2SEXUAL REPRODUCTIONSex cells formed by meiosisRequires 2 parentsOffspring not identicalMeiosisThe process that results in the formation of gametesGametesSex cells, egg or sperm, which have haploid chromosomesHaploidHaploid means HALF, as in the number of chromosomes,written as n
3The union of an egg and a sperm Fertilizationn2nThe union of an egg and a spermZygoteThe cell that is formed when the egg becomes fertilized by the sperm. This results in a full set of chromosomes.Somatic CellsAll body cells except for sex cells that have diploid chromosomesDiploidDiploid means they have chromosome found in pairs,written as 2n
4Homologous Chromosomes Similar chromosomes that are found in pairs. The paired chromosomes come from the mother and father.* Human body cells have 46 chromosomes each* Human body cells have 23 homologous pairsMeiosis and FertilizationImportant for survival of many species, because these processes result in genetic variation of offspring.MeiosisA kind of cell division that results in gametes (sex cells) with half the number of chromosomes.
5A. Gametes do not have homologous pairs, but instead they have only half the chromosomes from each pair. Gametes are referred to as haploid (n).B. When fertilization occurs, the zygote will have a normal number of chromosomes (2n).C. In meiosis, specialized cells divide 2 times producing daughter cells.
6Phases of Meiosis(2 Cell divisions – Meiosis I & Meiosis II)MEIOSIS IProphase IHomologous chromosomes pair together to form tetradsDuring Prophase I, crossing over occurs between homologous chromosomes and genes are exchanged causing genetic variation
7Metaphase ISpindle fibers attach to chromosomes and line them up in the middle of the cell.Anaphase IThe fibers pull the homologous chromosomes apart. This is called disjunction. Chromosomes move toward opposite sides of the cell.
8Telophase IDuring Telophase I and cytokinesis, the nuclear membrane forms and the cell splits into two new cells.**** The Chromosome number is reduced to half during Meiosis I forming haploid (n) daughter cells
9MEIOSIS IIProphase IISpindle fibers form and attach to double stranded chromosomesMetaphase IIChromosomes line up in the middle of the cell
10Anaphase IISister chromatids separate and move towards opposite sides of the cellTelophase IICells divide, resulting in 4 haploid cells – gametes (sex cells)
11Males form 4 haploid cells which develop into 4 sperm KFemales form 4 haploid cells which develop into 1 egg and 3 small polar bodiesEggPolar BodiesSmall polar bodies are not used and break down.
12Meiosis Parent cell – chromosome pair Chromosomes copied 1st division - pairs split2nd division – produces 4 gamete cells with ½ the original no. of chromosomes
14Figure 13.7 The stages of meiotic cell division: Meiosis I
15Figure 13.7 The stages of meiotic cell division: Meiosis II
16One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes.In humans, n = 23 and 223 = 6,000,0000.That’s a lot of diversity by this mechanism alone.
25Meiosis error - fertilization Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’.In humans this often occurs with the 21st pair – producing a child with Downs Syndrome
2621 trisomy – Downs Syndrome Can you see the extra 21st chromosome?Is this person male or female?