Meiosis

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Meiosis and Sexual Reproduction
Sex has existed on Earth for many thousands of years. It is very important, because through sex is that organisms inherit variations in their DNA. These variationsmake it more likely for living things to survive when environment has drastic changes. The mystery of how sexual reproduction appeared is still unsolved. However, it is interesting to compare sexual andasexual reproduction:
Asexual reproduction is given through mitosis, where the cell of an organism divides to form two new cells that are completely identical. Asexual reproduction spends lessenergy, so it is easier and faster. Many organisms that have short life cycles usually have more asexual reproduction; although, they may still reproduce sexually when they perceive environmental changes.Sexual reproduction is the main source of variation in living things. Mutations are not so important for variation in species, because they occur very sporadically: approximately 1/10000 times.
TheChromosomes
DNA (Deoxyribonucleic acid), as you remember, is a molecule composed of sugars, hydrogenated bases and phosphates. The DNA strands are complex. When we study genetic traits we observethe sequence of the area that tends to vary in the DNA molecule: the hydrogenated bases. These bases are named Adenine, Thymine, Guanine and Cytosine. The sequences in the molecule give each organismits unique characteristics.
Chromatin is the molecule of DNA in its extended form. You find chromatin like that when the cells are in their Interphase, not reproducing. When cells prepare for theirdivision (Mitosis or Meiosis) they begin condensing their DNA, forming what we know as chromosomes.

In a diploid cell you have one pair of homologous chromosomes. In a haploid cell you only have oneof those chromosomes (there is no homologue).
Chromosomes have different parts:
* The chromatids (2 of them): are two strands of condensed DNA. One strand is inherited by each parent cell....
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