Chapter 11: Introduction to Genetics

Chapter 11:Introduction to Genetics Section 11-4: Meiosis

Introduction • In the early 1900s, geneticists worked to apply Mendel’s principles • They knew that the structure that carried genes was in the cell, but which structure? • What process was responsible for allele segregation?

Chromosome Number • Chromosomes – strands of DNA in the nucleus – carry genes • Genes are located in specific positions on chromosomes • We will begin to explore this idea using another model system – the fruit fly • The body cell of a fruit fly contains 8 chromosomes – 4 from the male parent and 4 from the female parent • The two sets are homologous - meaning that they correspond, or match, in terms of their size and gene position

Chromosome Number • Cells that contain both sets homologous chromosomes are diploid, meaning “two sets” - represented as 2N • Cells that contain only one set of chromosomes are haploid, meaning “one set” – represented by N • Gametes – egg and sperm – are haploid • For a fruit fly, 2N=8 (diploid number) and N=4 (haploid number)

Phases of 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 • Involves two distinct divisions, called meiosis I and meiosis II • By the end of meiosis II, the diploid cell becomes four haploid cells

Meiosis I • Right before meiosis I, the cell goes through interphase where the chromosomes are replicated, so that they consist of 2 identical chromatids joined at a centromere

Prophase I • The cells begin to divide, and the chromosomes pair up, forming a structure called a tetrad, which contains four chromatids • When tetrads are formed, they undergo a process called crossing-over, which produces new combinations of alleles in the cell. • Chromatids of the homologous chromosomes cross over one another • Crossed sections of the chromatids are exchanged

Metaphase I • As prophase I ends, a spindle forms and attaches to each tetrad • During metaphase I, paired homologous chromosomes line up across the center of the cell

Anaphase I • Spindle fibers pull each homologous chromosome pair toward opposite ends of the cell • Separated chromosomes cluster at opposite ends of the cell

Telophase I and Cytokinesis • A nuclear membrane forms around each cluster of chromosomes • Cytokinesis follows, forming two new cells

Meiosis I • Results in two daughter cells, each of which has four chromatids, as it would after mitosis • Because each pair of homologous chromosomes was separated, neither daughter cell has two complete sets of chromosomes • The cells have sets of chromosomes /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 • Neither cell goes through a round of chromosome replication before entering meiosis II

Prophase II • Chromosomes—each consisting of two chromatids—become visible • Chromosomes do not pair to form tetrads, because the homologous pairs were already separated during meiosis I

Metaphase II • Chromosomes line up in the center of each cell

Anaphase II • Paired chromatids separate

Telophase II and Cytokinesis • In our example, each of the four daughter cells produced in meiosis II receives two chromatids • The four daughter cells are now haploid (N) with just two chromosomes each

Gametes to Zygotes • The four cells produced by meiosis are gametes • In male animals, gametes are called sperm (some pollen grains also contain haploid sperm cells) • In female animals, only one of the cells produced by meiosis becomes an egg (egg cell in plants) • Fertilization generates new combinations of alleles in a zygote, which undergoes cell division by mitosis and eventually forms a new organism

Comparing Mitosis and Meiosis • Mitosis is a form of asexual reproduction, while meiosis is the first step in sexual reproduction • There are 3 other big differences: 1. Replication and separation of genetic material • In mitosis, each daughter cell receives a complete set of chromosomes • In meiosis, homologous chromosomes separate to different daughter cells - the two alleles for each gene segregate randomly and end up in different gametes • The sorting and recombination of genes in meiosis result in a greater variety of possible gene combinations

Comparing Mitosis and Meiosis 2. Changes in chromosome number • Mitosis does not normally change the chromosome number of the original cell (diploid -> diploid) • Meiosis reduces the chromosome number by half (diploid -> haploid) 3. Number of divisions • Mitosis is a single cell division, resulting in two genetically identical diploid daughter cells • Meiosis requires two rounds of cell division, and produces four genetically different haploid daughter cells

Gene Linkage and Gene Maps • Gregor Mendel outlined the Law of Segregation, and the law of Independent Assortment

Chapter 11: Introduction to Genetics