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Chapter 13 Meiosis and Sexual Life Cycles ~ Gene – segment of DNA that code for basic units of heredity Gametes – (sperm, egg) reproductive cells that transmit genes from one generation to the next Locus (loci) – location of gene on a chromosome Types of Reproduction Asexual single parent passes all of its genes to the offspring offspring arises from mitosis exact copy of parent offspring referred to as clone Sexual two parents contribute to the genes of offspring Results in greater genetic variation Chromosomes Homologous chromosomes Pair of 2 chromosomes Carry genes that control same characteristics ie. If gene for eye color is at specific locus of a chromosome, it’s homologous will have the same gene at the same locus Similar length Same centromere position One set (23 chromosomes) is inherited from mom and the other (23) are inherited from dad (making 46) Chromosomes The only exceptions to the homologous rule is with sex chromosomes the 23rd pair of chomosomes are the sex chromosomes and determine gender If XX, then female. If XY then male. (offspring will always receive X from mom, and X or Y can come from dad) Autosomes – refers to the other 22 kinds of chromosomes that are not sex chromosomes Karyotype Picture of the homologous chromosomes Shows all 46 chromosomes paired up with the chromosome that is like them notice that this is a karyotype for a female (XX) Fertilization During fertilization, one haploid gamete from each parent will fuse The result is a zygote (fertilized egg) The zygote is diploid, arising from two haploids Meiosis Meiosis and mitosis are similar in that both are preceded by DNA being replicated Key differences is that mitosis involves 1 cell division, where meiosis involves 2 cell divisions Meiosis 1 The 1st cell division is referred to as meiosis 1 Prophase I Chromosomes condense Synapsis occurs – joining of homologous chromosomes forming tetrad Crossing over occurs – “mixing genes” . DNA from one homologous is exchanged with another (results in variation…no chromosome is identical to the other anymore) Chiasmata – area of crossing over Nucleus disappears, spindles appear and attach tot eh chromosomes Metaphase I Homologous pairs of chromosomes are lined up in the middle Anaphase I Spindle fibers pull chromosomes to opposite ends (chromosomes are still intact) Telophase 1 &Cytokinesis Chromosomes are at opposite ends and the cell splits into 2 not identical because of crossing over each cell now has 46 chromosomes In between Meiosis 1 and Meiosis 2 Chromosomes in the 2 cells DO NOT replicate (duplicate), but they now have 46 chromosomes a piece Meiosis II Prophase II Spindle fibers form again chromosomes start to move toward metaphase plate (equator) Metaphase II Individual chromosomes line up in the middle (no longer homologous pairs) Anaphase II Centromeres of sister chromatids separate Sister chromatids move toward opposite poles Telophase II & Cytokinesis Sister chromatids are now completely separated Nucleus appears The result is 4 haploid cells (sperm or egg) *** for females, only one egg is made the other 3 cells are called polar bodies which serve to give the egg cell energy as it stays in body for about 28 days Meiosis vs. mitosis 3 Events in Meiosis I that do not occur in Mitosis 1) synapsis and crossing over do not occur in mitosis 2) at metaphase I, homologous pairs are lined up, but in mitosis invidivual chromosomes lined up 3) Anaphase 1 chromosomes move to opposite poles, but in mitosis, the sister chromatids move toward poles Genetic Variation Crossing over – during prophase I the enchange of genes between homologous chromosomes (this will make each sister chromatid in the tetrad different) Independent assortment – in metaphase I the chromosomes can pair up with another chromosome (not their homolog) before the split Random Fertilization – n because each egg can be fertilized by any of a number of sperm, this lead to variation . Origins of Genetic Variation, II Crossing over (prophase I): • the reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I (recombinant chromosomes) Random fertilization: • 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations!