Fertilization and Meiosis alternate in sexual life cyclesA Life Cycle: the generation-to-generation sequence of stages in the reproductive history of an organism
In humans Each somatic cell has 46 chromosomes, made up of two sets One set of chromosomes comes from each parent When chromosomes are arranged in pairs starting with the longest are called a karyotype. Homologous chromosomes are two chromosome that are the same in length, centromere position and staining pattern. Females XX Males XY Small parts of XY are homologous. They are called sex chromosome.
Pair of homologous 5 µm chromosomes CentromereFigure 13.3
We inherit one set of chromosomes from each parent. One set is 23 chromosomes in humans. One set is represented by the letter "n“. Any cell with 2 sets of chromosome is called a diploid cell, which has ‘2n’ so a normal human would have 46 chromosomes. Gametes contain only one set so they are called haploid cells. This set contains 22 autosomes and 1 gender chromosome. A unfertilized egg only has a X chromosome but sperm cells can have X or Y chromosomes.
-The life cycle begins with a haploid sperm cell specifically from the father fuses with a haploid ovum from the mother. (sperm + egg = life)
-The process of gametes, culminating in fusion of their nuclei, is called fertilization. -The resulting fertilized egg, or zygote, is diploid because it contains 2 haploid sets of chromosomes bearing genes representing the maternal and paternal family lines. -As the human develops from a zygote to a sexually mature adult, mitosis generates all the somatic cells of the body. Both chromosomes sets in the zygote and all the genes they carry are passed with precision to our somatic cells.
-The only cells of the human body not produced by mitosis are the gametes, which develop in the gonads- ovaries in the females and testes in the males. -Then in fertilization once 2 gametes fuse, the normal chromosomes number of 46 would 2x to 96 and continual sequence. -Meiosis is a type of cell division reduce the number of sets of chromosomes from tow to one in the gametes, compensating for the doubling that occurs at fertilization. -Fertilization restores the diploid condition by combining 2 haploid sets of chromosomes and the human life cycle is repeated, generation after generation
The human life cycle Key Haploid gametes (n = 23) Haploid (n) Ovum (n) Diploid (2n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid Figure 13.5 adults (2n = 46)
Alternation of Generations- The second life cycle exhibited by plants and algae Sporophyte- the multicellular diploid stage in alternation of generations. Spores- haploid cells produced in the sporophyte. Gametophye- the multicellular haploid stage in alternation of generations.
In animals Meiosis occurs during gamete formation Gametes are the only haploid cells Key Haploid Diploid n n Gametes n MEIOSIS FERTILIZATION Zygote 2n 2n Diploid Mitosis multicellular organism Figure 13.6 A (a) Animals
Plants and some algae Exhibit an alternation of generations The life cycle includes both diploid and haploid multicellular stages Haploid multicellular organism (gametophyte) Mitosis n Mitosis n n n n Spores Gametes MEIOSIS FERTILIZATION Diploid 2n multicellular 2n organism Zygote (sporophyte) Mitosis Figure 13.6 B (b) Plants and some algae
In most fungi and some protists Meiosis produces haploid cells that give rise to a haploid multicellular adult organism The haploid adult carries out mitosis, producing cells that will become gametes Haploid multicellular organism n Mitosis Mitosis n n n Gametes n MEIOSIS FERTILIZATION 2n Zygote Figure 13.6 C (c) Most fungi and some protists