Chapter 11:Introduction to GeneticsSection 11-4: Meiosis

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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?

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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

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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)

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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

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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

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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

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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

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Anaphase I• Spindle fibers pull each homologous chromosome pair toward

opposite ends of the cell• Separated chromosomes cluster at opposite ends of the cell

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Telophase I and Cytokinesis• A nuclear membrane forms around each cluster of

chromosomes• Cytokinesis follows, forming two new cells

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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

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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

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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

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

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Anaphase II• Paired chromatids separate

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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

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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

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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

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Comparing Mitosis and Meiosis2. 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

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

of Independent Assortment

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