Parents move to encoded information in the form of children hereditary units called genes, and tens of thousands of genes that each individual receives from his father and mother constitute its genome.
Genes are segments of DNA, which in the eukaryotic cell is localized in the nucleus chromosomes.
Each chromosome is formed by a long DNA molecule wound in a spiral and associated with various proteins, also each chromosome includes hundreds / thousands of genes, each of which occupies a particular portion of the DNA molecule.
The specific location of a gene on the chromosome is called locus.
The genetic link with parents gives rise to the similarities and the hereditary information is transmitted in the form of specific nucleotide sequence that constitutes each gene.
The majority of genes induces the cells to synthesize specific enzymes or other proteins, and only the combined action of these gives rise to a particular hereditary character.
The DNA replication allows to produce copies of genes that can be transmitted through the sperm and the egg cell.
The riproduzzione asexual occurs when an individual is the only parent and forward a copy of all the genes to their offspring, through mitosis, where the genome of the offspring is an exact copy of the parent's.
Only rarely there are differences with this riproduzzione, due to genetic mutations, otherwise the copy of the parent is defined clone.
The riproduzzione instead sexed, takes place by means of 2 parents and determines a greater genetic variability.
Meiosis
The life cycle of an organism is the complete sequence of the various phases of growth and development, from fertilization to play the gametes.In humans, each somatic cell, or any other cell by the sperm and egg cells, is equipped with 46 chromosomes.
The karyotype is a picture ordered colored chromosomes of an individual, where there are pairs of chromosomes or chromosomes of the same length and the same position of the centromere and an equal part of colored bands, called homologous chromosomes.
The two chromosomes of a pair of homologous carry genes that control the same hereditary characters.
The female individuals possess a pair of homologous chromosomes XX, while males have XY, where only small segments of these 2 chromosomes are homologous, in fact most of the genes that are in X, are not in Y, and are defined sex chromosomes, while the other chromosomes are called autosomes.
Every individual inherits 23 chromosomes from one parent and 23 on the other.
The sperm and the egg cells are called gametes have 22 autosomes and one sex chromosome X or Y.
A cell characterized by a single set of chromosomes is called the haploid cell, and in humans the haploid number n equals 23.
The union of gametes is called fertilization or syngamy, where the fertilized egg resulting zygote, haploid chromosome contains 2 kits and for this reason is called diploid cell, and man the diploid number 2n is equal to 46.
During the development of the zygote genes are reproduced through mitosis, and the only cells in the body that do not arise by mitosis are the gametes, which are produced by the gonads, the ovaries in women and testes in males.
If the gametes were produced by mitosis, at each generation would double the number of chromosomes.
The riproduzzione so sexual, maintains constant the number of chromosomes in half the gametes chromosomes in a process known as meiosis, which occurs only in animals in the gonads.
So mitosis provides for keeping constant the number of chromosomes, meiosis instead halves them.
There are 3 main bilogici sexed cycles in animals, fungi and plants and algae.
In the gametes animal cycle are the only haploid cells of the organism and meiosis it occurs during their production.
The diploid zygote is then divided by mitosis giving rise to a multicellular diploid organism.
Depending on the cycle (animals, fungi ...) haploid cells can divide by mitosis or meiosis, but the diploid cells can divide only by meiosis.
Haploid cells produced by meiosis in the plant cycle are the spores.
In all cycles, each event of halving or doubling of the number of chromosomes contributes to increase the genetic variability between descendants.
Stages of meiosis:
And 'it preceded by DNA replication, which is followed by two consecutive cell divisions known as meiosis I and meiosis II.
These divisions lead to the production of four daughter cells, each with half the chromosomes.
The 2 chromosomes that constitute a pair of homologous chromosomes are single, one of which is the other of maternal and paternal origin, which are very similar to each other but not identical.
The stages of meiosis are:
Interphase: DNA replication occurs
Meiosis I: Prophase I Metaphase I Anaphase I Telophase I and cytokinesis.
Meiosis II: Prophase II, Metaphase II, Anaphase II Telophase II and cytokinesis.
Meiosis in detail:
Interphase: precedes meiosis real and happens DNA duplication of each chromosome, at the end of this process each chromosome consists of two sister chromatids are genetically identical, which are mutually joined at the level of the centromere, centrosomes also duplicate forming 2 pairs of centrioles.
Prophase I: the chromosomes begin to condense and counterparts join in pairs, with the synapse homologous chromosomes come together, forming tetrads. Chromatids of chromosomes intersect keeping them united to form said heat-exchange points chiasmus, then the pairs of centrioles move away from each other and form the spindle microtubules, dissolves the nuclear envelope and nucleoli, and microtubules make contact with the kinetochores starting to migrate toward the metaphase plate. Prophase I can last days and represents over 90% of the total time of meiosis.
Metaphase I: The pairs of homologous chromosomes are arranged on piatra metaphase and microtubule kinetochore of one pole become attached to a chromosome of each pair, while the opposite pole of the spindle fibers attach to the homologous chromosome.
Anaphase I: The spindle apparatus guides the movement of chromosomes toward the poles (the brothers are always united to the centromere).
Telophase I and cytokinesis: Continue the migration of chromosomes, and eventually to each pole there is a haploid set of chromosomes, both of cytokinesis (division of the cytoplasm) which gives rise to two daughter cells, in animal cells where it forms a cleavage furrow, while in plant cells to buy a cell plate. Before the second meiotic division never occurs further replication of the genetic material.
Prophase II: Compare the spindle apparatus and chromosomes are close to the plate of metaphase II.
Metaphase II: The chromosomes are arranged on the metaphase plate with kinetochores of sister chromatids of each chromosome facing the opposite poles.
Anaphase II: The centromeres of the brothers are separated and the sister chromatids of each chromosome migrate to opposite poles of the cell as individual chromosomes.
Telophase II and cytokinesis: At opposite poles of the cell begin to reform nuclei and cytokinesis occurs. At the end of the process there are 4 daughter cells, each with a haploid number of chromosomes consisting of a single chromatid.