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Sexual reproduction is a union that results in increasing genetic diversity of the
offspring. It is characterized by two processes: meiosis, involving the halving of the number of
chromosomes; and fertilisation, involving the fusion of two gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually chromosomal crossover to achieve genetic recombination.
The
evolution of sex is a major puzzle. The first fossilized evidence of sexually reproducing
organisms is from
eukaryotes of the
Stenian period, about 1.2 to 1 billion years ago. 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.
A major question is why sexual reproduction persists when
parthenogenesis appears in some ways to be a superior form of
reproduction. Contemporary evolutionary thought proposes some explanations. It may be due to selection pressure on the
clade itself—the ability for a population to radiate more rapidly due to a changing environment through sexual recombination than parthenogenesis allows. Alternatively, sexual reproduction may allow for the 'ratcheting' of evolutionary speed as one clade competes with another for a limited resource.
Reproduction in plants
In flowering plants, the Stamen produces male gametophytes, the sperm is produced in pollen, which attach to the stigma on top of a carpel, in which the female gametophytes (inside ovules) are located. After the pollen tube grows through the carpel's style, the sperm cell nuclei from the pollen grain migrate into the ovule to fertilize the egg cell and endosperm nuclei within the female gametophyte in a process termed
double fertilization. The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus two female cells) and female tissues of the ovule give rise to the surrounding tissues in the developing seed. The ovary, which produced the female gametophyte(s), then grows into a
fruit, which surrounds the seed(s). Plants may either Self-pollination or
pollination. Nonflowering plants like ferns, moss and liverworts use other mean of sexually reproduction.
Reproduction in archosaurs (reptiles and birds)
Male and female birds and reptiles both have cloacae, an opening through which eggs, sperm, and wastes pass. Intercourse is performed by pressing the lips of the cloacae together, during which time the male transfers his sperm to the female. The female lays
amniote eggs in which the young gestate. Nevertheless, a few species, including most waterfowl and ostriches, have a phallus shaped organ analogous to the mammals' penis.
Reproduction in mammals
There are three kind of mammals; Monotremes, Placentals and Marsupials, all with internal fertilisation yet do still differ from each other. In
placental mammals, offspring are born as juveniles: complete animals with the
sex organs present although not reproductively functional. After several months or years, the sex organs develop further to maturity and the animal becomes Sexual maturity. Most female mammals are only
fertility 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
The male reproductive system contains two main divisions: the penis, and the testicle, the latter of which is where sperm are produced. 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
Thermoregulation 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. Prior to ejaculation the produced sperm are stored in the seminal vesicle, a small gland that is located just behind the bladder.
A sperm cell is motile and swims via
chemotaxis, using its mitochondria to propel itself towards the ovum.
The mammalian female
The female reproductive system likewise contains two main divisions: the
vagina and uterus, which act as the receptacle for the sperm, and the
Ovary, which produce the female's
Ovum. 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, which passes through the fallopian tube into the uterus.
If, in this transit, it meets with Spermatozoon, the sperm penetrate and merge with the egg, fertilisation it. The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then implantation 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
menstrual cycle in humans and Hominidae and reabsorbed in all other mammals in the estrus cycle.
Gestation
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
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 (physiology) 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
egg (biology)s. 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 Joey (marsupial) crawl into their mother’s pouch and attach to a teat, where they receive nourishment and finish developing into self-sufficient animals.
See also
References
Pang, K. "Certificate Biology: New Mastering Basic Concepts", Hong Kong, 2004
Journal of Biology of Reproduction, accessed in August 2005.
Sexual reproduction is a union that results in increasing
genetic diversity of the offspring. It is characterized by two processes:
meiosis, involving the halving of the number of chromosomes; and fertilisation, involving the fusion of two gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually chromosomal crossover to achieve genetic recombination.
The evolution of sex is a major puzzle. The first
fossilized evidence of sexually reproducing organisms is from eukaryotes of the Stenian period, about 1.2 to 1 billion years ago. 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.
A major question is why sexual reproduction persists when parthenogenesis appears in some ways to be a superior form of
reproduction. Contemporary evolutionary thought proposes some explanations. It may be due to selection pressure on the clade itself—the ability for a population to radiate more rapidly due to a changing environment through sexual recombination than parthenogenesis allows. Alternatively, sexual reproduction may allow for the 'ratcheting' of evolutionary speed as one clade competes with another for a limited resource.
