Review of Terms Used in MeiosisMeiosis nuclear cell
division in sexually reproducing organisms
Sexual reproduction involves gamete and zygote formation
Chromosomes and homologous chromosomes
Chromatids and sister chromatids
Genes (DNA) and alleles, locus
Diploid and haploidAutosomes and sex
chromosomeskaryotype
Meiosis-Main Source of Genetic Variability• Formation of gametes (eggs and sperm)• Sexual reproduction (Why?)• Fertilization-union of sperm nuclei with egg
nuclei=diploid zygote (fertilized egg)• Produces new cells with different DNA than
parent cell• The gametes formed are haploid, they
contain half as much DNA as the parent cell
Brief Overview of Meiosis• Chromosome movement
and division of cells is just like mitosis
• Diploid germ cell starts off in interphase, DNA duplicated in S phase
• 2 sets of divisions (2 PMATs) called Meiosis I and II
• No DNA duplication after Meiosis I
• Homologous chromosomes separate in Meiosis I
• Sister chromatids separate during Meiosis II
Genetic Variation
Crossing Over-exchange of genetic material between nonsister chromatids of homologous chromosomes prophase
Independent assortment (random alignment) metaphase, 223 combos
Fertilization male and female gamete fuse, (223)2,
Plus 1 cross over: (423)2
different zygotes = 5 and 27 zeros
Meiosis I: reduction DNA duplicated in Interphase (S) Prophase I-chromosomes condense, centrioles duplicated and
migrate to opposite poles, microtubules grab on to chromosomes, nuclear envelope disappears, crossing over occurs
Metaphase I-microtubules align chromosomes at equator; chromosome organization is called Independent Assortment, the 2 kinetochores of the duplicated chromosome are attached to the same kinetochore spindle fiber
Anaphase I- spindle apparatus (microtubules contracting) separates homologous chromosomes; each chromosome still has 2 chromotids
Meiosis I: reductionTelophase I-no
formation of nuclear envelope, cell separates by cytokinesis
Interkinesis similar to interphase, no DNA replication
Meiosis II: divisionProphase II-centrioles
duplicate, move to opposite pole, microtubules grab chromosomes
Metaphase II-chromosomes align at metaphase plate, like mitosis except….?
Anaphase II-spindle apparatus pulls sister chromatids apart
Meiosis IITelophase II-nuclear
envelope forms, cytokinesis results in 4 distinctly unique new cells
Animals gametesPlants sporesPlants have an
Alternation of Generations: multicellular haploid and diploid forms for plants
Mitosis vs MeiosisMeiosis= 2 chromosomal divisions; Mitosis= 1Meiosis=4 daughter nuclei; Mitosis=2Following meiosis, 4 daughter cells are haploid
(1/2 chromosome # of parent cell; following mitosis the daughter cells have same chromosome #
Following meiosis daughter cells are genetically distinct form each other and parent cell; following mitosis daughter cells are genetically identical to each other and parent cell.
Where are Gametes Produced? In Humans:Sperm testis, eggs
ovaries In Plants:Sperm anthers, eggs
ovariesLife cycle of plants and
animals consists of both mitosis and meiosis
Sperm + egg zygote Adult gametes
SpermatogenesisFormation of sperm
through meiosisSpermatogonium
primary spermatocyte secondary spermatocyte spermatids sperm
Produces 4 genetically distinct gametes
Sperm production begins with male puberty and continues
Oogenesis Formation of eggs through
meiosis Oogonium primary oocyte
secondary oocyte + 1 polar body ovum + 3 polar bodies
Polar bodies are smaller than ovum and are the result of unequal cytoplasmic division
Egg production in utero (fetus has many primary oocytes), mature at puberty
Secondary oocyte arrests at metaphase 2, leaves ovary, enters oviduct
Entry of sperm triggers completion of meiosis II
Why must an egg be large?
Mitosis vs. MeiosisMitosissomatic cellsgrowth and repairdiploid diploid1 cell 2 cellssame DNA as
parent cell1 set of divisions (1
PMAT)
Meiosisgerm (sex) cellsgamete formationdiploid haploid1 cell 4 cellsDNA different than
parent cell2 sets of divisions (2
PMATs)
Changes in Chromosome NumbersNondisjunction-one
or more pairs of chromosomes fail to separate during anaphase of mitosis or meiosis
Aneuploidy-one extra or one less chromosome
Polyploidy-having 3 or more chromosomes
If nondisjunction occurs in Anaphase II of meiosis, how many chromosomes will each gamete have?
Changes in Autosome Number: Down’s Syndrome
Nondisjunction in reproductive cells that give rise to gametes
Extra chromosome #21-trisomy visible in karyotype
mildsevere mental retardation, heart defects, abnormal skeletal development
1 out of 1100 births Increased risk as age of
mother at conception increases
Changes in Sex Chromosome Number
Nondisjunction during gamete formation
Turner’s Syndrome-XO, female, short, sterile, nonfunctional ovaries, 1 out of 10,000 newborn girls
Klinefelter Syndrome-XXY, male, taller, sterile, abnormal testis, mild retardation, can be asymptomatic, 1 out of 5001000 newborn boys
Jacob’s Syndrome-XYY, male nondisjunction in sperm, taller, mild retardation, phenotypically normal, asymptomatic, 1 out of 1000 newborn boys
Poly-X females-usually no mental retardation, 1 out of 1500
Barr bodies- inactive X chromosomes
SRY gene – male determining factor
Changes in Chromosome StructureChange in physical structure of chromosomeRare spontaneous occurrenceCan lead to genetic disorder or abnormalityCan be caused by viruses, chemicals,
radiation
Types of Chromosome Structure Changes
Duplication-parts of chromosome repeated many times
Inversion-alteration of position and sequence of genes in a chromosome
Translocation-part of one chromosome is transferred to a non homologous chromosome
Deletion-loss of part of a chromosome
Deletion SyndromesWilliams Syndrome - chromosome 7 loses part of its end
“pixie” looking, turned up noses, wide mouths, small chin, large ears
Cri-du-Chat Syndrome- “cry of the cat” loss of part of chromosome #5 results in abnormal larynx and mental retardation