A reproduction is a copy or the act of making copies. In biology, reproduction refers to the production of a new member of a population from existing members. This can refer to individual cells or an entire organism, and it does not necessarily mean that an exact copy of the original member has been made. The process fundamentally consists of segregation of a portion of the parental body, and its subsequent growth and differentiation into a new individual. For most unicellular organisms, reproduction is a simple matter of cell duplication, also known as replication. But for multicellular organisms, cell replication and reproduction are two separate processes.
Most unicellular organisms create their next generation by replicating all of their parts and then splitting into two cells, a type of asexual reproduction called binary fission. This process spawns not just two new cells, but also two new organisms. Multicellular organisms replicate new cells in much the same way. For example, we produce new skin cells and liver cells by replicating the DNA found in that cell through mitosis. Yet, producing a whole new organism requires sexual reproduction, at least for most multicellular organisms. In the first step, specialized cells called gametes—eggs and sperm—are created through a process called meiosis. Meiosis serves to reduce the chromosome number for that particular organism by half. In the second step, the sperm and egg join to make a single cell, which restores the chromosome number. This joined cell then divides and differentiates into different cell types that eventually form an entire functioning organism.
- Main Article: Sexual reproduction
Sexual reproduction is a biological process by which organisms create descendants by combining their genetic material. These organisms have two different adult sexes (male and female). New individuals are produced by the fusion of haploid gametes (sperm, ovum, pollen), formed through a unique form of cell division called meiosis. Fertilization occurs when the two gametes fuse to form a diploid zygote. The zygote is a single cell which contains all the information required to build the adult organism. The growth from zygote to adult is achieved through somatic cell division called mitosis.
- Main Article: Asexual reproduction
Asexual reproduction is a mode of having offspring that does not involve sex. Offspring resulting from this method come from a single parent and inherit the genes from that parent only. In genetics terms, it is a method of reproduction where a 1N cell produces two to four cells with the same chromosome number. It does not involve meiosis or the the union of gametes such as ovum, sperm, or pollen. Therefore, offspring are identical to the parent, unlike those produced through sexual reproduction. This form of reproduction (also known as agamogenesis) is common among plants and single-celled organisms such as amoeba.
- Main Article: Binary Fission
Binary fission is a method of asexual reproduction used by most prokaryotes (i.e. bacteria). It is a fairly simple process whereby a living cell divides into two equal, or near equal, parts. Asexual reproduction theoretically results in two identical cells. However, bacterial DNA has a relatively high mutation rate. This rapid rate of genetic change is what makes bacteria capable of developing resistance to antibiotics and helps them exploit invasion into a wide range of environments.
- Main Article: Vegetative Reproduction
Although sexual reproduction is common in plants, most also reproduce asexually through what is commonly called vegetative propagation. There are a number of plant tissues capable of this process, for example rhizomes (potato), runners (strawberry), stem pieces (willow), and seed (dandelion).
Budding is a form of asexual reproduction where a copy of the parental body forms as a protrusion. The bud stays attached to the primary organism for a while, before becoming free. Like all forms of asexual reprodution, the new organism is genetically identical to the parent. Budding is very common in plants, but is also found in some animals, such as the hydra, sponge, and corals.
- What is a Cell? by the National Center for Biotechnology Information