Meiosis

• Gamete production, takes place in reproductive organs/parts in both plants and animals

• Gametes have half the number of chromosomes (haploid), so when they come together the zygote will be diploid

• If gametes weren’t haploid the offspring would have twice the correct number of chromosomes in every cell

• Corn plant has 20 chromosomes (10 pairs) in every somatic cell, so its gametes have 10 single chromosomes

• If it didn’t than the offspring would end up with 40 chromosomes (10 quads!)

• 20 + 20– 40 + 40

• 80 + 80– 160 + 160

» 320 the 5th generation would have 32 of each of the 10 chromosomes!!

Meiosis, cont.• This is how the correct number of

chromosomes is maintained from generation to generation

• Called REDUCTION DIVISION— as cells divide, the number of chromosomes is reduced

• Consists of 2 successive nuclear and cell divisions

• Before Meiosis I, DNA replicates into fuzzy X’s

• Meiosis I— homologous chromosomes pair up and then separate into 2 cells

• Meiosis II—just like mitosis, but it happens to 2 cells simultaneously

• Chromosomes line up at the center and sister chromatids are pulled apart

• One “2n” (diploid) cell produces 4 “n” (haploid) cells

Interesting occurrences during meiosis…..

• Crossing over—when homologous chromosomes exchange chromosome fragment

• Causes genetic recombination and provides genetic variation (good thing!)

• Happens during Prophase I, when chromosomes are coiling up and homologous chromosomes are pairing up

• Shown happening at both ends of chromosomes, but can happen at just one

• This is why you are not identical to your brothers/sisters

Another interesting occurrence….• Nondisjunction—homologous chromosomes do not separate properly• Happens during Anaphase I or II• End result is two gametes end up with both chromatids (of that specific

chromosome) and the other 2 cells end up without a copy of that chromosome

• So if the gamete that has both of the chromatids is fertilized then that zygote will be trisomy for that chromosome

• If the gamete that is missing that chromosome is fertilized, it will be monosomy for that chromosome

• Relatively common-- 1 in 5 normal human pregnancies miscarry w/in the first 2 months due to the baby having too many or too few chromosomes