5: Multicellularity and Asexual Reproduction - Biology

5: Multicellularity and Asexual Reproduction - Biology

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Learning Objectives

Content Objectives

  • Learn the process of mitosis and how this type of cell division is used for growth, repair, and asexual reproduction
  • Understand that mitosis is a type of cell division that results in clones of the original cell
  • Learn the two phases of cell division: mitosis is the division of the genetic material, while cytokinesis is the division of the rest of the cytoplasm and the cell wall
  • Understand that prokaryotes cannot undergo mitosis because they lack a nucleus

Skill Objectives

  • Differentiate between chromatin and chromosomes in a prepared slide
  • Locate and identify important cell structures, including the nucleus, nucleoli, nuclear envelope, and cell wall
  • Identify cells in different stages of mitosis based on chromosomal organization

The Biology of Reproduction

This book has been cited by the following publications. This list is generated based on data provided by CrossRef.
  • Publisher: Cambridge University Press
  • Online publication date: September 2019
  • Print publication year: 2019
  • Online ISBN: 9781108758970
  • DOI:
  • Subjects: Plant Sciences, Life Sciences, Zoology

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Book description

Reproduction is a fundamental feature of life, it is the way life persists across the ages. This book offers new, wider vistas on this fundamental biological phenomenon, exploring how it works through the whole tree of life. It explores facets such as asexual reproduction, parthenogenesis, sex determination and reproductive investment, with a taxonomic coverage extended over all the main groups - animals, plants including 'algae', fungi, protists and bacteria. It collates into one volume perspectives from varied disciplines - including zoology, botany, microbiology, genetics, cell biology, developmental biology, evolutionary biology, animal and plant physiology, and ethology - integrating information into a common language. Crucially, the book aims to identify the commonalties among reproductive phenomena, while demonstrating the diversity even amongst closely related taxa. Its integrated approach makes this a valuable reference book for students and researchers, as well as an effective entry point for deeper study on specific topics.


'Fusco’s and Minelli’s The Biology of Reproduction is impressive in scope. Rather than adopting a more restricted perspective on reproduction - be it on reproduction in mammals, animals, or plants - this book provides a comprehensive overview of the various similarities and variations of this central biological phenomenon across the whole tree of life. In an easily accessible style and exemplified through a wide range of illustrations, it offers the reader a great stepping stone to more in-depth comparative studies. Its greatest strengths are twofold. First, through its impressive taxonomic coverage it directly counteracts longstanding biases in our understanding of reproduction imposed through the selective use of a few model organisms. Second, the authors nicely link empirical findings with conceptual discussions on biological individuality and the boundaries between reproduction and development. Thus, this book is of use not only for biology students and professors but also for philosophers of biology. Highly recommended.'

Jan Baedke - Ruhr-Universität Bochum, Germany

'Crucially, The Biology of Reproduction successfully identifies the commonalties among reproductive phenomena, while demonstrating the diversity even amongst closely related taxa. Its integrated approach makes The Biology of Reproduction a valuable reference book for students and researchers, as well as an effective entry point for deeper study on specific topics.'

James A. Cox Source: Midwest Book Review

‘[Giuseppe Fusco and Alessandro Minelli] have created an excellent new resource on a subject that is almost impossible to define, bringing together diagrams, photographs, and illustrations from many sources. This is truly a reference work, yet it prominently features accessibility … Readers will no doubt find the coverage of this interplay interesting.’

‘Overall, The Biology of Reproduction offers readers a very comprehensive review of reproductive biology that cuts across all clades. This will be especially valuable for biologists who do most of their work within a relatively small subset of organisms, and for whom many novel reproductive strategies may be unknown … the book will be valuable to anyone seeking a detailed reference for comparative reproductive biology, where it makes sense to prioritize breadth ahead of depth. Moreover, it would also be a suitable choice as a textbook for a course on reproductive biology or the evolution of reproductive systems (at either the undergraduate or postgraduate level), since the text is organized well and easy to read.’

P. William Hughes Source: Evolution

‘… places reproduction at the forefront and beautifully summarizes the vast array of reproductive strategies from a diverse range of organisms, including bacteria, plants and animals. This book is unparalleled in scope and in addition to covering the natural history of reproduction and highlighting fascinating life-history strategies … covers the fundamental aspects of reproduction including key definitions, genetics and cytogenetics, and sex determination … well-written and organized with excellent taxonomic and subject indexes … this book is beautifully illustrated with informative and well-thought-out diagrams. The Biology of Reproduction is, therefore, suitable as an introductory or a more advanced text … is also a valuable primer for students and researchers interested in comparative and evolutionary reproductive biology … provides a comprehensive introduction to the diverse range of reproductive strategies found in nature and in doing so clarifies key terminology and concepts in a text which will be equally valuable to the student and expert alike.’

