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

2. 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?

3. 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

4. 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)

5. 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

6. 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

7. 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

8. 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

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

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

11. 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

12. 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

13. 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

14. Metaphase II
Chromosomes line up in the center of each cell

15. Anaphase II
Paired chromatids separate

16. 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

17. 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

18. 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

19. 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

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