Meiosis

Cell Division Review: Mitosis has 4 stages- 1. Prophase: The chromatin condenses into chromosomes. The centrioles separate and the spindle fiber begins to form. The nuclear envelope breaks down. 2. Metaphase: The chromosomes line up across the middle of the cell. Each chromosome is connected to a spindle fiber at its centromere. 3. Anaphase: The sister chromatids separate into individual chromosomes and are moved apart. 4. Telophase: The chromosomes gather at opposite ends of the cell and lose their distinct shape. Two new nuclear envelopes will form. Cytokinesis: the division of the cell cytoplasm

Chromosom e: made up of tightly coiled DNA Chromatid: one-half of two identical copies of DNA

Karyotype A karyotype is the number and visual appearance of the chromosomes in the cell nuclei of an organism.

All organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes in each of their somatic cells (also known as body cells). Of each pair, you get one from your mom and one from your dad. These sets of chromosomes are homologous. Fruit flies- 8 chromosomes

Homologous Chromosomes These chromosomes have the same structural features and pattern of genes. A cell that contains both sets of homologous chromosomes is said to be diploid. This is represented by the symbol 2N. For humans, the diploid number is 46, which can be written as 2N = 46

Gametes Each organism must inherit a single copy of every gene from each of its parents. Gametes are formed by a process that separates the two sets of genes so that each gamete ends up with just one set. This process is called meiosis. In humans, the male gamete is called sperm, and the female gamete is called an egg.

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. This is represented by the symbol N. For humans, the haploid number is 23, which can be written as N = 23.

Mitosis

Meiosis 4 haploid cells

Meiosis Meiosis is a process in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell to produce haploid gametes. Meiosis involves two divisions 1. Meiosis I 2. Meiosis II By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells.

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

Prophase I Each homologous chromosomes pair up to form a tetrad. There are 4 chromatids in a tetrad.

Prophase I 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 and contributes to genetic variation.

Metaphase I Chromosomes line up in the middle. Spindle fibers attach to the chromosomes.

Anaphase I The fibers pull the homologous chromosomes toward opposite ends of the cell. Independent assortment occurs. Chromosomes are randomly separated contributing to genetic variation.

Telophase I & Cytokinesis Nuclear membranes form. The cell separates into two cells through division of the cytoplasm. 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 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.

Prophase II The nuclear envelope breaks down and the spindle apparatus forms.

Metaphase II The chromosomes line up in the center of the cells.

Anaphase II The sister chromatids separate and move toward opposite ends of the cell.

Telophase II &Cytokinesis The nuclear envelope forms. Each cell separates into two cells through the division of the cytoplasm. This results in four haploid (N) daughter cells.

Gamete formation in males In male animals, meiosis results in four equal-sized gametes called sperm.

Gamete formation in females In many female animals, meiosis results in only one gamete called an egg. The other three cells, called polar bodies, are usually not involved in reproduction.

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

Comparing Mitosis & Meiosis Chart - does it produce daughter cells that are the same as the parent genetically? - how many cells does it produce and are they diploid or haploid? - what is the process used for? - when does the process occur? - what type of reproduction can it be used for?

Mitosis Meiosis Produces genetically identical cells. Same number of chromosomes and alleles as the original cell. Results in 2 diploid cells. Used for growth and repair. Takes place throughout an organism’s lifetime. Can be used by some organisms for asexual reproduction. Produces cells that are genetically unique. These cells are genetically different from the original parent cell & from each other. Results in 4 haploid cells Used for production of gametes. Takes place only at certain times in an organism’s life cycle. Is used for sexual reproduction.