Gametogenesis to mid-blastula transition Flashcards
steps of fertilisation
fusion of gametes, creation of first zygotic cell, restores diploid chromosomes, initiates cell cycle
steps of gametogenesis
germ cell vs somatic cell, halving of genetic content, homologous recombination
what is the outcome of germ cell development
a male gamete or female gamete
what to germ cells give rise to
all cells in an organism - only cells to undergo meiosis
why is there a reduction division in formation of the germ line
if there was no reduction then the no of chromosomes in somatic cells would double with each generation
what are the precursor germ cells
primordial germ cells - formed in locations that seem to protect them from inductive signals that determine the fate of somatic cells and shut down the somatic development programme
in drosophila how is movement of transposons prevented
by an rna silencing pathway
what is a germplasm
a special cytoplasm that specifies primordial germ cells
what is an example of a germplasm? (drosophila)
primordial germ cells (pole cells) become distinct at the posterior pole of the egg 90 mins after fertilisation, the cytoplasm at the posterior end is the pole plasm which is distinguished by large organelles
polar granules contain proteins and rnas
what happens when the posterior end of a drosophila egg is irridated by UV light
it destroys pole plasm activity so no germ cells develop, somatic cells in this region still develop
what happens if the pole plasm from an egg is transferred to another embryos anterior pole?
nuclei in the area are specified as germ cell nuclei - if this is then transplanted to a 3rd embryos posterior end the cells develop as fucntional germ cells
what does the oskar gene play a role in?
organisation and assembly of the pole plasm, only gene involved in pole plasm formation that has mRNA localised at the posterior pole
the signal for this localisation is in the 3’ untranslated region of mRNA
what protein is required for oskars localisation
staufen protein, it links mRNA to the microtubule system polarised along the antero-posterior axis
what happens if oskars genes for 3’ localisation are replaced by bicoid 3’ localisation?
oskar mRNA will be localised at the anteriors of the egg - this is manipulation of DNA
when is germline lineage set up in the nematode?
at the end of the 4th cleavage division with all germ cells derived from the p4 blastomere - p4 cells are derived from 3 stem cell like divisions of p1 cells
what happens at each division of the nematode
1 daughter produces somatic cells and the other divides again to produce a somatic cell progenitor (P cell)
where does the nematode egg produce P granules and how are they distributed
produces p granules in its cytoplasm then asymmetrically distributed before the first cleavage division - then subsequently confined to the p cell lineage
what are p granules suggested to have a role in
due to their association with germ cell formation they may have a role in germ cell specification
what is pg 1-1 gene needed for?
germ cell development it is a p granule component
in the fly and nematode what is transcription repression needed for?
specification of germ cell fate
when are p granules localised
after fertilisation - to the posterior end of the cell
when is diploid chromosome reinstated
after fertilisation
when does meiosis occur
when the germ cells reach the gonad
when are chromosomes replicated in meiosis
1st division not 2nd
what happens when cell divisions are unequal
a small structure called a polar body is formed
what happens during prophase of 1st division
replicated homologous pair undergo division and dna sequences are exchanged
what does meiosis result in
gametes with variations of alleles compared to the parents which is the main source of genetic variation
draw out meiosis
see notes
in oogenesis what are the diploid germline cells called
oogonia
after entry into meiosis what are cells called in oogenesis?
primary oocytes
how are oocytes found in the ovary
enclosed in a sheath of somatic ovarian cells called the follicle
how are mammalian primary oocytes arrested in 1st prophase
dependant on cAMP levels within the oocyte generated from signalling of g protein coupled receptors
when is the first mitotic division completed for oocytes
when does the second division occur
after ovulation of the adult, the second meiotic division occurs after fertilisation
in humans when do most oocytes degenerate
during puberty
when does meiosis occur in mammals
during ovulation, egg is released and the oocyte undergoes and completes its 1st meiotic division to produce a polar body it then proceeds until metaphase of the second meiotic division
what are polar bodies
small cells formed by meiosis during the development of an oocyte into an egg
what is a major error in human oocyte production
extra chromosome 21 leading to downs syndrome
most embryos with too few or too many chromosomes are non viable
what happens to diploid germ cells that give rise to sperm
they do not undergo meiosis at the embryo but they become arrested at an early stage of mitotic ell cycle in embryonic testis - they assume mitotic proliferation after birth
in sexually mature animals what to spermatogonial stem cells give rise to
differentiating spermatocytes which undergo meiosis to form 4 haploid spermatids that mature into sperm
what is the difference between female egg production and male sperm production?
