gametogenisis - overview
- meiosis is just one stage in the fromation of gametes the whole process is called gametogenisis
- Gamete production in mammals - sperm cells are produced in the testes and are continually produced after puberty, ova are produced in the ovaries and are limeted in number produced during embryonic development
- special cells called the primordial germ cells in the gonads divide, grow, divide again then differentiate.
- the formation of spermatozoa. Each primordial germ cells in the testes results in many sperm cells
- he formation of ova. Each primordial germ cell results in one ovum in the ovary. This small number is because it wouldn't make sense to waste the bodies resources when only one can be fertilized at a time
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- one primordial diploid (2n) germ cell duplicates itself via mitosis, it then goes through a second mitosis to produce 4 diploid (2n) cells - this happens in he overy of the fetus and is called Oogonia.
- a growth period follows during childhood,
- once puberty is reached one of the diploid cells from mitosis a the primary oocyte (2n) goes throught a stage of meiosis, which happens in a cycle to form a 2 secondary oocyte containing 23 chromosomes hapliod (n) as a biproduct one may not go on to divide again in meiosis so its called a polar body.
- during ovulation the second stage of meiosis happens producing 4 daughter cells 3 of which are polar bodies (not used) and die. the is the ovum or female gamete
- the final stage is fertilisation
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- takes place in the testes from puberety onwards
- starts with a primordial diploid germ cell with 48 chromosomes this goes through the a first then second set of mitosis to produce 4 spermatogonia (2n)
- each has the potential to become sperm, they are called primary spermatocytes these go through a growth stage
- once puberty is reached the first stage of meiosis occurs producing two haploid cells (n) then the second stage of meiosis happens producing 4 haplois cells (n) called spermatids each goes on to become sperm or spermatozoa
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mitosis and meiosis plays a role in gametogenesis
- Mitosis provides the precurser cells Oogonia in females spermatogonia in males
- Meiosis brings about the reduction divisions that results in gametes
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spermatozoa : many, mini, motile
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Ova : few, fat, fixed
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Gametogenesis in plants
plants are more complicated as there is two phases to their life cycles :
- the sporophyte generation is diploid and produces spores by meiosis
- the gametophyte generation which is the product of the sporophyte generation is haploid and gives rise to the gametes by mitosis.
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The formation of pollen, the microgamete
- meisis occurs here resulting in vast numbers of pollen grains, that carry the male gametes
- each anther contains four pollen sacs where the pollen grains develope. Each pollen sac there are large numbers of microspores or mother cells - theses are diploid (2n)
- they divide by meiosis to form haploid microspores which are the second gametophyte generation
- gametes themselves are formed by the microspores in mitosis, with one cell enveloping the other to form a pollen grain containing 2 haploid nuclei, the tube nucleus and the generative nucleus
- Tube nucleus has the function of producing pollen tube which penetrate through the overy and down into the ovule.
- the generative nucleus then fuses with the nucleus of the ovule to form a new individual.
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pollen formation microgametogenesis stages
- in the male anther, the pollen mother cell (microspore) is fromed by mitosis it id a diploid cell (2n).
- it goes throught the firsts stage of meiosis giving 2 (n) cells then second stage of meiosis to result in 4 haploid (n)cells. each are a pollen grain.
- from this mitosis happens which didvides the nucleus this results in a pollen gain microspore with a generative nucleus and pollen tube nucleus.
- after pollinisation ( pollen grain on stigam) make two male gametes produced by mitosis of the generative nucleus each haploid (n).
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formation of egg cells megagametes
- meisosi results in a small number of ova contained within the ovules inside the overy
- The ovule is attatched to the wall of the ovary by a pad of special tissue called the placenta.
- structure of integuments (coverings form around the tissue known as nucellus.
