Meiosis Flashcards
what is a haploid?
only 1 set of chromosomes, symbol showed as an “N”
What is the definition of meiosis
type of nuclear division that results in the formation of 4 haploid gametes containing half the number of chromosomes in the parent cell
what is a homologous pair
set of one maternal and paternal chromosome, which pair during fertilisation.
they are two matching chromosomes with different alleles
what is crossing over
is the process by which non-sister chromatids exchange alleles by swapping sections of chromatids.
Explain the process of crossing over
Homologous pairs can cross and become entangled
The entanglement places stress on the DNA molecules
this causes chromatids in the chromosome to break and re-join to the other chromosome
what does the crossing over allow to happen
create a new combination of alleles on the two chromosomes- increases genetic diversity
What is a bivalent?
a pair of a homologous chromosomes which come together in meiosis to create a homologous pair and are lying alongside eachother
They are crossed over, but there alleles are not yet shuffled
what is a diploid zygote?
a cell that forms when two haploid gametes fuse during fertilisation.
2n- 2 sets of chromosomes
how many chromosomes are in the human body
46
23 from each parent
What happens in prophase 1 of meiosis 1
chromatin condenses each chromsome becomes supercoiled- visible under microscope with stain.
nuclear envelope breaks down
Spindle threads form from the centriole
Homologous pairs form
Crossing over occurs- non sister chromatids entangle and swaps sections of chromatid to shuffle alleles
In what phase of meiosis are bivalents/ homologous pairs
formed
Prophase 1
how many chromatids is in a bivalent
4 chromatids in a bivalent
what is the name given to the area where homologous pairs cross over
Chiasmata
What happens in metaphase 1
Bivalents attach to the spindle at the equator, each attached by centromere
The maternal and paternal chromosomes assort themselves independently on the equator- INDEPENDENT ASSORTMENT
anaphase 1
motor proteins separate the homologous pairs, dragging them across the spindle fibres
NO CENTROMERE DIVISION.
Crossed over areas separate resulting in swapped areas of chromosome and allele shuffling
telophase 1-
two new nuclear envelopes form around each set of chromosomes, cell divides by cytokinesis
Each new nucleus contains half the original number of chromosomes, 23 OR “n”
what is meiosis 2
Just mitosis-
remember the result is 4 haploid gametes.
where as mitosis is two identical cells
WHAT IS INDEPENDENT ASSORTMENT and when does it occur in meiosis
random arrangement on chromosomes/ chromatids along the equator.
decides which chromosome/ chromatids ends at which pole of the cell
why do we need genetic diversity
it increases genetic variation- increases organism survivial chances when faced with environmental change
is there nay genetic variation processes in mitosis
no because all cells will be identical to parent, the reproduction is asexual so no crossing over will occur.
where does an interphase occur in meiosis
before meiosis1
there is also a short interphase between meiosis 1 and 2.
what cells are produced by meiosis
haploid cells
sperm
ova
germline cells
where does genetic variation occur outside of meiosis
when gametes fuse with another gamete of another organism
describe telophase 2
fourth step in meiosis 2
chromatids have reached opposite poles.
spindle breaks down
nucleolus reforms
nuclear envelope reforms around each new set of chromatids.
result is 4 haploid gamete cells
cytokinesis occurs straight afrer
