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