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Chapter 7
Meiosis
Mitosis vs. Meiosis
• What are the similarities and differences
between mitosis and meiosis?
– Similarities:
• Nucleus divides
• New cells are formed
• Spindle is used
– Differences:
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Meiosis = PMAT twice
Crossing-over
Mitosis forms 2 cells (diploid)
Meiosis forms 4 cells (haploid)
Objectives
• Describe the similarities and differences
between mitosis and meiosis.
• Describe the differences and similarities
between meiosis I and meiosis II.
Meiosis
• Cell division that halves the # of
chromosomes when forming specialized
reproductive cells, such as gametes
– 8 steps (PMAT twice)
• 2 Divisions of Nucleus:
– Meiosis I
– Meiosis II
• Produces 4 haploid cells
Prophase 1
• Chromosomes become
visible
• Nuclear envelope breaks
down
• Crossing-over occurs
– Portions of chromatids are exchanged
between chromosomes
Metaphase 1
• Pairs of homologous chromosomes move
to the equator of the cell.
Anaphase 1
• Homologous chromosomes move to
opposite poles
Telophase 1 and Cytokinesis
• Chromosomes gather at the poles of the
cell
• Cytoplasm divides
• ***2 cells with 2 sets
of chromosomes
Prophase 2
• New spindle forms around the
chromosomes
Metaphase 2
• Chromosomes line up at the equator of the
cell
• Centromeres attached
to spindle fibers
Anaphase 2
• Centromeres divide
• Chromatids move to opposite poles
Telophase 2 and Cytokinesis
• Nuclear envelope forms around each set
of chromosomes
• Cytoplasm divides
• ***Forms 4
haploid cells
Meiosis Flip Book
• Using the small notepads, you will be creating a
flip book of meiosis.
– You may want to sketch in pencil before you begin!!!
• Each of the 8 steps must be accurately drawn
on separate pages.
– ALSO…describe what is happening during each
stage.
• Be sure to begin your Flip Book with a title page
that includes your name.
Review 
• List the differences and similarities
between meiosis 1 and meiosis 2.
– Similarities:
• Both go through PMAT
• Nucleus divides
– Differences:
• Crossing-over occurs during Prophase 1
• 2 cells at the end of meiosis 1
• 4 cells at the end of meiosis 2
Objectives
• Discuss the importance of independent
assortment, crossing-over, and random
fertilization to genetic variation.
Meiosis and Genetic Variation
• Meiosis creates new genetic variations
• 3 mechanisms that create genetic
variation:
– Independent assortment
– Crossing-over
– Random fertilization
Independent Assortment
• Random distribution of homologous
chromosomes during meiosis
– Fig. 2 – p. 146
• Each of the 23 pairs of chromosomes
separate independently
• Creates 223 (8 million) different gene
combinations
Crossing-Over
• Occurs when portions of chromatids are
broken and exchange places
– Occurs during Prophase I
• Crossing-over and independent
assortment create a nearly unlimited # of
genetic combinations
• Zygote forms from 2 gametes
– 223 x 223 = 64 trillion genetic possibilities
– Random Fertilization
Crossing-Over
Importance of Genetic Variation
• Essential to evolution
– Every organism of the same species would be
the same
• Speeds up evolution
– Offspring not the exact same as parents
Review
• Take 5 minutes to write a short paragraph
that discusses how independent
assortment, crossing-over and random
fertilization create new genetic variations.
Objective
• Compare the meiosis process in males
and females.
Meiosis and Gamete Formation
• Meiosis occurs in all sexually reproducing
organisms
– May not be the same between males and
females
• Meiosis is the primary event that forms
gametes
– Gametogenesis
Meiosis in Males
• Spermatogenesis
– Production of sperm
• Sperm
– Male gametes
• Follows basic meiosis stages
– p. 148 – fig. 3
Meiosis in Females
• Oogensis
– Production of eggs
• Ovum
– Female gametes
• Cell division is not equal
– p. 148 – fig. 3
Meiosis in Females
• Cytoplasm divides unequally during meiosis 1
• One cell gets almost all of the cytoplasm
– The other is called a polar body
• Can divide again, but will not survive
• Larger cell undergoes meiosis 2
– Unequal cell division again
– Forms another polar body
• Only one useable egg cell is produced
Chapter 7 Quiz Activity
• Write a quiz using the material we covered
during Chapter 7.
– Use the objectives to help you.
• Your quiz must include:
– 5 multiple choice questions
– 5 matching questions
– 1 short answer question
• You must provide the correct answer for
each question you create.
Meiosis Simulation
• Vocab.
– Homologous Chromosomes
• Pairs of chromosomes that are identical in size and
appearance
• Have the same genetic content
– Tetrad
• Formed when 2 homologous pairs
of chromosomes are aligned
side by side