Cell Division

Meiosis Meiosis occurs in sexual

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

Meiosis Goal: reduce genetic material by

halfWhy? n (mom) + n (dad) = 2n

(offspring)from mom from dad child

meiosis reducesgenetic content

toomuch!

Just right!

Meiosis: 2 part cell division

Homologous Chromosomes pairs separate

Sister chromatidsseparate

Result: one copy of each numbered chromosome in a gamete.

Haploid

Diploid

Meiosis I Meiosis II

Haploid

2n

nn

Meiosis I

Prophase I(early)

(diploid)

Prophase I(late)

(diploid)

Metaphase I(diploid)

Anaphase I(diploid)

Telophase I(diploid)

Nucleus Spindlefibers

Nuclearenvelope

Prophase I Early

Early prophase Chromosomes

condenseSpindle forms

Nuclear envelope breaks apart

.

Prophase I Late

Late prophase Chromatids pair (synapse)

Crossing over occurs between chromatids of the homologous chromosome pair

Metaphase I

Homologous chromosome pairs line up on the cell’s equator

Anaphase I

Chromosomes separate andmove to opposite poles.

Sister chromatids remain Attached at their centromeres.

Telophase I

Nuclear envelopes reforms

Spindle disappears.

Cytokinesis divides cell into two

Meiosis II

End product of meiosis is Each Sex cell /gamete has :1 copy of each chromosome1 copy of each gene segment

Sister chromatids have carry different genetic information due to crossing over

Meiosis II

Prophase II(haploid)

Metaphase II(haploid)

Anaphase II(haploid)

Telophase II(haploid)

Four Non-identical

haploid daughter cells

Prophase II

Nuclear envelope fragments.

Spindle forms.

Metaphase II

Chromosomes align along equator of cell.

Anaphase II

Sister chromatids separateand move to opposite poles.

Telophase IINuclear envelope assembles.

Chromosomes unravel.

Spindle disappears.

Cytokinesis divides cell into two.

Results of meiosis

• Four haploid cells

• One copy of each chromosome

•Not the same genes on each chromosome

“Putting It All Together” - Fertilization

What Meiosis is About

Meiosis allows the creation of unique individuals through sexual reproduction.

Meiosis Video Link

Meiosis Video Link #1

Meiosis Video Link #2

Meiosis Video Link#3

2 diploid 4 haploid Crossing over between nonsister

chromatids Reduction divisiion Increases genetic variability vs. clones Meiosis 1 Homologous chrom separate Meiosis 2 Sister chromatids separate

Mitosis MeiosisNumber of

divisions 1 2

Number of daughter cells made through

process2 4

Are the new cells Genetically

identical?Yes No

# of Chromosomes

compared to parent cell

Same as parent Half of parent

What cell go thru this

division processSomatic cells Sex cells

When do cells got thru this

division processThroughout life At sexual maturity

Role Growth and repair Sexual reproduction

MITOSIS MEIOSISOccurs 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

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