONCE but divides TWICE, whereas in mitosis, DNA replicates ONCE but divides only ONCE.
Although mitosis and meiosis are similar in many ways, they have different outcomes. The main difference is in the type of cell produced: Mitosis produces identical cells, allowing growth or repair of tissues; meiosis generates reproductive cells, or gametes. Gametes, often called sex cells, unite with other sex cells to produce new, unique organisms.
Genetic variation occurs during meiosis I, in which homologous chromosomes pair and exchange nonsister chromatid segments, or crossover. Here the homologous chromosomes separate into different nuclei, causing a reduction in ploidy. During meiosis II—which is more similar to a mitotic division—the chromatids separate and segregate into four haploid sex cells. However, because of crossover, the resultant daughter cells do not contain identical genomes. As in mitosis, external factors and internal signals regulate the meiotic cell cycle. As we will explore in more detail in a later chapter, errors in meiosis can cause genetic disorders, such as Down syndrome.
Information presented and the examples highlighted in the section support concepts and Learning Objectives outlined in Big Idea 3 of the AP® Biology Curriculum Framework. The Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP® Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A Learning Objective merges required content with one or more of the seven Science Practices.
Big Idea 3 |
Living systems store, retrieve, transmit, and respond to information essential to life processes. |
Enduring Understanding 3.A |
Heritable information provides for continuity of life. |
Essential Knowledge
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3.A.2
In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.
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Science Practice |
6.2
The student can construct explanations of phenomena based on evidence produced through scientific practices.
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Learning Objective
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3.9
The student is able to construct an explanation, using visual representations or narratives, as to how DNA in chromosomes is transmitted to the next generation via mitosis, or meiosis followed by fertilization.
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Essential Knowledge
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3.A.2
In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization.
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Science Practice |
7.1
The student can connect phenomena and models across spatial and temporal scales.
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Learning Objective
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3.10
The student is able to represent the connection between meiosis and increased genetic diversity necessary for evolution.
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The Science Practices Assessment Ancillary contains additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards:
- [APLO 1.9]
- [APLO 2.15]
- [APLO 2.39]
- [APLO 3.11]
- [APLO 3.9]
In addition, content from this chapter is addressed in the AP Biology Laboratory Manual in the following lab(s):
You read that fertilization is the union of two sex cells from two individual organisms. If these two cells each contain one set of chromosomes, the resulting fertilized cell contains two sets of chromosomes. Haploid cells contain one set of chromosomes. Cells containing two sets of chromosomes are called diploid. The number of sets of chromosomes in a cell is called its ploidy level. If the reproductive cycle is to continue, a diploid cell must reduce the number of its chromosome sets before fertilization can occur again. Otherwise, the number of chromosome sets would double and continue to double in every generation. So, in addition to fertilization, sexual reproduction includes a nuclear division that reduces the number of chromosome sets.
Most animals and plants are diploid, containing two sets of chromosomes. In an organism’s somatic cells, sometimes referred to as body cells—all cells of a multicellular organism except the reproductive cells—the nucleus contains two copies of each chromosome, called homologous chromosomes. Homologous chromosomes are matched pairs containing the same genes in identical locations along their length. Diploid organisms inherit one copy of each homologous chromosome from each parent; all together, they are considered a full set of chromosomes. Haploid cells, containing a single copy of each homologous chromosome, are found only within an organism's reproductive structures, such as the ovaries and testes. Haploid cells can be either gametes or spores. Male gametes are sperm and female gametes are eggs. All animals and most plants produce gametes. Spores are haploid cells that can produce a haploid organism or can fuse with another spore to form a diploid cell. Some plants and all fungi produce spores.
As you have learned, the nuclear division that forms haploid cells—meiosis—is closely related to mitosis. Mitosis is the part of a cell reproduction cycle that results in identical daughter nuclei that are also genetically identical to the original parent nucleus. In mitosis, both the parent and the daughter nuclei are at the same ploidy level—diploid for most plants and animals. Meiosis employs many of the same mechanisms as mitosis. However, the starting nucleus is always diploid and the nuclei that result at the end of a meiotic cell division are haploid. To achieve this reduction in chromosome number, meiosis consists of one round of chromosome duplication and two rounds of nuclear division. Because the events that occur during each of the division stages are analogous to the events of mitosis, the same stage names are assigned. However, because there are two rounds of division, the major process and the stages are designated with a “I” or a “II.” Thus, meiosis I is the first round of meiotic division and consists of prophase I, prometaphase I, and so on. Meiosis II, in which the second round of meiotic division takes place, includes prophase II, prometaphase II, and so on.