Sperm & Eggs & Variation..OH MY! 1

 What if a new individual was formed through mitosis? 2

 allele  amniocentesis  asexual reproduction  autosome  binary fission  chorionic villi sampling  crossing over  diploid  exon  fertilization  gamete  genome  haploid  homologues  intron  karyotype  nondisjunction  placenta  reduction division  sexual reproduction  zygote 3

 Genome: Complete complement of an organism’s DNA.  includes genes (control traits) and non-coding DNA organized in chromosomes 4

 Eukaryotic DNA is organized in chromosomes  genes have specific places on chromosomes  exon: portion of a gene that is translated into protein  intron: non-coding segment of DNA, often found within an exon; removed before transcription 5

 Heredity – way of transferring genetic information to offspring  Chromosome theory of heredity: chromosomes carry genes  Gene – “unit of heredity” 6

 Asexual  splitting  budding  parthenogenesis (egg develops w/o fertilization) occurs naturally in some invertebrate animal species (e.g., water fleas, aphids, nematodes, some bees, some scorpion species, and parasitic wasps) and a few vertebrates (e.g., some fish, amphibians, reptiles, and very rarely birds) 7

 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. 8

 Karyotype:  ordered display of an individual’s chromosomes  collection of chromosomes from mitotic cells  staining can reveal visible band patterns, gross anomalies  Make a Karyotype Make a Karyotype 9

 Obtain some cells from the individual  Culture them in a test tube with nutrients  Treat cells w/chemical that stops them exactly midway through cell division (chromosomes are coiled thickly and more visible than usual)  Cells are placed on a microscope slide and a stain is added that binds to the chromosomes, making them visible  Chromosomes are arranged by size and shape and displayed on a monitor or in a photograph 10

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 Chromosomes exist in homologous pairs in diploid (2n) cells.  One chromosome of each homologous pair comes from the mother (called a maternal chromosome) and one comes from the father (paternal chromsosome).  Homologous chromosomes are similar but not identical. Each carries the same genes in the same order, but the alleles (alternative form of a gene) for each trait may not be the same.  Exception: sex chromosomes (X, Y) 14

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 23 chromosomes donated by each parent (total = 46 or 23 pairs).  Gametes (sperm/ova):  contain 22 autosomes and 1 sex chromosome  haploid (haploid number “n” = 23 in humans)  Fertilization results in zygote with 2 sets of chromosomes - now diploid (2n).  Most cells in the body produced by mitosis.  Only gametes are produced by meiosis. 16

All are even numbers – diploid (2n) sets of homologous chromosomes. 17

 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)  metaphase I: homologous pairs line up at metaphase plate  anaphase I: sister chromatids do NOT separate  overall, separation of homologous pairs of chromosomes, rather than sister chromatids of individual chromosome 18

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 Prophase 1  each chromosome duplicates and remains closely associated (sister chromatids)  crossing-over can occur during the latter part of this stage  Metaphase 1  homologous chromosomes align at the equatorial plate 23

 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 24

Second division of meiosis: Gamete formation  Prophase 2  DNA does not replicate  Metaphase 2  chromosomes align at the equatorial plate 25

 Anaphase 2  centromeres divide  sister chromatids migrate separately to each pole  Telophase 2  cell division is complete  4 haploid daughter cells 26

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 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 29

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Number of combinations: 2 n e.g. 2 chromosomes in haploid 2n = 4; n = 2 2 n = 2 2 = 4 possible combinations 31

e.g. 23 chromosomes in haploid 2n = 46; n = 23 2 n = 2 23 = ~ 8 million possible combinations! 32

Chiasmata – sites of crossing over, occur in synapsis. Exchange of genetic material between non-sister chromatids. Crossing over produces recombinant chromosomes. 33

Females don’t have a Y chromosome in any of their cells, yet they are able to develop and live normal, healthy lives. For this reason, we know that nothing on the Y chromosome is absolutely necessary. 34

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At least 8 million combinations from Mom, and another 8 million from Dad … >64 trillion combinations for a diploid zygote!!! 36

 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 37

Generalized animal life cycle 38

Unequal distribution of chromosomes during meiosis Resulting gametes zero or two copies of a chromosome instead of a single copy 39

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 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, syphilis, 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. 42

 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 43

 In what cellular processes is mitosis involved? In what cellular processes is meiosis involved?  In what type of cells does mitosis occur? In what type of cells does meiosis occur?  How many times does DNA replicate in mitosis? How many times does DNA replicate in meiosis?  How many cellular divisions occur in mitosis? How many cellular divisions occur in meiosis? 44

 How many daughter cells are formed by mitosis? How many daughter cells are formed by meiosis?  What is the chromosome number in daughter cells formed by mitosis from diploid parent cells? What is the chromosome number in daughter cells formed by meiosis from diploid parent cells?  In mitosis, are daughter cells identical to or different from parent cells? In meiosis, are daughter cells identical or different from parent cells?  In mitosis, when do synapsis and crossing over occur? In meiosis, when do synapsis and crossing over occur? 45

 cchs.churchill.k12.nv.us/marshk/Notes/meio sis.ppt  Genetic Science Learning Center, University of Utah,  place/labbench/lab3/concepts2.html place/labbench/lab3/concepts2.html 46