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  • Consider the greater variation with 23 pairs of chromosomes = mixing and matching
  • Consider the greater variation with 23 pairs of chromosomes = mixing and matching
  • Use 2 n to determine how many different gametes are possible In humans, n=23 so and 2 23 = 8,388,608

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  • Meiosis & Sexual Reproduction
  • Sexual Reproduction►2 parents► Exchange of DNA► Offspring genetically different from parents► Involves specialized sex cells (gametes)► Creates diversity
  • Sexual Reproduction in Simple Organisms ► Conjugation –  Cytoplasm bridge forms between 2 cells and an exchange of DNA takes place  Occurs between two cells with different mating types  Mating types are usually called plus (+) and minus (-)
  • Sexual Reproduction in Animals► Each parent produces a GAMETE  Female gamete – egg  Male gamete - sperm► The two gametes combine and produce a zygote  Called fertilization► Thezygote then divides by mitosis, grows and process begins again  Zygote has the same number of chromosomes as the parents
  • Somatic cells vs. Gametes► Somatic cells are body cells  2 set of chromosomes  Humans have 46 chromosomes or 23 pairs► Gametes are sex cells  1 set of chromosomes  Humans have 23 chromosomes
  • How are gametes produced? ► Eggs & sperm need to be made and joined… ► Do we make egg & sperm by mitosis? No!What if we did, then… 46 + 46 92 egg sperm zygote Doesn’t work!
  • diploid = 2 copies 2n Human female karyotype 46 chromosomes 23 pairs XX
  • diploid = 2 copies 2n Human male karyotype 46 chromosomes 23 pairs XY
  • Paired chromosomes► Homologous chromosomes  both chromosomes of a pair carry “matching” genes ►control same inherited characters ►homologous = same information diploid 2n eye color eye color 2n = 4 (brown?) (blue?) homologous double stranded chromosomes homologous chromosomes
  • How do we make sperm & eggs? ► Must reduce 46 chromosomes → 23  must half the number of chromosomes  haploid zygote46 23 23 meiosis egg 46 2346 23 fertilization sperm gametes
  • Meiosis makes sperm & eggs► 46 chromosomes to 23 chromosomes  half the number of chromosomes 46 23 egg meiosis 46 23diploid sperm haploid
  • Meiosis► Specialcell division in sexually reproducing organisms► Makes gametes  Sperm & egg► Reduction division  Reduce number of chromosomes  2n  1n  diploid  haploid
  • Interphase ► DNA replicates ► Cell prepares for division
  • Prophase I ► Homologous chromosomes pair up and form a tetrad  This is called synapsis ► Crossing over occurs during synapsis  Homologous chromosomes exchange segments of DNA
  • Metaphase I ► Tetrads line up at the equator. ► Centromeres attach chromosomes to the spindle fibers
  • Anaphase I ► Homologous chromosomes separate and move to the poles  Called disjunctions  Sister chromatids remain attached
  • Telophase I ► Cytoplasm divides, forming two new daughter cells  Cells are haploid
  • Cytokinesis I► Occurs simultaneously with telophase I  Forms 2 daughter cells► Plantcells – cell plate► Animal cells – cleavage furrow► There is no further replication of genetic material prior to meiosis II
  • Prophase II ► Spindle forms ► Sister chromatids move toward the equator
  • Metaphase II ► Sister chromatids line up at the equator  Similar to mitosis ► Spindle fibers attach at the centromere
  • Anaphase II ► Sisterchromatids separate and move to opposite poles
  • Telophase II ► Nuclei form at poles and cytokinesis occurs ► After completion of cytokinesis there are 4 daughter cells  Each cell is haploid (n)
  • double Meiosis 1 overview stranded ► 1st Divide 1 division of meiosis Copy DNA Line Up 14 prophase 1 metaphase 1 chromosomes telophase 1 diploid 2n  2 chromosomes  haploid gamete  1n
  • Bye Bye 2 Meiosis 2 overview telophase 2 telophase 1 Line Up 2 4► 2nd division of meiosis  looks like mitosis metaphase 2  2 chromosomes  haploid  1n gametes
  • ► Meiosis Square Dance Let’s Dance!!
  • Sexual reproduction lifecycle  2 copies  diploid  2n 1 copy  1 copy fertilization meiosis haploid  haploid 1n  1n We’re mixing things up here! A good thing?
  • Putting it all together…meiosis → fertilization → mitosis + development gametes46 23 46 23 46 46 meiosis egg 46 4646 23 46 46 4646 23 zygote fertilization mitosis & mitosis sperm development
  • The value of meiosis 1 ► Consistency over time  meiosis keeps chromosome number same from generation to generation from MomMomDad offspring from Dad
  • The value of meiosis 2 We’re mixing things up here! ► Change over time  meiosis introduces genetic variation ►gametes of offspring do not have same genes as gametes from parents ►new combinations of traitsfrom Dad variationfrom Mom offspring new gametes made by offspring
  • Fertilization► Joining of the sperm & egg► Internal Fertilization  Gametes fuse inside the body of a female  Occurs in many land animals  Offspring have a better chance of survival  Fewer offspring are produced► External Fertilization  Gametes fuse outside the body of a female  Occurs in many water animals  Many offspring produced  Fewer offspring survive
  • Fertilization► Parthenogenesis  Development of an unfertilized egg into an adult animal  E.g. rotifers, some insects such as bees, wasps,  E.g. Queen bee can produce fertilized or unfertilized eggs. ►Fertilized eggs become female workers ►Unfertilized eggs become male drones
  • Independent Assortment► Chromosomes are separated and combined independently of each other during meiosis  E.g. all your mom’s chromosomes don’t end up together in your gametes. They are randomly mixed.
  • Independent Assortment► Independent assortment produces many different gametes  In humans, n=23 so over 8 million different gametes can be produced…
  • Crossing Over► Occurs in prophase I, during synapsis► Homologous chromosomes twist and exchange pieces of genetic material► This results in even more different gametes
  • The Key Difference Between Mitosis and Meiosis isthe Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis
  • Problems in Meiosis► Sometimes chromosomes don’t separate properly  This is called nondisjunction and result in a cell with too many or too few chromosomes
  • Problems in Meiosis
  • Problems in MeiosisShould the gamete with thechromosome pair be fertilizedthen the offspring will not be‘normal’.In humans this often occurswith the 21st pair – producinga child with Downs Syndrome
  • Trisomy 21– Downs Syndrome Can you see the extra 21st chromosome? Is this person male or female? •This picture of a person’s chromosomes is called a karyotype. The chromosomes are organized from largest to smallest. •How can karyotypes be helpful in identifying diseases?
  • Gametogenesis► Gametes develop within specialized organs  Female gametes – egg cells  Male gametes – sperm cells  Female gonads – ovaries  Male gonads - testes
  • Gametogenesis►Gametogenesis – the formation of gametes in the gonads  Two types: ►Oogenesis – formation of eggs by meiosis ►Spermatogenesis – formation of sperm by meiosis
  • Spermatogenesis► Produces 4 haploid sperm
  • Oogenesis► Produces 1 haploid egg and 3 haploid polar bodies
  • Hermaphrodites► Hermaphrodites have both testes and ovaries  E.g. earthworm, snails, and hydra