Lesson 1: Sexual reproduction Fusion of two gametes (sex cells) to produce a single zygote. Introduces greater genetic variation, allows genetic recombination. With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents. Meiosis KM

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The process resulting in the creation of sex cells (GAMETES) MEIOSIS The process resulting in the creation of sex cells (GAMETES) Meiosis KM

Haploid = 1 set of chromosomes Meiosis KM

Chromosome numbers: Ploidy = number of copies of each chromosome. Haploid = 1 set of chromosome Diploid = 2 sets of each chromosome All sexually reproducing organisms usually have even numbers of chromosomes : diploid (2n) sets of homologous chromosomes! All even, as all are diploid, contain pairs of chromosomes. Meiosis KM

Chromosomes exist in homologous pairs in diploid cells. Homologues Chromosomes exist in homologous pairs in diploid cells. Exception: Sex chromosomes (X, Y). Other chromosomes are known as autosomes, they have homologues. Meiosis KM

Sockosome Example Meiosis KM

Homologues Chromosomes in Humans Meiosis KM

In humans cont’d… 23 chromosomes donated by each parent (total = 46 or 23 pairs). Gametes (sperm & ova or egg): Contain 22 autosomes and 1 sex chromosome. Are haploid (haploid number “n” = 23 in humans). Fertilization results in zygote with 2 haploid sets of chromosomes - now diploid. Diploid cell; 2n = 46. (n=23 in humans) Most cells in the body produced by mitosis. Only gametes are produced by meiosis. Meiosis KM

Are There Advantages of Diploidy in Sexually Reproducing Organisms? Back up instructions if a problem arises with the first set Greater diversity of genes that may impact the development of the organism (greater diversity = greater chance for survival in changing environmental conditions – EVOLUTION) Meiosis KM

Roadblock activity… Meiosis KM

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Sockosome Example – Mitosis Vs. Meiosis Meiosis KM

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Line Up and Expect to Be Divided!!! Mitosis and Meiosis Comparison… Meiosis KM

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MEIOSIS II Resembles MITOSIS 2 CELLS (2 N EACH) MEIOSIS II Resembles MITOSIS 4 CELLS (1 N EACH) Meiosis KM

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Sockosome Example – Mitosis Vs. Meiosis Meiosis KM

Animation Meiosis KM

MITOSIS VS. MEIOSIS Meiosis KM

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2N or 4N?? Meiosis KM

Mitosis vs. meiosis Meiosis KM

Meiosis – key differences from mitosis Meiosis reduces the number of chromosomes by half. Daughter cells differ from parent, and each other. Meiosis involves two divisions, Mitosis only one. Meiosis I involves: Synapsis – homologous chromosomes pair up. Chiasmata form (crossing over of non-sister chromatids). In Metaphase I, homologous pairs line up at metaphase plate. In Anaphase I, sister chromatids do NOT separate. Overall, separation of homologous pairs of chromosomes, rather than sister chromatids of individual chromosome. Meiosis KM

MEIOSIS AND VARIATION! During normal cell growth, mitosis produces daughter cells identical to parent cell (2n to 2n) Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over. No daughter cells formed during meiosis are genetically identical to either mother or father During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring. Meiosis KM

Independent assortment Meiosis KM

Independent assortment Number of combinations: 2n e.g. 2 chromosomes in haploid 2n = 4; n = 2 2n = 22 = 4 possible combinations Meiosis KM

In humans e.g. 23 chromosomes in haploid 2n = 46; n = 23 2n = 223 = ~ 8 million possible combinations! Meiosis KM

>64 trillion combinations for a diploid zygote!!! Random fertilization At least 8 million combinations from Mom, and another 8 million from Dad … >64 trillion combinations for a diploid zygote!!! Meiosis KM

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Meiosis KM

Crossing over Chiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids. Crossing over produces recombinant chromosomes. Meiosis KM

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Harlequin chromosomes Meiosis KM

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>>>64 trillion combinations for a diploid zygote!!! Add the variability of crossing over to the variable of all of the different combinations and sexual reproduction equals… >>>64 trillion combinations for a diploid zygote!!! SEXUAL REPRODCTION = DIVERSITY Meiosis KM

Meiosis & sexual life cycles Life cycle = sequence of stages in organisms reproductive history; conception to reproduction. Somatic cells = any cell other than gametes, most of the cells in the body. Gametes produced by meiosis. Generalized animal life cycle Meiosis KM

Meiosis KM

Sex is costly! Large amounts of energy required to find a mate and do the mating: specialized structures and behavior required Intimate contact provides route for infection by parasites (AIDS, syphillis, etc.) Genetic costs: in sex, we pass on only half of genes to offspring. Males are an expensive luxury - in most species they contribute little to rearing offspring. Meiosis KM

But … More genetic diversity: more potential for survival of species when environmental conditions change. Shuffling of genes in meiosis Crossing-over in meiosis Fertilization: combines genes from 2 separate individuals DNA back-up and repair. Asexual organisms don't have back-up copies of genes, sexual organisms have 2 sets of chromosomes and one can act as a back-up if the other is damaged. Sexual mechanisms, especially recombination, are used to repair damaged DNA - the undamaged chromosome acts as a template and eventually both chromosomes end up with the correct gene. Meiosis KM

Problems! Oh No! Meiosis KM

Down's Syndrome Meiosis KM

Meiosis KM

2. How does metaphase of mitosis differ from metaphase I of meiosis? Study Questions 1. What happens as homologous chromosomes pair up during prophase I of meiosis? 2. How does metaphase of mitosis differ from metaphase I of meiosis? 3. What is the sole purpose of meiosis? 4. What specific activities, involving DNA, occur during interphase prior to both mitosis and meiosis? Meiosis KM

5. Compare mitosis and meiosis on the following points: a. number of daughter cells produced. b. the amount of DNA in the daughter cells in contrast to the original cell. c. mechanism for introducing genetic variation. 6.  What is a zygote and how is it formed? 7. Draw a nucleotide and then draw a 10 nucleotide linear sequence of DNA. Meiosis KM

References http://www.google.com/search?q=meisosis+powerpoint&ie=utf-8&oe=utf-8&aq=t&client=firefox-a&rlz=1R1GGGL_en___US350 http://umhs.eduhsd.k12.ca.us/departments/science/freimann/AP%20Bio%20ppt/Meiosis%20AP.pdf Meiosis KM