Plate Tectonics: Shaping Earth’s Crust
The study of the processes that shape the broad outlines of the Earth’s crust through the creation of continents and oceans, mountains and sea trenches, and so on.
The plates are pieces (more or less rigid) in the most superficial part of the Earth that move about with respect to others. For example, imagine chunks of melon peel slipping on the fruit’s flesh. The interactions between these plates result in tectonic processes. The theory of plate tectonics elegantly explains a wide range of geological, geophysical, geographical, botanical, and zoological observations, which previously had no satisfactory explanation.
The theory of plate tectonics is important not only for scientists, as it helps in the understanding of seismic and volcanic processes and the calculation of the risks associated with them. In addition, its principles are used in the search for methods of earthquake prediction and in the search for economic mineral deposits.
However, the theory of plate tectonics provides no solution to all the unknowns of the Earth. Although in its current form it explains the overall picture quite well, it is still necessary to refine it enough to be applicable for special cases that apparently do not obey the rules of a simple large-scale model. These unknowns represent a challenge and motivation for current geophysics.
Meiosis: Cell Division for Sexual Reproduction
Meiosis is cell division that allows sexual reproduction. It comprises two successive divisions:
- First meiotic division: A reduction division because a diploid (2n) stem cell produces two haploid (n) daughter cells.
- Second meiotic division: An equational division because daughter cells have the same number of chromosomes as the parent cell (like mitotic division).
Thus, two n cells from the first meiotic division result in four n cells. As in mitosis, before the first meiotic division, there is a period of interphase in which DNA is duplicated. However, at the interphase of the second meiotic division, there is no duplication of DNA.
First Meiotic Division
- Prophase I: This is the longest and most complex phase, and can take months or even years depending on the species. It is subdivided into:
- Leptotene: Chromosomes with two chromatids form.
- Zygotene: Each chromosome is intimately united with its counterpart.
- Pachytene: Homologous chromosomes remain together as a bivalent or tetrad.
- Diplotene: The homologous chromosomes begin to separate, having chiasmata.
- Diakinesis: Chromosome condensation increases, distinguishing the two sister chromatids in a bivalent.
- Metaphase I: The nuclear envelope and the nucleoli have disappeared, and the bivalents are arranged in the equatorial plate.
- Anaphase I: The two homologous chromosomes that form the bivalent split, with each chromosome containing two chromatids going to each pole.
- Telophase I: Depending on the species, either the chromosomes decondense and a nuclear envelope is formed, or the second meiotic division begins directly.
Second Meiotic Division
It is preceded by a brief interphase, called interkinesis, where there is never a duplication of DNA. It is similar to a mitotic division, consisting of:
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II