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 half Why? n (mom) + n (dad) = 2n (offspring) from mom from dad child too much! meiosis reduces genetic content 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!

HAPLOID HAPLOID DIPLOID

Meiosis – Making the Sperm 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) (late) Metaphase I Anaphase I Telophase I Nucleus Spindle fibers Nuclear envelope

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

Occurs during Prophase I when parts of homologous chromosomes overlap 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 Meiosis II : (the equational division) -Sister Chromatids Separate *****very similar to Mitosis***** Prophase II (haploid) Metaphase II Anaphase II Telophase II 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 4 Results identical 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

Parthogenesis spontaneous reproduction where the egg develops into an adult without ever being fertilized by a male.