CELL REPRODUCTIONCELL REPRODUCTION

MEIOSIS 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

http://youtu.be/CCqAri4wtjQ

BACK

Formation of homologous chromosomes called a tetrad

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 ASSORTMENTRETURN

MEIOSIS II• Prophase II• Metaphase II• Anaphase II• Telophase II & Cytokinesis II

Note: DNA is not replicated again between Meiosis Iand 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.