2.4 Flashcards
oogenices
-diploid primordial germ cell divides several times by mitosis to form diploid oogonia
-most of the time only one develops to form primary oocyte
-meioisis occurs- 2 uneven cells
large -secondary oocyte
small- polar body
after firtilisation second meiotic division occurs ovum is formed and another polar body
-first polar body makes 2 more
spermatogenisis
-diploid primal germ cell divides several times by mitosis to form spermatogonia
-the spermatogonia then grow without further division until big enough to be called primary spermatocytes
-undergo meiosis -secondary spermatocytes- haploid
-2nd division results in 4 haploid cells (spermatids)
-differentiate in testes to form spermatoza
meiosis
many stages are similar to mitosis
chormasomes replicate to form chromatids
in meoisis chromasomes of each pair stay close (homologus pairs)
-crossing over takes place where some of the chromasomes break off and switch
-point where is breaks is chiasmata
nondisjunction
one of the homologous pairs fail to separate
so 1 has 2 copies of that chromosome and another has 0
can lead to
-monomsy
absence of one member of a pair of chromosomes
-polysomy
at least more than one chromosome than normal
-aneuploidy
total no of chromosomes dosent equal 46
double fertilisation
one male nuclei fuses with two polar nucli to form triploid
other male fuses with egg to form diploid zygote
human fertilisation
when sperm heads meet the ovum an acrosome reaction is triggered
enzymes are released which digest the zona pellucuda
eventually one sperm will get through and touch the oogycte which triggers a cortical reaction which means cortical granuels are released which harden the membrane so no more sperm can get through
head of sperm enters tail is left
down syndrome
polysomy
non disjunction occurs
one gamete will contain two copies of a chromosome
after fertilisation the zygote will have polysomy (3 copies)
heart abnormalities
learning difficulties
turners syndrome
monosomy
non disjunction occurs and gamete has no sex chromosomes so baby has monosomy just x chromasome
genotype xo resulting in turners syndrome
female- not fertile
types of fertilisation
internal -transfer of male gametes to female
more likely
external - occurs outside the body
e.g water
large amounts released
less likely
formation of egg cells in plants
in the plant ovary
ovule is attached to the wall of the ovarie
megaspore mother divides by meiosis to form 4 haploid megaspores
only one develops
3 mitotic divisions making egg cell
2 polar nuclei and other small cells
after fertilisation
clevage 5-6 days
zygote divides by mitosis several times to form blastocyst
first being a morlula
outerlayer forms placenta
to make identical copies of chromatids so all cells are diploid
mutations
can be caused by random assortment and crossing over
can be balanced
however some are unbalanced and can lead to changes in phenotype
formation of pollen
anthers are equivelent of testes
meiosis occurs
4 pollen sacs containing microspore mother cells which form microspores
which forms gametes by mitosis
contains two nuclei
-tube
-generative
plant firtilisation
pollen lands on stigma
pollen tube begins to grow from haploid tube cell through stigma into style
enzymes released (tissues act as a nutrient source)
as tube grows haploid generative cells moves down
nucleus divides by mitosis to form 2 cells
passes through ovule
2 male nuclei enter
double firtilisation occurs
stages of meiosis
prophase 1 - crossing over occurs
metaphase 1 - random assortment
anaphase 1 - centromeres do not divide ( homologus paits split )
telophase 1 - membranes re form
no further replication
prophase 2
metaphase 2
anaphase 2
telophase 2
cytokensis
