11.2: Sexual Reproduction - Biology

11.2: Sexual Reproduction

Chapter Summary

Sexual reproduction requires that diploid organisms produce haploid cells that can fuse during fertilization to form diploid offspring. As with mitosis, DNA replication occurs prior to meiosis during the S-phase of the cell cycle. Meiosis is a series of events that arrange and separate chromosomes and chromatids into daughter cells. During the interphases of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four daughter cells, each with half the number of chromosomes as the parent cell. The first separates homologs, and the second—like mitosis—separates chromatids into individual chromosomes. During meiosis, variation in the daughter nuclei is introduced because of crossover in prophase I and random alignment of tetrads at metaphase I. The cells that are produced by meiosis are genetically unique.

Meiosis and mitosis share similarities, but have distinct outcomes. Mitotic divisions are single nuclear divisions that produce daughter nuclei that are genetically identical and have the same number of chromosome sets as the original cell. Meiotic divisions include two nuclear divisions that produce four daughter nuclei that are genetically different and have one chromosome set instead of the two sets of chromosomes in the parent cell. The main differences between the processes occur in the first division of meiosis, in which homologous chromosomes are paired and exchange non-sister chromatid segments. The homologous chromosomes separate into different nuclei during meiosis I, causing a reduction of ploidy level in the first division. The second division of meiosis is more similar to a mitotic division, except that the daughter cells do not contain identical genomes because of crossover.

11.2 Sexual Reproduction

Nearly all eukaryotes undergo sexual reproduction. The variation introduced into the reproductive cells by meiosis appears to be one of the advantages of sexual reproduction that has made it so successful. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. Fertilization, the fusion of haploid gametes from two individuals, restores the diploid condition. Thus, sexually reproducing organisms alternate between haploid and diploid stages. However, the ways in which reproductive cells are produced and the timing between meiosis and fertilization vary greatly. There are three main categories of life cycles: diploid-dominant, demonstrated by most animals haploid-dominant, demonstrated by all fungi and some algae and the alternation of generations, demonstrated by plants and some algae.

Sexual Reproduction in Flowering Plants Class 12 Notes offer students with a simple way to study or revise for the chapter since it is prepared by our panel of qualified teachers strictly according to the NCERT Syllabus. This not only reduces the pressure on the students but also, offer them a simple way to study or revise the chapter. CBSE Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants Notes would fuel your exam preparation and increase your self-confidence which would help you score maximum marks in the exam.

Define Sexual Reproduction in Flowering Plants.

Events that take place before fertilization are termed as Pre-fertilization events. Following events would occur before actual fertilization takes place in a plant: Gametogenesis-Development of male & female reproductive structures. Formation of male & female gametes. Gamete transfer-Bringing together male & female gamete. Pollination occurs in plants to facilitate gamete transfer.

Know more about this in Sexual Reproduction in Flowering Plants Class 12 Notes pdf.

11.2: Sexual Reproduction - Biology

The ability to reproduce “in kind” is a basic characteristic of all living things. “In kind” means that the offspring of an organism closely resembles its parent or parents. Hippopotamuses give birth to hippopotamus calves, Joshua trees produce Joshua tree seedlings, and flamingos lay eggs that hatch into flamingo chicks. In kind can mean exactly the same. Many unicellular organisms, such as yeast, and a few multicellular organisms, such as sponges, can produce genetically identical clones of themselves through cell division. However, many single-celled organisms and most multicellular organisms reproduce regularly using a method requiring two parents. Sexual reproduction occurs through the production by each parent of a haploid cell (containing one half of an offspring’s required genetic material) and the fusion of these two haploid cells to form a single, unique diploid cell with a complete set of genetic information. In most plants and animals, through multiple rounds of mitotic cell division, this diploid cell will develop into an adult organism. Haploid cells that are necessary for sexual reproduction are produced by a type of cell division called meiosis. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring. Variation is an important component of a species evolutionary success. The vast majority of eukaryotic organisms employs some form of meiosis and fertilization to reproduce.

Not all sexually reproducing eukaryotes reproduce solely by sexual reproduction. For example, an Asian termite species, Reticulitermes speratus, can reproduce sexually or asexually. In a young colony, a single termite pair—the king and queen—produce worker offspring sexually by the union of haploid cells. However, after several years, as the queen begins to age, she produces some offspring asexually in a process called parthenogenesis. These offspring, which are destined to become new queens, are not fertilized by the king. They are genetic clones of the queen. More information about parthenogenesis in these termites can be found at this article.

