Meiosis Flashcards
(13 cards)
Describe features of homologous chromosomes:
Same length, same genes at same loci, but may have different alleles.
Describe the difference between diploid and haploid cells:
- Diploid - has 2 complete sets of chromosomes, represented as 2n
- Haploid - has a single set of unpaired chromosomes, represented as n
Describe how a cell divides by meiosis:
In interphase, DNA replicates → 2 copies of each chromosome (sister chromatids), joined by a centromere
1. Meiosis I (first nuclear division) separates homologous chromosomes. Chromosomes arrange into homologous pairs.Crossing over between homologous chromosomes Independent segregation of homologous chromosomes
2. Meiosis II (second nuclear division) separates chromatids
Explain why the number of chromosomes is halved during meiosis:
Homologous chromosomes are separated during meiosis I (first division).
Explain how crossing over creates genetic variation:
- Homologous pairs of chromosomes associate / form a bivalent
- Chiasmata form (point of contact between (non-sister) chromatids)
- Alleles / (equal) lengths of (non-sister) chromatids exchanged between chromosomes
- Creating new combinations of (maternal & paternal) alleles on chromosomes
Explain how independent segregation creates genetic variation:
- Homologous pairs randomly align at equator → so random which chromosome from each pair
goes into each daughter cell - Creating different combinations of maternal & paternal chromosomes / alleles in daughter cells
Other than mutation and meiosis, explain how genetic variation within a
species is increased:
- Random fertilisation / fusion of gametes
- Creating new allele combinations / new maternal and paternal chromosome combinations
Explain the different outcomes of mitosis and meiosis:
- Mitosis produces 2 daughter cells, whereas meiosis produces 4 daughter cells. As 1 division in mitosis, whereas 2 divisions in meiosis
- Mitosis maintains the chromosome number (eg. diploid → diploid or haploid → haploid) whereas meiosis halves the chromosome number (eg. diploid → haploid) As homologous chromosomes separate in meiosis but not mitosis
- Mitosis produces genetically identical daughter cells, whereas meiosis produces
genetically varied daughter cells. As crossing over and independent segregation happen in meiosis but not mitosis
Explain the importance of meiosis:
- Two divisions creates haploid gametes (halves number of chromosomes)
- So diploid number is restored at fertilisation → chromosome number maintained between generations
- Independent segregation and crossing over creates genetic variation
How can you recognise where meiosis and mitosis occur in a life cycle?
● Mitosis occurs between stages where chromosome number is maintained
○ Eg. diploid (2n) → diploid (2n) OR haploid (n) → haploid (n)
● Meiosis occurs between stages where chromosome number halves
○ Eg. diploid (2n) → haploid (n)
Describe how mutations in the number of chromosomes arise:
- Spontaneously by chromosome non-disjunction during meiosis
2..Homologous chromosomes (meiosis I) or sister chromatids (meiosis II) fail to separate during meiosis - So some gametes have an extra copy (n+1) of a particular chromosome and others have none (n-1)
Suggest how the number of possible combinations of chromosomes in
daughter cells following meiosis can be calculated
2 to the power of n where n = number of pairs of homologous chromosomes (half the diploid number)
Suggest how the number of possible combinations of chromosomes
following random fertilisation of two gametes can be calculated
(2 to the power of n) squared where n = number of pairs of homologous chromosomes (half the diploid number)
