3: Meiosis - Sexual Reproduction
Learning Objectives
Course Outcomes for this section:
Apply biological theories and concepts to solve problems related to classical and molecular genetics
- Describe the molecular basis of inheritance.
The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resembles its parent or parents. Hippopotamuses give birth to hippopotamus calves; Monterey pine trees produce seeds from which Monterey pine seedlings emerge; and adult flamingos lay eggs that hatch into flamingo chicks. In kind does not generally mean exactly the same . While many single-celled organisms and a few multicellular organisms can produce genetically identical clones of themselves through mitotic cell division, many single-celled organisms and most multicellular organisms reproduce regularly using another method.
Sexual reproduction is the production by parents of sex cells and the fusion of two sex cells to form a single, unique cell. In multicellular organisms, this new cell will then undergo mitotic cell divisions to develop into an adult organism. A type of cell division called meiosis leads to the cells that are part of the sexual reproductive cycle. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms can or must employ some form of meiosis and fertilization to reproduce.
Sexual reproduction was an early evolutionary innovation after the appearance of eukaryotic cells. The fact that most eukaryotes reproduce sexually is evidence of its evolutionary success. In many animals, it is the only mode of reproduction. And yet, scientists recognize some real disadvantages to sexual reproduction. On the surface, offspring that are genetically identical to the parent may appear to be more advantageous. If the parent organism is successfully occupying a habitat, offspring with the same traits would be similarly successful. There is also the obvious benefit to an organism that can produce offspring by asexual budding, fragmentation, or asexual eggs. These methods of reproduction do not require another organism of the opposite sex. There is no need to expend energy finding or attracting a mate. That energy can be spent on producing more offspring. Indeed, some organisms that lead a solitary lifestyle have retained the ability to reproduce asexually. In addition, asexual populations only have female individuals, so every individual is capable of reproduction. In contrast, the males in sexual populations (half the population) are not producing offspring themselves. Because of this, an asexual population can grow twice as fast as a sexual population in theory. This means that in competition, the asexual population would have the advantage. All of these advantages to asexual reproduction, which are also disadvantages to sexual reproduction, should mean that the number of species with asexual reproduction should be more common.
However, multicellular organisms that exclusively depend on asexual reproduction are exceedingly rare. Why is sexual reproduction so common? This is one of the important questions in biology and has been the focus of much research from the latter half of the twentieth century until now. A likely explanation is that the variation that sexual reproduction creates among offspring is very important to the survival and reproduction of those offspring. The only source of variation in asexual organisms is mutation. This is the ultimate source of variation in sexual organisms. In addition, those different mutations are continually reshuffled from one generation to the next when different parents combine their unique genomes, and the genes are mixed into different combinations by the process of meiosis . Meiosis is the division of the contents of the nucleus that divides the chromosomes among gametes. Variation is introduced during meiosis, as well as when the gametes combine in fertilization.
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- 3.1: Overview of Meiosis
- This page explains sexual reproduction, highlighting the union of haploid cells from two organisms to form a diploid cell. To maintain chromosome numbers across generations, diploid cells undergo meiosis, which reduces chromosome sets to haploid levels. Most animals are diploid, with gametes as the only haploid cells.
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- 3.2: Meiosis I
- This page describes meiosis, initiated after interphase with DNA replication during the S phase. It outlines the key stages of meiosis I, including prophase I with crossing over for genetic diversity, and independent assortment in metaphase I. The process results in two haploid cells with sister chromatids. Meiosis II then separates these sister chromatids, ultimately producing four haploid daughter cells, enhancing genetic variation through these processes.
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- 3.3: Meiosis II
- This page explains meiosis II, which follows meiosis I and divides two haploid cells without DNA replication. It includes prophase II, prometaphase II, metaphase II, anaphase II, telophase II, and cytokinesis. The process results in the separation of sister chromatids, producing four genetically unique haploid cells due to random assortment and recombination during crossover.
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- 3.4: Meiosis
- Most eukaryotes replicate sexually - a cell from one individual joins with a cell from another to create the next generation. For this to be successful, the cells that fuse must contain half the number of chromosomes as in the adult organism. Otherwise, the number of chromosomes would double with each generation! The reduction in chromosome number is achieved by the process of meiosis.
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- 3.5: Comparing Meiosis and Mitosis
- This page details the differences between mitosis and meiosis, two types of nuclear division in eukaryotic cells. Mitosis produces two genetically identical diploid cells for growth and replacement, while meiosis results in four genetically diverse haploid cells essential for sexual reproduction.
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- 3.6: Errors in Meiosis
- This page covers inherited disorders caused by abnormal chromosome behavior during meiosis, categorized into numerical and structural abnormalities. Key syndromes like Down Syndrome, Turner Syndrome, and Klinefelter syndrome are discussed, alongside their implications on development and fertility. Cytogenetic techniques like karyotyping help identify these conditions, revealing issues such as aneuploidy and structural rearrangements (e.g., deletions and translocations).
References
Unless otherwise noted, images on this page are licensed under CC-BY 4.0 by OpenStax.
OpenStax, Biology. OpenStax CNX. May 27, 2016 http://cnx.org/contents/s8Hh0oOc@9.10:qOUtHXNY@3/Sexual-Reproduction
OpenStax, Biology. OpenStax CNX. May 27, 2016 http://cnx.org/contents/s8Hh0oOc@9.10:1Q8z96mT@4/Meiosis
OpenStax, Biology. OpenStax CNX. May 27, 2016 http://cnx.org/contents/s8Hh0oOc@9.10:6-3MVU-j@4/Errors-in-Meiosis
Thumbnail: Pollen mother cells dividing during meiosis. (CC BY-SA 3.0; Josef Reischig via Wikimedia Commons ).