  • You are here:  
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This text is based on Openstax Biology for AP Courses, Senior Contributing Authors Julianne Zedalis, The Bishop's School in La Jolla, CA, John Eggebrecht, Cornell University Contributing Authors Yael Avissar, Rhode Island College, Jung Choi, Georgia Institute of Technology, Jean DeSaix, University of North Carolina at Chapel Hill, Vladimir Jurukovski, Suffolk County Community College, Connie Rye, East Mississippi Community College, Robert Wise, University of Wisconsin, Oshkosh

This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 Unported License, with no additional restrictions

11.2: Sexual Reproduction - Biology

  • You are here:  
  • Home
  • Andover Biology Department Textbooks
  • Openstax Biology for AP Courses (textbook for Bio58x sequence)
  • Bio581
  • Chapter 11 Meiosis and Sexual Reproduction
  • 11 Meiosis and Sexual Reproduction

This text is based on Openstax Biology for AP Courses, Senior Contributing Authors Julianne Zedalis, The Bishop's School in La Jolla, CA, John Eggebrecht, Cornell University Contributing Authors Yael Avissar, Rhode Island College, Jung Choi, Georgia Institute of Technology, Jean DeSaix, University of North Carolina at Chapel Hill, Vladimir Jurukovski, Suffolk County Community College, Connie Rye, East Mississippi Community College, Robert Wise, University of Wisconsin, Oshkosh

This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 Unported License, with no additional restrictions


The first fossilized evidence of sexual reproduction in eukaryotes is from the Stenian period, about 1 to 1.2 billion years ago. [10]

Biologists studying evolution propose several explanations for the development of sexual reproduction and its maintenance. These reasons include reducing the likelihood of the accumulation of deleterious mutations, increasing rate of adaptation to changing environments, [11] dealing with competition, DNA repair and masking deleterious mutations. [12] [13] [14] All of these ideas about why sexual reproduction has been maintained are generally supported, but ultimately the size of the population determines if sexual reproduction is entirely beneficial. Larger populations appear to respond more quickly to some of the benefits obtained through sexual reproduction than do smaller population sizes. [15]

Maintenance of sexual reproduction has been explained by theories that work at several levels of selection, though some of these models remain controversial. [ citation needed ] However, newer models presented in recent years suggest a basic advantage for sexual reproduction in slowly reproducing complex organisms.

Sexual reproduction allows these species to exhibit characteristics that depend on the specific environment that they inhabit, and the particular survival strategies that they employ. [16]

In order to sexually reproduce, both males and females need to find a mate. Generally in animals mate choice is made by females while males compete to be chosen. This can lead organisms to extreme efforts in order to reproduce, such as combat and display, or produce extreme features caused by a positive feedback known as a Fisherian runaway. Thus sexual reproduction, as a form of natural selection, has an effect on evolution. Sexual dimorphism is where the basic phenotypic traits vary between males and females of the same species. Dimorphism is found in both sex organs and in secondary sex characteristics, body size, physical strength and morphology, biological ornamentation, behavior and other bodily traits. However, sexual selection is only implied over an extended period of time leading to sexual dimorphism. [17]

13.1. Asexual Reproduction

Asexual Reproduction

Although nearly all multicellular organsims have sexual reproduction, some plants and animals will also engage in asexual reproduction, producing identical copies of the parent, usually the mother. There are several types of asexual reproduction, we will consider two of these, vegetative propagation and parthenogenesis.

Figure 13.2. The Kalanchoe plant, sometimes called mother-of-thousands, with plantlets that will eventually drop off and grow independantly.

During vegetative propagation, plants develop miniature versions of themselves, usually on specialized leaves. The Kalanchoe leaf above has several plantlets that are developing asexually. Note that only one leaf of the plant has plantlets. When large enough, these plantlets will drop of and continue growing independently of the parent plant.

Other plants such as the tulip, will develop bulbs asexually by vegetative propagation. Many grasses and several species of trees also propagate asexually, often through the roots. The famous Pando, a clonal colony of quaking aspen trees in Utah, has grown from 1 male tree to 47,000 through asexual reproduction through the roots. This tree system is estimated to be 80,000 years old and is among the oldest known living organisms.

Parthenogenesis is another way in which organisms can reproduce asexually, usually through an unfertilized egg, so only females can asexually reproduce this way. Plants, invertebrates including insects often undergo parthenogenesis, along with a few species of lizards will reproduce parthenogenically. There are a number of different methods by which the egg is diploid and is stimulated to develop, depending on the species. In some organisms haploid eggs fuse to become diploid in others there is doubling of the chromosomes without cytoplasmic division either before or after meiosis. Regardless of the method, all organisms which are produced by parthenogenesis are identical clones of their mother, as is the aphid being born from an unfertilized egg in Figure 13.3.

