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Meiosis Formation of Gametes (Eggs & Sperm). Important Terminology. Diploid (2N): “two sets”; a cell that has 1 set of chromosomes from its female parent and 1 set from its male parent (=2 copies of each chromosome)

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Meiosis Formation of Gametes (Eggs & Sperm)

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MeiosisFormation of Gametes(Eggs & Sperm)


Important Terminology

  • Diploid (2N): “two sets”; a cell that has 1 set of chromosomes from its female parent and 1 set from its male parent (=2 copies of each chromosome)

  • Homologous: each chromosome from one parent has a corresponding chromosome from the other parent.

  • Haploid (N or 1N):

    • the number of chromosomes in a gamete (sex cell – egg or sperm)

    • Only one copy of each chromosome


Polypoidy

  • What would this mean???

  • More than diploid (more than 2 sets of chromosomes)

    • Triploid (3N)

    • Tetraploid (4N)

  • Not viable (“survivable”) in humans

  • Seedless watermelons are triploid, formed from a cross between diploid and tetraploid watermelons.

  • Because the 3 copies of the chromosomes won’t line up correctly during meiosis, meiosis is halted, so no gametes (seeds) are formed.


Definition of Meiosis

  • The process of reduction division in which the number of chromosomes per cell is cut in half (reduction) through the separation (division) of homologous chromosomes in a diploid cell

  • Homologous chromosomes (“homologs”): “matched” chromosomes (not necessarily identical); both before and after replication!


Facts About Meiosis

  • Preceded by interphase which includeschromosome replication

  • Twomeiotic divisions --- Meiosis I and Meiosis II

  • Called reduction division

  • Original cell is diploid (2n)

  • Four daughter cellsproduced are monoploid (1n) (AKA, haploid)


Facts About Meiosis

  • Daughter cellscontain half the number of chromosomesas the original cell.

  • Producesgametes(eggs & sperm)

  • Occurs in the

    • testes in males (Spermatogenesis)

    • ovaries in females (Oogenesis)


More Meiosis Facts

  • Human cells start with 46 doublestranded chromosomes (2n).

  • After 1st division - 23 double stranded chromosomes (n) in each of 2 cells

  • After 2nd division - 23 single stranded chromosomes (n) in each of 4 cells

  • Occurs in our germ cells that produce gametes


Why Do We Need Meiosis?

  • It is the fundamental basis of sexual reproduction.

  • Two haploid (1n) gametes are brought together through fertilization to form a diploid (2n) zygote.


Fertilization – “Putting it all together”

2n = 6

1n =3


Replication of Chromosomes

  • Replication is the process of duplicating a chromosome

  • Occurs prior to division (interphase)

  • Replicated copies are called sister chromatids.

  • Held together at centromere

Occurs in Interphase


SisterChromatids

(same genes,

same alleles)

Homologs

(same genes, different alleles)

A Replicated Chromosome

Gene X

Homologs separate in meiosis I and therefore different alleles separate.


from mom

from dad

child

too

much!

meiosis reduces

genetic content

Meiosis Forms Haploid Gametes

  • Meiosis must reduce the chromosome number by half.

  • Fertilization then restores the 2n number.

The right number!


Meiosis I

MeiosisII

Meiosis: Two Part Cell Division

Sister

chromatids

separate

Homologs

separate

Diploid

Diploid

Haploid


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