Meiosis Part 2 SBI 3U November 4th, 2012
Recall • What is the function of meiosis? • How many cell divisions are there in the process of meiosis?
Random Assortment of Chromosomes • Random assortment of chromosomes during Anaphase I • 1 of the ways by which meiosis creates genetic variability • 2n=46 how many variations can we have?
Crossing Over • Genetic recombination • During prophase of Meiosis I • Breakage and reunion of chromatids • Large genetic diversity
Sperm & Egg Cells • Spermatids: in male animals. 4 functional cells later become sperm cells -Have a head (containing nucleus) -Have a tail (flagellum) -Middle piece (containing mitochondria)
Sperm & Egg Cells • In meiosis I division of the cytoplasm is unequal = one large cell (a secondary oocyte) and one small cell (the first polar body) • Polar body contains only the nucleus • Oogenesis
Sperm & Egg Cells • Meiosis II secondary oocyte divides unequally again = one large ovum/egg and a small polar body • End of meiosis for females: 1 egg
Mistakes in Meiosis • Mistakes may be in separation of the chromosomes during division or an incorrect exchange of genetic information • Mistake in separation of chromosomes during division: Can result in an abnormal # of chromosomes in an egg or sperm cell
Mistakes in Meiosis • Resulting zygote will have too few or too many chromosomes: aneuploidy • Down syndrome • Mistake in exchange of information during crossing over: correct # of chromosomes but altered genetic information
Mistakes in Meiosis • Altered chromosomes may contain duplications or deletions
Abnormal Chromosome Number • Nondisjunction occurs when the homologous chromosomes do not split properly during meiosis I or the sister chromatids do not separate during meiosis II • What will happen to the resulting gametes?
Abnormal Chromosome Number • Monosomy: when a child only has 1 copy of a chromosome • Polysomy: when a child has 3 copies of a particular chromosome • Polyploidy: 3 sets of chromosomes! Very rare.
Abnormal Chromosomal Structure • Occurs during crossing over • Deletions and duplications • If a segment reattaches to a complete homolog, there will be a duplication • If a segment reattaches to the correct homolog inversion
Abnormal Chromosomal Structure • Translocation: segment attaches to a non-homologous chromosome