How do Amoeba Reproduce

3 Key excerpts on "How do Amoeba Reproduce"

• 

  eBook - ePub

  Animals Without Backbones

  An Introduction to the Invertebrates

  • Ralph Buchsbaum, Mildred Buchsbaum, John Pearse, Vicki Pearse(Authors)
  • 2013(Publication Date)
  • University of Chicago Press

    (Publisher)

  4: The vacuole forms again and increases in size. The pore is a temporary structure, formed anew each time the vacuole empties.

  An ameba divides. 1: A large, wellfed ameba. 2: The ameba rounds up, and the nucleus enters upon the first stages of mitosis. 3: Both nucleus and cytoplasm divide. 4: Two small amebas result, each with a nucleus and half the cytoplasm of the original ameba. (Based on Dawson, Kessler, and Silberstein.)

  Amebas grow by increasing in size and also by increasing in number through asexual reproduction, a process of replication whereby each ameba divides into two. After feeding and increasing in size for some time, an ameba rounds up into a spherical mass, the nucleus divides by mitosis (see chapter 2 ), the cytoplasm constricts, and finally the slender strand that connects the two halves breaks. The entire process requires less than an hour. Each half behaves just like the original ameba and soon increases to maximum size to divide again. Under favorable conditions, a single ameba can soon number in the thousands of nearly identical replicates. No sexual recombination has ever been seen in amebas. However the mutations in individual cells may lead to new varieties, and those varieties that survive and replicate may replace earlier forms; we can be nearly certain that no modern strain of amebas is exactly like the ancestral forms.

  Amebas carry on their routine activities only when immersed in water that contains a plentiful supply of food. If the water dries up or the food supply runs low, some amebas can round up and secrete around themselves a hard and impervious protective shell called a cyst. Within the cyst the ameba’s respiration rate falls to nearly zero so that very little energy is used. Living cysts can survive for years in the soil, or be blown about in the wind before returning to a suitable environment where they break open and the enclosed amebas resume their usual activities.

  An encysted ameba

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Order Out of Chaos

  Man’s New Dialogue with Nature

  • Isabelle Stengers, Ilya Prigogine(Authors)
  • 2018(Publication Date)
  • Verso

    (Publisher)

  Figure A .) Starting as a population of isolated cells, they join to form a mass composed of several tens of thousands of cells. This “pseudoplasmodium” then undergoes differentiation, all the while changing shape. A “foot” forms, consisting of about one third of the cells and containing abundant cellulose. This foot supports a round mass of spores, which will detach themselves and spread, multiplying as soon as they come in contact with a suitable nutrient medium and thus forming a new colony of amoebas. This is a spectacular example of adaptation to the environment. The population lives in one region until it has exhausted the available resources. It then goes through a metamorphosis by means of which it acquires the mobility to invade other environments.

  The aggregation of cellular slime molds furnishes a particularly remarkable example of a self-organization phenomenon in a biological system in which a chemical clock plays an essential role. See Figure A .

  Figure A

  When coming out of spores the amoebae grow and multiply as unicellular organisms. This situation extends until food, principally furnished by bacteria, becomes scarce. Then the amoebae cease to reproduce and enter into an interphase that lasts some eight hours. At the end of this period the amoebae begin to aggregate around cells that behave as aggregation centers. The aggregation occurs in response to chemotactic signals emitted by the centers. The aggregate thus formed migrates until the conditions for the formation of a fruiting body are satisfied. Then the mass of cells differentiates to form a stalk surmounted by a mass of spores.

  In Dictyostelium discoideum, the aggregation proceeds in a periodic manner. Movies of aggregation process show the existence of concentric waves of amoebae moving toward the center with a periodicity of several minutes. The nature of the chemotactic factor is known: it is cyclic AMP (cAMP), a substance involved in numerous biochemical processes such as hormonal regulations. The aggregation centers release the signals of cAMP in a periodic fashion. The other cells respond by moving toward the centers and by relaying the signals to the periphery of the aggregation territory. The existence of a mechanism of relay of the chemotactic signals allows each center to control the aggregation of some 105

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  The Evolution of Man — Volume 1

  • Ernst Haeckel(Author)
  • 2004(Publication Date)
  • Perlego

    (Publisher)

  If you touch one of these creeping amoebae with a needle, or put a drop of acid in the water, the whole body at once contracts in consequence of this mechanical or physical stimulus. As a rule, the body then resumes its globular shape. In certain circumstances—for instance, if the impurity of the water lasts some time—the amoeba begins to develop a covering. It exudes a membrane or capsule, which immediately hardens, and assumes the appearance of a round cell with a protective membrane. The amoeba either takes its food directly by imbibition of matter floating in the water, or by pressing into its protoplasmic body solid particles with which it comes in contact. The latter process may be observed at any moment by forcing it to eat. If finely ground colouring matter, such as carmine or indigo, is put into the water, you can see the body of the amoeba pressing these coloured particles into itself, the substance of the cell closing round them. The amoeba can take in food in this way at any point on its surface, without having any special organs for intussusception and digestion, or a real mouth or gut.

  The amoeba grows by thus taking in food and dissolving the particles eaten in its protoplasm. When it reaches a certain size by this continual feeding, it begins to reproduce. This is done by the simple process of cleavage (Figure 1.17). First, the nucleus divides into two parts. Then the protoplasm is separated between the two new nuclei, and the whole cell splits into two daughter-cells, the protoplasm gathering about each of the nuclei. The thin bridge of protoplasm which at first connects the daughter-cells soon breaks. Here we have the simple form of direct cleavage of the nuclei. Without mitosis, or formation of threads, the homogeneous nucleus divides into two halves. These move away from each other, and become centres of attraction for the enveloping matter, the protoplasm. The same direct cleavage of the nuclei is also witnessed in the reproduction of many other protists, while other unicellular organisms show the indirect division of the cell.

  Hence, although the amoeba is nothing but a simple cell, it is evidently able to accomplish all the functions of the multicellular organism. It moves, feels, nourishes itself, and reproduces. Some kinds of these amoebae can be seen with the naked eye, but most of them are microscopically small. It is for the following reasons that we regard the amoebae as the unicellular organisms which have special phylogenetic (or evolutionary) relations to the ovum. In many of the lower animals the ovum retains its original naked form until fertilisation, develops no membranes, and is then often indistinguishable from the ordinary amoeba. Like the amoebae, these naked ova may thrust out processes, and move about as travelling cells. In the sponges these mobile ova move about freely in the maternal body like independent amoebae (Figure 1.17). They had been observed by earlier scientists, but described as foreign bodies—namely, parasitic amoebae, living parasitically on the body of the sponge. Later, however, it was discovered that they were not parasites, but the ova of the sponge. We also find this remarkable phenomenon among other animals, such as the graceful, bell-shaped zoophytes, which we call polyps and medusae. Their ova remain naked cells, which thrust out amoeboid projections, nourish themselves, and move about. When they have been fertilised, the multicellular organism is formed from them by repeated segmentation.

  Sign up to read

  Learn more about book