Spermatogenesis, Oogenesis and Ploidy

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  • Created by: rosieevie
  • Created on: 23-05-17 11:31

Spermatogenesis

Continous once puberty reached = sperm constantly produced until death

Produces 4 identical haploid cells, 8 with mitotic multiplication stage - spermatozoa cannot divide

Spermatids mature to spermatozoa

Significant loss of cytoplasm = little cytoplasmic inheritance from father

Mitochondria in spermatozoa tail - stay behind during fertilisation = no mitchondrial DNA passing on

In tubule, physical division of each developmental stage of sperm - cells move along conveyor belt of development into spermatozoa

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Oogenesis

Oogenesis in females produces ova - not continous

Limited number of oogonia which produce diploid primary oocytes

Oogonis undergo mitosis 6 months after contraception and halt at meiosis diplotene

Remain as primary oocytes until puberty - one follicle matures and completes meiosis 1 and produces secondary oocyte and then ova

Ovum released from follice and travels down fallopian tubes

Fertilisation = second meiotic division - produces 3 polar bodies

Both meiosis 1 and 2 produce polar bodies

Polar bodies - genetically identical to oocytes (minus crossing over) but less cytoplasm - not fertile so discarded

Ova - large cytoplasm store w/ mitochondria  = mirochondrial traits inherited from mother

Female age increase - chance of successful fertilisation and genetic problems increases

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Ploidy

Ploidy - number of homologous chromsome sets present in organism

Genome - basic chromosome number

Diploid - 2 sets of chromosome/genomes, somatic cells

Haploid - 1 set of chromsome/1 genome, gametes

Polyploid - cells containing more than 2 genomes

Tetraploid - contain 4 homologous chromosome sets

Octoploid - contain 8 homologous chromsome sets

Autopolyploid - multiple of identical genome e.g. 4X

Allopolyploid - multiple genomes of different species, created by selective breeding

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Natural Haploidy

Include fungi, bees, wasps, ants

In fungi - cell nuclei divide

In bees:

  • Haploid male drones produce haploit sperm by mitosis
  • Diploid queens produce haploid eggs by meiosis
  • Unfertilised eggs form drones
  • Fertillized eggs form female 
  • Worker or queen, depending on feeding in developmental stage 
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Artificial Haploidy

Rare in animal eukaryotes - die during embryonic development

Induced in plants to produce viable offspring using anther/pollen culture

Plants treated w/ colochicine (disrupts chromosome movement) to induce diploidy for homozygosity and stability in plant breeding

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Aneuploidy

Aneuploidy - changes in number of single chromsomes

Caused by non-disjunction of individual chromosomes = laggards form in meiosis 1 or 2 resulting in aneuploid gametes

Also occur during mitosis

  • -2 = Nullisomic organisms (no homologous)
  • -1 = Monosomic oragnism (1 homologues)
  • 0 (2n) = Disomic organism (normal diploid)
  • +1 = Trisomic organism (3 homologoues)
  • +2 = Tetrasomic organism (4 homologues)
  • +3 = Pentasomic organism (5 homologues

Aneuploidy often lethal while survival common in plants

Monosomics rarely natural but occurs in insect sex chromosmes - females **, males xo e.g. locusts

Downs Syndrome - Trisomy 21 (1/2 normal and 1/2 disomic gametes)

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Euploidy

Euploidy - changes in entire chromosome number (genome)

Rare in animals

Human monoploids never observed, triploids rare, tetraploids even rarer

Common in lizards, amphibians and fish as well as plants

Odd genome sets not maintained over generations because of genetically unbalanced gametes

Euploidy caused by non-disjunction of whole genome/failure of cell division or fertilization by more than one gamete

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Autopolyploidy

Autoployploidy - having multiple of identical genome

Doubling of genome results in autotetraploid (AAAA) cells which form diploid gametes - self-fertilise to form new stable autotetraploid plants

Autotetraploid plants have larger cell size/plants - used in agriculture

Autotriploids - diploid gametes fertilised by haploid gametes or two haploid sperm fertilising an haploid ovum = STERILE

Sterile = no viable seeds = 'seedless' fruits

Seedless fruits - reproduced using vegetative propagation                                                                                                                         

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Allopolyploid

Allopolyploid - have multiple genomes of different species, created by selective breeding

Species AA combines with species BB, resulting in AABB hybrid = allotetraploiod - two complete diploid genomes from known species

Example - wiild American cotton and cultivated Asiatic cotton forming large cotton w/ AABB - used in production

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