Figure 13.3. Female aphid giving birth to a clone derived from an unfertilized egg by parthenogenesis.

Asexual reproduction is less common than sexual reproduction in flowering plants and rare in vertebrates, making sexual reproduction much more common among these organisms. Yet sexual reproduction is often very risky business for the individuals involved by involving large outlays of energy. Take the peacock for example (Figure 13.4) This impressive display of tail feathers with their distinctive eyes, require a lot of energy to produce each spring. In addition, having such a large and cumbersome tail makes it difficult for the peacock to fly away from predators and even to capture food. Yet the peacock flaunts its feathers with great pride, shaking them at nearby pea hens to get the female’s attention.

Why would the peacock invest so much energy at so much risk of injury or even death to attract a female? The answer, of course, is sexual reproduction. There is among nearly all sexually reproductive organisms a competition for reproductive success. In many species, males are driven to want to produce as many offspring as he can from as many females as possible, hence the annual fights for females among such diverse species as deer, frogs, and even round worms. The better the peacock is at acquiring food the more extensive his display and the more likely he will be to fertilize her eggs.

The female, however, is looking for the fittest male to fertilize her eggs, thereby helping to ensure their survival. Studies have shown that the peahen, for example, counts the number of eyes on the feathers of the peacock’s tail. The male with the most eyes, and therefore the most healthy, will usually win the female.

Figure 13.4. The peacock in full springtime display

Populations that reproduce asexually are at the whim of a variable environment. Since they are clones of each other, they are equally suseptable to environment factors such as disease or changing climate conditions, and could easily become extinct. Populations that reproduce sexually are much more diverse and so are more likely to have at least some in the population who will survive environmental changes. This “drive to survive” is at the heart of why the large investment of energy in sexual reproduction is worth the effort.


  1. The female tiger shark has occasionally produced offspring from unfertilized eggs. What type of asexual reproduction is this?
  1. Sexual reproduction involves fewer steps.
  2. There is a lower chance of using up the resources in a given environment.
  3. Sexual reproduction is more cost-effective in terms of energy utilization.
  4. Sexual reproduction results in variation in the offspring.

Visual Connection

Which of the following statements is true?

  1. A basidium is the fruiting body of a mushroom-producing fungus, and it forms four basidiocarps.
  2. The result of the plasmogamy step is four basidiospores.
  3. Karyogamy results directly in the formation of mycelia.
  4. A basidiocarp is the fruiting body of a mushroom-producing fungus.

5.12 Review Questions

Meiosis: Where the Sex Starts – Crash Course Biology #13, CrashCourse, 2012.

Mitosis vs Meiosis Comparison, Amoeba Sisters, 2018.

Figure 5.12.1

Figure 5.12.2

Human Life Cycle by Christine Miller is used under a CC BY-NC-SA 4.0 ( license.

Figure 5.12.3

MajorEventsInMeiosis_variant_int by PatríciaR (i nternationalization) on Wikimedia Commons is used and adapted by Christine Miller. This image in the public domain. ( Original image from NCBI original vector version by Jakov.)

Figure 5.12.4

Meiosis 1/ Meiosis Stages by Ali Zifan on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 4.0 ( license.

Figure 5.12.5

Meiosis 2/ Meiosis Stages by Ali Zifan on Wikimedia Commons is used and adapted by Christine Miller under a CC BY-SA 4.0 ( license.

Figure 5.12.6

Crossover/ Figure 17 02 01 by CNX OpenStax on Wikimedia Commons is used under a CC BY 4.0 ( license.

Figure 5.12.7

Independent_assortment by Mtian20 on Wikimedia Commons is used under a CC BY-SA 4.0 ( license.

Figure 5.12.8

sperm fertilizing egg by AndreaLaurel on Flickr is used under a CC BY 2.0 ( license.

