meiosis Flashcards
define diploid
contains homologous chromosome pairs - two complete sets of chromosomes, one from each parent
define haploid
containing one chromosome from each homologous pair
define gamete
haploid sex cell produced by meiosis in organisms that reproduce sexually
define zygote
initial diploid cell formed when two gametes are joined by sexual reproduction
define meiosis
nuclear division for the purpose of gamete production
- nucleus divides twice = four haploid cells from one diploid cell
define reduction division
where the chromosome number is halved from diploid to haploid (first meiotic division)
explain 3 roles of meiosis in life cycles
- production of gametes to allow of sexual reproduction
- ensures organisms produced via sexual reproduction contain correct number of chromosomes (number of chromosomes would double with every round of reproduction otherwise)
- produces genetic variation
state 3 ways in which meiosis produces variation
- crossing over during prophase 1
- independent assortment of chromosomes in metaphase 1
- independent assortment of chromatids in metaphase 2
suggest the importance of the creation of different allele combinations in populations
- phenotypes will be produced
- reduces chances of a population going extinct - it is more likely an individual will possess advantageous characteristics
define homologous chromosomes
matching pair of chromosomes - one inherited from each parent
define bivalent
a pair of homologous chromosomes during crossing over
define crossing over
exchange of genes between homologous chromosomes, resulting in a mixture of parental characteristics in offspring
define chiasmata
points at which chromatids break and rejoin
define recombinant chromatid
chromatid with a combination of DNA from both homologous chromosomes,
- formed by crossing over and chiasmata in meiosis
state the stages of meiosis in order
- prophase 1
- metaphase 1
- anaphase 1
- telophase 1
- prophase 2
- metaphase 2
- anaphase 2
- telophase 2
what happens at P1?
- chromosomes condense, nuclear envelope disintegrates, nucleolus disappears
- spindle formation begins
- homologous chromosomes pair up, forming bivalents & crossing over occurs
what happens at M1?
- homologous pairs assemble along metaphase plate
- independent assortment occurs
what happens at A1?
- homologous chromosomes pulled to opposite poles, chromatids stay joined to each other
- entangled sections of DNA on non-sister chromatids from crossing over break off + rejoin at chiasmata = formation of recombinant chromatids
what happens at T1?
- chromosomes assemble at each pole, nuclear membrane reforms
- chromosomes uncoil
- cytokinesis occurs, cell divides into two
- reduction of chromosome number from diploid to haploid is complete
what happens at P2?
- chromosomes that still consist of two chromatids condense and become visible
- nuclear envelope breaks down, spindle formation begins
what happens at M2?
- individual chromosomes assemble on metaphase plate
- due to crossing over, chromatids are not identical - independent assortment occurs
what happens at A2?
chromatids of individual chromosomes are pulled to opposite poles after division of centromeres
what happens at T2?
- chromatids assemble at poles, uncoil and form chromatin
- nuclear envelope reforms, nucleolus is visible again
- cytokinesis results in division of the cells forming four daughter cells in total
- daughter cells will be haploid and genetically different from each other & from parent cell
describe the process of crossing over and how it produces genetic variation
- homologous chromosomes pair up, forming a bivalent
- non-sister chromatids become entangled at chiasmata
- they break off and re-join, sometimes resulting in an exchange of DNA forming recombinant chromatids with different combinations of alleles = genetic variation