Reproduction in plants
In flowering plants, the Stamen produces male gametophytes, the sperm is produced in pollen, which attach to the stigma on top of a carpel, in which the female gametophytes (inside ovules) are located. After the pollen tube grows through the carpel's style, the sperm cell nuclei from the pollen grain migrate into the ovule to fertilize the egg cell and endosperm nuclei within the female gametophyte in a process termed
double fertilization. The resulting zygote develops into an embryo, while the triploid endosperm (one sperm cell plus two female cells) and female tissues of the ovule give rise to the surrounding tissues in the developing seed. The ovary, which produced the female gametophyte(s), then grows into a
fruit, which surrounds the seed(s). Plants may either Self-pollination or
pollination. Nonflowering plants like
ferns, moss and liverworts use other mean of sexually reproduction.
Reproduction in archosaurs (reptiles and birds)
Male and female birds and reptiles both have cloacae, an opening through which eggs, sperm, and wastes pass. Intercourse is performed by pressing the lips of the cloacae together, during which time the male transfers his sperm to the female. The female lays
amniote eggs in which the young gestate. Nevertheless, a few species, including most waterfowl and ostriches, have a phallus shaped organ analogous to the mammals' penis.
Reproduction in mammals
There are three kind of mammals; Monotremes, Placentals and Marsupials, all with internal fertilisation yet do still differ from each other. In
placental mammals,
offspring are born as juveniles: complete animals with the sex organs present although not reproductively functional. After several months or years, the sex organs develop further to maturity and the animal becomes Sexual maturity. Most female mammals are only
fertility 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
The male reproductive system contains two main divisions: the
penis, and the testicle, the latter of which is where sperm are produced. 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 Thermoregulation 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. Prior to ejaculation the produced sperm are stored in the seminal vesicle, a small gland that is located just behind the bladder.
A sperm cell is motile and swims via chemotaxis, using its mitochondria to propel itself towards the ovum.
The mammalian female
The female reproductive system likewise contains two main divisions: the vagina and
uterus, which act as the receptacle for the sperm, and the
Ovary, which produce the female's
Ovum. 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, which passes through the fallopian tube into the uterus.
If, in this transit, it meets with
Spermatozoon, the sperm penetrate and merge with the egg, fertilisation it. The fertilization usually occurs in the oviducts, but can happen in the uterus itself. The zygote then
implantation 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 menstrual cycle in humans and Hominidae and reabsorbed in all other mammals in the estrus cycle.
Gestation
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
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 (physiology) 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
egg (biology)s. 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 Joey (marsupial) crawl into their mother’s pouch and attach to a teat, where they receive nourishment and finish developing into self-sufficient animals.
See also
- Asexual reproduction
- Biological reproduction
- Bird
- Flowering plant
- Mammal
- Reptile
- Sex organ
- Sexual intercourse
- Anisogamy
- Isogamy
References
Pang, K. "Certificate Biology: New Mastering Basic Concepts", Hong Kong, 2004
Journal of Biology of Reproduction, accessed in August 2005.
Sexual reproduction - Wikipedia, the free encyclopedia
Sexual reproduction is a union that results in genetic diversity of the offspring. It is characterized by two processes: meiosis, involving the halving of the number of chromosomes ...
Reproduction - Wikipedia, the free encyclopedia
Reproduction is the biological process by which new individual organisms are produced. Reproduction is a fundamental feature of all known life; each individual organism exists as ...
BBC - GCSE Bitesize - Biology | Variation | Sexual reproduction in ...
Many plants reproduce sexually. The advantage to the plant is that its offspring have a selection of genes from two parents, so each individual's genes are different. The offspring ...
BBC - GCSE Bitesize - Science | Edexcel | Genes | Sexual reproduction
Organisms have sex cells called gametes. In human beings, the male sex cells are called sperm and the female sex cells are called eggs, or ova.
Definition: sexual reproduction from Online Medical Dictionary
The Online Medical Dictionary is a searchable dictionary of definitions from medicine, science and technology.
sexual reproduction
Reproductive process in organisms that requires the union, or fertilization, of gametes (such as eggs and sperm)
GO:0019953 sexual reproduction - QuickGO
The European Bioinformatics Institute ... You can reach the search box using alt+9 (Win/IE), alt+shift+9 (Win/Firefox) or ctrl+9 (Mac)
Why Sexual Reproduction? by Brig Klyce
When Maynard Smith reverse-engineered sex... he created a paradox. Sex should not exist; natural selection will favor asexual reproduction. The solution to the paradox is almost ...
Sexual vs Asexual Reproduction
Slide 1 of 26
sexual reproduction - Hutchinson encyclopedia article about sexual ...
Hutchinson encyclopedia article about sexual reproduction. sexual reproduction. Information about sexual reproduction in the Hutchinson encyclopedia.
