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| Cromosomas X e Y |
Sexual reproduction is the most
common form of reproduction among eukaryotes, including multi-cellular living
beings. After billions of years of asexual reproduction among prokaryotes, who
share genetic information by exchanging plasmids (small DNA fragments), a new
type of reproduction suddenly appeared. As it was successful, we must assume
that it must have provided some advantage over the alternative procedure.
Sexual reproduction can be defined
as the alternation between two life cycles for the same type of organism:
- Haploid cycle: each cell has a single copy of every chromosome.
- Diploid cycle: each cell has two copies of every chromosome.
In eukaryotes, the haploid cycle is
always unicellular; the diploid cycle may be unicellular (in unicellular
eukaryotes) or multi-cellular (in multi-cellular eukaryotes). The individuals in
the haploid cycle are called gametes.
How was this alternation
established? It could have been caused by an alternation between two different
environments. Haploid and diploid cells do not have the same properties. For
example, diploid cells are more voluminous, having many duplicate organelles,
so that the ratio of surface to volume is usually larger than in haploid cells (about
1.25 times). As the absorption of nutrients by the cells depends on their surface,
haploids tend to grow faster than diploids.
