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Meiosis (Gametogenesis) 6.1 Chromosomes and Meiosis TEKS 6A, 6G KEY CONCEPT Gametes have half the number of chromosomes that body cells have. 6.1 Chromosomes and Meiosis TEKS 6A, 6G You have body cells and gametes. • Body cells are also called somatic cells. • Germ cells develop into gametes. – Germ cells are located in the ovaries and testes. – Gametes are sex cells: egg and sperm. – Gametes have DNA that can be passed to offspring. body cells sex cells (sperm) sex cells (egg) 6.1 Chromosomes and Meiosis TEKS 6A, 6G Your cells have autosomes and sex chromosomes. • Your body cells have 23 pairs of chromosomes. – Homologous pairs of chromosomes have the same structure. – For each homologous pair, one chromosome comes from each parent. • Chromosome pairs 1-22 are autosomes. • Sex chromosomes, X and Y, determine gender in mammals. Chromosomes • You have 23 different pairs of chromosomes, for a total of 46 chromosomes. • One chromosome in each pair came from your mother and one from your father. Each chromosome in a pair is said to be homologous, meaning that the chromosome from the father has a corresponding chromosome from the mother. 6.1 Chromosomes and Meiosis TEKS 6A, 6G Body cells are diploid; gametes are haploid. • Fertilization between egg and sperm occurs in sexual reproduction. • Diploid (2n) cells have two copies of every chromosome. – Body cells are diploid. – Half the chromosomes come from each parent. 6.1 Chromosomes and Meiosis TEKS 6A, 6G • Haploid (n) cells have one copy of every chromosome. – Gametes are haploid. – Gametes have 22 autosomes and 1 sex chromosome. • Cells that contain both sets of homologous chromosomes are called diploid (2N). • All of your cells except the sex cells (sperm and eggs; also called gametes) are diploid (2N). • Gametes are haploid (N), meaning they contain only one copy of each chromosome. • When one sperm and one egg combine their DNA, there are two versions of each chromosome. haploid haploid diploid We use “N” to represent the haploid number of chromosomes and “2N” to represent the diploid number of chromosomes. 1. For humans, the haploid number is 23. 2. We write this as N = 23. 3. The diploid number is 46, which we write as 2N = 46. 2.In fruit flies, N = 4 and 2N = 8. Practice with haploid and diploid: haploid = 4 and In fruit flies, N (______) diploid = ___8. 2N (______) If elephants have a diploid number of 56, what is their haploid number? 28 6.1 Chromosomes and Meiosis • Meiosis makes haploid cells from diploid cells. – Meiosis occurs in sex cells. – Meiosis produces gametes. TEKS 6A, 6G 6.2 Process of Meiosis The student is expected to: 6G recognize the significance of meiosis to sexual reproduction TEKS 6G Meiosis Meiosis is the process that divides one diploid (2N) cell to form four haploid (N) cells. This process is a reductional division because the number of chromosomes per cell are cut in half. Meiosis is how gametes are formed. 6.2 Process of Meiosis Cells go through two rounds of division in meiosis. • Meiosis reduces chromosome number and creates genetic diversity. TEKS 6G 6.2 Process of Meiosis TEKS 6G • Meiosis I and meiosis II each have four phases, similar to those in mitosis. – Pairs of homologous chromosomes separate in meiosis I. – Homologous chromosomes are similar but not identical. – Sister chromatids divide in meiosis II. – Sister chromatids are copies of the same chromosome. homologous chromosomes sister chromatids sister chromatids There are two divisions that occur in meiosis: Meiosis I and Meiosis II. 1.Before meiosis I begins, cells go through Interphase I. 2.This involves DNA replication, forming a duplicate copy of each chromosome. 3.Each chromosome is made of two sister chromatids. centrioles Nucleus with duplicated DNA that is not condensed. cell 6.2 Process of Meiosis TEKS 6G • Meiosis I occurs after DNA has been replicated. • Meiosis I divides homologous chromosomes in four phases. Meiosis I is similar to mitosis. a. Prophase I: i. ii. iii. Centrioles are visible on opposite sides of the nucleus Nucleolus disappears Nuclear envelope breaks down spindle centrioles DNA condensed as chromosomes iv. Homologous chromosomes pair 1. When a pair of chromosomes aligns a tetrad is formed. 2. When chromosomes form a tetrad, they exchange portions of their chromatids in a process called crossing-over. 3. Crossing-over produces new combinations of DNA. tetrad b. Metaphase I: i. Spindle fibers attach to the chromosomes at the centromere spindle centromere centrioles chromosomes (DNA) c. Anaphase I: i. Spindle fibers pull the homologous chromosomes toward opposite ends of the cell. ii. Chromosomes move and separate into two groups near the spindle iii. Anaphase ends when the chromosomes stop moving spindle centrioles chromosomes (DNA) d. Telophase I i. ii. e. Nuclear membranes form around chromosomes Daughter nuclei form Cytokinesis i. Cytoplasm divides Nucleus centrioles chromosomes (DNA) Nucleus Meiosis I produces two diploid (2N) daughter cells. Each chromosome is made of two sister chromatids. 6.2 Process of Meiosis • Meiosis II divides sister chromatids in four phases. • DNA is not replicated between meiosis I and meiosis II. TEKS 6G Meiosis II After Meiosis I, cells enter Meiosis II. No DNA replication occurs between Meiosis I and Meiosis II. Meiosis II separates the sister chromatids. a. Prophase II: i. Centrioles are visible on opposite sides of the nucleus centrioles ii. Nucleolus disappears iii. Nuclear envelope breaks down chromosome b. Metaphase II: i. Chromosomes align like they do in mitosis ii. Chromosomes are attached to the spindle at the centromere. centrioles spindle chromosome c. Anaphase II: Sister chromatids separate and move towards opposite ends of the cell. centrioles spindle chromosome d. Telophase II: i. Nuclear membranes form around chromosomes ii. Daughter nuclei form e. Cytokinesis: centrioles nucleus i. Division of the cytoplasm f. Meiosis II - produces four haploid (N) daughter cells chromosome Gamete formation • In male animals, the haploid gametes are called sperm. • In female animals, the haploid gametes are called eggs. 6.2 Process of Meiosis TEKS 6G • Meiosis differs from mitosis in significant ways. – Meiosis has two cell divisions while mitosis has one. – In mitosis, homologous chromosomes never pair up. – Meiosis results in haploid cells; mitosis results in diploid cells. • When the male and female gametes join, this is called fertilization and a zygote is formed. 6.2 Process of Meiosis Haploid cells develop into mature gametes. • Gametogenesis is the production of gametes. • Gametogenesis differs between females and males. – Sperm become streamlined and motile. – Sperm primarily contribute DNA to an embryo. – Eggs contribute DNA, cytoplasm, and organelles to an embryo. – During meiosis, the egg gets most of the contents; the other cells form polar bodies. TEKS 6G Comparing mitosis and meiosis • Mitosis produces two genetically identical diploid (2N) somatic (body) cells. Mitosis allows an organism’s body to grow and to replace cells. In organisms that reproduce asexually, new organisms are produced by mitosis. • Meiosis (or gametogenesis) produces four genetically different haploid (N) cells. Meiosis produces gametes for use in sexual reproduction.