Cell cycles and reproduction

Cell division is the fundamental process that allows a cell replication. In unicellular organisms this process obviously also corresponds to the reproduction of the whole organism. In prokaryotes (e.g. bacteria) reproduction simply occurs via binary fission. In a point of the circular chromosome the double helix begins to separate and the two separate strands act each as a template for a new complementary strand. In this way two chromosomes are formed, each of which migrates to opposite sides of the cell wall. Then the cell stretches itself and separates giving rise to two new sister cells with the same genetic heritage.

Figure 10: The stages of the cell cycle (left panel) and those of mitosis (right). During the G1 phase there occur cell growth and organelles replication; during the S phase the cell duplicates the chromosomal material; during the G2 phase are assembled the necessary structures for mitosis and cytokinesis, namely the duplication of the cytoplasm.

More complex is the cell cycle in eukaryotes. It consists of five stages, summarized in Fig. 10. The fundamental phase is of course the division of the nucleus, which is termed mitosis. It is divided into four sub-phases: prophase, metaphase, anaphase and telophase, also summarized in Fig. 10. In the prophase chromosomes condense and become visible under a microscope. Each of the chromosomes consists of two replicates, named sister chromatids, still joined in a ​​narrow common region, called centromere. Subsequently, the nuclear membrane disintegrates while a spindle-shaped structure forms which consists of microtubule fibres, whose ends are grouped around two pairs of centrioles. Some microtubules connect each pair of centrioles to centromeres. In the metaphase chromatid pairs move back and forth within the spindle until they are arranged symmetrically in the equatorial plane of the cell. In the anaphase centromeres separate completely in all pairs of sister chromatids. The chromatids of each pair move far away from each other, so that that each chromatid becomes an independent chromosome attracted to its pair of centrioles. In the telophase the two identical sets of chromosomes reach the two opposite ends of the cell while the spindle fibres degenerate. Subsequently, around each set of chromosomes a nuclear membrane is formed while the chromosomes unwind and again become non visible and the cytoplasm begins to divide.

While binary fission in prokaryotes coincides with reproduction and involves a single circular chromosome, things are more complicated in eukaryotes which possess more chromosomes with a linear structure. In this regard, we have to first specify that chromosomes may or may not be organized in pairs of homologous chromosomes. The two homologues are usually similar in shape and size. When chromosomes are organized in pairs one speaks of diploid cells while in the other case the cells are termed haploid. The clearest example is actually that of man, whose cells are diploid as they have 46 chromosomes arranged in 23 pairs (see Fig. 11). There is an exception for special cells, called gametes (eggs in women and spermatozoa in men), which are haploid, as they have only 23 chromosomes. The gametes are produced from diploid cells, which, through a process called meiosis, double their genetic material and then divide into four daughter cells each containing only one of the homologous chromosomes. There exist, although they are not very frequent, fully haploid eukaryotes. For example amoebae (single-celled protists) have more than 500 small chromosomes which are not arranged in pairs of homologous chromosomes. Reproduction in this case occurs through a process substantially similar to binary fission in prokaryotes. The two daughter amoebae have the same genome, that is they are clones of their mother.

Figure 11: The 23 pairs of chromosomes in humans. X and Y are the sex chromosomes.

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Amoeba is an example of asexual reproduction. In fact, the concept of sex is closely linked to the concepts of haploid and diploid. In the case of humans, for example, the two haploid gametes unite during fertilization to form a single diploid cell (the zygote) which then will develop into an embryo. Among the 23 pairs of human chromosomes there exists a pair of sex chromosomes (conventionally indicated with the letters X and Y). The male sex corresponds to the presence of the XY pair, the female sex to the presence of the XX pair. In the process of sperm formation, male diploid cells, through meiosis, give rise to haploid cells (spermatozoa) half of which contain the X chromosome and half the Y chromosome. All the eggs, instead, since they originate from diploid cells of the type XX, contain the chromosome X. If the zygote arises from the encounter of an egg with a spermatozoon containing the X chromosome, the offspring will be a female, if it originates from a type Y spermatozoon the offspring will be a male. The genetic make-up of children is half inherited from mother and half from father, therefore children are not clones of their parents. However, not necessarily must organisms with diploid cells have sexual reproduction, like in men. In particular, reproduction can occur asexually in the following ways (see Fig. 12):

• via budding: attached to the mother's body develops a complete individual, which, once it reaches the appropriate size, detaches and becomes independent; an example is that of hydra, a freshwater polyp;
• via vegetative propagation: many plants give origin to a clone of the same individual by simple division into two parts, usually genetically identical, or because of simple detachment of body portions;
• via fragmentation: one part of the body that is detached regenerates a complete individual, as for example in many polychaete annelids.

Figure 12: Examples of asexual reproduction: budding in hydra (left), vegetative reproduction in a plant (center), fragmentation in an annelid polychaete (right).

Sexes are not always separate. There are hermaphrodites (e.g. slugs, see Fig. 13(B)) which have both male and female reproductive organs. Even many plants have hermaphroditic flowers possessing both stamens (male reproductive organs) and pistils (female reproductive organs). If male and female reproductive organs are instead separated the plant bears unisexual flowers or cones. In that case they can be:

• located on the same individual (for example in larch, see Fig. 13(C)): the species is then called monoecious (i.e. with one house, oikos in Greek);
• carried by two different individuals (e.g. in laurel and in holly, see Fig. 13(A)): the species is called dioecious.

In hermaphroditic or monoecious species, self-fertilization (i.e. the zygote is formed by the fusion of male and female gametes from the same organism) is very rare. In fact, hermaphroditic animals mate anyway with another partner, while male and female parts of hermaphroditic flowers or male and female flowers of monoecious species generally mature at different times. Therefore, this guarantees the genetic reshuffling since the two halves of the chromosomal make-up come from two different individuals. A special case that is a mixture between sexual and asexual reproduction is parthenogenesis (virginal reproduction). This is quite common for example in rotifers, which are one of the most important components of plankton. Females lay diploid eggs, which do not require fertilization. Therefore, there are only females that produce daughters that are clones of themselves.

There actually exist even more complex cases than those described above. In particular, in addition to haploid and diploid cells there exist cells with triplets, quadruplets, etc. of homologous chromosomes. For example, salmon are tetraploid (quadruplets of chromosomes), wheat and kiwi are hexaploid (sextuplets). Moreover, the same species can reproduce either sexually or asexually, depending on the conditions. For example, many species of rotifers are normally parthenogenetic, but under stress they can have sexual cycles: special eggs are produced that are haploid and develop into males which in turn fertilize other haploid eggs thus producing diploid eggs that carry the genetic heritage of both the mother and the father.

Figure 13: (A) Holly as an example of dioecious species. The twig on the left comes from a male plant and displays flowers with stamens which carry pollen; the twig on the right comes from a female plant and displays flowers with pistils. (B) Coupling of black slugs, hermaphroditic gastropod molluscs; the white gelatinous mass is the sperm exchange between the two slugs. (C) Twig of a larch, a monoecious conifer: the male cones, yellow, and the female ones, purple, are carried by the same individual.