Quiz 7: The Reproductive Process
Asexual reproduction is the production of offspring without the involvement of gametes. Offspring produced by asexual reproduction have same genotype and hence are called clones. The four forms of asexual reproduction are as follows: • Binary fission - It is common among bacteria and protozoa. Here the unicellular parent divides by mitosis in two equal halves, each of which grows into an individual similar to the parent. It may be lengthwise or transverse. In multiple fission or schizogony, the nucleus divides repeatedly before division of the cytoplasm, producing many daughter cells simultaneously. • Budding - It is an unequal division of an organism. A new individual arises as an outgrowth from its parent, develops organs like those of the parent, and then detaches itself from the parent. • Gemmulation - It is the formation of a new individual from a gemmule. A gemmule is an aggregation of cells surrounded by a resistant capsule. Gemmules develop in the fall and survive the winter in the dried or frozen body of the parent. In spring, the enclosed cells become active, emerge from the capsule, and grow into the new offspring. • Fragmentation - A multicellular animal breaks into two or more parts, with each fragment capable of becoming a complete individual. Many invertebrates can reproduce asexually by simply breaking into two parts and then regenerating the missing parts of the fragments which is seen in most anemones and hydroids.
Sexual reproduction is the production of individuals from the fusion of gametes from two genetically different parents. Offspring thus produced is genetically different from its parents. The crucial feature of sexual reproduction is the production of male and female gametes by meiosis which is a distinctive type of gamete-producing nuclear division. There are two types of meiosis: meiosis I (reductional division) and meiosis II (equational division). Chromosomes split once but the cell divides twice thus producing four cells, each with half the original number of chromosomes (haploid). Meiosis is followed by fertilization of two haploid gametes which helps in establishing the diploid condition of the individual.
Mechanisms for interchange of genes between individuals are more limited in organisms with only asexual reproduction. Since asexual organisms are haploid, genetic mutations are immediately expressed and evolution proceeds quickly resulting in faster evolutionary changes. In sexual organisms, on the other hand, a genetic mutation is often not expressed immediately, since it may be masked by its normal partner on the homologous chromosome. There is remote chance that both members of a gene pair will mutate in the same way at the same moment and thus be expressed immediately. Harmful mutations are more deleterious to asexual organisms compared to sexual organisms as they have only one copy of the mutated gene which would result in the death of the organism.