Reproduction without sex (Asexual Reproduction)

Learning Objectives

  1. Know which domains have asexual reproduction and how asexual reproduction occurs by binary fission of cells.
  2. Know and provide biological examples of the types of asexual reproduction: binary fission, budding, fragmentation, and parthenogenesis.
  3. Know that faithful replication of DNA is the essential element in asexual reproduction and in cell division, both of which produce daughter cells identical to the parent cell.
  4. Compare and contrast binary fission with mitosis and cytokinesis.

Reproduction, the production of offspring, is an essential step in the life cycle. It can occur with or without sex! The tricky bit in reproduction is to make sure that each offspring has a full set of DNA, which contains all the genes that encode life. 

Who reproduces asexually? Almost everyone but us [Adapted from Openstax Biology]

Sexual reproduction, like we and most animals have, is actually pretty unusual. Bacteria and Archaea reproduce asexually, by simply dividing a parent cell into two new cells in a process called binary fission. Reproduction in bacteria and archaea is asexual and usually takes place by binary fission. The circular chromosome (the DNA) is replicated and the cell grows, separating the two resulting chromosome copies from one another. The cell, now enlarged, pinches inward at its equator and separates into two cells, which are clones of each other, genetically identical to the parent.

The key ingredient in asexual reproduction is the faithful replication of the DNA into each new cell, because DNA contains the full set of instructions to make everything that new cell will need for life. Binary fission does not provide an opportunity for genetic recombination, a benefit of sex that we will read about next time, but bacteria and archaea can share genes by three other mechanisms: transformation, transduction, and conjugation.

In transformation, the prokaryote takes in DNA found in the environment; this DNA has been shed by other organisms. If a nonpathogenic bacterium takes up DNA for a toxin gene from a pathogen and incorporates the new DNA into its own chromosome, it too may become pathogenic. In transduction, bacteriophages, the viruses that infect bacteria, sometimes also move short pieces of chromosomal DNA from one bacterium to another. Transduction results in a recombinant organism. Archaea are not affected by bacteriophages but instead have their own viruses that translocate genetic material from one individual to another. In conjugation, DNA is transferred from one prokaryote to another by means of a pilus, which brings the organisms into contact with one another. The DNA transferred can be in the form of a small circular plasmid or a hybrid of both plasmid and chromosomal DNA. Plasmid transfer is a common vehicle for antibiotic resistance to spread between bacterial species.

Besides binary fission, there are three other mechanisms by which prokaryotes can exchange DNA. Cell membrane (orange), chromosomal DNA (purple), plasmid DNA (green), pilus (blue) [Image credit: Openstax Biology.]

Reproduction in bacteria and archaea can be very rapid: a few minutes for some species. This short generation time coupled with the three mechanisms of DNA transfer and high rates of mutation result in the rapid evolution of prokaryotes, allowing them to respond to environmental changes and challenges (such as the introduction of an antibiotic) very quickly.

 

The Eukaryotic Cell Cycle

Cell_Cycle_2-2

The cell cycle shows interphase (I) and mitosis (M). Interphase consists of G1, DNA synthesis (DNA), and G2 phases. Cells that stop dividing exit the G1 phase of the cell cycle into a so-called G0 state. Mitosis results in two new cells with identical DNA to each other, where the entire cell cycle begins again. [Image credit: Wikipedia.]

Eukaryotic cells reproduce genetically identical copies of themselves by cycles of cell growth and division, instead of by binary fission. The cell cycle diagram shows that a cell division cycle consists of 4 stages:

  • G1 is the period after cell division, and before the start of DNA replication. Cells grow and monitor their environment to determine whether they should initiate another round of cell division.
  • S is the period of DNA synthesis, where cells replicate their chromosomes.
  • G2 is the period between the end of DNA replication and the start of cell division. Cells check to make sure DNA replication has successfully completed, and make any necessary repairs.
  • M is the actual period of cell division, consisting of prophase, metaphase, anaphase, telophase, and cytokinesis.

Chromosomes 

Chromosomes condense and become visible by light microscopy as eukaryotic cells enter mitosis or meiosis (which we’ll learn about in the next reading). During interphase (G1 + S + G2), chromosomes are decondensed in the form of chromatin, which consists of DNA wound around proteins called histones.

chromatin_lg

DNA is packaged with histones to form nucleosomes, that resemble “beads on a string” when fully decondensed chromatin is visualized by electron microscopy. Chromatin condenses and becomes chromosomes visible by light microscopy during mitosis or meiosis. Figure from https://www.genome.gov/Glossary/index.cfm?id=32

As a result of DNA replication during S phase, each prophase chromosome consists of a pair of sister chromatids. Each chromatid contains a linear DNA molecule that is identical to the sister chromatid it is joined to. The sister chromatids are joined at their centromeres, shown in the diagram below as the left and right halves of an X shape.

