- Slides: 49
Meiosis • A type of cell division that results in gametes(sperm and egg) being created with half the number of chromosomes(haploid-n) as the parent.
Purpose of Meiosis • genetic continuity • Genetic variability
Where does meiosis occur? • It occurs in the gonads( testes and Ovaries)
Two Types of Meiosis • Oogenesis • spermatogenesis
Meiosis Haploid cells Diploid cells Somatic cells Gametes Role of Meiosis To produce haploid gametes from diploid germ cells. Occurs only in the gonads • Plays an important role in creating genetic variability through the processes of: • Segregation • Independent assortment • Crossing over
Meiosis • Interphase • S phase—replication • Cell is now diploid with all chromosomes being mitotic • Meiosis I—reductive division • Creates 2 haploid cells with mitotic chromsomes • Consists of several stages • Prophase I—Like prophase in mitosis. • Synapsis—homologous chromosomes attach to form tetrads. • Crossing over occurs. • Reshuffling of genes. Genetic recombination. Important for creating variability.
The stages of meiosis • Meiosis 1 • Meiosis 2 • • • Four stages Prophase 1 Metaphase 1 Anaphase 1 Telophase 1 Four stages Prophase 2 Metaphase 2 Anaphase 2 Telophase 2
Stages of meiosis 1 • Meiosis one is called the reduction division • The process of reducing the number of chromosomes in a cell by half (1/2). In other words, going from diploid (2 n) to haploid(n) • Ex. 46 chromosomes to 23 chromosomes
Prophase 1 • During prophase 1 of meiosis, homologous chromosomes(similar chromosomes) line up next to each other as pairs • Pairs of homologous chromosomes are called Tetrads. • Crossing over of non-sister chromatids occurs. • Crossing over : the overlapping of non-sister chromatids resulting in an exchange of genes.
Metaphase 1 • During this phase homologous chromosomes line up at the middle of the cell. • tetrads are aligned in such a way that the homologous chromosomes are situated across from each other • The spindle from one end of the cell attaches to one pair of sister chromatids while a spindle fiber from the other end attaches to the other pair of sister chromatids. • Independent Assortment—maternal and paternal chromosomes are line up on the opposite sides of the metaphase plate in a random fashion. When these homologues split during anaphase I. The new nuclei consist of a mixture of maternal and paternal
Anaphase 1 • Spindle fibers pull homologous chromosomes to separate ends of the cell
Telophase 1 • Nuclear membrane reappears around each set of separated chromosomes • Spindle disappears • Cytokinesis (division of the cytoplasm) occurs • Two new cells are created with ½ (haploid) the chromosomes number of the original cell • 46 DS chromosomes – 23 DS chromosomes per cell or 96 chromatids- 46 chromatids per cell
Meiosis II • Almost the same as Mitosis. • 2 cells are formed from each of the cells from Meiosis I. • These cells are haploid and have chromosomes formed by only one chromatid. • These cells develop into gametes • • • Prophase II Metaphase II Anaphase II Telophase II Cytokinesis Results in 4 haploid cells that will eventually form gametes
Stages of Meiosis II • Stages of meiosis 2 are identical to the stages of Mitosis • Prophase II • Metaphase. II • Anaphase II • Telophase II
Prophase II • Nuclear membrane disappears • Spindle fibers form • Chromosomes become visible
Metaphase II • Chromosomes line up at the middle of the cell. • Spindle attaches to centromere of each chromosome
Anaphase II • Spindle pulls one chromatid from each chromosome to opposite ends of cell
Telophase II • Nuclear membrane reappears around each group of chromatids (now called SS chromosomes) • Spindle disappears • Cytokinesis occurs • two new cells are created each having the haploid number of chromosomes
Gametogenesis • In males: • spermatogenesis • 1 diploid germ cell goes through meiosis to form 4 haploid cells. These develop in the testes to form sperm. • In females: • Oogenesis • 1 diploid germ cell split unequally into 4 haploid cells. • The large cell develops into an egg. • The small cells are know as polar bodies and usually die. • Why does this unequal division occur?
Oogenesis • This is the meiosis that occurs in the ovaries of females • Resulr in the creation of An egg with ½ the normal number of chromosomes
Explanation of Oogensis • Within the ovaries of females, a diploid 2 n cell called an oogonium undergoes meiosis • Before meiosis begins the oogonium replicates(doubles) its chromosomes so that it has 46 DS chromosomes or 92 sister chromatids • During the first meiotic division the Homologous (similar chromosomes are separated. During this stage cytokinesis occurs unevenly and as a result one large and one samll cell are created. The large cell is called the primary Oocyte and the small one is a polar body. • Each of these chromosomes will have 23 chromosomes or 46 SS chromatids
• During the second metiotic division the cells divide again creating 4 cells, but this time the new cells created have 23 SS chromosomes, Again cytokinesis has resulted in an unequal distribution of the cytoplasm • Note: The three polar bodies will die because they do not have enough cytoplasm to keep them running
Spermatogensis • This is meiosis that occurs in the testes of males • Results in the creation of 4 sperm each with ½(haploid) the number of chromosomes as the original cell
Explanation of Spermatogenesis • Within the testicles of a male, a spermatogonium replicates its chromosomes to contain 46 Ds chromosomes or 92 chromatids. • During the first meiotic division, the cell divides and the homologous chromosomes separate. This results in 2 new cells each with 23 DS chromosomes or 46 chromatids each • During the second division two new cells are created, each with 23 SS chromosomes. These cells are called Primary spermatids will mature into sperm • Each spermatid matures into sperm in the Epididymis
Definitions • • • Diploid Haploid/ monoploid Tetrad Synapsis Crossing Over Nondisjunction Reduction Division Spermatids Primary body Oognium
End Result of Meiosis • 4 new cells are created • Each cell has ½ or haploid (n) the number of chromosomes as the parent /original cell • Crossing over has resulted in a new combination of genes on chromosomes leading to variety in orgasnisms
Meiosis / Mitosis Compared Event Where does it occur? Mitosis Meiosis Somatic cells Gonads Number of cells created per complete cycle. 2 4 Number of divisions 1 2 Number of Chromosomes per new cell created Same as parent ½ parent cell Does crossing over occur? No Yes possibly Do homologous chromosomes line up? No Yes Genetic continuity, growth Genetic variability Purpose