11.1 |The Process of Meiosis

By the end of this section, you will be able to:

  • Describe the behavior of chromosomes during meiosis
  • Describe cellular events during meiosis
  • Explain the differences between meiosis and mitosis
  • Explain the mechanisms within meiosis that generate genetic variation among the products of meiosis

Sexual reproduction requires fertilization, the union of two cells from two individual organisms. If those two cells each contain one set of chromosomes, then the resulting 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, 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. 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 each somatic cell of the organism (all cells of a multicellular organism except the gametes or reproductive cells), the nucleus contains two copies of each chromosome, called homologous chromosomes. Somatic cells are sometimes referred to as “body” cells. 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 structures that give rise to either gametes or spores. Spores are haploid cells that can produce a haploid organism or can fuse with another spore to form a diploid cell. All animals and most plants produce eggs and sperm, or gametes. Some plants and all fungi produce spores.

The nuclear division that forms haploid cells, which is called meiosis, is related to mitosis. As you have learned, 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.

Meiosis I

Meiosis is preceded by an interphase consisting of the G1, S, and G2 phases, which are nearly identical to the phases preceding mitosis. The G1 phase, which is also called the first gap phase, is the first phase of the interphase and is focused on cell growth. The S phase is the second phase of interphase, during which the DNA of the chromosomes is replicated. Finally, the G2 phase, also called the second gap phase, is the third and final phase of interphase in this phase, the cell undergoes the final preparations for meiosis.

During DNA duplication in the S phase, each chromosome is replicated to produce two identical copies, called sister chromatids, that are held together at the centromere by cohesin proteins. Cohesin holds the chromatids together until anaphase II. The centrosomes, which are the structures that organize the microtubules of the meiotic spindle, also replicate. This prepares the cell to enter prophase I, the first meiotic phase.

Early in prophase I, before the chromosomes can be seen clearly microscopically, the homologous chromosomes are attached at their tips to the nuclear envelope by proteins. As the nuclear envelope begins to break down, the proteins associated with homologous chromosomes bring the pair close to each other. Recall that, in mitosis, homologous chromosomes do not pair together. In mitosis, homologous chromosomes line up end-to-end so that when they divide, each daughter cell receives a sister chromatid from both members of the homologous pair. The synaptonemal complex, a lattice of proteins between the homologous chromosomes, first forms at specific locations and then spreads to cover the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis. In synapsis, the genes on the chromatids of the homologous chromosomes are aligned precisely with each other. The synaptonemal complex supports the exchange of chromosomal segments between non-sister homologous chromatids, a process called crossing over. Crossing over can be observed visually after the exchange as chiasmata (singular = chiasma) (Figure 11.2).

In species such as humans, even though the X and Y sex chromosomes are not homologous (most of their genes differ), they have a small region of homology that allows the X and Y chromosomes to pair up during prophase I. A partial synaptonemal complex develops only between the regions of homology.

Figure 11.2 Early in prophase I, homologous chromosomes come together to form a synapse. The chromosomes are bound tightly together and in perfect alignment by a protein lattice called a synaptonemal complex and by cohesin proteins at the centromere.

Located at intervals along the synaptonemal complex are large protein assemblies called recombination nodules. These assemblies mark the points of later chiasmata and mediate the multistep process of crossover—or genetic recombination—between the non-sister chromatids. Near the recombination nodule on each chromatid, the double-stranded DNA is cleaved, the cut ends are modified, and a new connection is made between the non-sister chromatids. As prophase I progresses, the synaptonemal complex begins to break down and the chromosomes begin to condense. When the synaptonemal complex is gone, the homologous chromosomes remain attached to each other at the centromere and at chiasmata. The chiasmata remain until anaphase I. The number of chiasmata varies according to the species and the length of the chromosome. There must be at least one chiasma per chromosome for proper separation of homologous chromosomes during meiosis I, but there may be as many as 25. Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata (Figure 11.3) and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.

The crossover events are the first source of genetic variation in the nuclei produced by meiosis. A single crossover event between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal chromosome. Now, when that sister chromatid is moved into a gamete cell it will carry some DNA from one parent of the individual and some DNA from the other parent. The sister recombinant chromatid has a combination of maternal and paternal genes that did not exist before the crossover. Multiple crossovers in an arm of the chromosome have the same effect, exchanging segments of DNA to create recombinant chromosomes.

Figure 11.3 Crossover occurs between non-sister chromatids of homologous chromosomes. The result is an exchange of genetic material between homologous chromosomes.

