CELL DIVISION: MEIOSIS

How do Organisms Reproduce? • Option 1: Asexual Reproduction

•  Can be done by a single organism without the involvement of gametes (sperm or egg)

•  Offspring are clones of the parent, meaning they are genetically

identical •  There are many different types of asexual reproduction.

•  Example: Bacteria undergo binary fission

Types of Asexual Reproduction 1.  Vegetative Propagation 2.  Fragmentation 3.  Budding

How do Organisms Reproduce? • Option 2: Sexual Reproduction

•  Requires two parents who produce gametes that unite to form an embryo or zygote

•  Offspring may be somewhat similar to the parents, but are genetically unique

•  Many eukaryotic organisms reproduce sexually •  Example: plants and animals

Chromosome Pairs • An embryo receives one set of chromosomes from its

mother and one set from its father

•  This means that sexually reproducing organisms have 2 sets of chromosomes that are arranged in pairs.

•  These chromosome pairs are called homologous •  They are similar but not identical

From “Dad” From “Mom”

Homologous Pair

Two Basic Types of Cells 1.  Diploid:

•  Cells that are body cells such as skin, nerve, and bone that have the full number of chromosomes (2 sets = 2N)

2.  Haploid: •  Sex cells such as egg and sperm that have half the number of

chromosomes (1 set = 1N)

Gamete Production • Gametes are produced by the process of meiosis

•  A series of cell divisions (with similar phases as mitosis – PMAT)

• Meiosis starts with one diploid cell and creates four haploid gametes.

•  The term for creating gametes is gametogenesis •  The term for creating eggs is oogenesis •  The term for creating sperm is spermatogenesis

Where does Meiosis Occur? • Meiosis occurs in the reproductive organs

• Spermatogenesis occurs in the testes • Oogenesis occurs in the ovaries

• A human female is born with all of the primary oocytes she will ever have ~1-2 million

• Males continually produce primary spermatocytes throughout their lifetime.

What is Meiosis? •  The goal of meiosis is to reduce the chromosome number

by half! •  Humans: 46 chromosomes à 23

• Meiosis also creates genetic diversity through a process called crossing-over •  This is the reason why siblings have different DNA (except for

identical twins)

What is Meiosis?

Meiosis I

• Prophase I • Metaphase I • Anaphase I •  Telophase I • Cytokinesis

Meiosis II

• Prophase II • Metaphase II • Anaphase II •  Telophase II • Cytokinesis

Meiosis I • Begins with interphase:

1.  Cells increase in size 2.  Cells produce RNA 3.  Cells synthesize (create) proteins 4.  Cells replicate DNA

Phases of Meiosis I

Prophase I •  The nuclear envelope breaks down

• Spindle fibers & centrioles appear

•  The chromosomes pair up with their homologous partner •  This form a tetrad

• Because the homologous chromosome pairs are in close proximity, an exchange of chromosome genetic material between the pairs often occurs •  This is called “Crossing Over”

Prophase I • Crossing Over results in a new combination of alleles.

Metaphase I • Spindle fibers attach to the centromeres

• Homologous chromosomes line up in pairs across the middle of the cell

Anaphase I •  The chromosome pairs separate, one chromosome to

each side of the cell •  Each daughter cell will receive only one chromosome from each

homologous chromosome pair •  Sister chromatids remain attached to each other

•  There are more than 8 million different configurations for how our chromosomes can be “independently assorted”

Telophase I & Cytokinesis • Spindle fibers disappear and the nuclear membrane

reforms.

•  The cell membrane pinches inward and two haploid cells are formed.

•  These two cells do not enter interphase •  They will both proceed directly to Meiosis II

Phases of Meiosis II • Meiosis II is just like Mitosis

Prophase II •  The nuclear envelope breaks down

• Spindle fibers & Centrioles appear

Metaphase II • Spindle fibers attach to the centromeres

•  The chromosomes line up singularly across the middle of the cell

Anaphase II • Spindle fibers shorten and separate the chromosomes

• Sister chromatids are pulled apart and move to opposite ends of the cell

Telophase II & Cytokinesis • Spindle fibers disappear and the nuclear membrane

reforms.

•  The cell membrane pinches inward and two cells are formed.

•  4 Haploid cells are produced •  2 from each division

Interphase

Meiosis II

Meiosis I

1 diploid cell

4 haploid cells

Meiosis Animation

Mitosis vs. Meiosis

Mitosis

•  Occurs in somatic cells

•  Makes 2 diploid cells

•  Daughter cells are genetically identical to parent

Meiosis

•  Occurs in gametic cells

•  Makes 4 haploid cells

•  Produces gametes that are genetically unique