In biology, the process by which a living organism produces other organisms more or less similar to itself. The ways in which species reproduce differ, but the two main methods are by asexual reproduction and sexual reproduction. Asexual reproduction involves only one parent without the formation of gametes: the parent's cells divide by mitosis to produce new cells with the same number and kind of chromosomes as its own. Thus offspring produced asexually are clones of the parent and there is no variation. Sexual reproduction involves two parents, one male and one female. The parents' sex cells divide by meiosis producing gametes, which contain only half the number of chromosomes of the parent cell. In this way, when two sets of chromosomes combine during fertilization, a new combination of genes is produced. Hence the new organism will differ from both parents, and variation is introduced. The ability to reproduce is considered one of the fundamental attributes of living things.
Sexual reproductive systems
The plant organs concerned with sexual reproduction are found in the flowers. These consist of the stamens (male organ) and carpels (female organ). In male mammals the reproductive system consists of the testes, which produce sperm, epididymis, sperm duct, and penis, and in the females the ovaries, which produce eggs, Fallopian tubes, and uterus.
These are bisexual organisms, such as earthworms, that have both male and female reproductive organs, or plants whose flowers contain both stamens and carpels. This is the normal arrangement in most plants. Some plant species, such as maize and birch, which have separate male and female flowers on the same plants are described as monoeious; in dioecious species, such as willow and holly, the male and female flowers are on separate plants.
This occurs mainly in lower animals, micro-organisms, and plants. In lower animals and micro-organisms, the chief methods of reproduction are by binary fission, fragmentation, and budding. Binary fission occurs in unicellular organisms, such as protozoans and bacteria: the nucleus of the parent cell divides to form two new daughter cells. Where more than two new cells are formed, this is termed ‘multiple fission’. Fragmentation occurs in some invertebrates, such as jellyfish: parts of the organisms break away and subsequently differentiate to form new organisms. Regeneration may sometimes occur before separation, producing chains of offspring budding from the parent organism.
The main methods of asexual reproduction in plants are by vegetative reproductionand the formation of sexual spores. In vegetative reproduction, or propagation, new plants are produced from the outgrowth of the old ones, such as by runners, bulbs, cuttings, and grafting. Spore formation occurs in plants such as mosses and ferns; it also occurs in fungi, bacteria, and some protozoans. The spore may develop into an organism resembling the parent, or into another stage in the life cycle (see alternation of generations.) Some organisms, such as aphids, reproduce by parthenogenesis; this is a degenerate form of sexual reproduction in which the unfertilized female's eggs develop directly into new organisms without contribution from the male.
The fusion of the sperm nucleus and egg nucleus in the female's oviduct produces a zygote, which divides by mitosis to form an embryo. Most mammals produce several young at a time; humans are unusual in normally producing only one offspring at a time. Occasionally two or more young may be produced at the same time. When a zygote divides after fertilization to produce separate embryos with genetically identical characteristics, these are described as monozygotic. Dizygotic embryos form from separately fertilized eggs and therefore differ genetically.
Sexual reproduction in plants
In flowering plants, the function of a flower is reproduction. Despite the enormous variety of flowers, they all contain parts that produce gametes for sexual reproduction. On the outside of the flower are sepals, which provide protection while it is in bud. Inside the sepals lie the petals, usually brightly coloured in order to attract insects to the nectary. This is a gland at the base of the petals which produces a sugary liquid, nectar, on which the insects feed. Inside the petals are the stamens; these are long filaments, each with an anther at the top containing pollen grains; this is where the male gametes are found. The female part of the flower, the carpel, lies at the centre; it consists mainly of an ovary within which are many ovules containing the female gametes. Before fertilization can take place, pollination must occur - that is, the pollen containing the male gametes must be transferred from the anther to the stigma in order to reach the ovules inside the carpel. This may happen by the wind blowing the pollen onto the stigma, or by the acidental transfer of pollen when insects are feeding on the flower's nectar. Once fertilization has taken place, the zygote divides and forms an embryo inside the ovule, which increases in size as the embryo develops and becomes a seed. Upon germination, the embryonic shoot and root emerge from the seed to develop into a new plant.
The principal gymnosperms are cycads and conifers, the cone-bearing plants. The plants carry both male cones and female cones and, like flowering plants, pollen is transferred both by the wind and by insects. Gymnosperm means ‘naked seed’, and in these plants the ovules, and the seeds into which they develop, are not enclosed in ovaries but are borne unprotected on the scales of their cones.
Sexual reproduction in fish
Reproduction in fish is mainly by external fertilization, or spawning. The females, which carry enormous numbers of eggs in their ovaries, lay their eggs in the water, and the male releases sperm onto them. The eggs contain yolk which supplies the embryo with food, and albumen - a protein- which protects it. The embryo obtains its oxygen from the water by diffusion, and the young fish, or larvae, hatch after a few days. In cartilaginous fish, such as sharks, the eggs are fertilized internally and hatch inside the body.
Sexual reproduction in amphibians
Like fish, fertilization in amphibians (such as frogs, toads, salamanders) is usually external. Their larvae are aquatic, having gills for respiration. Upon metamorphosis to adult terrestrial form, the tail is absorbed into the body and the gills are replaced by lungs.
Sexual reproduction in birds
All birds use internal fertilization. The sperm is passed from the male bird into the female's oviduct. The fertilized egg then travels down the oviduct and, as it does so, the walls of the oviduct secrete a layer of albumen which surrounds the yolk. Just before the egg is laid, the oviduct walls secrete a calcium-rich substance over it which hardens into the shell. The embryo obtains its oxygen by diffusion through the shell and membranes.
Sexual reproduction in mammals
Fertilization in all mammals is internal. The embryo may develop within a soft-shelled egg, as in the platypus; or, as in the case of marsupials, immature live offspring or embyros are produced which complete their development within a marsupium, or pouch. However, in the majority of mammals, the embryos develop within the mother's uterus, where they are protected and nourished by embryonic membranes and a placenta until birth. Such mammals, including humans, are termed ‘viviparous’; after birth the young mammals are fed on milk produced by the mother's mammary glands.
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