Meiosis Flashcards
4.8
what is the purpose of meiosis?
- increasing genetic variation by producing gametes that are genetically different
- produces haploid cells so that at fertilisation the diploid number is restored
- chromosome number stays constant in each generation
are maternal and paternal chromosomes genetically identical?
no since they have different alleles for certain genes
describe what happens in meiosis one and two
meiosis one the homologous chromosomes pair up and their chromatids wrap around each other. at the end of meiosis one the homologous chromosomes have separated so the chromosome number of daughter nuceli is halved
meiosis two the sister chromatids from each chromosome move apart. at the end of this stage haploid (n) gametes have been formed. each of these cells only contain a single chromosome from each homologous pair.
how does genetic variation occur in meiosis?
- recombination by crossing over of homologous pairs
- independent segregation of homologous chromosomes
what happens in recombination by crossing over of homologous chromosomes?
- in meiosis 1, the homologous chromosomes pair up - these are known as bivalents
- the non-sister chromatids of homologous chromosomes wrap around each other, crossing over at the chiasmata
- these portions of the chromatids can be exchanges, the broken-off portions of non-sister chromatids recombine with the other chromatid, so this process in called recombination
- this produced new combinations of alleles
how does independent segregation of homologous chromosomes occur?
in meiosis one the homologous chromosomes pair up, how they line up is random. the different combinations of maternal and paternal chromosomes line up is a matter of chance, this is known as independent segregation
whats the formula to determine how many possible chromosome combinations produced from the random fertilisation of two gametes from separate individuals
(2n)²
what are the two forms a chromosome mutation can occur?
- changes in whole sets of chromosomes
- changes in the number of individual chromosomes
what happens when there are changes in whole sets of chromosomes?
occurs when organisms have three or more sets of chromosomes rather than the usual two. this condition is called polyploidy and occurs mostly in plants. (result of total non-disjunction)
what happens when there is a change in the number of individual chromosomes?
sometimes individual homologous pairs of chromosomes fail to separate during meiosis. this is known as non-disjunction and usually results in a gamete having either one more or one fewer chromosomes, on fertilisation the resultant offspring have more or fewer chromosomes than normal in all their body cells
compare meiosis and mitosis
- four daughter cells produced, two daughter cells produced
- two divisions , one division
- genetically different cells, genetically identical cells
- homologous chromosomes aren’t separated, homologous chromosomes are separated
- recombination by crossing over and independent segregation occur, recombination and segregation don’t occur
- diploid parents produce haploid daughter cells, diploid parents produce diploid cells
describe the process of meiosis
- In meiosis 1, homologous chromosomes pair up forming bivalents.
- Non-sister chromatids of homologous chromosomes cross over, break off and recombine, exchanging alleles at points called chiasmata, this creates new combinations of alleles
- Each pair of homologous chromosomes lines up independently of any other pair, independent segregation of homologous chromosomes therefore occurs
- Each homologous chromosome is pulled to the opposite pole of the cell via spindle fibres attached to their centromere
- in meiosis 2, the sister chromatids line up on the equator, the centromere divides, spindle fibres pull the sister chromatids to opposite poles of the cell
- this produces 4 genetically different daughter cells
