Reproduction: Asexual and Sexual

The mitotic cell cycle repeats itself over and over. By this process, a single cell can give rise to a vast number of other cells. Meiosis, on the other hand, results in only four progeny cells which usually do not undergo further duplications.

These two methods of nuclear and cell division are both involved in reproduction but they have different reproductive roles.

Reproduction by mitosis results in genetic constancy

A cell undergoing mitosis may be an entire single-celled organism reproducing itself with each cell cycle. Alternatively, it may be a cell produced by a multicellular organism that divides further to produce a new multicellular organism. Some multicellular organisms can reproduce themselves by releasing cells derived from mitosis and cytokinesis or by having a multicellular piece break away and grow on its own.

Asexual reproduction, sometimes called vegetative reproduction, is based on mitotic division of the nucleus.

Accordingly, it produces a clone of offspring that are genetically identical to the parent. If there is any variation among the offspring, it is likely to be due to mutations or changes in the genetic material. Asexual reproduction is a rapid and effective means of making new individuals, and it is common in nature.

Sexual reproduction, which involves meiosis, is very different. In sexual reproduction, two parents each contribute one cell, a gamete, to their offspring. This method produces offspring that differ genetically from each parent as well as from one another. This genetic variation among the offspring means that some of them may be better adapted than others to survive and reproduce in a particular environment. Thus this genetic diversity provides the raw material for natural selection.

Reproduction by meiosis results in genetic diversity

Sexual reproduction, which combines genetic information from two different cells, generates genetic diversity.

All sexual life cycles have certain hallmarks:

There are two parents, each of which provides chromosomes to the offspring in the form of a gamete produced by meiosis.

Each gamete contains a single set of chromosomes.

The two gametes — often identifiable as a female egg and a male sperm — fuse to produce a single cell, the zygote or fertilized egg. The zygote thus contains two sets ofchromosomes.

In multicellular organisms, somatic cells—those body cells that are not specialized for reproduction—each contain two sets of chromosomes, which are found in pairs. One chromosome of each pair comes from each of the organism’s two parents. The members of such a homologous pairare similar in size and appearance (except for the sex chromosomes found in some species). The two chromosomes (the homologs) of a homologous pair bear corresponding, though generally not identical, genetic information. Gametes, on the other hand, contain only a single set of chromosomes—that is, one homolog from each pair. The number of chromosomes in such a cell is denoted by n, and the cell is said to be haploid. Two haploid gametes fuse to form a new organism in a process called fertilization. The resulting zygote thus has two sets of chromosomes, just as somatic cells do. Its chromosome number is denoted by 2n, and the zygote is said to be diploid.

Different kinds of sexual life cycles exhibit different patterns of development after zygote formation:

In haplontic organisms, such as protists and many fungi, the tiny zygote is the only diploid cell in the life cycle; the mature organism is haploid. The zygote undergoes meiosis to produce haploid cells or spores. These spores form the new organism which may be single-celled or multicellular by mitosis. The mature haploid organism produces gametes by mitosis, which fuse to form the diploid zygote.

Most plants and some protists have alternation of generations. Here, meiosis does not give rise to gametes but to haploid spores. The spores divide by mitosis to form an alternate, haploid life stage (the gametophyte). It is this haploid stage that forms gametes by mitosis. The gametes fuse to form a diploid zygote which divides by mitosis to become the diploid sporophyte.

In diplontic organisms, including animals and some plants, the gametes are the only haploid cells in the life cycle and the mature organism is diploid. Gametes are formed by meiosis, which fuse to form a diploid zygote.

The organism is formed by mitosis of diploid cells. The essence of sexual reproduction is the random selection of half of a parent’s diploid chromosome set to make a haploid gamete followed by the fusion of two such haploid gametes to produce a diploid cell that contains genetic information from both gametes. Both of these steps contribute to a shuffling of genetic information in the population, so that no two individuals have exactly the same genetic constitution. The diversity provided by sexual reproduction opens up enormous opportunities for evolution.

The number, shapes, and sizes of the metaphase chromosomes constitute the karyotype

When nuclei are in metaphase of mitosis, it is often possible to count and characterize the individual chromosomes. This is a relatively simple process in some organisms, thanks to techniques that can capture cells in metaphase and spread out the chromosomes. A photograph of the entire set of chromosomes can then be made, and the images of the individual chromosomes can be placed in an orderly arrangement. Such a rearranged photograph reveals the number, shapes and sizes of chromosomes in a cell which together constitute its karyotype. Individual chromosomes can be recognized by their lengths, the positions of their centromeres and characteristic banding when they are stained and observed at high magnification. When the cell is diploid, the karyotype consists of homologous pairs of chromosomes — 23 pairs for a total of 46 chromosomes in humans and greater or smaller numbers of pairs in other diploid species. There is no simple relationship between the size of an organism and its chromosome number .