Everything about Sexual Reproduction totally explained
Sexual reproduction is characterised by processes that pass a
combination of
genetic material to
offspring, resulting in
diversity. The main two processes are:
meiosis, involving the halving of the number of
chromosomes; and
fertilization, involving the fusion of two
gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually
cross over 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
anther produces male gametophytes, the sperm is produced in
pollen grains, 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-pollinate or
cross-pollinate. Nonflowering plants like
ferns,
moss and
liverworts use other means of sexual reproduction.
Ferns typically produce large diploid
sporophytes with
rhizomes, roots and leaves; and on fertile leaves called
sporangium,
spores are produced. The spores are released and germinate to produce smaller gametophytes that are typically heart shaped, small and green in color. The gametophytes or
thallus, produce both motile sperm in the
antheridia and egg cells in separate
archegonia. After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produce on the top side of the thallus, and swim in the film of water to the antheridia and fertilize the egg. To promote out crossing or cross fertilization the sperm are released before the egg is receptive of sperm, making it more likely that the sperm will fertilize the eggs of another thallus. A
zygote is formed after fertilization, which grows into a new sporophytic plant. The condition of having separate sporephyte and gametophyte plants is call
alternation of generations. Other plants with similar reproductive means include the
Psilotum,
Lycopodium,
Selaginella and
Equisetum.
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
sexually mature. Most female mammals are only
fertile during certain periods and during those times, they're 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
testes, 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
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. Prior to ejaculation the produced sperm are stored in the seminal vesicle, a small gland that's located just behind the bladder.
A sperm cell is motile and swims via
chemotaxis, using its flagellum 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
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, 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, which are the female sex cells, are much larger than the sperm and are normally formed with in the ovaries of the fetus before its birth. They are mostly fixed in location with in the ovary until their transit to the uterus, and contain nutrients for the later
zygote and
embryo. Over a regular interval, in response to hormonal signals, a process of
oogenesis matures one ovum which is released and sent down the Fallopian tube. If not fertilized, this egg is flushed out of the system through
menstruation in humans and other
great apes and reabsorbed in 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 slightly 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's 40 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's 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's 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.
Fish
The vast majority of fish species lay eggs that are then fertilized by the male, some species lay their eggs on a substrate like a rock or on plants, while others scatter their eggs and the eggs are fertilized as they drift or sink in the water column.
Some fish species use internal fertilization and then disperse the developing eggs or give birth to live offspring. Fishes that have live-bearing offspring include the
Guppy and Mollies or
Poecilia. Fishes that give birth to live young can be
ovoviviparous, were the eggs are fertilized within the female and the eggs simply hatch within the female body, or they can be
viviparous, were the female supplies nourishment to the internally growing offspring. Some fish are hermaphrodites, where a single fish is both male and female and can produce eggs and sperm. In hermaphroditic fish, some are male and female at the same time while in other fish they're serially hermaphroditic; starting as one sex and changing to the other. In at least one hermaphroditic species, self-fertilization occurs when the eggs and sperm are released together. Internal self-fertilization may occur in some other species. One fish species doesn't need sexually reproduction to produce offspring;
Poecilia formosa can use
parthenogenesis for reproduction, were unfertilized eggs develop into embryos that produce female offspring.
Further Information
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