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Meiosis Formation of Gametes (Eggs & Sperm)

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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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important terminology
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
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
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
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 meiosis1
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
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
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.
replication of chromosomes
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

a replicated chromosome
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.

meiosis forms haploid gametes
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 two part cell division
Meiosis I

MeiosisII

Meiosis: Two Part Cell Division

Sister

chromatids

separate

Homologs

separate

Diploid

Diploid

Haploid

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