Reproduction of living organisms |
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La reproducción es el proceso por el cual los seres vivos forman nuevas generaciones de individuos de su misma especie. Las células son las unidades a partir de las cuales se desarrollan los nuevos seres vivos. Hay dos tipos de reproducción, siendo cada una adecuada a diferentes ambientes:
Reproduction is the process by which living organisms form new generations of individuals of the same species. Cells are the units from which they develop new living beings. There are two types of reproduction, each appropriate to different environments:
- Sexual reproduction: in this type of reproduction, a special kind of cells, from both parents, called gametes (or reproductive cells). It is energetically expensive, but the mix of maternal and paternal characteristics implies a greater variety of individuals, something that provides a high adaptation.
- Asexual reproduction: in this kind of reproduction, a single parent produces new generations of beings without the involvement of gametes. It is cheap in terms of energy and very fast, allowing a quick expansion of the species. Unfortunately, the offsprings are clones of the parent and have virtually no ability to adapt to environmental changes.
Sexual Reproduction: cell cycle
El ciclo celular comprende todos los acontecimientos que tienen lugar desde la formación de la célula hasta la división de ésta. Se divide en:
The cell cycle includes all the events that have occurred since the formation of the cell to its division. It Is divided into
- M or Split Phase: period of cell division. It covers the division of the nucleus (mitosis) and cytoplasmic division (cytokinesis).
- Interface: period between two divisions. We can distinguish:
- G1 phase: the cell stores proteins and other components, and checks whether the internal and external conditions are appropriate for the next phase.
- S phase: the genetic material is duplicated.
- G2 phase: the manufacture of components ends. Cell control necessary for M phase continues.
1.- El núcleo en división. Los cromosomas:
When the process of division begins, the nucleus changes its appearance. The chromatin (or genetic material) inside condenses, rolling on itself to form chromosomes, a staff-shaped bodies. So we can say that a chromosome is an elongated body formed by the chromatin inside the nucleus, which has been condensed into a state of packing almost 10,000 times higher than normal, becoming visible under the microscope.
Chromosome structure:
- Chromatids: they are the "arms" of the chromosome.
- Telomere: the end of the chromatid.
- Centromere: alsco called primary constriction. It is the junction between the chromatids, where they sag resulting in the characteristic shape of a double X chromosomes.
- Secondary constrictions: they do not always appear.
- Satellite: it is produced when the telomere takes the shape of a "peninsula", and joins the rest of the arm by a small "isthmus" formed by a secondary constriction.
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2.-Types of cells depending on number of chromosomes:
- Diploid cells: forty-six chromosomes are distributed in two identical series, in which pairs of chromosomes can be formed depending on their shape, size ... The chromosomes of these pairs are called homologous chromosomes, and each one comes from a different parent.
- Haploid cells: they only have one set of chromosomes (twenty-three non-homologous chromosomes). They are usually reproductive cells.
3.- The cycle of sexual reproduction: meiosis
Meiosis is a type of cell division that gives rise to four haploid cells from a diploid. Thus, when the two reproductive cells (gametes) come together, you get the original diploid number. It begins with a diploid cell, whose genetic material is duplicated. Then it gets split four times, resulting in cells with half the genetic load than the original. Meiosis is very important in sexual reproduction, since, if two diploid cells joined, the chromosome number would double with each generation. During meiosis, chromosomes of homologous pairs exchange genes, resulting in almost inimitable combinations. This means that two identical offspring can not arise from two independent fertilizations.
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