Unit 9 – Cell Division

What is Mitosis is used for?

a. Growth in multicellular organisms

b. Repair and regeneration of tissues/organs

c. Asexual repro in multicellular organisms (ex: spores, veg propagation)

d. Asexual Reproduction of unicellular organisms

What are the steps of mitosis?

Prophase, Metaphase, Anaphase, Telophase, (Cytokinesis)

Interphase

Nuclear membrane visible; no chromosomes present

Prophase

Nuclear membrane disappears, chromosomes appear

mitotic spindle begins to appear; centrioles on animals appear and migrate towards poles of cell

Metaphase

Chromosomes in single line along equator of cell; mitotic spindle formed

Anaphase

Sister chromatids separate; migrate to poles of cell

Telophase

Sister chromatids (now called individual chromosomes) clustered at poles of cell; nuclear membrane begins to reform, mitotic spindle disappearing

Cytokinesis

Cleavage furrow (animal) or cell plate (plant) visible; chromosomes disappearing or invisible; nuclear membrane reformed

Results of Mitosis

1 diploid (2N) somatic cell -----> 2 diploid (2N) somatic cells

Both cells should be identical except for point mutations

What is meiosis used for?

Used by multi celled organisms for: production of gametes only!

Only occurs in gonadal tissues

Prophase I

the nuclear membrane disappears, chromosomes appear, centrioles appear 

and begin to migrate to the poles; “Tetrads” form and “crossing over” occurs

during synapsis

Synapsis

Homologous chromosomes “find” each other; form tetrads

Crossing over

Form of genetic variation in which non sister chromatids in the same tetard exchange genes

Metaphase I

Homologous pairs line up as partners at equator; mitotic spindle fully formed; maternal/paternal chromo. randomly situated 

Anaphase I

Homologous pairs separate; whole chromosomes migrate to poles

Random distribution of maternal/paternal chromosomes to poles ensures “independent assortment” of genetic information

Telophase I

Chromosomes collected at poles; mitotic spindle disappears

Nuclear membrane reappears

Cytokinesis I

 Cleavage furrow/cell plate appears, cell divides into two

Prophase II

nuclear membrane disappears, chromo appear, centrioles appear and migrate to poles of cell

Metaphase II

mitotic spindle fully formed, single chromo line up at equator; centrioles at poles of cell

Anaphase II

sister chromatids separate, migrate to opposite poles of cell

Telophase II

sister chromatids (now individual chromosomes) clustered at poles of cell, nuclear membrane reappears, mitotic spindle disappears

Cytokinesis II

 Cleavage furrow/cell plate appears, cell membrane/cytoplasm splits

Result of meiosis

1 diploid (2N x 2) cell --------> 4 haploid (1N) gametes

All 4 gametes are unique due to:

Point mutations

Independent assortment

Crossing over

Independent Assortment

- random distribution of maternal and paternal chromosomes to gametes during meiosis

- homologous pairs in tetrads line up randomly

Spermatogenesis

- occurs in the male gonads (testes)

- produces four haploid sperm of equal size

Oogenesis

- occurs in the female gonads (ovaries)

- 1 large haploid cell (ootid) and 3 small polar bodies

- Only the ootid may be fertilized

Fertilization

union of an egg and sperm

Zygote

result of fertilization; full set of genetic information