Meiosis

Introduction to Meiosis

Meiosis is the nuclear division of diploid cells into haploid cells, which is a necessary step in sexual reproduction.

Learning Objectives

Describe the importance of meiosis in sexual reproduction

Key Takeaways

Key Points

  • Sexual reproduction is the production of haploid cells and the fusion of two of those cells to form a diploid cell.
  • Before sexual reproduction can occur, the number of chromosomes in a diploid cell must decrease by half.
  • Meiosis produces cells with half the number of chromosomes as the original cell.
  • Haploid cells used in sexual reproduction, gametes, are formed during meiosis, which consists of one round of chromosome replication and two rounds of nuclear division.
  • Meiosis I is the first round of meiotic division, while meiosis II is the second round.

Key Terms

  • haploid: of a cell having a single set of unpaired chromosomes
  • gamete: a reproductive cell, male (sperm) or female (egg), that has only half the usual number of chromosomes
  • diploid: of a cell, having a pair of each type of chromosome, one of the pair being derived from the ovum and the other from the spermatozoon

Introduction: Meiosis and Sexual Reproduction

The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resemble their parent or parents. Sexual reproduction requires fertilization: the union of two cells from two individual organisms. 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, then the diploid cell must somehow reduce its number of chromosome sets before fertilization can occur again or there will be a continual doubling in the number of chromosome sets in every generation. Therefore, sexual reproduction includes a nuclear division that reduces the number of chromosome sets.

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Offspring Closely Resemble Their Parents: In kind means that the offspring of any organism closely resemble their parent or parents. The hippopotamus gives birth to hippopotamus calves (a). Joshua trees produce seeds from which Joshua tree seedlings emerge (b). Adult flamingos lay eggs that hatch into flamingo chicks (c).

Sexual reproduction is the production of haploid cells (gametes) and the fusion (fertilization) of two gametes to form a single, unique diploid cell called a zygote. All animals and most plants produce these gametes, or eggs and sperm. In most plants and animals, through tens of rounds of mitotic cell division, this diploid cell will develop into an adult organism.

Haploid cells that are part of the sexual reproductive cycle are produced by a type of cell division called meiosis. 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, so the resulting cells have half the chromosomes as the original. To achieve this reduction in chromosomes, 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, the second round of meiotic division, includes prophase II, prometaphase II, and so on.

Comparing Meiosis and Mitosis

Mitosis and meiosis share some similarities, but also some differences, most of which are observed during meiosis I.

Learning Objectives

Compare and contrast mitosis and meiosis

Key Takeaways

Key Points

  • For the most part, in mitosis, diploid cells are partitioned into two new diploid cells, while in meiosis, diploid cells are partitioned into four new haploid cells.
  • In mitosis, the daughter cells have the same number of chromosomes as the parent cell, while in meiosis, the daughter cells have half the number of chromosomes as the parent.
  • The daughter cells produced by mitosis are identical, whereas the daughter cells produced by meiosis are different because crossing over has occurred.
  • The events that occur in meiosis but not mitosis include homologous chromosomes pairing up, crossing over, and lining up along the metaphase plate in tetrads.
  • Meiosis II and mitosis are not reduction division like meiosis I because the number of chromosomes remains the same; therefore, meiosis II is referred to as equatorial division.
  • When the homologous chromosomes separate and move to opposite poles during meiosis I, the ploidy level is reduced from two to one, which is referred to as a reduction division.

Key Terms

  • reduction division: the first of the two divisions of meiosis, a type of cell division
  • ploidy: the number of homologous sets of chromosomes in a cell
  • equatorial division: a process of nuclear division in which each chromosome divides equally such that the number of chromosomes remains the same from parent to daughter cells

Comparing Meiosis and Mitosis

Mitosis and meiosis are both forms of division of the nucleus in eukaryotic cells. They share some similarities, but also exhibit distinct differences that lead to very different outcomes. The purpose of mitosis is cell regeneration, growth, and asexual reproduction,while the purpose of meiosis is the production of gametes for sexual reproduction. Mitosis is a single nuclear division that results in two nuclei that are usually partitioned into two new daughter cells. The nuclei resulting from a mitotic division are genetically identical to the original nucleus. They have the same number of sets of chromosomes, one set in the case of haploid cells and two sets in the case of diploid cells. In most plants and all animal species, it is typically diploid cells that undergo mitosis to form new diploid cells. In contrast, meiosis consists of two nuclear divisions resulting in four nuclei that are usually partitioned into four new haploid daughter cells. The nuclei resulting from meiosis are not genetically identical and they contain one chromosome set only. This is half the number of chromosome sets in the original cell, which is diploid.

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Comparing Meiosis and Mitosis: Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell.

The main differences between mitosis and meiosis occur in meiosis I. In meiosis I, the homologous chromosome pairs become associated with each other and are bound together with the synaptonemal complex. Chiasmata develop and crossover occurs between homologous chromosomes, which then line up along the metaphase plate in tetrads with kinetochore fibers from opposite spindle poles attached to each kinetochore of a homolog in a tetrad. All of these events occur only in meiosis I.

When the tetrad is broken up and the homologous chromosomes move to opposite poles, the ploidy level is reduced from two to one. For this reason, meiosis I is referred to as a reduction division. There is no such reduction in ploidy level during mitosis.

Meiosis II is much more similar to a mitotic division. In this case, the duplicated chromosomes (only one set, as the homologous pairs have now been separated into two different cells) line up on the metaphase plate with divided kinetochores attached to kinetochore fibers from opposite poles. During anaphase II and mitotic anaphase, the kinetochores divide and sister chromatids, now referred to as chromosomes, are pulled to opposite poles. The two daughter cells of mitosis, however, are identical, unlike the daughter cells produced by meiosis. They are different because there has been at least one crossover per chromosome. Meiosis II is not a reduction division because, although there are fewer copies of the genome in the resulting cells, there is still one set of chromosomes, as there was at the end of meiosis I. Meiosis II is, therefore, referred to as equatorial division.