Chapter 12: Meiosis
Meiosis involves the production of 4 unique daughter cells
Gamete- a haploid (n) reproductive cell that has one set of chromosomes
Fertilization- occurs during the fusion of two gametes (one from each parent; sperm and egg)
Colchicine- a chemical used to stop the cell cycle to take a digital picture of the chromosomes
that they can be analyzed and matched up (staining them makes the matching up easy;
matching allows scientists to understand what chromosomes cause what diseases).
Sexual Reproduction in Humans
Brings 2 gametes (one from the male and one from the female) together
2 parents produce offspring (progeny)
Parents have 2n (diploid) nuclei
Gametes are n (haploid) and differ one from another gene in composition
Gametes have half the DNA content of each parent nucleus (n is 0.5 of 2n)
Each parent passes on half of its genes to its offspring
Offspring inherit a unique combination (mixture) of genes from both parents= Genetic
Composed of DNA and proteins
Carry genes (hereditary information)
Chromosomes of the same type, size and with the same genes at identical locations are
called homologs or homologous chromosomes.
Table 12.1 The Number of Chromosomes Found in Some Familiar Organisms
The number of chromosomes varies for different species.
Organism # of Different Types of Diploid Chromosome
Chromosomes (haploid #) Number (2n)
Human 23 46
Domestic Dog 36 72
Fruit fly 4 8
Chimpanzee 24 48
Bulldog ant 1 2
Garden pea 7 14
Corn (maize) 10 20
Units of heredity
Made of DNA (nucleic acid) located on chromosomes
Have specific sequences of nucleotides (monomers)
Most genes program cells to synthesize proteins
The actions of these proteins produce the organism’s inherited traits
Alleles- are the different versions of a specific gene
Sex chromosomes- are chromosomes that determine an individual’s sex
Autosomes- are non-sex chromosomes Karyotype- a display of an individual’s chromosomes that is organized in terms of chromosome
number, size and type.
Box 12.1 Karyotyping Techniques
To determine the karyotype the biologist would first obtain a sample of cells from the
patient and grow them within a culture. When the culture cells begin to divide rapidly, they are
treated with a compound called colchicine. This compound stops mitosis at metaphase by
disrupting the formation of the mitotic spindle. By this point, the chromosomes are relatively
easy to study, because they are condensed and consist of sister chromatids. These
chromosomes are then stained and examined under a light microscope. Chromosomes can be
distinguished by size, position of the centromere, the stripping or banding patterns that appear
in response to some stains.
Fig. 12.6 Human Chromosomes
Unlike the orderly arrangement of chromosomes seen in a), condensed chromosomes
that are undergoing mitosis are arranged randomly when first observed with the microscope. To
determine a karyotype, a computer is used to separate the image of each condensed
chromosome, place homologous pairs side by side, and arrange the homologs by number.
b) shows the swapping of parts in the chromosome 9 and chromosome 22; this defect is
called a translocation. This translocation causes a genetic change that leads to uncontrolled
c) shows the sex chromosomes seen in someone who has Klinefelter’s syndrome (two X
chromosomes and one Y chromosome).
Real World Examples
Klinefelter Syndrome- XXY, XXXY, XXXXY, XXXXXY
CML (Chronic Myelogenous Leukemia)
Form of cancer
Portion of chromosome 22 (autosomal) switches with a small piece from chromosome 9
Treatment is available
Why are Karyotypes Useful?
For genetic screening to identify chromosome defects in their number, size and type.
Locus- position of a gene along a chromosome (or along a DNA double helix)
Summary Table 12.2 Vocabulary Used to Indicate the Chromosomal Makeup of a Cell
1. Prior to Meiosis
During Interphase, each chromosome in the 2n parent cell replicates
Fig. 12.2 The Major Events of Meiosis
Meiosis reduces chromosome number by half. In diploid organisms, the products of
meiosis are haploid. Meiosis
Occurs in sexually reproducing individuals
Consists of two consecutive cell divisions, Meiosis 1 and Meiosis 2
Homologs from each pair of homologous chromosomes, separate and move to 2
different daughter cells.
Sister chromatids of each chromosome separate and move to each of the daughter cells
At the end of meiosis 2 and cytokinesis, there are 4 haploid gametes each with one copy
of each chromosome produced.
Fig. 12.4 The Phases of Meiosis
The phases of Meiosis are: Prophase 1, Metaphase 1, Anaphase 1, Telophase1 and
2. Early Prophase 1
Nuclear envelope breaks down and chromosomes condense
Replicated homologous chromosomes pair to form tetrads, this process is called
Homologous chromosomes align gene by gene
The exact mechanism which causes synapsis is still unknown, but it may involve a
protein complex called the synaptonemal complex.
Synaptonemal complex- proteins draw the chromosomes together to cross over
segments of the chromosomes; denature protein to determine what it effects.
3. Late Prophase 1
Crossing Over (recombination) occurs between two non-sister chromatids
Chiasma (singular)- one crossing over
Chiasmata (plural)- several crossing overs
Fig. 12.7 A Closer Look at Chromosome Replication, Synapsis & Crossing Over
Long chromosome 9, with knob
Kernels are coloured and waxy