Meiosis PDF

3. MEIOSIS = produces new gametes ◦ number of chromosomes ◦ there is a fixed number of chromosomes in each species (46 in humans, 48 in chimpanzees, 56 in elephants, 78 in dog........) ◦ the basic number of chromosomes in a cell is called a chomosome set (sada chr...

3. MEIOSIS = produces new gametes ◦ number of chromosomes ◦ there is a fixed number of chromosomes in each species (46 in humans, 48 in chimpanzees, 56 in elephants, 78 in dog........) ◦ the basic number of chromosomes in a cell is called a chomosome set (sada chromozómov) - represented by n. ◦ a cell that has just one chromosome set is called a haploid cell (haploidná bunka) and has a haploid number (haploidný počet) of chromosomes, abbreviated 1n. (for humans 1n=23) ◦ any cell with two chromosome sets is called a diploid cell (diploidná bunka) and has a diploid number (diploidný počet) of chromosomes, abbreviated 2n. (for humans 2n=46) ◦ the number of chromosomes in gametes is haploid (1n). fertilization (oplodnenie) is the process in which male and female gametes combine. nuclei of two gametes fuse together to form a single cell called a zygote (zygota). ◦ each gamete brings a set of chromosomes (1n) to the zygote (2n). ◦ the zygote divides by mitosis and creates an embryo (zárodok) that develops into an offspring. ◦ the offspring has a diploid number of chromosomes in each body cell. ◦ the chromosomes in body cells are always in pairs. paired chromosomes are called homologous chromosomes (homologické chromozómy). one chromosome of each pair comes from the male gamete (paternal) and one from the female gamet (maternal). homologous chromosomes  ◦ meiosis is a type of cell division that only occurs in reproductive organs (testes and ovaries in humans) ◦ it produces gametes (sperm and egg cells) ◦ it is a reduction division. it reduces the number of chromosomes from diploid to haploid ◦ a diploid cell divides twice to produce 4 haploid cells which are genetically different ◦ the purpose of meiosis is sexual reproduction ◦ it is fundamental in the passing of genetic information from parent to offspring, while also creating genetic diversity in future generations ◦ meiosis begins in a specific cell called a germ cell (zárodočná bunka), which like most of the cells is diploid ◦ germ cells undergo the G1, S and G2 phases of interphase ◦ Cells undergoing meiosis divide twice: meiosis I: meiosis II: ⁃ prophase I ⁃ prophase II two pairs of ⁃ metaphase I ⁃ metaphase II chromosomes ⁃ anaphase I ⁃ anaphase II before S phase ⁃ telophase I ⁃ telophase II after S phase meiosis I prophase I ◦ homologous chromosomes move close together and pair up. ◦ at this point the process of crossing – over occurs: the non-sister chromatides break, overlap and exchange segments (recombination, rekombinácia = výmena genetickej informácie). ◦ crossing-over is responsible for a new combination of genes. it provides variability for the species of population metaphase I ◦ each chromosome is attached to the spindle fibers. ◦ the chromosomes move to the middle of the cell and the homologous pairs are arranged in such a way that half are on one side and half on the other. ◦ how the chromosomes are arranged at the middle of the cell is random, which is an additional way to make sure that the maternal and paternal genetic material is ‘mixed up’. this is known as random assortment (náhodné roztriedenie).  two pairs would give four possible orientations, but as humans have 23 pairs, this equates to more than eight million possibilities anaphase I ◦ each homologous chromosome (consisting of two chromatides) moves to an opposite pole of the dividing cell ◦ each new cell will have only one member of the homologous pair telophase I ◦ the chromosomes reach the opposite ends of the cell ◦ cytokinesis begins and the cell splits in two the two new cells contain the haploid number of chromosomes meiosis II a) spindle forms and captures the chromosomes, which line up individually at the cell’s centre. b) the spindle contracts and the sister chromatids are pulled apart to opposite ends of the cell. c) The daughter cells constrict and divide. d) Four new cells called gametes are produced. differences between the sexes in meiosis ◦ the formation of a sperm and egg is similar, although there are some differences. ◦ for instance, the production of sperm in males, called spermatogenesis, begins at the onset of puberty and follows the processes described above, resulting in four gametes each time. ◦ the production of eggs in females, called oogenesis, on the other hand begins when the individual is still in the uterus, but then stops at the early stages of