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 (fertilized egg) will be diploid• If gametes weren’t haploid the offspring would have twice the correct

number of chromosomes in every cell , for example:• 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 (3 of one chromosome) for that chromosome

• If the gamete that is missing that chromosome is fertilized, it will be monosomy (one chromosome) 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