females do not produce more eggs throughout their lives, males produce sperm throughout lives
what is different about drosophila sperm and egg production to mammals
they have a continuous production of both eggs and sperm in females and males respectively
what is genomic imprinting and what evidence is there of it
where certain genes are switched off in either the sperm or egg during development and remain silenced in the genome of the early embryo
evidence in mammals - demonstration of maternal and paternal genomes making different contributions to embryonic development
how can mouse genomic imprinting be manipulated
manipulated to have 2 paternal or 2 maternal genomes and can be re-implanted at a later stage - both embryos would be diploid but development is abnormal
what happens to mice with 2 paternal genomes
they dont proceed beyond several somites
what happens to mice with 2 maternal genomes
they have relatively well developed embryos but placenta and yolk sac are not well developed
why does imprinting need to be irreversible
so that the next generation can become male or female
when is inherited imprinting erased and re-established
inherited imprinting erased during early germ cell development and later established during germ cell differentiation
what happens to imprinting when mammals are cloned
donor nuclei will not have gone through normal reprogramming and imprinting so abnormalities are shown
what are spermatozoon designed to do
activate egg cells and deliver their nucleus into the egg cytoplasm
what is the anterior end of the spermatozoon specialised for
penetration of the physical barriers of the egg
what are the cumulus cells of the egg
a sticky layer of hyaluronic acid embedded in somatic follicle cells
how do sperm penetrate the cumulus layer of the egg
hylauronidase on its surface allows penetration
what is the zona pellucida
a layer of fibrous glycoproteins secreted by the oocyte, it is a physical barrier
how do sperm penetrate the zona pellucida
they secrete enzymes from the acrosome released by exocytosis
what are the 3 regions of the spermatozoon and what do they each contain
head - acrosome, nucleus (contains DNA)
mid piece - centrosome and mitochondria
tail - flagellum
all surrounded by the plasma membrane
how are other sperm blocked from entering an egg post fertilisation
resting membrane potential rapidly increased then slowly returns to normal - this depolarisation provides block to polyspermy and ensures only one sperm reaches the egg cell membrane
how is the vitelline membrane lifted after fertilisation
cortical granules release their content between the egg plasma membrane and the vitelline membrane
how is the hardened fertilisation membrane produced
and what happens to the remaining cortical granule
follow the lifting of the vitelline membrane, cortical granules then cross link the molecules in the vitelline membrane to produce the hardened membrane
the remaining cortical granule material forms the hyaline layer
how does the calcium wave at fertilisation occur and what does it trigger
it is triggered by release of stores at sperm entry (from sperm enzymatic activity of phospholipase C) and triggers the completion of meiosis in the fertilised egg
when do maternal effect genes have effects in drosophila
at the start of the mid-blastula transition
when are nurse cells in drosophila development produced
a germline cell undergoes an asymmetrical division to produce another stem cell and a cystoblast
the cystoblast undergoes 4 mitotic divisions to give 16 cells with cytoplasmic bridges between them this is a germline cyst
one of these 16 cells becomes an oocyte and the other 15 become nurse cells
what do nurse cells do
produce protein and RNA that are exported into the oocyte through the cytoplasmic bridges
how is the egg chamber formed
soamtic ovarian cells forma sheath of follicle cells around the nurse cells and oocyte = egg chamber
what do follicle cells play a key role in
forming eggs axis
