Unit ThreeUnit Three“Cell Proliferation and “Cell Proliferation and

Genetics”Genetics”

““Meiosis”Meiosis”

History of MeiosisHistory of Meiosis In the mid 1880’s after Walter Fleming In the mid 1880’s after Walter Fleming

discovered chromosomes, Belgian discovered chromosomes, Belgian cytologist Pierre-Joseph van Beneden cytologist Pierre-Joseph van Beneden found that different types of cells from the found that different types of cells from the same organisms had two different same organisms had two different numbers of chromosomesnumbers of chromosomes

He further documented the “Gametes” He further documented the “Gametes” (sperm and egg) each had half the (sperm and egg) each had half the number of chromosomes of the somatic number of chromosomes of the somatic cells (non-sex cells) cells (non-sex cells)

History of MeiosisHistory of Meiosis In 1887, van Beneden proposed an egg In 1887, van Beneden proposed an egg

and sperm fuse to produce a single cell and sperm fuse to produce a single cell called the “Zygote”called the “Zygote”

The Zygote contains two copies of each The Zygote contains two copies of each chromosomechromosome

The fusion of the gametes is called The fusion of the gametes is called “Fertilization” or “Syngamy”“Fertilization” or “Syngamy”

It was clear to van Beneden and his It was clear to van Beneden and his contemporaries some mechanism must contemporaries some mechanism must be involved in reducing the number of be involved in reducing the number of chromosomes by half in preparation for chromosomes by half in preparation for fertilizationfertilization

MeiosisMeiosis Meiosis is the process by which the Diploid Meiosis is the process by which the Diploid

number of chromosomes (2n) is reduced to number of chromosomes (2n) is reduced to the Haploid number of chromosomes (n) in the Haploid number of chromosomes (n) in preparation for fertilizationpreparation for fertilization

Because Meiosis is the halving of Because Meiosis is the halving of chromosomes, it is sometimes referred to as chromosomes, it is sometimes referred to as “Reduction Division”“Reduction Division”

Meiosis is a process that occurs only in Meiosis is a process that occurs only in species that participate in “Sexual species that participate in “Sexual Reproduction”Reproduction”

Species that reproduce asexually via cloning, Species that reproduce asexually via cloning, fission etc. participate in “Asexual fission etc. participate in “Asexual Reproduction”Reproduction”

MeiosisMeiosis Meiosis produces viable sperm and egg Meiosis produces viable sperm and egg

that are ready to participate in fertilizationthat are ready to participate in fertilization Upon fertilization, the single cell that is Upon fertilization, the single cell that is

the result of the sperm and egg the result of the sperm and egg combining, begins to divide mitotically to combining, begins to divide mitotically to produce the entire fully functioning produce the entire fully functioning eukaryotic organismeukaryotic organism

Both somatic cells and primordial gamete Both somatic cells and primordial gamete cells are diploid, however, the sperm and cells are diploid, however, the sperm and egg cells are haploidegg cells are haploid

MeiosisMeiosis Meiosis is divided into: Meiosis is divided into:

– Meiosis I Meiosis I Duplicated chromatin (2 sets of 46 Duplicated chromatin (2 sets of 46

chomosomes) from interphase is divided chomosomes) from interphase is divided between two daughter cells between two daughter cells

– Meiosis IIMeiosis IIEach daughter cell divides further to reduce Each daughter cell divides further to reduce

the diploid # of 46 to the haploid # of 23the diploid # of 46 to the haploid # of 23

Meiosis I and IIMeiosis I and II

How Meiosis differs from MitosisHow Meiosis differs from Mitosis PMAT occurs twice instead of oncePMAT occurs twice instead of once In conjunction with the above statement, In conjunction with the above statement,

the resulting 4 cells in Meiosis have a the resulting 4 cells in Meiosis have a Haploid number of chromosomes, Haploid number of chromosomes, whereas the resulting 2 cells in Mitosis whereas the resulting 2 cells in Mitosis have a Diploid number of chromosomeshave a Diploid number of chromosomes

To encourage variation in individuals of a To encourage variation in individuals of a species, two mechanisms have evolved species, two mechanisms have evolved which are unique to Meiosis:which are unique to Meiosis:– ““Crossing Over”Crossing Over”– ““Independent Assortment”Independent Assortment”

Synapsis & Crossing OverSynapsis & Crossing Over During Metaphase I, two similar sized During Metaphase I, two similar sized

chromosomes with variations of the same gene chromosomes with variations of the same gene line up next to each other forming a “Tetrad”line up next to each other forming a “Tetrad”

The tetrad is made of two sets of “Sister The tetrad is made of two sets of “Sister Chromatids”Chromatids”

When the tetrad has formed during Metaphase When the tetrad has formed during Metaphase I, equal segments of two “Nonsister I, equal segments of two “Nonsister Chromatids” are exchanged amongst each Chromatids” are exchanged amongst each other other

Synapsis & Crossing OverSynapsis & Crossing Over

Independent AssortmentIndependent Assortment Occurs during Meiosis I Occurs during Meiosis I Sister chromatids are pulled to either Sister chromatids are pulled to either

end of the cell in a random fashionend of the cell in a random fashion This situation results in additionally This situation results in additionally

variation in the genetic code each variation in the genetic code each egg or sperm cell containsegg or sperm cell contains

Independent AssortmentIndependent Assortment

Consequences of Meiosis and Consequences of Meiosis and Sexual ReproductionSexual Reproduction

Genetic diversity is favored due to an Genetic diversity is favored due to an ever changing environment with new ever changing environment with new abiotic variables coming to the forefront abiotic variables coming to the forefront constantlyconstantly

The best mode of ensuring diversity is The best mode of ensuring diversity is via sexual reproduction as the result of via sexual reproduction as the result of MeiosisMeiosis

It should be noted, changes in the It should be noted, changes in the genetic code are not always beneficial; genetic code are not always beneficial; they can be harmful toothey can be harmful too

Consequences of Meiosis and Consequences of Meiosis and Sexual ReproductionSexual Reproduction

Main benefit to sexual reproduction Main benefit to sexual reproduction is variation between offspring which is variation between offspring which allows for better chance of survival of allows for better chance of survival of speciesspecies

Main cost is the energy requirement Main cost is the energy requirement of entire process:of entire process:– Nucleic acid and cellular divisionNucleic acid and cellular division– Search for a suitable mateSearch for a suitable mate– Physical act of intercoursePhysical act of intercourse

Sex Determination within Sexual Sex Determination within Sexual ReproductionReproduction

Sex of offspring is determined in a Sex of offspring is determined in a variety of waysvariety of ways– humans: 23humans: 23rdrd pair/46 pair/46thth chomosome chomosome– Reptiles: temperature of nestReptiles: temperature of nest– Insects: number of chromosomes – Insects: number of chromosomes –

haploid or diploidhaploid or diploid

Meiotic AbnormalitiesMeiotic Abnormalities Errors during Meiosis I and II result in Errors during Meiosis I and II result in

an incorrect number of chromosomes an incorrect number of chromosomes - Nondisjunction - Nondisjunction

Turner’s Syndrome, Klinefelter’s Turner’s Syndrome, Klinefelter’s Syndrome, Metafemales, Metamales, Syndrome, Metafemales, Metamales, Trisomy 21/Down Syndrome etc.Trisomy 21/Down Syndrome etc.