Elizabeth J. Duncan Source: Invertebrate Reproduction & Development

‘Fusco and Minelli’s The Biology of Reproduction is a much-needed and welcome addition. It allows readers to place whatever model system and aspect of reproductive biology they seek into a broader context, across levels of biological organization but also in relation to the breathtaking diversity that exists among living systems in when, where, and by what means the continuity of life is made possible.’

Classification and microbiota

Prior to the recognition of microbial life, the living world was too easily divided into animals that moved in pursuit of food and plants that produced food from sunlight. The futility of this simplistic classification scheme has been underscored by entire fields of science. Many bacteria both swim (like animals) and photosynthesize (like plants), yet they are best considered neither. Many algae (e.g., euglenids, dinomastigotes, chlorophytes) also swim and photosynthesize simultaneously. Molecular biological measurements of the DNA that codes for components of the ribosomes (organelles that are universally distributed) consistently find fungi to be extremely different from plants. Indeed, fungi genetically resemble animals more than plants.

Modern biology, following the lead of the German biologist Ernst Haeckel and the American biologists Herbert F. Copeland and Robert H. Whittaker, has now thoroughly abandoned the two-kingdom plant-versus-animal dichotomy. Haeckel proposed three kingdoms when he established “Protista” for microorganisms. Copeland classified the microorganisms into the Monerans (prokaryotes) and the Protoctista (which included fungi with the rest of the eukaryotic microorganisms). His four-kingdom scheme (Monera, Protoctista, Animalia, and Plantae) had the advantage of clearly separating microbes with nuclei (Protoctista) from those without (Monera: the prokaryotes—that is, the bacteria and archaea) and of distinguishing the two embryo-forming groups—plants and animals—from the rest of life. Whittaker, on ecological grounds, raised the fungi to kingdom status. The modified Whittaker five-kingdom classification system is perhaps the most comprehensible and biologically based way to unambiguously organize information about all groups of living beings. American microbiologist Carl Woese has offered still another classification scheme, in which all organisms are placed in either the Archaea (prokaryotes that include some salt lovers, acid lovers, and methane producers), the Bacteria (all other prokaryotes, including obligate anaerobic bacteria as well as photosynthetic and chemoautotrophic bacteria), or the Eukarya (all eukaryote forms of life). Woese’s scheme is based on molecular biological criteria that focus on the RNA sequence of morphological factors to classify new or disputed organisms. Although Woese’s three-domain system is very popular, a potential problem with it is that RNA, one characteristic among thousands, does not consistently correlate with many others.

Microbes (or microbiota) are simply all those organisms too small to be visualized without some sort of microscopy. Bacteria, the smaller fungi, and the smaller protists are undoubtedly microbes. Some scientists classify tiny animals, worms, and rotifers as microbes as well. Like weed, a plant not wanted in a garden, microbe is often a more useful term than one with a precise scientific meaning.

Plant Life Spans

The life cycles and life spans of plants vary and are affected by environmental and genetic factors.

Learning Objectives

Explain the process of aging in plants

Key Takeaways

Key Points

  • The life span of a plant is the length of time it takes from the beginning of development until death, while the life cycle is the series of stages between the germination of the seed until the plant produces its own seeds.
  • Annuals complete their life cycle in one season biennials complete their life cycle in two seasons and perennials complete their life cycle in more than two seasons.
  • Monocarpic plants flower only once in their lifetime, while polycarpic plants flower more than once.
  • Plant survival depends on changing environmental conditions, drought, cold, and competition.
  • Senescence refers to aging of the plant, during which components of the plant cells are broken down and used to support the growth of other plant tissues.

Key Terms

  • annual: a plant which naturally germinates, flowers, and dies in one year
  • biennial: a plant that requires two years to complete its life cycle
  • perennial: a plant that is active throughout the year or survives for more than two growing seasons
  • monocarpic: a plant that flowers and bears fruit only once before dying
  • polycarpic: bearing fruit repeatedly, or year after year
  • senescence: aging of a plant accumulated damage to macromolecules, cells, tissues, and organs with the passage of time

Plant Life Spans

The length of time from the beginning of development to the death of a plant is called its life span. The life cycle, on the other hand, is the sequence of stages a plant goes through from seed germination to seed production of the mature plant. Some plants, such as annuals, only need a few weeks to grow, produce seeds, and die. Other plants, such as the bristlecone pine, live for thousands of years. Some bristlecone pines have a documented age of 4,500 years. Even as some parts of a plant, such as regions containing meristematic tissue (the area of active plant growth consisting of undifferentiated cells capable of cell division) continue to grow, some parts undergo programmed cell death (apoptosis). The cork found on stems and the water-conducting tissue of the xylem, for example, are composed of dead cells.

