Cell DivisionCell DivisionSOL: BIO 6 a-cSOL: BIO 6 a-c

MeiosisMeiosis Meiosis occurs in sexual Meiosis occurs in sexual

reproduction when a diploid reproduction when a diploid germ cell produces four haploid germ cell produces four haploid daughter cells that can mature daughter cells that can mature to become gametes (sperm or to become gametes (sperm or egg).egg).

MeiosisMeiosis

Goal: reduce genetic material by halfGoal: reduce genetic material by halfWhy? Why? n (mom) + n (dad) = 2n (offspring)

from mom from dad child

meiosis reducesgenetic content

toomuch!

Just right!

Meiosis: cell division in Meiosis: cell division in two partstwo parts

Chromatidsseparate

Sister chromatidsseparate

Result: one copy of each chromosome in a gamete.

Haploid

Diploid

Meiosis I(reductiondivision)

Meiosis II

Haploid

(equationaldivision)

2n

nn

Meiosis I : the reduction Meiosis I : the reduction divisiondivision

Prophase I(early)

(diploid)

Prophase I(late)

(diploid)

Metaphase I(diploid)

Anaphase I(diploid)

Telophase I(diploid)

Nucleus Spindlefibers

Nuclearenvelope

Prophase IProphase I

Early prophase

Chromatids pair. Crossing over occurs.

Late prophase

Chromosomes condense.Spindle forms.

Nuclear envelope fragments.

Metaphase IMetaphase I

Chromatid pairs alignalong the equator of the cell.

Anaphase IAnaphase I

Chromosomes separate andmove to opposite poles.

Sister chromatids remain Attached at their centromeres.

Telophase ITelophase I

Nuclear envelopes reassemble.

Spindle disappears.

Cytokinesis divides cell into two.

Meiosis IIMeiosis II

Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene.

Sister chromatids carry identical genetic

information.

Gene X

Meiosis II : the equational Meiosis II : the equational divisiondivision

Prophase II(haploid)

Metaphase II(haploid)

Anaphase II(haploid)

Telophase II(haploid)

Four Non-identical

haploid daughter cells

Prophase IIProphase II

Nuclear envelope fragments.

Spindle forms.

Metaphase IIMetaphase II

Chromosomes align along equator of cell.

Anaphase IIAnaphase II

Sister chromatids separateand move to opposite poles.

Telophase IITelophase II

Nuclear envelope assembles.

Chromosomes unravel.

Spindle disappears.

Cytokinesis divides cell into two.

Results of meiosisResults of meiosis

• Four haploid cells

• One copy of each chromosome

“Putting It All Together” - Fertilization

What Meiosis is About

Meiosis allows the creation of unique individuals through sexual reproduction.

Mitosis Meiosis

Number of divisions 1 2

Number of daughter cells

2 4

Genetically identical?

Yes No

Chromosome # Same as parent Half of parent

Where Somatic cells Sex cells

When Throughout life At sexual maturity

Role Growth and repair Sexual reproduction

MITOSIS MEIOSIS

Occurs in somatic (body) cells Occurs only in gonads (sex organs: ovary/testes)

Produces cells for repair,maintenance, growth, asexual reproduction

Only produces gametes (sex cells: egg/sperm)

Results in identical diploid (2n) daughter cells

Reduction division results in haploid (n) cells