190 likes | 509 Views
Meiosis. Overview of meiosis Homologous chromosomes Stages of meiosis Spermatogenesis & oogenesis. A. Overview of Meiosis. Meiosis: Specialized nuclear division in which the number of chromosomes is reduced by half Purpose of meiosis: the formation of gametes
E N D
Meiosis • Overview of meiosis • Homologous chromosomes • Stages of meiosis • Spermatogenesis & oogenesis
A. Overview of Meiosis • Meiosis: • Specialized nuclear division in which the number of chromosomes is reduced by half • Purpose of meiosis: the formation of gametes • Occurs only in germ-line tissue • Diploid number: the number of chromosomes in a germ-line cell • Haploid number: the number of chromosomes in a gamete; ½ The diploid number
A. Overview of Meiosis • Zygote: • During the process of fertilization, a haploid gamete from one parent fuses with the haploid gamete from the other parent • The resulting diploid cell is called a zygote
A. Overview of Meiosis • Gametes: • In most sexually reproducing species, there are two distinct types of gametes • Spermatozoa (or pollen in plants) are compact, highly motile gametes that contribute their chromosomes to the zygote; “Male” gametes • Ova (or ovules in plants) are much larger and contribute both chromosomes and cytoplasm (the bulk of the cell mass) to the zygote; “Female” gametes
A. Overview of Meiosis • Gonads • In most multicellular species, germ-line tissue is found in organs called gonads • Spermatozoa are produced in gonads called testes • Ova are produced in gonads called ovaries • Sexually dimorphic species: two separate genders, with each individual having either male or female gonads • Sexually monomorphic species (hermaphroditic species): each individual contains both male and female gonads
B. Homologous Chromosomes • Homologous chromosome pairs: • For each chromosome in a diploid nucleus, there is another very similar chromosome in the same nucleus • This pair of very similar chromosomes is called a homologous chromosome pair • One chromosome in each pair comes from one parent, and the other chromosome comes from the other parent
B. Homologous Chromosomes • Homologous chromosomes are similar in: • Size • Position of the centromere • Banding patterns in staining procedures • The type of genetic information they contain • During meiosis, the homologous chromosomes are separated, so that a gamete receives only one member of each homologous chromosome pair
C. Stages of Meiosis • Prior to meiosis: • The diploid germ-line cell goes through a complete interphase, including an S phase • Therefore, at the start of meiosis, each of the chromosomes is in a replicated state (consisting of sister chromatids connected at the centromere) • Meiosis is accomplished in two divisions: meiosis I and meiosis II
C. Stages of Meiosis • Meiosis I: • At the start of meiosis I, the two chromosomes in each homologous chromosome pair line up along their lengths • During meiosis I the homologous chromosomes separate and move to opposite sides of the cell. (Note that the chromatids stay together at this point.) • At the end of meiosis I, the cell divides into two • Note that each daughter cell formed by meiosis I is haploid, but the chromosomes are still in their replicated state
C. Stages of Meiosis • Meiosis II: • Each of the cells from meiosis I can undergo meiosis II • During meiosis II, the centromeres split, the sister chromatids separate and become daughter chromosomes, and the daughter chromosomes move to opposite sides of the cell • New nuclei form, and the cell divides • Therefore, meiosis (I and II together) has the potential of forming four haploid cells, with the chromosomes in an unreplicated state at the end of the process
C. Stages of Meiosis • Meiosis I is divided into four stages: prophase I, metaphase I, anaphase I, and telophase I • Prophase I: during prophase I, the nuclear membrane & nucleolus disperse, and a spindle forms. The homologous chromosomes condense and pair in five steps: • Leptonema: the chromosomes begin to condense and have the appearance of slender threads • Zygonema: the homologous chromosomes align completely along their lengths, forming paired chromosomes called bivalents. The connection between the chromosomes is called the synaptonemal complex
C. Stages of Meiosis • Prophase I (continued): • Pachynema: the bivalent chromosomes continue to condense, becoming very short & thick • Diplonema: the chromosomes in each bivalent begin to partially separate. The two chromosomes in the bivalent remain connected at X-shaped regions called chiasmata (singular: chiasma) • Diakinesis: the chiasmata migrate to the ends of the bivalents
C. Stages of Meiosis • Metaphase I: • The bivalents align at the equator of the spindle • Anaphase I: • The homologous chromosomes separate and move to opposite poles of the spindle • Note that the the chromatids do not separate at this time • Telophase I: • The chromosomes at each pole may decondense, and new nuclei form • Cytokinesis takes place, resulting in two cells
C. Stages of Meiosis • Meiosis II is divided into four stages: prophase II, metaphase II, anaphase II, and telophase II • Prophase II • Chromosomes condense; Membrane disperses; Spindle forms
C. Stages of Meiosis • Metaphase II • Chromosomes align at equator of spindle • Anaphase II • The sister chromatids separate and move to opposite poles of the spindle • Telophase II • Chromosomes decondense; New nuclei form • Cytokinesis takes place
D. Gametogenesis • Spermatogenesis • The cytokinesis divisions (cell divisions) after meiosis I and meiosis II are equal • This means that one germ line cell in the testes divides by meiosis to produce four cells of equal size • Each of these four cells develops into a spermatozoan
D. Gametogenesis • Spermatogenesis • Stages • Spermatogonium • Primary spermatocyte • Secondary spermatocyte • Spermatid • Mature spermatozoan
D. Gametogenesis • Oogenesis • When the primary oocyte undergoes meiosis I, the cytokinesis is unequal, resulting in one very large cell (the secondary oocyte) and one much smaller cell (the first polar body) • When the secondary oocyte divides in meiosis II, again the division is unequal. The result is one very large gamete (the ovum) and a second polar body • Therefore, a single diploid germ-line cell in an ovary will produce only one gamete
D. Gametogenesis • Oogenesis • Stages • Oogonium • Primary oocyte • Secondary oocyte • Ootid • Ovum