- in the centre of the embryo sacs forms the gametophyte generation
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megagametes formation of ova stages
- Diploid megaspore mother cell divide by meiosis to give rise to 4 daughter haploid (n) megaspores
- 3 of which degenerate leaving one to continu to develope
- the mega spore undergoes 3 mitotic divisions which result in an embryo sac containing an egg cell (the megagamete), 2 polar bodies and various other small cells , some of which degenerate
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sexual reproduction fertilisation
- some flowers attract other organisms such as insects, birds or mammels to transfer the pollen from one plant to another called pollination. Others rely on wind to carry pollen from plant to plant
animals - two main categories of fertilisation
- external fertilisation: happens outside the body with gametes shed directly into the environment common in aquatic species due to spermatoza and ova being vulnerable to drying in air. it is very wasteful and not an option for land animals
- internal fertilisation: involves the transfer of male gametes dirctly into the female. Does not guarentee fertilisation but makes it more likely. mating ensures the gametes are placed as close as possible and kept in a moist environment maximising chances of fertilisation.
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Fertilisation in humans
- As the sperm moves through the reproductive track the acrosome region matures so it can release the enzymes used to digest the outer membrane and penetrate the egg
- the ova realeased at ovulation has not fully compleated meiosis and is called a secondary oocyte, its surrounded by a protective jelly like layer known as the zona pellucida and cells called follicle cells.
- sperm cluster around the ovum, as soon as the sperm head touches the ovum surface the acrosome reaction takes place. Acrosomes digestive enzymes, are released which digest the follicle cells and the zona pellucia.
- (one sperm doesn't produce sufficient enzymes to prnetrate protective layer another reason form many sperm)
- eventually one sperm gets throught the weakened protective layer and touches the membrane of the oocyte.
- this causes the oocyte to undergo the secondar meiosis division providing a haploid egg nucleus to fuse with the haploid male nucleus
- to stop multiple fertilization (polyspermy)
- the ion channels in the cell membrane of the ovum op[en and close so inside the cell, instead of being electronegatively charged as usual it becomes positively charged which blocks entery of other sperm - this is tempory until a tough fertilisation membrane forms around the fertilized ovum which take over job of sperm repellent
- the head of the sperm enters oocyte, tail left behind, head absorbs water and swells, releasing its chromosomes to fuse with the ovums chromosomes froming a diploid zygote. called fertlization or conception in humans.
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Stages of human fertilisation
- sperm approaches the oocyte in the ovduct, acrosomes have matured since leaving the male
- the front of the sperm touches the zona pellucida of the oocyte and the acrosomes reaction triggerd release of digestive enzymes to digest the zona pellucida.
- fertilisation occurs when one sperm touches the surface of the oocyte and the membranes fuse (ion channels open causing the oocyte to become positivly charged to repell other sperm preventing polyspermy)
- the sperm nucleus rapdily condenses and releases its chromosmoes into the oocyte
- meiosis 2 is compleate for the oocyte forming an ovum
- male and female chromosomes group together
- cornicle granules are released combining with the zona pallucida forming tough membrane which takes the place of polarisation in repelling other sperm
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- pollen grain lands on the surface of the stigma
- the molecules on the surface of the pollen grain and stigma interact (and recognise if they are from the same plant species but different plants to stop self fertilisation)
- begin to grow and germinate
- a pollen tube grows out of the tube cell of the pollen grain throught the stigma into the hollow style
- grows down towards the overy
- generative cell containing the generative nucleus travells down it
- nucleus of the cell divides va mitosis to from 2 male nuclei
- tip of the pollen tube passes through the micropyle of the ovule ( growth of the pollen tube is fast due to rapid elongation of the cells)
- 2 male nuclei are passed into the ovule so fertilisation can happen
- flowering plants undergo double fertilisation -
- one male nuclei fuses with the nuclei of the two polar bodies to form endosperm nucleus, which is triploid
- the other amle nucleus fuses with the egg cell to form a diploid zygote
- fertilistaion is compleate development of seed and embryo can begin
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- usually happens due to:
- no sperm produce
- low sperm count
- sperm which isn't mobile and doesn't swim stongly
- abnormal sperm with 2 heads or broken necks
- eggs harvested from women and injected with a healthy sperm ivf treatments
- should everyone be able to have child
- genetic conditions such as CF should they be allowed to bring a child into the world with this condition
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