Meiosis and Genetic Variation 1 Genome Genome Complete

Meiosis and Genetic Variation 1

Genome • Genome: Complete complement of an organism’s DNA. – Includes genes (control traits) and noncoding DNA organized in chromosomes. 2

Genes • Eukaryotic DNA is organized in chromosomes. – Genes have specific places on chromosomes. 3

Heredity • Heredity – way of transferring genetic information to offspring • Chromosome theory of heredity: chromosomes carry genes. • Gene – “unit of heredity”. 4

Reproduction • Asexual – Many single-celled organisms reproduce by splitting, budding, parthenogenesis. – Some multicellular organisms can reproduce asexually, produce clones (offspring genetically identical to parent). 5

Sexual reproduction • Fusion of two gametes 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. 6

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Chromosomes • Karyotype: – ordered display of an individual’s chromosomes. – Collection of chromosomes from mitotic cells. – Staining can reveal visible band patterns, gross anomalies. 8

Karyotyping 9

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Homologues • Chromosomes exist in homologous pairs in diploid cells. Exception: Sex chromosomes (X, Y). Other chromosomes are known as autosomes, they have homologues. 13

In humans … • 23 chromosomes donated by each parent (total = 46 or 23 pairs). • Gametes (sperm/ova): – Contain 22 autosomes and 1 sex chromosome. – Are haploid (haploid number “n” = 23 in humans). • Fertilization/syngamy results in zygote with 2 haploid sets of chromosomes - now diploid. – Diploid cell; 2 n = 46. (n=23 in humans) • Most cells in the body produced by mitosis. • Only gametes are produced by meiosis. 14

Chromosome numbers All are even numbers – diploid (2 n) sets of homologous chromosomes! Ploidy = number of copies of each chromosome. Diploidy 15

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. 16

https: //www. youtube. com /watch? v=M Nq 015 d 03 M U 17

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Animation 20

Meiosis 1 First division of meiosis • Prophase 1: All pairs of replicated homologous chromosomes attach at their centromeres in a process called synapsis forming a tetrad. These are called sister chromatids. Crossing-over can occur during the latter part of this stage. • Metaphase 1: Homologous chromosomes align at the equatorial plate. • Anaphase 1: Homologous pairs separate with sister chromatids remaining together. • Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair. 21

Meiosis II Second division of meiosis: Gamete formation • Prophase 2: DNA does not replicate. • Metaphase 2: Chromosomes align at the equatorial plate. • Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole. • Telophase 2: Cell division is complete. Four haploid daughter cells are obtained. 22

Nondisjunction leads to abnormal chromosome numbers in gametes 23

Mitosis vs. meiosis Meiosis KM 24

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Meiosis creates genetic variation • During normal cell growth, mitosis produces daughter cells identical to parent cell (2 n to 2 n) • 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. 26

Independent assortment 27

Independent assortment Number of combinations: 2 n https: //ww w. youtube. c om/watch? v =c 5 h. A 0 WC v 1 lg e. g. 2 chromosomes in haploid 2 n = 4; n = 2 2 n = 22 = 4 possible combinations 28

In humans e. g. 23 chromosomes in haploid 2 n = 46; n = 23 2 n = 223 = ~ 8 million possible combinations! 29

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 30

Harlequin chromosomes 31

Random fertilization At least 8 million combinations from Mom, and another 8 million from Dad … >64 trillion combinations for a diploid zygote!!! 32

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 Generalized animal life cycle meiosis. 33

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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. 36

But … • More genetic diversity: more potential for survival of species when environmental conditions change. – Shuffling of chromosomes in meiosis (independent assortment) – 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. 37

Zygote Formation and Early Development • Zygote Formation 1. When the nuclei of the egg and sperm meet (fertilization) a zygote is formed. This zygote has the full complement of chromosomes needed by the offspring. 2. Zygote formation is also called recombination because the genes from both parents recombine when fertilization occurs. 3. A genetically unique egg and a genetically unique sperm meet randomly leading to an offspring different from each parent. 4. This variation in offspring plays a key role in evolutionary change and species survival. 5. The zygote divides by mitosis to form a multicellular organism. 38

Early Development • Cells forming by mitotic division begin to specialize in their roles and differentiate in physical appearance from each other. The shape of the cell corresponds to the role of the cell. Meiosis KM 39

Shape Determines Function • Specialized cells form tissues and then organs in a multicellular organism. • In an embryo, all the genetic information starts out the same. Differentiation occurs when different genes are activated or deactivated in certain cells, causing them to make only some of the many proteins they are capable of synthesizing. This activation of only certain genes results in the variety of cells in our body. RBCs carry O 2 Meiosis KM nerve cells- transmit impulses WBCs fight disease 40

The Environment Effects Gene Expression • Environmental conditions in the cell, from surrounding cells or outside the organism, are responsible for affecting the activation or deactivation of certain genes in certain organisms. • When a gene is actively making a certain protein the gene is expressed. • Examples of environment affecting gene expressiona. Low temp. cause black hair growth in the Himalayan rabbit. b. Plants grown in the dark are white not green because sunlight is needed to activate the gene which codes for the synthesis of the protein pigment chlorophyll. 41

Vocabulary • • • centriole centromere: crossing over: gamete: meiosis: zygote: 42

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? 43

• • 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? 44