Biology PPT Notes Ch 11 Meiosis & Sexual Reproduction PDF

Meiosis and Sexual Reproduction  Mitosis vs. Meiosis MiTosis happens in my Toes. – all cells except gametes – each parent cell produces two new cells that are identical to the original cell MeiOsis happens in my Ovaries. – gametes only (sperm and egg...

Meiosis and Sexual Reproduction  Mitosis vs. Meiosis MiTosis happens in my Toes. – all cells except gametes – each parent cell produces two new cells that are identical to the original cell MeiOsis happens in my Ovaries. – gametes only (sperm and egg) – each parent cell produces four genetically different haploid cells (tetrad)  Asexual Reproduction a single parent passes copies of all of its genes to each of its offspring – clone – genetically identical to its parent types of asexual reproduction: – fission – fragmentation – budding advantages: – the simplest and most primitive method of reproduction – allows organisms to produce many offspring in a short period of time, without using energy disadvantages: – the DNA of these organisms does not vary much between individuals – may not be able to adapt to a new environment  Sexual Reproduction only occurs in eukaryotic organisms advantages: – provides a powerful means of quickly making different combinations of genes among individuals disadvantages: – takes a lot of energy – must find a compatible mate life cycle – the entire span in the life of an organism from one generation to the next  Sets of Chromosomes homologous chromosomes – chromosomes that are similar in size, shape, and genetic content – each homologue comes from one of the two parents haploid – when a cell, such as a gamete, contains one set of chromosomes (N = haploid) diploid – when a somatic cell contains two sets of chromosomes (2N = diploid) fertilization – the gametes join (sperm & egg cells), which results in a diploid zygote – zygote – a fertilized egg cell, the first cell of a new individual  Chromosomes Determine Your Sex autosomes – chromosomes that are not directly involved in determining the sex of an individual (22 of the 23 pairs) sex chromosomes – one of the 23 pairs of chromosomes in humans that contain genes which will determine the sex of the individual – the sex of an offspring is determined by the male – the father (XY) can contribute either an X or a Y chromosome thus the offspring could be XY = male or XX = female  Meiosis a form of cell division that halves the number of chromosomes when forming specialized reproductive cells before meiosis begins, DNA in the original cell is replicated meiosis starts with homologous chromosomes  Stages of Meiosis Prophase I chromosomes become visible and the nuclear envelope breaks down crossing-over occurs when portions of a chromatid on one homologous chromosome are broken and exchanged with the corresponding portions on one of the chromatids of the other homologous chromosome  Meiosis I (con’t) Metaphase I pairs of homologous chromosomes are moved by the spindle to the equator of the cell Anaphase I homologous chromosomes move to opposite poles of the cell the chromatids do not separate at their centromeres each chromosome is still made up of two chromatids genetic material has recombined Telophase I chromosomes gather at the poles of the cell cytokinesis occurs both cells contain one chromosome from each pair of homologous chromosomes The chromosomes do not replicate between meiosis I and meiosis II.  Meiosis II Prophase II a new spindle forms around the chromosomes Metaphase II chromosomes line up at the equator Anaphase II centromeres divide chromatids (now called chromosomes) move to opposite poles of the cell Telophase II a nuclear envelope forms around each set of chromosomes cytokinesis occurs  Meiosis Contributes to Genetic Variation Independent Assortment – the random distribution homologous chromosomes during meiosis – each of the 23 pairs of chromosomes separates independently – 223 gametes (about 8 million) with different gene combinations can be produced from one original cell Crossing-over and Random Fertilization – the DNA exchanged during crossing-over adds even more recombination to independent assortment – random joining of two gametes during fertilization squares the number of possible outcomes (223 x 223 = 64 trillion)  Importance of Generating Variation DON’T WRITE THESE DOWN, JUST LISTEN the pace of evolution increases as the level of genetic variation increases as the existing genetic combinations become used up, the ability to obtain a desired trait (such as larger animals) slows down. further progress must wait for the formation of new gene combinations genetic diversity is the raw material for evolution  Gamete Formation in Male and Female Animals gametogenesis – meiosis is the primary event in the formation of gametes Meiosis in Males spermatogenesis – the process by which sperm are produced – occurs in the testes (male reproductive organs) – a diploid cell increases in size and becomes a large immature cell (germ cell) – the large cell undergoes meiosis to form four haploid cells sperm – the four cells change in form and develop a tail  Meiosis in Females oogenesis – the process by which an ovum (egg) is produced – occurs in the ovaries (female reproductive organs) – during cytokinesis following meiosis I, the cytoplasm divides unequally one of the resulting cells gets nearly all of the cytoplasm and will ultimately give rise to an egg cell – the other cell, called a polar body, will not survive – in meiosis II, the cytoplasm is again divided unequally the larger cell develops into an ovum (plural ova) spermatogenesis & oogenesis