Plant life spans: The bristlecone pine, shown here in the Ancient Bristlecone Pine Forest in the White Mountains of eastern California, has been known to live for 4,500 years.

Annuals, Biennial, and Perennials

Plant species that complete their life cycle in one season are known as annuals, an example of which is Arabidopsis, or mouse-ear cress. Biennials, such as carrots, complete their life cycle in two seasons. In a biennial’s first season, the plant has a vegetative phase, whereas in the next season, it completes its reproductive phase. Commercial growers harvest the carrot roots after the first year of growth and do not allow the plants to flower. Perennials, such as the magnolia, complete their life cycle in two years or more.

Monocarpic and Polycarpic Plants

In another classification based on flowering frequency, monocarpic plants flower only once in their lifetime examples of monocarpic plants include bamboo and yucca. During the vegetative period of their life cycle (which may be as long as 120 years in some bamboo species), these plants may reproduce asexually, accumulating a great deal of food material that will be required during their once-in-a-lifetime flowering and setting of seed after fertilization. Soon after flowering, these plants die. Polycarpic plants form flowers many times during their lifetime. Fruit trees, such as apple and orange trees, are polycarpic they flower every year. Other polycarpic species, such as perennials, flower several times during their life span, but not each year. By this method, the plant does not require all its nutrients to be channeled towards flowering each year.

Genetics and Environmental Conditions

As is the case with all living organisms, genetics and environmental conditions have a role to play in determining how long a plant will live. Susceptibility to disease, changing environmental conditions, drought, cold, and competition for nutrients are some of the factors that determine the survival of a plant. Plants continue to grow, despite the presence of dead tissue, such as cork. Individual parts of plants, such as flowers and leaves, have different rates of survival. In many trees, the older leaves turn yellow and eventually fall from the tree. Leaf fall is triggered by factors such as a decrease in photosynthetic efficiency due to shading by upper leaves or oxidative damage incurred as a result of photosynthetic reactions. The components of the part to be shed are recycled by the plant for use in other processes, such as development of seed and storage. This process is known as nutrient recycling. However, the complex pathways of nutrient recycling within a plant are not well understood

The aging of a plant and all the associated processes is known as senescence, which is marked by several complex biochemical changes. One of the characteristics of senescence is the breakdown of chloroplasts, which is characterized by the yellowing of leaves. The chloroplasts contain components of photosynthetic machinery, such as membranes and proteins. Chloroplasts also contain DNA. The proteins, lipids, and nucleic acids are broken down by specific enzymes into smaller molecules and salvaged by the plant to support the growth of other plant tissues. Hormones are known to play a role in senescence. Applications of cytokinins and ethylene delay or prevent senescence in contrast, abscissic acid causes premature onset of senescence.

Plant senescence: The autumn color of these Oregon Grape leaves is an example of programmed plant senescence.

Vegetative Propagation and Strawberries

Many plants reproduce asexually through vegetative propagation, which can either be naturally occurring or produced artificially. In vegetative propagation, plant tissues and organs are regenerated from another part of the plant, and a new organism can be produced. Strawberries naturally reproduce via this method. The horizontal stems -- called runners or stolens -- of strawberries grow from parent plants, and tiny plantlets form along these runners, forming roots along the way. When there is a break in the connection to the parent plant, the plantlets become new independent organisms.

Asexual reproduction


…female gametes (sex cells), by asexual reproduction, or by both ways.


Asexual reproduction (i.e., reproduction not involving the union of gametes), however, occurs only in the invertebrates, in which it is common, occurring in animals as highly evolved as the sea squirts, which are closely related to the vertebrates. Temporary gonads are common among lower animals…


Asexual reproduction is by binary or multiple fission (schizogony).