The key event in prometaphase I is the attachment of the spindle fiber microtubules to the kinetochore proteins at the centromeres. Kinetochore proteins are multiprotein complexes that bind the centromeres of a chromosome to the microtubules of the mitotic spindle. Microtubules grow from centrosomes placed at opposite poles of the cell. The microtubules move toward the middle of the cell and attach to one of the two fused homologous chromosomes. The microtubules attach at each chromosomes' kinetochores. With each member of the homologous pair attached to opposite poles of the cell, in the next phase, the microtubules can pull the homologous pair apart. A spindle fiber that has attached to a kinetochore is called a kinetochore microtubule. At the end of prometaphase I, each tetrad is attached to microtubules from both poles, with one homologous chromosome facing each pole. The homologous chromosomes are still held together at chiasmata. In addition, the nuclear membrane has broken down entirely.

During metaphase I, the homologous chromosomes are arranged in the center of the cell with the kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. For example, if the two homologous members of chromosome 1 are labeled a and b, then the chromosomes could line up a-b, or b-a. This is important in determining the genes carried by a gamete, as each will only receive one of the two homologous chromosomes. Recall that homologous chromosomes are not identical. They contain slight differences in their genetic information, causing each gamete to have a unique genetic makeup.

This randomness is the physical basis for the creation of the second form of genetic variation in offspring. Consider that the homologous chromosomes of a sexually reproducing organism are originally inherited as two separate sets, one from each parent. Using humans as an example, one set of 23 chromosomes is present in the egg donated by the mother. The father provides the other set of 23 chromosomes in the sperm that fertilizes the egg. Every cell of the multicellular offspring has copies of the original two sets of homologous chromosomes. In prophase I of meiosis, the homologous chromosomes form the tetrads. In metaphase I, these pairs line up at the midway point between the two poles of the cell to form the metaphase plate. Because there is an equal chance that a microtubule fiber will encounter a maternally or paternally inherited chromosome, the arrangement of the tetrads at the metaphase plate is random. Any maternally inherited chromosome may face either pole. Any paternally inherited chromosome may also face either pole. The orientation of each tetrad is independent of the orientation of the other 22 tetrads.

This event—the random (or independent) assortment of homologous chromosomes at the metaphase plate—is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different. The number of variations is dependent on the number of chromosomes making up a set. There are two possibilities for orientation at the metaphase plate the possible number of alignments therefore equals 2n, where n is the number of chromosomes per set. Humans have 23 chromosome pairs, which results in over eight million (2 23 ) possible genetically-distinct gametes. This number does not include the variability that was previously created in the sister chromatids by crossover. Given these two mechanisms, it is highly unlikely that any two haploid cells resulting from meiosis will have the same genetic composition (Figure 11.4).

To summarize the genetic consequences of meiosis I, the maternal and paternal genes are recombined by crossover events that occur between each homologous pair during prophase I. In addition, the random assortment of tetrads on the metaphase plate produces a unique combination of maternal and paternal chromosomes that will make their way into the gametes.

Figure 11.4 Random, independent assortment during metaphase I can be demonstrated by considering a cell with a set of two chromosomes (n = 2). In this case, there are two possible arrangements at the equatorial plane in metaphase I. The total possible number of different gametes is 2n, where n equals the number of chromosomes in a set. In this example, there are four possible genetic combinations for the gametes. With n = 23 in human cells, there are over 8 million possible combinations of paternal and maternal chromosomes.

In anaphase I, the microtubules pull the linked chromosomes apart. The sister chromatids remain tightly bound together at the centromere. The chiasmata are broken in anaphase I as the microtubules attached to the fused kinetochores pull the homologous chromosomes apart (Figure 11.5).

Telophase I and Cytokinesis

In telophase, the separated chromosomes arrive at opposite poles. The remainder of the typical telophase events may or may not occur, depending on the species. In some organisms, the chromosomes decondense and nuclear envelopes form around the chromatids in telophase I. In other organisms, cytokinesis—the physical separation of the cytoplasmic components into two daughter cells—occurs without reformation of the nuclei. In nearly all species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow (constriction of the actin ring that leads to cytoplasmic division). In plants, a cell plate is formed during cell cytokinesis by Golgi vesicles fusing at the metaphase plate. This cell plate will ultimately lead to the formation of cell walls that separate the two daughter cells.

Two haploid cells are the end result of the first meiotic division. The cells are haploid because at each pole, there is just one of each pair of the homologous chromosomes. Therefore, only one full set of the chromosomes is present. This is why the cells are considered haploid—there is only one chromosome set, even though each homolog still consists of two sister chromatids. Recall that sister chromatids are merely duplicates of one of the two homologous chromosomes (except for changes that occurred during crossing over). In meiosis II, these two sister chromatids will separate, creating four haploid daughter cells.

Watch the video: Mitosis vs Meiosis Rap Battle! SCIENCE SONGS (January 2022).