Meiotic Cell Division
Meiotic Cell DivisionObectivesDescribe the result of meiotic
division in terms of sexual reproductionDiscuss the structure of
homologous chromosomesDescribe chromosomes in terms of
ploidyDistinguish between sexual and asexual reproductionDiscuss
genetic variationIntroductionChromosomes occur in pairsEach
chromosome may contain ~ 1000 genesDiploid cells contain two of
each kind of chromosome (2n)Somatic or body cells (46 chromosomes
in humans)Produced by mitosisHaploid cells contain one of each kind
of chromosome (n)Gametes or sex cells (23 chromosomes in
humans)Produced by meiosisIntroductionHomologous Chromosomes
(Homolog's)Homo Greek word for the sameRepresents the two
chromosomes of each pair in a diploid cellEach homologous pair has
genes for the same traitTall or shortHomolog's are not always
identical
IntroductionWhy is meiosis needed to produce gametes?Meiosis
form of cell division which produces the number of chromosomes as a
somatic cellMeiosis divided into two phasesMeiosis I begins with
one diploid cellMeiosis II ends with four haploid gametesGametes
must be haploid in order to continually produce a diploid
zygoteSperm (n) + Egg (n) = Zygote (2n)Introduction
IntroductionSexual reproductionProduction and fusion of haploid
gametesn + n = 2nGenetic information is exchangedAsexual
reproductionSingle parent produces one or more identical offspring
by dividing into two cellsNo exchange of genetic materialBinary
Fission, Parthenogenesis Budding, Fragmentation Introduction
Introduction
Introduction
Introduction
Phases of MeiosisInterphaseCell replicates its chromosomesEach
chromosome consists of two sister chromatids
Phases of MeiosisProphase IDNA coils as the spindle formsTwo
homologous chromosomes line up gene by gene to form a tetradTetrad
forms so tightly that crossing over occursExchange of genetic
materialCan occur at any location along the chromosomeResults in
new combinations of allelesPhases of Meiosis
Phases of Meiosis IMetaphase 1Centromere of each chromosome
becomes attached to a spindle fiberTetrads move to the equatorial
plane of the spindleUnique to MeiosisAnaphase IHomologous
chromosomes separate and move to opposite ends of the
cellCentromeres holding sister chromatids together do not
splitPhases of MeiosisTelophase 1Spindle breaks downChromosomes
uncoilNucleus ReappearsCytoplasm dividesOne more cell division is
needed b/c each chromosome is still doubled (2 sister
chromatids)Phases of Meiosis iiMitotic division of products of
Meiosis IProphase 2Spindle forms in each cellNucleus
disappearsChromatin coils into chromosomesCentrioles migrate to
opposite sides of cell
Phases of Meiosis IIMetaphase 2Chromosomes line up on the
equatorial plane of the spindleEach centromere connected to two
spindle fibersAnaphase 2Centromeres splitChromatids
separateChromatids migrate to opposite ends of the cellTelophase
2Nuclei reformSpindle breaks downCytoplasm dividesChromosomes
uncoil into chromatin
Phases of Meiosis IIEnd result of MeiosisFour haploid cells
formed from one diploid cellFour haploid cells become gametes
Genetic VariabilityGenetic Recombination re-assortment of
chromosomes and the genetic information they carryIndependent
segregation and crossing over increase genetic variability and
drive evolution
Genetic VariabilityGenetic RecombinationIndependent
SegregationGene combinations vary depending on how each pair of
homolog's lines up during Metaphase 1Random ProcessNumber of
combinations increases as chromosome increasesEach 23 pairs of
chromosomes may align independently in a gamete223 = 8 million
types of egg or sperm a person can produceWhen fertilization occurs
223 x 223 = 70 trillion possible zygote combinations
Genetic VariabilityGenetic RecombinationCrossing overMay occur
at any location when tetrads are formedVariation is the raw
material that forms the basis of evolution
Genetic Variability
Genetic Variability
Genetic VariabilityNondisjunctionFailure of homologous
chromosomes to separate properly during meiosisFour basic
typesTrisomyOne gamete with an extra chromosomeOne gamete missing a
chromosomen + 2n = 3nTrisomy 21 (down syndrome)Gamete with an extra
chromosome is fertilized by a normal gameteResulting zygote has 47
chromosomesGenetic Variability
Genetic VariabilityNondisjunctionMonosomyGamete missing a
chromosome fuses with a normal gamete0 + n = nMost zygotes with
monosomy do not surviveTurner syndromeHuman females have only one X
chromosome instead of twoGenetic Variability
Genetic VariabilityNondisjunctionTetraploidyFusion of gametes,
each with a complete set of chromosomes2n + 2n = 4nResults from a
total lack of separation of homologous chromosomesCommon in
plantsChrysanthemum
Genetic VariabilityNondisjunctionPolyploidyOrganisms with more
than usual number of chromosomesRare in animals, usually results in
death of zygoteFrequently occurs in plantsErrors in meiosis can be
beneficial for agriculture6n Wheat3n ApplesResulting plants are
usually larger, healthier and more disease resistantGenetic
Variability