Asexual reproduction in echinoderms usually involves the division of the body into two or more parts (fragmentation) and the regeneration of missing body parts. Fragmentation is a common method of reproduction used by some species of asteroids, ophiuroids, and holothurians, and in some…


Typically in asexual reproduction, a single individual gives rise to a genetic duplicate of the progenitor without a genetic contribution from another individual. Perhaps the simplest method of reproduction of fungi is by fragmentation of the thallus, the body of a fungus. Some…

Growth and development

…in plants that reproduce by vegetative division, the breaking off of a part that can grow into another complete plant. The possibilities for debate that arise in these special cases, however, do not in any way invalidate the general usefulness of the distinctions as conventionally made in biology.

Major references

In asexual reproduction the new individual is derived from a blastema, a group of cells from the parent body, sometimes, as in Hydra and other coelenterates, in the form of a “bud” on the body surface. In sponges and bryozoans, the cell groups from which new…

Multicellular organisms also reproduce asexually and sexually asexual, or vegetative, reproduction can take a great variety of forms. Many multicellular lower plants give off asexual spores, either aerial or motile and aquatic (zoospores), which may be uninucleate or multinucleate. In some cases the reproductive body is multicellular, as in…

…higher plants also reproduce by nonsexual means. Bulbs bud off new bulbs from the side. Certain jellyfish, sea anemones, marine worms, and other lowly creatures bud off parts of the body during one season or another, each thereby giving rise to populations of new, though identical, individuals. At the microscopic…


Both homosporous and heterosporous life histories may exhibit various types of asexual reproduction (vegetative reproduction, somatic reproduction). Asexual reproduction is any reproductive process that does not involve meiosis or the union of nuclei, sex cells, or sex organs. Depending on the type of…

Asexual reproduction involves no union of cells or nuclei of cells and, therefore, no mingling of genetic traits, since the nucleus contains the genetic material (chromosomes) of the cell. Only those systems of asexual reproduction that are not really modifications of sexual reproduction are considered…

Population ecology

In sexual populations, genes are recombined in each generation, and new genotypes may result. Offspring in most sexual species inherit half their genes from their mother and half from their father, and their genetic makeup is therefore different from either parent or any other…


Asexual reproduction is the most common means of replication by protozoans. The ability to undergo a sexual phase is confined to the ciliates, the apicomplexans, and restricted taxa among the flagellated and amoeboid organisms. Moreover, sexual reproduction does not always result in an immediate increase…


Spores are agents of asexual reproduction, whereas gametes are agents of sexual reproduction. Spores are produced by bacteria, fungi, algae, and plants.

Differences between Asexual and sexual reproduction

Differences between Asexual and sexual reproduction:– asexual mode of reproduction is the formation of new individual without involving the formation and fusion of gametes whereas sexual reproduction involves fusion of two sex gametes to form new individual.


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Differences between Asexual and sexual reproduction

Differences between Asexual and sexual reproduction:– asexual mode of reproduction is the formation of new individual without involving the formation and fusion of gametes whereas sexual reproduction involves fusion of two sex gametes to form new individual.

Sexual reproduction is also known as syngamy, it involves complete and permanent fusion of two hapload gametes to form diploid zygote, it is the most common method of sexual reproduction. Gametes involves are of two types of sperm in ovum, sperm also called microgamete is minute microscopic and motile gametes formed by spermatogenesis in reproductive organ called testis. Ovum is macro gemete is large spherical and non motile and is formed by oogenesis in reproductive organ called ovary.

Differences between Asexual and sexual reproduction are given in the following table:-

★ Asexual reproduction:-

1) there is only one parents are involved in asexual reproduction that’s why it is uniparental reproduction

2) gametes are not formed and fuse in this reproduction

3) in Asexual mode of reproduction only mitotic cell division is occurs

4) generally somatic cells of parents are involved in reproduction

5) rate of reproduction is faster

6) unit of asexual reproduction is whole parent body or bud or body fragment

7) it is found in only lower invertebrates and lower chordates.

8) nature of daughter cells involved in asexual reproduction are genetically similar to parents

★ Sexual reproduction:-

1) in this reproduction there is two parents involved, so it is biparental

2) there is formation of male and female gametes and their fusion to form diploid zygote

3) in sexual mode of reproduction meiosis occurs at the time of gametogenesis, mitosis occurs after fertilization

4) nature of cells that are involved in sexual reproduction are germ cells of parent

5) rate of reproduction is slower

6) unit of reproduction in sexual mode of reproduction is gametes

7) it is found in higher plants and animals

8) nature of daughter which are produced in sexual reproduction are genetically different from parents

How Did Multicellular Life Evolve?

Scientists are discovering ways in which single cells might have evolved traits that entrenched them into group behavior, paving the way for multicellular life. These discoveries could shed light on how complex extraterrestrial life might evolve on alien worlds.

