Review of Terms Used in Meiosis Meiosis  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 haploid Autosomes and sex chromosomes karyotype

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, 2 23 combos Fertilization  male and female gamete fuse, (2 23 ) 2, Plus 1 cross over: (4 23 ) 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: reduction Telophase I-no formation of nuclear envelope, cell separates by cytokinesis Interkinesis  similar to interphase, no DNA replication

Meiosis II: division Prophase 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 II Telophase II-nuclear envelope forms, cytokinesis results in 4 distinctly unique new cells Animals  gametes Plants  spores Plants have an Alternation of Generations: multicellular haploid and diploid forms for plants

Mitosis vs Meiosis

Meiosis= 2 chromosomal divisions; Mitosis= 1 Meiosis=4 daughter nuclei; Mitosis=2 Following 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  ovaries Life cycle of plants and animals consists of both mitosis and meiosis Sperm + egg  zygote  Adult  gametes

Spermatogenesis Formation of sperm through meiosis Spermatogonium  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. Meiosis Mitosis somatic cells growth and repair diploid  diploid 1 cell  2 cells same DNA as parent cell 1 set of divisions (1 PMAT) Meiosis germ (sex) cells gamete formation diploid  haploid 1 cell  4 cells DNA different than parent cell 2 sets of divisions (2 PMATs)

Changes in Chromosome Numbers Nondisjunction-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 mild  severe 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 500  1000 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 Structure Change in physical structure of chromosome Rare spontaneous occurrence Can lead to genetic disorder or abnormality Can 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 Syndromes Williams 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 Syndromes Alagille 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