Meiosis

I Definition: *The formation of Gametes(sex cells) that separates the two sets of genes so that each gamete ends up with just one set. *Each organism must inherit a single copy of every gene from each of its “parents.”

II Chromosome Number These sets of chromosomes are homologous. All organisms have different numbers of chromosomes. These sets of chromosomes are homologous. Each of the chromosomes that came from the male parent has a corresponding chromosome from the female parent. These chromosomes are from a fruit fly. Each of the fruit fly’s body cells has 8 chromosomes.

Common Name--Diploid Chromosome #--Do not copy this down FYI Buffalo—60 Fruit fly--8 Cat--38 Cattle-60 Dog--78 Donkey--62 Goat--60 Horse--64 Human--46 Pig--38 Sheep--54

What are the minimum and maximum number of chromosomes What are the minimum and maximum number of chromosomes? Do not copy down, just FYI The record for minimum number of chromosomes belongs to a subspecies of the ant Myrmecia pilosula, in which females have a single pair of chromosomes. This species reproduces by a process called haplodiploidy, in which fertilized eggs (diploid) become females, while unfertilized eggs (haploid) develop into males. Hence, the males of this group of ants have, in each of their cells, a single chromosome.

The record for maximum number of chromosomes is found in found in the fern family. Polyploidy is a common conduction in plants, but seemingly taken to its limits in the Ophioglossum reticulatum. This fern has roughly 630 pairs of chromosomes or 1260 chromosomes per cell. The fact that these cells can accurately segregate these enormous numbers of chromosomes during mitosis is truly remarkable

Ex. Fruit Fly, the diploid number is 8, which can be written as 2N=8. Chromosome Number A cell that contains both sets of homologous chromosomes is said to be diploid. The number of chromosomes in a diploid cell is sometimes represented by the symbol 2N. Ex. Fruit Fly, the diploid number is 8, which can be written as 2N=8.

Ex. Fruit FLy, the haploid number is 4, which can be written as N=4. Chromosome Number The gametes of sexually reproducing organisms contain only a single set of chromosomes, and therefore only a single set of genes. These cells are haploid. Haploid cells are represented by the symbol N. Ex. Fruit FLy, the haploid number is 4, which can be written as N=4.

Phases of Meiosis III Phases of Meiosis Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.

Meiosis involves two divisions, meiosis I and meiosis II. Phases of Meiosis Meiosis involves two divisions, meiosis I and meiosis II. By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.

Telophase I and Cytokinesis Phases of Meiosis Meiosis I Meiosis I Interphase I During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell. Prophase I Metaphase I Anaphase I Telophase I and Cytokinesis

First division of meiosis Prophase 1: Each chromosome duplicates and remains closely associated. These are called sister chromatids. Crossing-over can occur during the latter part of this stage. Metaphase 1: Homologous chromosomes align at the equatorial plate. Anaphase 1: Homologous pairs separate with sister chromatids remaining together. Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

Phases of Meiosis Interphase I Cells undergo a round of DNA replication, forming duplicate chromosomes. Interphase I - Cells undergo a round of DNA replication, forming duplicate chromosomes.

There are 4 chromatids in a tetrad. Phases of Meiosis MEIOSIS I Prophase I Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. There are 4 chromatids in a tetrad. MEIOSIS I Prophase I - Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.

Crossing-over produces new combinations of alleles. Phases of Meiosis When homologous chromosomes form tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over. Crossing-over produces new combinations of alleles. Nondisjunction may also occur, chromosomes fail to separate properly, causing extra or missing crhomosomes within the gametes. Crossing-over occurs during meiosis. (1) Homologous chromosomes form a tetrad. (2) Chromatids cross over one another. (3) The crossed sections of the chromatids are exchanged.

Spindle fibers attach to the chromosomes. Phases of Meiosis MEIOSIS I Metaphase I Spindle fibers attach to the chromosomes. MEIOSIS I Metaphase I - Spindle fibers attach to the chromosomes.

Phases of Meiosis MEIOSIS I Anaphase I The fibers pull the homologous chromosomes toward opposite ends of the cell. MEIOSIS I Anaphase I - The fibers pull the homologous chromosomes toward opposite ends of the cell.

Nuclear membranes form. The cell separates into two cells. Phases of Meiosis MEIOSIS I Telophase I and Cytokinesis Nuclear membranes form. The cell separates into two cells. The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I. MEIOSIS I Telophase I and Cytokinesis - Nuclear membranes form. The cell separates into two cells.

Phases of Meiosis Meiosis II The two cells produced by meiosis I now enter a second meiotic division. Unlike meiosis I, neither cell goes through chromosome replication. Each of the cell’s chromosomes has 2 chromatids.

Telophase II and Cytokinesis Phases of Meiosis Meiosis II During meiosis, the number of chromosomes per cell is cut in half through the separation of the homologous chromosomes. The result of meiosis is 4 haploid cells that are genetically different from one another and from the original cell. Meiosis II Telophase I and Cytokinesis I Metaphase II Anaphase II Telophase II and Cytokinesis Prophase II

Phases of Meiosis MEIOSIS II Prophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell. MEIOSIS II Prophase II - Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.

The chromosomes line up in the center of cell. Phases of Meiosis MEIOSIS II Metaphase II The chromosomes line up in the center of cell. MEIOSIS II Metaphase II - The chromosomes line up in a similar way to the metaphase state of mitosis.

Phases of Meiosis MEIOSIS II Anaphase II Centromeres divide and sister chromatids separate and move toward opposite ends of the cell. MEIOSIS II Anaphase II - The sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (N) daughter cells. Phases of Meiosis MEIOSIS II Telophase II and Cytokinesis Meiosis II results in four haploid (N) daughter cells. MEIOSIS II Telophase II and Cytokinesis - Meiosis II results in four haploid (N) daughter cells.

IV Gamete Formation In male animals, meiosis results in four equal-sized gametes called sperm. Spermatogenesis Meiosis produces four genetically different haploid cells. In males, meiosis results in four equal-sized gametes called sperm.

In many female animals, only one egg results from meiosis In many female animals, only one egg results from meiosis. The other three cells, called polar bodies, are usually not involved in reproduction. Oogenesis Meiosis produces four genetically different haploid cells. In females, only one large egg cell results from meiosis. The other three cells, called polar bodies, usually are not involved in reproduction.

V Comparing Mitosis and Meiosis Mitosis results in the production of two genetically identical diploid cells. Meiosis produces four genetically different haploid cells.

Comparing Mitosis and Meiosis Cells produced by mitosis have the same number of chromosomes and alleles as the original cell. Mitosis allows an organism to grow and replace cells. Some organisms reproduce asexually by mitosis.

Comparing Mitosis and Meiosis Cells produced by meiosis have half the number of chromosomes as the parent cell. These cells are genetically different from the diploid cell and from each other. Meiosis is how sexually-reproducing organisms produce gametes.

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