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Sexual reproduction is one in which two sex cells, called gametes , unite through a process called fertilization, giving rise to a new cell called a zygote.
In addition to gametes, multicellular living things that reproduce sexually have a type of cell called somatic , which is any cell in the body except eggs and sperm.
somatic cells
Somatic cells are part of the tissues and organs of multicellular individuals. They are diploid , that is, they have two sets of chromosomes: one set comes from the sperm of the male parent and the other from the egg of the female parent.
For example, each of the at least 30 trillion cells that make up the human body has 46 chromosomes (except gametes). Of those 46, 23 come from the father and 23 from the mother. Each pair of chromosomes is known as homologous or sibling chromosomes ; each homologue is from a different parent. Because chromosomes come in pairs, cells have two copies of each gene. Alternative forms of a gene are also known as alleles . Examples of alleles are those that determine the blood group: A, B and O; Depending on the alleles inherited from the father and the mother, the child will have a certain blood type.
Somatic cells originate from a process called mitosis . During mitosis, the chromosomes of the parent cell are duplicated, its nucleus divides, and two daughter nuclei identical to each other and to the original are formed. In this way, somatic cells only generate copies of themselves. This happens unless a DNA alteration or mutation occurs, which can cause diseases such as tumor formation; mutations in somatic cells are not passed on to children.
The genetic makeup of diploid cells is represented as 2n , where n is the number of chromosome pairs. In humans, diploid cells have a chromosome number of 2n = 46 or 2(23) = 46, which means they have 46 chromosomes, arranged in 23 pairs.
gametes
Sex cells, or gametes, are haploid cells , meaning they contain only one set of chromosomes. Human haploid cells have 23 chromosomes, half of which are found in somatic cells. In humans, the sex cells are the eggs and sperm.
Gametes originate through a process known as meiosis , from a type of cell called the germ cell . During meiosis, the chromosomes of the diploid germ cell are duplicated, two successive nuclear divisions occur, and four n haploid nuclei are formed, each belonging to a gamete. In this way, the four resulting gametes will contain half the genetic information of the germ cell from which they were formed.
Gametes are not the same as the cell from which they were formed, not just because they have fewer chromosomes, but because the genes on their chromosomes are different: Before the first of two successive divisions in meiosis, the chromosomes exchange genetic information, which allows each gamete to have chromosomes with different genes; this explains, for example, that each child of one parent is not identical to his siblings.
When a mutation occurs in a gene in an egg or sperm that gave rise to a zygote, this mutation is added to the DNA of all the cells of the offspring that developed from that zygote. Thus, germ cell mutations are passed from parent to child and are also called germline mutations, heritable mutations, and germline variants.
The genetic makeup of the haploid cells is indicated as n. In humans, haploid cells have chromosome number n = 23, which means they have 23 chromosomes that are not arranged in pairs.
Sources
Curtis, H., Barnes, N.S., Schnek, A., Massarini, A. Biology . 7th edition. Editorial Médica Panamericana., Buenos Aires, 2013.
National Cancer Institute. Germline mutation , sf
National Cancer Institute. Somatic mutation , nd
Paniagua, R., Nistal, M., Sesma, P., Álvarez-Uría, M., Fraile, B., Anadón, R., Sáez, F. Cellular Biology . 3rd edition. McGraw Hill Inter-American., Madrid, 2007.
