Obectives Describe the result of meiotic division in terms of sexual reproduction Discuss the structure of homologous chromosomes Describe chromosomes in terms of ploidy Distinguish between sexual and asexual reproduction ID: 340527
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Meiotic Cell DivisionSlide2
Obectives
Describe the result of meiotic division in terms of sexual reproduction
Discuss the structure of homologous chromosomes
Describe chromosomes in terms of ploidy
Distinguish between sexual and asexual reproduction
Discuss genetic variationSlide3
Introduction
Chromosomes occur in pairs
Each chromosome may contain ~ 1000 genes
Diploid
cells contain two of each kind of chromosome (2n)
Somatic or body cells (46 chromosomes in humans)
Produced by mitosis
Haploid
cells contain one of each kind of chromosome (n)
Gametes or sex cells (23 chromosomes in humans)
Produced by meiosisSlide4
Introduction
Homologous Chromosomes
(Homolog's)
“Homo” Greek word for the same
Represents the two chromosomes of each pair in a diploid cell
Each homologous pair has genes for the same trait
Tall or shortHomolog's are not always identicalSlide5
Introduction
Why is meiosis needed to produce gametes?
Meiosis
– form of cell division which produces ½ the number of chromosomes as a somatic cell
Meiosis divided into two phases
Meiosis I begins with one diploid cell
Meiosis II ends with four haploid gametesGametes must be haploid in order to continually produce a diploid zygote
Sperm (n) + Egg (n) = Zygote (2n)Slide6
IntroductionSlide7
Introduction
Sexual reproduction
Production and fusion of haploid gametes
n + n = 2n
Genetic information is exchanged
Asexual reproduction
Single parent produces one or more identical offspring by dividing into two cellsNo exchange of genetic material
Binary Fission,
Parthenogenesis
Budding, Fragmentation Slide8
IntroductionSlide9
IntroductionSlide10
IntroductionSlide11
IntroductionSlide12
Phases of Meiosis
Interphase
Cell replicates its chromosomes
Each chromosome consists of two sister chromatidsSlide13
Phases of Meiosis
Prophase I
DNA coils as the spindle forms
Two homologous chromosomes line up gene by gene to form a
tetrad
Tetrad forms so tightly that
crossing over occursExchange of genetic materialCan occur at any location along the chromosome
Results in new combinations of
allelesSlide14
Phases of MeiosisSlide15
Phases of Meiosis I
Metaphase 1
Centromere of each chromosome becomes attached to a spindle fiber
Tetrads move to the equatorial plane of the spindle
Unique to Meiosis
Anaphase I
Homologous chromosomes separate and move to opposite ends of the cellCentromeres holding sister chromatids together do not splitSlide16
Phases of Meiosis
Telophase 1
Spindle breaks down
Chromosomes
uncoil
Nucleus Reappears
Cytoplasm dividesOne more cell division is needed b/c each chromosome is still doubled (
2 sister chromatids)Slide17
Phases of Meiosis ii
Mitotic division of products of Meiosis I
Prophase 2
Spindle forms in each cell
Nucleus disappears
Chromatin coils into chromosomes
Centrioles migrate to opposite sides of cellSlide18
Phases of Meiosis II
Metaphase 2
Chromosomes line up on the equatorial plane of the
spindle
Each centromere connected to two spindle fibers
Anaphase 2
Centromeres splitChromatids separateChromatids migrate to opposite ends of the cell
Telophase 2
Nuclei reform
Spindle breaks down
Cytoplasm
divides
Chromosomes uncoil
into chromatinSlide19
Phases of Meiosis II
End result of Meiosis
Four haploid cells formed from one diploid cell
Four haploid cells become gametesSlide20
Genetic Variability
Genetic Recombination
– re-assortment of chromosomes and the genetic information they carry
Independent segregation
and
crossing over
increase genetic variability and drive evolutionSlide21
Genetic Variability
Genetic Recombination
Independent Segregation
Gene combinations vary depending on how each pair of homolog's lines up during Metaphase 1
Random Process
Number of combinations increases as chromosome increases
Each 23 pairs of chromosomes may align independently in a gamete2
23
= 8 million types of egg or sperm a person can produce
When fertilization occurs 2
23
x 2
23
= 70 trillion possible zygote combinationsSlide22
Genetic Variability
Genetic Recombination
Crossing over
May occur at any location when tetrads are formed
Variation is the raw material that forms the basis of evolutionSlide23
Genetic VariabilitySlide24
Genetic VariabilitySlide25
Genetic Variability
Nondisjunction
Failure of homologous chromosomes to separate properly during meiosis
Four basic types
Trisomy
One gamete with an extra chromosome
One gamete missing a chromosomen + 2n = 3n
Trisomy 21 (down syndrome)
Gamete with an extra chromosome is fertilized by a normal gamete
Resulting zygote has 47 chromosomesSlide26
Genetic VariabilitySlide27
Genetic Variability
Nondisjunction
Monosomy
Gamete missing a chromosome fuses with a normal gamete
0 + n = n
Most zygotes with monosomy do not survive
Turner syndromeHuman females have only one X chromosome instead of twoSlide28
Genetic VariabilitySlide29
Genetic Variability
Nondisjunction
Tetraploidy
Fusion of gametes, each with a complete set of chromosomes
2n + 2n = 4n
Results from a total lack of separation of homologous chromosomes
Common in plantsChrysanthemum Slide30
Genetic Variability
Nondisjunction
Polyploidy
Organisms with more than usual number of chromosomes
Rare in animals, usually results in death of zygote
Frequently occurs in plants
Errors in meiosis can be beneficial for agriculture6n Wheat3n Apples
Resulting plants are usually larger, healthier and more disease resistantSlide31
Genetic Variability