Researchers detailed these findings in the October 24, 2016 issue of the journal Science.

The first known single-celled organisms appeared on Earth about 3.5 billion years ago, roughly a billion years after Earth formed. More complex forms of life took longer to evolve, with the first multicellular animals not appearing until about 600 million years ago.

The evolution of multicellular life from simpler, unicellular microbes was a pivotal moment in the history of biology on Earth and has drastically reshaped the planet’s ecology. However, one mystery about multicellular organisms is why cells did not return back to single-celled life.

“Unicellularity is clearly successful — unicellular organisms are much more abundant than multicellular organisms, and have been around for at least an additional 2 billion years,” said lead study author Eric Libby, a mathematical biologist at the Santa Fe Institute in New Mexico. “So what is the advantage to being multicellular and staying that way?”

The answer to this question is usually cooperation, as cells benefitted more from working together than they would from living alone. However, in scenarios of cooperation, there are constantly tempting opportunities “for cells to shirk their duties — that is, cheat,” Libby said.

“As an example, consider an ant colony where only the queen is laying eggs and the workers, who cannot reproduce, must sacrifice themselves for the colony,” Libby said. “What prevents the ant worker from leaving the colony and forming a new colony? Well, obviously the ant worker cannot reproduce, so it cannot start its own colony. But if it got a mutation that enabled it to do that, then this would be a real problem for the colony. This kind of struggle is prevalent in the evolution of multicellularity because the first multicellular organisms were only a mutation away from being strictly unicellular.”

Experiments have shown that a group of microbes that secretes useful molecules that all members of the group can benefit from can grow faster than groups that do not. But within that group, freeloaders that do not expend resources or energy to secrete these molecules grow fastest of all. Another example of cells that grow in a way that harms other members of their groups are cancer cells, which are a potential problem for all multicellular organisms.

Indeed, many primitive multicellular organisms probably experienced both unicellular and multicellular states, providing opportunities to forego a group lifestyle. For example, the bacterium Pseudomonas fluorescens rapidly evolves to generate multicellular mats on surfaces to gain better access to oxygen. However, once a mat has formed, unicellular cheats have an incentive to not produce the glue responsible for mat formation, ultimately leading to the mat’s destruction.

To solve the mystery of how multicellular life persisted, scientists are suggesting what they call “ratcheting mechanisms.” Ratchets are devices that permit motion in just one direction. By analogy, ratcheting mechanisms are traits that provide benefits in a group context but are detrimental to loners, ultimately preventing a reversion to a single-celled state, said Libby and study co-author William Ratcliff at the Georgia Institute of Technology in Atlanta.

In general, the more a trait makes cells in a group mutually reliant, the more it serves as a ratchet. For instance, groups of cells may divide labor so that some cells grow one vital molecule while other cells grow a different essential compound, so these cells do better together than apart, an idea supported by recent experiments with bacteria.

Ratcheting can also explain the symbiosis between ancient microbes that led to symbionts living inside cells, such as the mitochondria and chloroplasts that respectively help their hosts make use of oxygen and sunlight. The single-celled organisms known as Paramecia do poorly when experimentally derived of photosynthetic symbionts, and in turn symbionts typically lose genes that are required for life outside their hosts.

These ratcheting mechanisms can lead to seemingly nonsensical results. For instance, apoptosis, or programmed cell death, is a process by which a cell essentially undergoes suicide. However, experiments show that higher rates of apoptosis can actually have benefits. In large clusters of yeast cells, apoptotic cells act like weak links whose death allows small clumps of yeast cells to break free and go on to spread elsewhere where they might have more room and nutrients to grow.

“This advantage does not work for single cells, which meant that any cell that abandoned the group would suffer a disadvantage,” Libby said. “This work shows that a cell living in a group can experience a fundamentally different environment than a cell living on its own. The environment can be so different that traits disastrous for a solitary organism, like increased rates of death, can become advantageous for cells in a group.”

When it comes to what these findings mean in the search for alien life, Libby said this research suggests that extraterrestrial behavior might appear odd until one better understands that an organism may be a member of a group.

“Organisms in communities can adopt behaviors that would appear bizarre or counterintuitive without proper consideration of their communal context,” Libby said. “It is essentially a reminder that a puzzle piece is a puzzle until you know how it fits into a larger context.”

Libby and his colleagues plan to identify other ratcheting mechanisms.

“We also have some experiments in the works to calculate the stability provided by some possible ratcheting traits,” Libby said.

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