Meiosis

What is it?

Mitosis occurs, producing two identical daughter cells◦Only eukaryotic cells◦In humans, all cells but one type◦Involve diploid cells (2n)

Humans reproduce sexually◦Sex cells = gametes = sperm & egg

Meiosis is the making of gametes

Where does it happen?Made in reproductive organs

◦Males: testes sperm◦Females: ovaries ova (eggs)

Gametes contain 23 chromosomes ◦Haploid cells (n)

Mitosis involves the halving of the normal chromosome number

Creation of offspring ◦Fertilization

◦23 + 23 = 46◦n + n = 2n

Chromosomes

Humans contain 23 pairs of chromosomes (46 total)

Half from each parentPairs 1 through 22 are homologous pairs

◦Same size◦Carry genes for same traits◦Not identical

AbnormalitiesNondisjunction- “failure to

separate”◦Trisomy: three of one type of

chromosome Trisomy 21: Down Syndrome

◦Monosomy: one of one type of chromosome

Klinefelter’s◦XXY (23rd pair)

Turner’s Syndrome ◦XO (23rd pair)

MeiosisServes to halve the number of

chromosomes, creating gametes◦In humans:

Male gametes – 4 haploid sperm Female gametes – 1 haploid egg, 3 polar

bodies

Meiosis I & Meiosis II

Meiosis I

Prophase I• Chromosome: sister chromatids• Spindle fibers appear around centrioles• Nucleus and nucleolus disappear• Homologous chromosomes synapse (pair up) forming tetrads• Crossing over occurs; tetrads exchange gene segments

Separates homologous pairs

Crossing OverIncreases Genetic Variability

Metaphase I• Tetrads are arranged at the Metaphase plate• Homologues are aligned• Kinetochore fibers are attached to each pair

Anaphase I• Homologous chromosomes are pulled apart by fibers• Moved towards poles

Telophase I• Two daughter cells are formed• Homologous chromosomes are separated - half original number• Not identical• Daughter cells are diploid (2n)

Meiosis IINo duplication of chromosomes

between Meiosis I & IIMeiosis II separates sister

chromatids

Anaphase II Telophase II• Fibers separate sister chromatids and pull them towards poles

• Nuclei reform at poles• Cells are divided

Prophase II Metaphase II• Fibers reform and attach to chromosomes at kinetochores• Centrioles move towards poles

• Chromosomes line up at metaphase plate

Meiosis II ResultsFour daughter cellsContain half as many (n)

chromosomes as parent (2n)Each cell is genetically

different

Fertilization restores chromosome number to 2n

Meiosis in HumansFormation of gametes

◦Males: Spermatogeneis

Meiosis in HumansFormation of gametes

◦Females: Oogeneis

◦Meiosis I 1 diploid cell 1 polar body

◦Meiosis II 1 haploid gamete 3 haploid polar bodies

◦Result 1 egg

Genetic VariationsCrossing over

◦Shuffle genes on chromosomesIndependent assortment of

homologous pairs◦Arrange randomly on metaphase

plate◦223 (or 8 million) possibilities

Random fertilization◦8 million egg x 8 million sperm = 64

trillion zygote