Meiosis

What is Meiosis and Why is it Important? To really understand basic concepts of genetics, you need to know how hereditary traits are passed from parents to offspring. Sexual reproduction – 2 parts: Fertilization – genetic material from parents brought together to form new genetic identity of offspring Meiosis – nuclear division helping to diversify genetics of the offspring Occurs in sex cells – sperm and egg cells

The Basics Genes located on chromosomes in the cell nucleus Chromosomes – each species has certain chromosome numbers per somatic (body) cells which are diploid (2n) Cats – 38, humans – 46, goldfish – 94, corn plants – 10 In every diploid cell, each chromosome has a partner resembling each other = homologous pairs/homologues. One homologue comes from one parent and the other comes from the other parent Diploid cells contain two complete sets of chromosomes and two complete sets of genes – one from “mom” and one from “dad”

The Basics (continued) Sex cells (gametes – egg and sperm) are “special” in that they have exactly half the number of chromosomes as the somatic cells – thus, sex cells are haploid (n) Gametes have one allele for each gene Cats – 19, humans – 23, goldfish – 47, corn plants – 5 Why haploid? When a male gamete (sperm) fuses with a female gamete (egg), the diploid number is restored = counterbalances effects of fertilization

The Phases of Meiosis There are two nuclear divisions during meiosis: Meiosis I Meiosis II Meiosis forms gametes and is a process where the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell – 2n to n

Overview of Meiosis

Stages in Meiosis – Meiosis I Meiosis I – homologous chromosomes pair and separate Interphase – chromosomes replicate Prophase I – chromosomes condense and each chromosomes pairs with its homologue, forming a tetrad (tetrads – 4 chromosomes). Once contact is made at any point, crossing over (exchanging genetic information) can occur = VERY important for genetic diversity Metaphase I – paired homologues line up in middle of cell Anaphase I – paired homologues separate to opposite ends of the cell (tetrad separates) Telophase I – two genetically different daughter cells form

A Closer Look at Crossing Over Section 11-4 Go to Section:

A Closer Look at Crossing Over Section 11-4 Go to Section:

A Closer Look at Crossing Over Section 11-4 Go to Section:

Stages in Meiosis – Meiosis II Meiosis II – chromatids of each homologue separate – no replication occurs, which allows for reduction in number of chromosomes Prophase II – cell prepares for Metaphase II (spindles form, nuclear envelope disappears) Metaphase II – chromatid pairs line up in middle of cell Anaphase II – chromatids separate into individual chromosomes Telophase II – 4 haploid cells form, each genetically different from one another and from the parent cell

Figure 11-15 Meiosis Meiosis I Interphase I Section 11-4 Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell. Go to Section:

Figure 11-15 Meiosis Meiosis I Interphase I Section 11-4 Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell. Go to Section:

Figure 11-15 Meiosis Meiosis I Interphase I Section 11-4 Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell. Go to Section:

Figure 11-15 Meiosis Meiosis I Interphase I Section 11-4 Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell. Go to Section:

Figure 11-17 Meiosis II Meiosis II Section 11-4 Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Go to Section:

Figure 11-17 Meiosis II Meiosis II Section 11-4 Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Go to Section:

Figure 11-17 Meiosis II Meiosis II Section 11-4 Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Go to Section:

Figure 11-17 Meiosis II Meiosis II Section 11-4 Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Go to Section:

Figure 11-17 Meiosis II Meiosis II Section 11-4 Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II results in four haploid (N) daughter cells. Go to Section:

Meiosis

Meiosis

Gamete Formation In males – haploid gametes = sperm In females – haploid gametes = egg

Because of Meiosis… We can get a great deal of genetic diversity Random Fertilization – 1 egg and 1 sperm each with such different genetic combinations Crossing Over – Prophase I, different combinations of parts of chromosomes Independent Assortment – what “side of cell” each chromosome is on – Mendel’s 2nd Law

Independent Assortment

Meiosis vs. Mitosis Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells Mitosis allows for multicellular organisms to grow Meiosis produces gametes with many different genetic possibilities