Chapter 11:
Introduction to Genetics
Section 11-4: Meiosis
• In the early 1900s, geneticists worked to apply Mendel’s
• 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
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
• 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
• Crossed sections of the chromatids are exchanged
Metaphase I
• As prophase I ends, a spindle forms and attaches to each
• 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
• 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
• 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
• 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
• 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 ->
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
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