Sexual reproduction
From Wikipedia, the free encyclopedia.
Sexual reproduction is a process of reproduction involving the merging of two sex cells or gametes from the same species to produce a new organism. One clear advantage of this form of reproduction over asexual reproduction is that the DNA or genetic makeup of the offspring produced can be significantly different from that of either of the parents that contributed the gametes. This fact allows for considerable phenotypic variation to exist within a population. The DNA of each offspring is different because each contributing organism (i.e., "parent") randomly and independently donates half of its DNA compliment to the sex cells through a process called meiosis.
The first fossilized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1.2 to 1 billion years before the present time. Sexual reproduction is the primary method of reproduction for the vast majority of visible organisms, including almost all animals and plants. Bacterial conjugation, the transfer of DNA between two bacteria, is often mistakenly confused with sexual reproduction, because the mechanics are similar.
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Sexual reproduction of protists and fungi
Many protists and fungi reproduce sexually. Although they are unicellular, at times of reproduction the "father" cell and the "mother" cell combines together. Next, their genetic information combines together into a new formation, and by cell division the offspring is born.
Reproduction in flowering plants
- Main articles: Plant sexuality
In flowering plants, a stamen produces gametes called pollen grains, which attach to a pistil, in which the female gametes (ovules) are located. Here, the female gamete is fertilized and develops into a seed. The ovary, which produced the gamete then grows into a fruit, which surrounds the seed(s). Plants may either self-pollinate or cross-pollinate.
Reproduction in reptiles
Female reptiles lay eggs, fertilized by the male, in which the young gestate.
Reproduction in birds
Male and female birds both have cloacas. The female lays eggs, fertilized by the male, in which the young gestate.
Reproduction in mammals
In placental mammals, offspring are born as juveniles: complete animals with the sex organs present although non-functional. After several months or years, the sex organs develop further to maturity and the animal becomes sexually mature. Most female mammals are only fertile during certain periods and during those times, they are said to be "in heat". At this point, the animal is ready to mate. Individual male and female mammals meet and carry out copulation. For most mammals, males and females exchange sexual partners throughout their adult lives.
The mammalian male
- For more details on this topic, see Male reproductive system (human).
The male reproductive system contains two main divisions: the penis, which is inserted into the female and carries the sperm inside it, and the testes, which produce the sperm. In humans, both of these organs are outside the abdominal cavity, but they can be primarily housed within the abdomen in other animals (for instance, in dogs, the penis is internal except when mating). Having the testes outside the abdomen best facilitates temperature regulation of the sperm, which require specific temperatures to survive.
Sperm are the smaller of the two gametes and are generally very short-lived, requiring males to produce them continuously from the time of sexual maturity until death. They are motile and swim by chemotaxis.
The mammalian female
- For more details on this topic, see Female reproductive system (human).
The female reproductive system likewise contains two main divisions: the vagina and uterus, which act as the receptacle for the male's sperm, and the ovaries, which produce the female's ova. All of these parts are always internal. The vagina is attached to the uterus through the cervix, while the uterus is attached to the ovaries via the Fallopian tubes. At certain intervals, the ovaries release an ovum (the singular of ova), which passes through the fallopian tube into the uterus.
If, in this transit, it meets with sperm, the sperm penetrate and merge with the egg, fertilizing it. The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then implants itself in the wall of the uterus, where it begins the processes of embryogenesis and morphogenesis. When developed enough to survive outside the womb, the cervix dilates and contractions of the uterus propel the fetus through the birth canal, which is the vagina.
The ova are larger than sperm and are generally all created by birth. They are for the most part stationary, aside from their transit to the uterus, and contain nutrients for the later zygote and embryo. Over a regular interval, a process of oogenesis matures one ovum to be sent down the Fallopian tube attached to its ovary in anticipation of fertilization. If not fertilized, this egg is flushed out of the system through menstruation in humans and great apes and reabsorbed in all other mammals in the estrus cycle.
Gestation
- Main articles: Mammalian gestation, Pregnancy
Gestation, called pregnancy in humans, is the period of time during which the fetus develops, dividing via mitosis inside the female. During this time, the fetus receives all of its nutrition and oxygenated blood from the female, filtered through the placenta, which is attached to the fetus' abdomen via an umbilical cord. This drain of nutrients can be quite taxing on the female, who is required to ingest significantly higher levels of calories. In addition, certain vitamins and other nutrients are required in greater quantities than normal, often creating abnormal eating habits. The length of gestation, called the gestation period, varies greatly from species to species; it is 38 weeks in humans, 56-60 in giraffes and 16 days in hamsters.
Birth
- Main articles: Childbirth, and [[]], and [[]], and [[]], and [[]]
Once the fetus is sufficiently developed, chemical signals start the process of birth, which begins with contractions of the uterus and the dilation of the cervix. The fetus then descends to the cervix, where it is pushed out into the vagina, and eventually out of the female. The newborn, which is called an infant in humans, should typically begin respiration on its own shortly after birth. Not long after, the placenta is passed as well. Most mammals eat this, as it is a good source of protein and other vital nutrients needed for caring for the young. The end of the umbilical cord attached to the young’s abdomen eventually falls off on its own.
Monotremes
Monotremes, only five species of which exist, all from Australia and New Guinea, lay eggs. They have one opening for excretion and reproduction called the cloaca. They hold the eggs internally for several weeks, providing nutrients, and then lay them and cover them like birds. After less than two weeks the young hatches and crawls into its mother’s pouch, much like marsupials, where it nurses for several weeks as it grows.
Marsupials
Marsupials reproduce in essentially the same manner, though their young are born at a far earlier stage of development than other mammals. After birth, marsupial joeys crawl into their mother’s pouch and attach to a teat, where they receive nourishment and finish developing into self-sufficient animals.
Origin of Sexual Reproduction
See also 'Evolution of sex'
There are three classes of hypothesis as to how sexual reproduction originated:
1) Sex originated from selfish parasitic genetic elements that used sexual reproduction for their own transmission and propagation.
2) Sex evolved as a byproduct of DNA repair of spontaneous mutations.
3) Sex evolved because it increases the efficiency of natural selection.
Computer simulations have shown that bacteria that reproduce asexually can be nudged to evolve into sexual reproduction if they are subject to high levels of mutation induced by environmental stress [1].
Sexual reproduction introduces new genetic material, increasing the chances of individuals surviving. Hermaphrodites capable of both sexual and asexual reproduction tend to be asexual when the surroundings are stable and suitable for their countinued survival. If the surroundings change drastically, these same hermaphrodites switch to sexual reproduction, improving their chances of surviving during the period of uncertainty.
See also
References
- Pang, K. "Certificate Biology: New Mastering Basic Concepts", Hong Kong, 2003.
- Journal of Biology of Reproduction, accessed in August 2005.
