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 Meiosis

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

Translocation SyndromesAlagille syndrome

transfer between 2 and 20, distinctive face, abnormalities of the eyes and internal organs

Chronic Myelogeneous Leukemia, transfer between #9 and #22, “Philadelphia Chromosome”

Burkitt lymphoma translocation of chromosome #8 and 14, tumors of the jaw