Sexual Reproduction - Gametes and Fertilization

Sexual Reproduction - Gametes and Fertilization Sexual Reproduction Individual organisms come and go, but, to a certain extent, organisms transcend time through producing offspring. Reproduction in animals occurs in two primary ways, through sexual reproduction and through asexual reproduction.  While most animal organisms reproduce by sexual means, some are also capable of reproducing asexually. Advantages and Disadvantages In sexual reproduction, two individuals produce offspring that inherit  genetic characteristics from both parents. Sexual reproduction introduces new gene combinations in a population through genetic recombination.  The influx of new gene combinations allows members of a species to survive adverse or deadly environmental changes and conditions. This is a major advantage that sexually reproducing organisms have over those that reproduce asexually. Sexual reproduction is also advantageous as it is a way to remove harmful gene mutations from a population through recombination. There are some disadvantages to sexual reproduction. Since a male and female of the same species are required to sexually reproduce, a considerable amount of time and energy is often spent in finding the right mate. This is especially important for animals that do not bear many young as the proper mate can increase the chances of survival for the offspring. Another disadvantage is that it takes longer for offspring to grow and develop in sexually reproducing organisms. In mammals, for example, it can take several months for offspring to be born and many more months or years before they become independent. Gametes In animals, sexual reproduction encompasses the fusion of two distinct gametes (sex cells) to form a zygote. Gametes are produced by a type of cell division called meiosis. In humans, gametes are produced in the male and female gonads.  When gametes unite in fertilization, a new individual is formed. Gametes are haploid containing only one set of chromosomes. For example, human gametes contain 23 chromosomes.  After fertilization, a  zygote is  produced from the union of an egg and sperm. The zygote is diploid,  containing two sets of 23 chromosomes for a total of 46 chromosomes. In the case of animals and higher plant species, the male sex cell  is relatively motile and usually has a flagellum. The female gamete is non-motile and relatively large in comparison to the male gamete. Types of Fertilization There are two mechanisms by which fertilization can take place. The first is external (the eggs are fertilized outside of the body) and  the second is internal (the eggs are fertilized within the female reproductive tract).  Ã‚  An egg is fertilized by a single sperm to ensure that the correct  chromosome  numbers are preserved.   In external fertilization, gametes are released into the environment (typically water) and are united at random. This type of fertilization is also referred to as spawning. In internal fertilization, gametes are united within the female. In birds and reptiles, the embryo matures outside of the body and is protected by a shell. In most mammals, the embryo matures within the mother. Patterns and Cycles Reproduction is not a continuous activity and is subject to certain patterns and cycles. Oftentimes these patterns and cycles may be linked to environmental conditions which allow organisms to reproduce effectively. For example, many animals have estrous cycles that occur during certain parts of the year so that offspring can typically be born under favorable conditions. Humans, however, do not undergo estrous cycles but menstrual cycles. Likewise, these cycles and patterns are controlled by hormonal cues. Estrous can also be controlled by other seasonal cues such as rainfall. All of these cycles and patterns allow organisms to manage the relative expenditure of energy for reproduction and maximize the chances of survival for the resulting offspring.