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Medicine LibreTexts

26.2A: 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 ParentsIn 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.