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Topic: Digestive System In Invertebrates.
Lesson Objectives: By the end of the lesson, the learners should be able to:
- Give the Meaning and Types of Reproduction
- State Forms of Asexual Reproduction
- Explain Sexual Reproduction (Conjugation and Fusion of Gametes)
- Meiosis and Importance of Meiosis
MEANING AND TYPES OF REPRODUCTION
Reproduction is the ability of an organism to give rise to new individuals of the same species in order to ensure continuity of life.
There are two types of reproduction;
- Asexual reproduction
- Sexual reproduction
Asexual Reproduction: is the process whereby an organism produces an offspring by itself. I.e. only one parent is presence. No gametes involved thus there is no fusion of nuclei, but the cells that give rise to the offspring usually divide by means of mitosis. Offspring produced are identical to the parent in all respect and are called clones
Sexual Reproduction: is a type of reproduction that involves two parents and the fusion of the male and the female gamete to form a zygote. Offspring produced show new variation. The sex cells (gametes) are produced by meiotic cell division and after fertilization the new individual continue to grow and produce new cells by mitosis.
FORMS OF ASEXUAL REPRODUCTION
Binary Fission: Fission is the simplest form and involves the division of a single organism into two complete organisms, each identical to the other and to the parent. Fission is common among unicellular organisms such as bacteria, many protists and some algae.
Budding: The parent organism develops an outgrowth which subsequently forms the new individual organism. These buds break off from the parent without causing any injury and live an independent life. Budding is common in yeast and hydra
Spore Formation: Spores are DNA-containing capsules capable of sprouting into new organisms; unlike most seeds, spores are produced without sexual union of gametes, when dispersed, each spore is capable of developing into a new organism. Spores are common in lower organisms especially fungi such as rhizopus and penicillum.
Fragmentation: A part of the parent organism breaks up and develops into a new x organism. This type of reproduction is also called regeneration. Fragmentation is common in spirogyra and coelenterates.
Vegetative Propagation: It occurs in higher plants. In this process, a new plant grows from any portion of an old one other than the seeds. There are two methods of vegetative propagation, this include natural and artificial vegetative propagations.
Natural vegetative propagation involves the use of vegetative parts such as stems, leaves, roots or buds. The part involved must have a store of food and sometimes able to act as a perennating organ i. e. enable the plant to survive from one growing season to the next.
Organs Of Vegetative Propagation
1. Bulbil: Axillary buds growing from the veins of leaves e. g. Bryophyllum and Begonia.
Runners or stolons: Stems that creep horizontally on soil surface. Buds and adventitious roots develop from the nodes of the parent plants e. g. sweet potato, grass.
2. Rhizomes: Underground horizontal stem. It has scaly leaves which cover lateral buds at the nodes. Lateral buds grow into new aerial shoots e. g. ginger, canna lily.
3. Corms: Underground stems which grow vertically in the soil. Buds develop from the axils of scale leaves, grow upward and form leaves and flowers e. g. cocoyam.
4. Stem tubers: Underground stems which have swollen tips. Axillary buds on tubers give rise to new aerial shoots e. g. yam, sweet potato
5. Suckers: Short underground horizontal branches e. g. banana, pineapple
6. Bulb: Underground condensed shoots with compressed stems and scaly leaves e. g. onion, garlic.
Artificial propagation is the use of parts of the parent plant to multiply the plants. Budding, grafting, layering, cutting and marcotting are types of artificial propagation.
There are two types of sexual reproduction, this include conjugation and fusion of gametes.
Conjugation: the simplest form of sexual reproduction, as contrasted with asexual reproduction. This is observed in some unicellular organisms e. g. paramecium, fungi e. g. rhizopus, algae e. g. spirogyra. Two similar organisms (conjugants) join together and exchange genetic material contained in their nuclei. After the exchange, the organisms separate. The zygote or zygosphere form is capable of developing into a new organism.
Fusion of Gametes: This occurs in higher organisms where gametes (male and female sex cells) are produced in special structures by a process known as gametogenesis which involves meiotic cell divisions. Fertilization, the process in which haploid male and female sex cells fuse together to producing a single diploid cell (zygote) that develops into an adult organism occurs after gamatogenesis.
Meiosis is the reduction method of division that leads to the formation of four daughter cells that are haploids. Meiosis differs from mitosis, in that it involves two consecutive cell divisions instead of one and the genetic material contained in chromosomes is not copied during the second meiotic division. Whereas mitosis produces identical daughter cells, meiosis randomly mixes the chromosomes, resulting in unique combinations of chromosomes in each daughter cell. Meiosis ensures the chromosome number of an individual remains the same from generation to generation.
Meiosis consists of two successive divisions:
a. First Meiotic Division
Interphase: resting phase, chromosomes are not seen.
Prophase I: At early prophase, chromosomes contract and become clearly visible. At middle prophase, homologous chromosomes come together and spindle is formed. At late prophase crossing over takes place between homologous chromosomes at a place called chiasma.
Metaphase: Nuclear membrane disappears, the bivalent chromosomes assemble at the equator and are attached to the spindle by their centromere.
Anaphase: Bivalent chromosomes separate completely and move to the opposite pole of the cell.
Telophase: bivalent chromosomes arrived at the pole, nuclear membrane are formed around the chromosomes at the two poles, two daughter cells result with half the number of chromosomes in the parent cell.
b. Second Meiotic Division : This consists of four stages similar to mitosis, no resting stage and no replication of chromosomes. At the end, four daughter cells are formed.