Spermatogenesis, Oogenesis and Ploidy
- 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
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
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
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
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
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)
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
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
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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