Explain the mechanisms of reproduction

Explain the mechanisms of reproduction that ensure the continuity of a species, by analysing sexual and asexual methods of reproduction in a variety of organisms, including but not limited to:

• animals: advantages of external and internal fertilisation
• plants: asexual and sexual reproduction
• fungi: budding, spores
• bacteria: binary fission (ACSBL075)
• protists: binary fission, budding

Reproduction in plants:

• • Asexual reproduction:
    • The type of reproduction which does not involve union of male and female gametes yet results in formation of new plants thus maintains continuity of plant species.
    • The different processes of asexual reproduction are discussed in brief below:
      • New plant parts can grow from vegetative organs of plants such as roots, leaves, stems etc. in a process called vegetative propagation.
      • Examples include: Sweet potato (propagation by roots), Ginger (propagation by stem), Bryophyllum (propagation by leaves) etc.
      • Vegetative propagation methods include:
        • Cutting:
          • Stem from the plant is cut and is planted in the soil that will gradually grow and turn into another plant.
          • Used for plants like rose, hibiscus, sugarcane etc.

• • • • • Layering:
          • A branch from the whole plant is selected, bent and the bent portion is covered with soil while still being attached to the mother plant.
          • The bent portion touching the soil develops roots and the part above the ground develops into individual plants.
          • Jasmine and bougainvillea (paper flower) can be propagated this way.

• • • • • • Grafting:
            • In this process two different plants are selected and joined so that they grow as one single plant.
            • One of the plants serves the purpose of being grounded by root formation which is termed as the stock.
            • A part from the other plant is cut and is joined with the stock. This part is known as scion and is the desired plant that we want to grow on the stock.
            • The type of fruits and flowers produced from the joined plant depends on the scion and not the stock.

• • • • Sexual Reproduction:
        • Sexual reproduction occurs when a male gamete and a female gamete unite to form a single celled zygote which after a certain period of time turns into a complete plant.
        • Sexual reproduction occurs in flowering plants and involve multiple steps which have been discussed in brief below:
          • Male gametophytes are produced inside anthers and when matured are termed as pollen grains which actively take part in reproduction.
          • Inside the pollen grains, two nuclei are present; the tube nucleus and the generative nucleus. The tube nucleus forms the pollen tube and the generative nucleus prior to reproduction produces two haploid male gametes.

• • • • • • The ovule is created in the female plants from the placenta which is surrounded by a layer of tissue known as Nucellus. Both the external and internal layers of the ovule have a pore known as micropile through which pollen tube will enter during fertilization.
          • The region inside the ovule surrounded by the nucellus where the female gametes reside is known as embryo sac. Inside the embryo sac, the following cells will be found:
            • A large nucleus near the micropyle which is the female gamete.
            • Two smaller nuclei situated on either side of the female gamete known as synergides.
            • A set of three nuclei situated at the other end of the embryo sac known as antipodal cells.
            • A diploid secondary nucleus situated at the center of the embryo sac.

• • • • • • Pollen grains are carried to the stigma of the female flower where they adhere to the stigma by interaction between different surface chemicals.
          • The intine elongates forming the pollen tube carrying the tube nucleus and the two male gametes.
          • The pollen tube enters the embryo sac through the micropyle.
          • The tube nucleus fuses with the synergides and they disintegrate.
          • One of the male gametes fuses with the ovum to produce zygote(2n).
          • The second male gamete (n) fuses with the secondary nucleus (2n) resulting in the formation of a triploid nucleus. This is known as Double Fertilization.

Reproduction in fungi:

• • • • Budding:
        • A form of asexual reproduction in which a new organism is developed from a small outgrowth in the parents’ vegetative body.
        • Commonly occurs in Yeast.
        • Process:
          • At first, a small irregular growth is seen on the parent’s body.
          • Next, in favourable condition, the growth becomes larger (by consecutive cell division) and when reaching a state where it can live without depending on the parent, the outgrowth detaches from the parent and leads its own life.

• • • • • •  Sporulation:
            • Sporulation is the most commonly used asexual reproduction method in fungi where reproduction occurs mainly by formation of spores in sacs called sporangium.
            • Spores maybe of the following types:
              • Asexual spores:
                • They are innumerable, non-motile, uninucleate and are produced on the diplont mycelium.
                • The spores are of diverse type and borne upon special structures called the sporophores.
              • Endogenous spores:
                • The endogenous spores are produced within the special spore producing cell the sporangium.
                • They may be branched or unbranched, motile or non-motile.
              • Exogenous spores:
                • The spores producing externally or exogenously are either called the exogenous spores or conidia.
                • They are produced externally on the branched or unbranched conidiophores and may be septate or aseptate.
                • They may be unicellular or multicellular, uninucleate or multinucleate. Different genera may be recognized only by the presence of various shaped and various coloured conidia

Reproduction in Bacteria

• • • • Binary Fission:
        • Process in which bacterial cells multiply in number.
        • Is very similar to mitosis cell division in eukaryotes but serves a different purpose because binary fission is a form of bacterial reproduction and is used for increasing number of bacteria in a population rather than promoting cell growth which is the purpose of mitosis in eukaryotic organisms.
        • Steps include:
          • DNA replication begins in the origin of replication. The origin is the first part of the DNA to be copied.
          • As replication continues, the two origins move towards opposite ends of the cell, pulling the rest of the chromosome along with them making the bacterial cell longer.
          • Once replication is complete and the new chromosomes have moved to opposite cell ends and cleared the center of the cell, division of the cytoplasm can take place.
          • In this process, the membrane pinches inward and a septum, or new dividing wall, forms down the middle of the cell. (Bacteria have a cell wall, so they must regenerate this wall when they undergo cell division.)
          • Finally, the septum itself splits down the middle, and the two cells are released to continue their lives as individual bacteria.

Reproduction in Protists:

• • • • Protists are eukaryotic organisms but are not animals, plants or fungus and can reproduce asexually by a number of process including budding and binary fission.
        • Binary Fission:
          • Similar to bacteria, protists also have the capability to multiply in number by binary fission a process by which one protist is split into two protists and keeps continuing this process for continuation of generation.
        • Budding:
          • udding occurs when a new organism grows from the body of the parent organism and is detached from the parent once it attains maturity to form its own colony.

Advantages of Internal and External Fertilization in animals:

• • • Internal Fertilization:
      • Increased possibilities of unions of gamete because all conditions required for fusion of gametes is maintained inside the body
      • More protection against outside environments and predators, and therefore a higher chance of surviving until birth.
      • More selective of their mates
      • Less chance of desiccation of gametes
    • External Fertilization:
      • Results in the production of a large number of zygotes and thus more offsprings can be produced
      • Easier to find mates as the gametes released can drift (wind, water etc).
      • More genetic variation

Extract from HSC Biology Stage 6 Syllabus. © 2017 Board of Studies NSW.