Human karyotype "painted" using fluorescent DNA probes. These mitotic chromosomes each consist of a pair of sister chromatids joined at their centromeres. The images of the homologous chromosome pairs (e.g., 2 copies of chromosome 1) have been lined up next to each other. Image from Bolzer et al., (2005) Three-Dimensional Maps of All Chromosomes in Human Male Fibroblast Nuclei and Prometaphase Rosettes. PLoS Biol 3(5): e157 DOI: 10.1371/journal.pbio.0030157

Image of human chromosomes “painted” using fluorescent DNA probes. These mitotic chromosomes each consist of a pair of sister chromatids joined at their centromeres. The images of the homologous chromosome pairs (e.g., 2 copies of chromosome 1) have been lined up next to each other. [Image credit: from Bolzer et al., (2005) Three-Dimensional Maps of All Chromosomes in Human Male Fibroblast Nuclei and Prometaphase Rosettes. PLoS Biol 3(5): e157 DOI: 10.1371/journal.pbio.0030157]

Asexual reproduction in animals [Adapted from Openstax Biology]

As in bacteria and archaea, some invertebrate animals reproduce by binary fission, splitting into two separate organisms after a period of growth. Some unicellular eukaryotic organisms undergo binary fission by mitosis. In other organisms, part of the individual separates and forms a second individual. This process occurs, for example, in many asteroid echinoderms through splitting of the central disk. Some sea anemones and some coral polyps (figure below) also reproduce through fission.

 Image shows many coral polyps clustered together. Each Polyp is cup-shaped, with tentacles radiating out from the rim.
Coral polyps reproduce asexually by fission. Image source. (credit: G. P. Schmahl, NOAA FGBNMS Manager)

Budding

Budding is a form of asexual reproduction that results from the outgrowth of a part of a cell or body region leading to a separation from the original organism into two individuals. Budding occurs commonly in some invertebrate animals such as corals and hydras. In hydras, a bud forms that develops into an adult and breaks away from the main body, as illustrated below, whereas in coral budding, the bud does not detach and multiplies as part of a new colony.

Illustration shows a hydra, which has a stalk-like body with tentacles growing out the top. A smaller hydra is budding from the side of the stalk.
Hydra reproduce asexually through budding. Image source.

 

Fragmentation

Fragmentation is the breaking of the body into two parts with subsequent regeneration. If the animal is capable of fragmentation, and the part is big enough, a separate individual will regrow.

For example, in many sea stars, asexual reproduction is accomplished by fragmentation. The figure below illustrates a sea star for which an arm of the individual is broken off and regenerates a new sea star. Fisheries workers have been known to try to kill the sea stars eating their clam or oyster beds by cutting them in half and throwing them back into the ocean. Unfortunately for the workers, the two parts can each regenerate a new half, resulting in twice as many sea stars to prey upon the oysters and clams. Fragmentation also occurs in annelid worms, turbellarians, and poriferans.

 Illustration shows a sea star with one long arm and four very short arms.
Sea stars can reproduce through fragmentation. The large arm, a fragment from another sea star, is developing into a new individual. Image source

Note that in fragmentation, there is generally a noticeable difference in the size of the individuals, whereas in fission, two individuals of approximate size are formed.

Parthenogenesis

Parthenogenesis is a form of asexual reproduction where an egg develops into a complete individual without being fertilized. The resulting offspring can be either haploid or diploid, depending on the process and the species. Parthenogenesis occurs in invertebrates such as water fleas, rotifers, aphids, stick insects, some ants, wasps, and bees. Honeybees use parthenogenesis to produce haploid males (drones). If an egg is fertilized, a female is produced, which can become a worker or a queen. The queen bee controls the reproduction of the hive bees to regulate the type of bee produced.

Some vertebrate animals—such as certain reptiles, amphibians, and fish—also reproduce through parthenogenesis. Although more common in plants, parthenogenesis has been observed in animal species that were segregated by sex in terrestrial or marine zoos. Two female Komodo dragons, a hammerhead shark, and a blacktop shark have produced parthenogenic young when the females have been isolated from males.

Mitosis

In eukaryotes, asexual reproduction uses the same process as when the cells in your body divide via mitosis, so we’ll learn that process now. Mitosis produces two daughter cells that are genetically identical to each other, and to the parental cell. All eukaryotic cells replicate via mitosis, except for cells that produce gametes (eggs and sperm).

  • prophase – chromosomes condense; each chromosome consists of a pair of identical sister chromatids joined at the centromere.
  • metaphase – chromosomes line up at the middle of the cell, along the plane of cell division, pushed and pulled by microtubules of the spindle apparatus
  • anaphase – sister chromatids separate and migrate towards opposite ends of the cell
  • telophase – chromatids cluster at opposite ends of the cell and begin to decondense
  • cytokinesis – the membrane pinches in to divide the two daughter cells

Here is a simplified diagram illustrating the overall process and products of mitosis:

File:Major events in mitosis.svg

DNA replication occurs during interphase before mitosis. Then mitosis divides the DNA into two new cells that are exact genetic replicas of the original cell. [Image credit: Wikimedia Commons]

 

The video gives an overview of mitosis:

The next short video reviews methods of asexual reproduction and gives some examples of organisms that reproduce asexually: bacteria, budding yeast, sea anemone, and clonal plant cuttings.