CELL REPRODUCTION
MEIOSIS
Lesson Objectives –Meiosis
• Describe asexual reproduction; explain the genetic
relationship between parent and offspring.
• Describe sexual reproduction; explain the genetic
relationship between parent and offspring.
• Describe gametogenesis and identify the key differences
between oogenesis and spermatogenesis.
• Identify and describe the main steps of meiosis,
distinguishing between the quantity of genetic material
in the parent and resulting cells.
ASEXUAL REPRODUCTION
•
•
•
•
One parent
Offspring genetically identical to parent
Does not happen through MEIOSIS!!!
Some modes of reproduction:
– Binary fission
– Budding
– Spore formation
SEXUAL REPRODUCTION
• Two parents
• Offspring NOT genetically identical to parents
• Happens through MEIOSIS..
SEXUAL REPRODUCTION AND
GAMETES
• Two different cells join together to form a new organism
• Gametes (sexual reproduction cells) only have one-half
of the chromosomes found in body cells
• Two types of gametes
– Sperm (male)
– Eggs or ova (female)
• The union of these two is called fertilization
• New diploid cell formed from two haploid cells is called a zygote
• Number of chromosomes present in each gamete
called haploid number (n)
• Number of chromosomes present in each body cell
called diploid number (2n)
GAMETOGENEIS: SPERMATOGENESIS
AND OOGENESIS
• Spermatogenesis produces four haploid sperm cells
• Oogenesis produces one mature ovum or egg cell
CROSSING_OVER
Formation of homologous chromosomes called a tetrad
http://youtu.be/CCqAri4wtjQ
BACK
MEIOSIS
• Form of cell division that produces daughter cells having
half the number of chromosomes of the parent cell
• Phases of meiosis similar to mitosis
– Mitosis produces two genetically identical diploid cells from
one diploid cell
• Four phases
• Cell divides once
– Meiosis creates four genetically different haploid cells from
one diploid cell
• Eight phases
• Cell divides twice
PHASES OF MEIOSIS
• Meiosis I
1. Prophase I
2. Metaphase I
3. Anaphase I
4. Telophase I and
Cytokinesis I
• Meiosis II
5. Prophase II
6. Metaphase II
7. Anaphase II
8. Telophase II and
Cytokinesis II
MEIOSIS I
• Prophase I  crossing-over occurs (genetic variation)
NEXT
• Metaphase I  independent assortment VIEW
• Anaphase I  homologous chromosomes separated
• Telophase I & Cytokinesis I  two daughter cells formed
CONTINUE
INDEPENDENT ASSORTMENT
RETURN
MEIOSIS II
•
•
•
•
Prophase II
Metaphase II
Anaphase II
Telophase II & Cytokinesis II
Note: DNA is not replicated
again between Meiosis I
and Meiosis II
Summary: At the end of meiosis, 4 daughter cells are formed = eggs or sperm.
Each contains half as many chromosomes as the parent cell (n); each cell is
genetically different from its parents and from its "siblings". Fertilization of
an egg by a sperm restores the chromosome number to 2n.
GENETIC VARIABILITY OF FOUR
HAPLOID CELLS
Lesson Summary—Meiosis
• Asexual reproduction produces a clone, an organism that is genetically identical to its
parent.
• Asexual reproduction includes binary fission, budding, and spore formation.
• Sexual reproduction involves haploid gametes and produces a diploid zygote through
fertilization.
• Meiosis is a type of cell division that halves the number of chromosomes. There are
eight stages of meiosis, divided into meiosis I and meiosis II. DNA is not replicated
between meiosis I and meiosis II.
• Crossing-over, the independent assortment of chromosomes during anaphase I, and
random fertilization result in genetic variation.
• Meiosis is a step during spermatogenesis and oogenesis. Spermatogenesis produces
four haploid sperm cells, while oogenesis produces one mature ovum.
• Eukaryotes have three different versions of the sexual life cycle: a haploid life cycle, a
diploid life cycle, and a life cycle known as the alternation of generations. The sexual
life cycle depends on when meiosis occurs and the type of cell that undergoes meiosis.