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Meiosis. Creation of Gametes. Asexual reproduction yields two identical organisms (with exception of mutations) Sexual reproduction merges the genes of the two parents Result: two versions of each chromosome Called homologous chromosomes Organism is diploid (2n=46)

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creation of gametes
Creation of Gametes
  • Asexual reproduction yields two identical organisms (with exception of mutations)
  • Sexual reproduction merges the genes of the two parents
  • Result: two versions of each chromosome
    • Called homologous chromosomes
  • Organism is diploid (2n=46)
  • All somatic cells are diploid
slide4

First 22 are autosomes

  • Chromosome 23 is the sex chromosome
  • Haploid (n=23) sex cells merge to create a zygote in sexual reproduction
  • Meiosis creates these sex cells (gametes)
  • The overall process is the sexual life cycle
slide6

Interphase I of Meiosis

Homologous chromosomes

Chromosomes replicate

Homologous chromosomes separate

Meiosis I

Meiosis II

Sister chromatids separate

interphase i
Interphase I
  • Similar to mitosis, chromatin replicates
  • Centrioles replicate, proteins form around to make the centrosomes
    • Also called the microtubule organizing centre
    • Collectively, with microtubules, is called mitotic spindle
prophase i
Prophase I
  • Longest part of meiosis
  • Chromatin condenses, chromosomes become visible
    • Homologous chromosomes come pair up, forming a tetrad – called synapsis
    • The chromatids can overlap, called a chiasmata
  • All other features are similar to mitosis
metaphase i
Metaphase I
  • Almost identical to metaphase in mitosis, with the tetrad exception
anaphase i
Anaphase I
  • Sister chromatids remain attached, tetrads separate (cells are now haploid)
telophase i
Telophase I
  • Homologous chromosomes are at opposite poles of cell
    • Each end is now haploid
  • Cytokinesis takes place simultaneously
  • Some cells proceed to interphase II, others directly to prophase II
  • No cells replicate chromatin again
prophase ii
Prophase II
  • Spindle forms again, chromosomes move towards equator
metaphase ii
Metaphase II
  • Closely resembling mitosis, metaphase plate is formed
anaphase ii
Anaphase II
  • Sister chromatids finally separate
telophase ii
Telophase II
  • Mitotic spindle breaks down, nucleus reforms around two daughter nuclei
errors
Errors
  • Some of the processes can go awry
  • If anaphase I or II does not proceed correctly, the chromosomes may not be shared evenly
  • This is called nondisjunction
  • More of a problem if it occurs during meiosis I
  • Can take place in mitosis, but not as much of a concern
slide20

An abnormal chromosome number is called aneuploidy

  • If faulty gamete succeeds in fertilization, the offspring with have health consequences
  • If a gamate with an extra chromosome fertilizes, the zygote is trisomic (2n+1)
  • If the gamete has a chromosome missing, the zygote is monosomic (2n-1)
  • Ex. Extra chromosome 21 – Down syndrome
slide22

If the extra chromosome is an X in a male (XXY), called Klinefelter syndrome

  • The individual will have male sex organs, but the testes will be abnormally small, and he will be sterile
  • As well, femine characteristics such as breast enlargement is common
slide24

Final example is Turner syndrome

  • Example of monosomyX
  • Individuals are classified X0, and are female
  • However, sex organs do not mature at adolescence and secondary sex characteristics fail to develop
polyploidy
Polyploidy
  • Also possible for anaphase I or II to completely fail and all chromosomes move to one cell
  • Referred to as polyploidy
  • Could have three chromosome sets (triploidy, 3n) or four (tetraploidy, 4n)