chromosomes n.
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  1. Chromosomes • Chromosomes are located in the _____ of the cell • Each chromosome is made up of segments called genes • Genes produce proteins that determine characteristics and functions of the cell.

  2. Homologous Chromosomes • Humans have 46 chromosomes • 23 from each parent • These are called homologous chromosomes because: • They are the same length • Have the centromere in the same location • Carry genes for the same trait • Same trait different characteristics

  3. Haploid and diploid cells • Every species has a different number of chromosomes • The letter n is used to represent the number of chromosomes in general terms • A gamete with half the number of chromosomes is called haploid and is represented by n • When two haploid cells come together during fertilization a diploid cell is formed and is represented by 2n

  4. Sex cells • Every cell contains a full set of chromosomes • Only half of these chromosomes are passed on to the offspring • Special cells called gametes (sex cells) are produced containing ½ of the original number of chromosomes. • Example: a human has 46 chromosomes in a normal cell. A gamete or sex cell only has 23 chromosomes

  5. The formation of gametes • Is called meiosis I and II • Meiosis is cell division that reduces the number of chromosomes and occurs only in the reproductive organs of organisms that reproduce sexually

  6. Phases of Meiosis I • Interphase – same as regular cell cycle • Prophase I - 1 cell • Pairing of homologous chromosomes • Crossing over – segments of homologous chromosomes are exchanged to increase genetic variation (not like mitosis) • Nuclear envelope breaks down • Spindles form

  7. Phases of Meiosis I • Metaphase I • Homologous chromosomes line up on the equator Spindle fibers attach to centromere • Anaphase I • Homologous chromosomes separate and move to opposite sides of the cell • Telephase I - 2 cells • Spindle fibers break down • Chromosomes uncoil • Two nuclei form • Cell divides (not like mitosis)

  8. Meiosis I is very much like mitosis • If it were a human the cell would have 46 chromosomes at the end of meiosis I • Cell may go through interphase after meiosis I but the DNA will NOT be replicated

  9. Meiosis II • Prophase II – 2 cells • Chromosomes condense • Spindle fibers form and attach to the centromere • Metaphase II • Chromosomes line up on the equator • Anaphase II • Sister chromatids are separated and moved to opposite sides of the cell

  10. Telephase II – 4 cells • Four nuclei form • Spindle fibers break down • Cells divide • Meiosis II is very much like mitosis • If it were a human the cells would have 23 chromosomes at the end of meiosis II

  11. Complete the following questions on white lined paper – write the question • Define gene • List the stages of meiosis I • Compute the number of chromosomes that the gametes of a cat (2n = 38) will have. Show your work • Compare and contrast anaphase I and anaphase II • Devise a theory that explains why the most complex animals reproduce sexually • Hand in your paper

  12. Diagram of Meiosis Start of Meiosis I 1 cell 2n chromosomes 46 End of Meiosis I 2 cells 2n chromosomes 46 46 Meiosis II 4 cell n chromosomes each 23 23 23 23

  13. 3 Ways Genetic Variation Occurs: 1. Independent Assortment– Random distribution of homologous chromosomes during meiosis. 2. The DNA exchange that occurs during crossing-overincreases variation. 3. Random Fertilization-The joining of 2 gametes is produced independently.

  14. Meiosis & Gamete Formation • In males this is called spermatogenesis. See p.148. • In females this is called oogenesis.

  15. Questions 1.What is the significance of meiosis in sexual reproduction? 2. How is gamete formation in male different in gamete formation in females? 3. Devise a theory that explains why the most complex animals reproduce sexually.

  16. Types of Reproduction • Asexual – A single parent passes copies of all of its genes to each of its offspring. The offspring are clones. • Ex. prokaryote –binary fission, like bacteria and amoebas • Many eukaryotes also asexually reproduce by fragmentation- body breaks into pieces. • Hydras and yeast undergo budding, new offspring break off from parent.

  17. Advantages to asexual reproduction: • Can produce many offspring in a short period of time. • Spend no energy on mating • Do not have to waste time looking for a mate.

  18. Sexual – There are two parents each donate half of their chromosomes (haploid) and give rise to a diploid offspring. • Each offspring has half the parents genes. • Ex. Human • Dogs • Protists can reproduce both ways.

  19. Advantages to sexual reproduction: • Can make many variations. • Genetic diversity increases organisms survival. • You can repair chromosomal damage.

  20. Questions • Identify the type of reproduction that results in offspring that are genetically identical to their parents? • Describe two types of eukaryotic reproduction?

  21. Haploid Life Cycle • The entire span in the life of an organism from one generation to the next. • The haploid cell fuses with another gamete and is diploid, it immediately undergoes meiosis and begins its haploid life cycle.

  22. Diploid Life Cycle • Diploid life cycle- The first cell of a gamete is called a germ cell. It is diploid and undergoes meiosis to become haploid. • Also when fertilization occurs the zygote is a diploid which undergoes mitosis.

  23. Alternation of Generation • Plants, algae, and some protists that regularly alternate between haploid and diploid phase. • Gametophyte (in a plant) is the haploid phase that produces gametes by mitosis. Two gametes fuse that give rise to a diploid.

  24. A sporophyte is a spore forming cell. Sporophyte is a Diploid undergoing meiosis to become haploid and capable of becoming an adult without fusing with another cell.