Meiosis

Science Department
Biology- Tenth Grade

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 variations make 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 and asexual 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 less energy, so it is easier and faster. Many organisms that have short life cycles usually have more asexual reproduction; although, they may still reproducesexually 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.
The Chromosomes
DNA (Deoxyribonucleic acid), as you remember, is a molecule composed of sugars, hydrogenated bases and phosphates. The DNA strands arecomplex. When we study genetic traits we observe the 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 organism its unique characteristics.
Chromatin is the molecule of DNA in its extended form. You find chromatin like that when the cells are in their Interphase, notreproducing. When cells prepare for their division (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 one of those chromosomes (there is no homologue).
Chromosomes have different parts:
* The chromatids (2 of them): are two strands of condensed DNA. Onestrand is inherited by each parent cell. They are connected.
* The centromere: The point where the two chromatids join.
* Gene: Sequence of DNA in a chromosome that determines certain aspects of the organism. For example, the eye color or the chin dimples. When a gene is activated in a chromosome, it is transcripted into RNA and later it is translated into certain proteins that determinecertain characteristics in an organism.
* Allele: Are the different versions of a gene. For example, the gene of eye color has many alleles: brown color, green color, blue color, etc. Each of the colors is one allele.

Mitosis and Meiosis
Mitosis is the reproduction of skin, heart and stomach cells, for example. This is also a form of "Asexual" reproduction, where one organism or cellreproduces itself. Some organisms that reproduce asexually are hydra, bacteria, and single celled organisms. The term comes from: "A" Greek meaning "without" - "Sex" Greek meaning "to cross." The cells produced through mitosis are called "Autosomal" or “Somatic” cells.
Autosomal cells are called DIPLOID, because they have a pair of each chromosome. They are also referred to as “2n”. Autosomal cells ofhumans have 46 chromosomes (one pair of each). In Mitosis, one cell becomes two cells: The new "daughter" cell has exactly the same number of chromosomes and information as the "parent" cell. Mitosis consists of 4 phases: Prophase, Metaphase, Anaphase and Telophase.
Meiosis is the production of gametes or sexual cells. Through Meiosis, sperm and egg cells are formed. These cells have half of thegenetic information in an autosomal cell. A gamete is referred to as HAPLOID, because the cell has only one chromosome of each kind. For example, human sexual cells have 23 chromosomes.
Meiosis produces 4 haploid cells out of one diploid cell. Each cell has to go through the nuclear division process twice in order for the cell to end up with half the number of chromosomes. The cells pass on...