Meiosis notes

Chapter 13 - Meiosis Meiosis – is nuclear division that leads to a halving of the chromosome number, and ultimately the formation of gametes (2n -> n) Mitosis – cellular asexual division, mother cell (2n) divides into two identical daughter cells (2n) Zygote – fertilized egg Male parents determine sex (X, Y Chromosome) Human Karyotype 22 pairs of homologous autosomal chromosome, 1 pair of sex chromosomes n=23, 2n=46 Male parents determine sex XY – Male XX – Female Diploid – two sets of chromosomes Gene – segment of DNA at a particular location on a chromosome that influences a hereditary trait Gene for eye color Allele – different forms of genes Allele for blue eyes Meiosis Overview Before meiosis begins, each chromosome in the diploid parent cell is replicated When replication is complete, each chromosome has two identical sister chromatids They remain attached along most of their length The two attached sister chromatids ae still considered a single replicated chromosome Meiosis I – the two homologs of each chromosome pair separate into two daughter cells Each daughter cell has one set of chromosomes A diploid parent produces two haploid daughter cells Is a reduction division Meiosis II – the sister chromatids of each chromosome separate into two daughter cells Phases of Meiosis I Interphase – cell increases its mass and duplicates its chromosomes, uncondensed, chromosomes replicate in parent cell Prophase I - Metaphase I Anaphase I – spindle fibers contract and pull chromosomes apart, cuts genetic information in half Telophase I – cell splits in two and produces two haploid cells Phases of Meiosis II Prophase II – Metaphase II – Individual chromosomes line up in the middle of the cell Anaphase II – Spindle apparatus contracts and pulls the chromatids apart Telephase II – Problems in Meiosis Errors in meiosis are common Meiotic error is called nondisjunction Over a third of conceptions are spontaneously terminated because of problems in meiosis One infant in every 691 live births in the USA have down syndrome Caused by an extra copy of chromosome 21 Called trisomy 21 Why do mistakes occur? Meiotic errors are a result of random errors Maternal age is an important factor in the frequency of trisomy Egg development, or oogenesis, in humans Primary oocytes enter meiosis I during female embryonic development