Meiosis

& Sexual Reproduction

• Remember that 1 chromosome is made when 2 chromatids join at the centromere.– These 2 chromatids are EXACT copies of each other

• Every human cell has 23 pairs of chromosomes– Total of _____– 1 set from Mom, 1 set from Dad (_______ from each!)– They each give us the same chromosomes.

• Same size, shape….

• They are only different in their GENES! (DNA that codes for a protein)

• Different versions of a gene are called ALLELES– Think flower color or hair color

46

23

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

from mom from dad child

meiosis reducesgenetic content

toomuch!

Just right!

Chromatids vs. Homologous Chromosomes

• Combining the 1 set from each parent gives us 23 pairs (46 total) – DIPLOID– Diploid just means 2 of

each chromosome• All human cells except

sex cells!

– HAPLOID means 1 of each chromosome

• In humans, only sex cells!

HAPLOIDHAPLOID

DIPLOID

Meiosis – Making theSperm and Egg

• Sperm and Egg are haploid

• Meiosis has 2 phases, Meiosis I and Meiosis II– Meiosis I = Homologous chromosomes

separate– Meiosis II = Sister chromatids separate

Meiosis I : (the reduction division)-Homologous Chromosomes Separate

Prophase I(early)

(diploid)

Prophase I(late)

(diploid)

Metaphase I(diploid)

Anaphase I(diploid)

Telophase I(diploid)

Nucleus Spindlefibers

Nuclearenvelope

Prophase I • Chomatids pair and condense.• Crossing over occurs• Spindle forms.• Nuclear envelope fragments.

Crossing Over• Occurs during Prophase I when parts of homologous

chromosomes overlap

Letters A/a and B/b represent alleles

Metaphase I• Chromatid pairs align along the equator of the cell.

Anaphase I• Chromosomes separate and move to opposite poles.• Sister chromatids remain attached at their centromeres.

Telophase I• Nuclear envelopes reassemble.• Spindle disappears.• Cytokinesis divides cell into two.

Meiosis II : (the equational division)-Sister Chromatids Separate

*****very similar to Mitosis*****

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 separate and move to opposite poles.

Telophase II

• Nuclear envelope assembles.• Chromosomes unravel.• Spindle disappears.• Cytokinesis divides cell into two.

Results of meiosis

• Four haploid cells• One copy of each chromosome

Animations

• http://www.johnkyrk.com/meiosis.html

Mitosis Meiosis

Number of divisions 1 2

Number of daughter cells

2 4

Resultsidentical diploid (2n) daughter cells

Reduction division results in haploid (n) cells

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

Mistakes in Meiosis

Nondisjunction = The failure of homologous chromosomes to separate properly during meiosis

• Trisomy: When a gamete with extra chromosomes is fertilized by a normal gamete, the zygote will have an extra chromosome

Ex. In human, if a gamete with an extra chromosome number 21 is fertilized by a normal gamete, the resulting zygote has 47 chromosomes instead of 46. This will develop into a baby with Down syndrome.

• Monosomy = When a gamete that is missing a chromosome fuses with a normal gamete during fertilization, the resulting zygote lacks a chromosome.

– Most often this is lethal. One non-lethal example is Turner syndrome, in which human females have only a single X chromosome instead of two.

Practice Questions• What do Haploid and Diploid mean?

• How are homologous chromosomes and sister chromatids different?

• Describe crossing over in your own words.

• Which phase of Meiosis (I or II) is most like Mitosis? Why?

• Polyploids = organisms with more than the usual number of chromosomes sets– Often results in death of the zygote. However,

polyploids often occur in plants

Hermaphrodites (yes, they DO exist in the human population)-Have both male and female genitals

Parthogenesisspontaneous reproduction where the egg develops into an adult without ever

being fertilized by a male.