3.3 MEIOSIS
DISCOVERY OF MEIOSIS
Discovered by microscope observation around
1880’s.
One species studied was Parascaris equorum
aka horse threadworm.
REVIEW
• A diploid human cell has 46 chromosomes.
These are arranged as 23 pairs of
chromosomes – homologous chromosomes
• The pair are not identical. Homologous
means similar not identical. One from father,
one from mother.
• n – number of unique chromosomes in an
organism
• Eukaryotes have n pairs of chromosomes = 2n
per cell
• Haploid cells = n
• Diploid cells = 2n
• This is shorthand for writing the number of
chromosomes in a haploid or diploid cell.
MEIOSIS
• A diploid nucleus divides producing four haploid nuclei
• Divided into meiosis I and meiosis II
• Meiosis I results in two haploid cells
• Meiosis II results in four haploid cells
• During prophase I, the chromosomes have doubled (2n
chromosomes).
• No replication after telophase I, nucleus has double
chromatids but one chromosome of each type.
CROSSING OVER
• Occurs during meiosis I
• Exchange of genetic material between bivalents
• Chromatids are a new combination of genes
• See pg. 86 in orange textbook.
• http://highered.mheducation.com/sites/007249585
5/student_view0/chapter28/animation__how_meios
is_works.html
TASK
• You must be able to recognise and draw diagrams
to represent the stages of meiosis. These are very
similar to mitosis, but with a few important
differences.
1. Crossing over during prophase I
2. Random orientation during metaphase I
3. Separation of chromatids in meiosis II
Notes/diagrams on pg. 164/65 will help you to do this.