Biology 120 Notes (Part 16) Starting and Finishing Meiosis II, Life Cycle, and Nondisjunction

Before Meiosis II starts, a process called ==interkinesis== occurs. It is a short resting period, similar to Interphase, but without DNA replication.
Meiosis II is very similar to Mitosis, in terms of cell division, and results in four haploid daughter cells.
In ==prophase II==, sister chromatids condense. A new spindle begins to form. The nuclear envelope starts to fragment.
In ==prometaphase II==, the nuclear envelope disappears, and the spindle fibers engage the individual kinetochores on the sister chromatids.
In ==metaphase II==, sister chromatids line up at the metaphase plate.
In ==anaphase II==, sister chromatids are pulled apart by the shortening of the kinetochore microtubules. Non-kinetochore microtubules lengthen the cell.
In ==telophase II== and ==cytokinesis,== chromosomes arrive at the poles of the cell and decondense.
Nuclear envelopes surround the four nuclei. Cleavage furrows divide the two cells into four haploid cells.

The number of nuclear divisions, number of daughter cells produced, genetic variability of daughter cells, the ploidy level of daughter cells compared to parent cells, and relative occurrence.

Meiosis I is a reduction division.
==Life cycle== is a term that describes how the gamete and zygote coming together to form a zygote. This process starts at fertilization and ends at the production of the offspring.
Meiosis produces gametes through spermiogenesis and oogenesis.
==Spermiogenesis== occurs in the testes of males.
==Oogenesis== occurs in the female ovaries.
An egg fertilized by a sperm produces a diploid zygote.
Euploidy vs. Aneuploidy.
==Monosomy== describes how there is only one of a chromosome.
==Trisomy== describes how there is three of a particular type of chromosome. The most common autosomal type of trisomy seen among humans is Trisomy 21, also known as Down Syndrome.
==Nondisjunction== describes how there is a failure of chromosomes to separate properly.
Sex chromosome abnormalities result from nondisjunction during oogenesis or spermiogenesis such as Klinefelter Syndrome and Turner Syndrome.
==Klinefelter Syndrome== describes how males can have underdeveloped testes and prostate; some breast underdevelopment. This affects 1 in 500 to 1,000 males.
==Turner Syndrome== affects 1 in 10,000 females. They have small, underdeveloped ovaries, oviducts, and uterus. Those with Turner Syndrome do not undergo puberty and never menstruate. Hormone therapy is usually required.