meiosis Flashcards
what is meiosis
the division of one cell into four cells each of which has half the chromosome number as the parent cell , each of the four cells is genetically different from each other
when does meiosis occur
during gametogenesis a process during which gametes are formed
where does meiosis occur in plants
in the anther to produce pollen grains and in the ovary to produce the ovule
where does meiosis occur in humans
in the testes to produce sperms and in the ovary to produce an ovum
where does meiosis occur in some lower organisms
immediately after fertilization
halving effect
meiosis allows for diploid cells in the sex organ to form haploid gametes
doubling effect
when two haploid gametes fuse , they form a diploid zygote
how is the number of chromosomes kept constant from one generation to the next
the halving effect of meiosis overcomes the doubling effect of fusion
when does meiosis take place
in most plants and animals including humans meiosis take place when gametes are formed (before fertilization )
what doe these gametes therefore have
half the chromosome number or one set of chromosomes ( haploid number or n ) compared to adults
what does the fusion of the gametes result in
the diploid chromosome number being restored in the zygote which undergoes mitosis to produce a multicellular adult
what results in diploid zygotes
the adults are haploid giving rise to haploid gametes which on fertilization results in diploid zygotes
what is the function of meiosis occurring in the zygote or zygospore ( after fertilization)
th alve the chromosome number so that the new offspring also have the haploid number of chromosomes
meiosis in moss and ferns
what do mosses and ferns show
they show an alternation generations whereby a haploid gametophyte ( gamete producing) generation alternates with a diploid sporophyte ( spore producing ) generation
what does haploid gametophytes produce
haploid gametes by mitosis
what does the diploid sporophyte produce
haploid spores by meiosis
what allows for such alternation of generations
the halving effect
what is meiosis
the division od one cell into four cells each of which only has half the chromosome number as the parent cell and each of the four cells is genetically different from each other and the parent cell
how many divisions does the nucleus undergo
two divisions
1) meiosis I
2) meiosis II
even though meiosis is a continuous process what various events in this process is it divided into
prophase , metaphase , anaphase ,telophase
what begins before meiosis
dna replication takes place before meiosis begins during a stage called interphase
interphase
dna replicates in the preparation for what
nuclear division
interphase
why does dna replicate
so that the genetic material is doubled in the chromatin network
interphase
what happens to the single stranded chromosome’s
they become double stranded
prophase I
what happens to the nuclear membrane
begins to disappear
prophase I
what does the chromatin network unwind to form
to form distinct chromosomes
prophase I
what is each chromosomes made up of
two identical chromatids (because of dna replication in interphase
prophase I
what do the chromosomes come together in
together in homologous pairs i.e. two identical chromosomes , one of paternal origin and one of maternal origin
prophase I
where does crossing over take place
between chromatids of homologous chromosomes leading to the exchange of genetic material
prophase I
what does the centrosome split into
into two centrioles which start moving in opposite directions
metaphase I
what forms between the centrioles
spindle threads
metaphase I
where does the chromosomes become arranged
along the equator in homologous pairs , attached to spindle threads
metaphase I
what happens to the spindle threads
they contract
anaphase I
what happens to the two chromosomes of each homologous pair ( each with two chromatids )
they are pulled to the opposite poles
telophase I
what appear forming a nucleus at each pole
two groups of chromosomes
telophase I
what happens the cytoplasm
divides to form two new cells
telophase I
thus what do we have resulting from the division of cytoplasm
we have a nucleus in each cell within which there is half the number of chromosomes as the original
telophase I
why are the two new cells genetically different
because of crossing over
