Meiosis Flashcards
Describe what happens during meiosis 1 (prophase 1)
- the chromatin condenses and undergoes supercoiling so that chromosomes shorten and thicken
- the chromosomes come together in their homologous pairs to form a bivalent
- the non-sister chromatids wrap around each other and attach at points called chiasmata
- they may cross over and swap sections with each other (crossing over)
- the nucleolus disappears and the nuclear envelope breaks down
- a spindle forms
What do homologous chromosomes have in common?
- similar size/shape
- same bonding pattern when stained
- centromeres in same position
- 1 maternal, 1 paternal
- same genes is same loci
- pair up to form bivalent in meiosis
Describe what happens during meiosis 1 (metaphase 1)
- bivalents line up across the equator of the spindle, attached to spindle fibres at the centromeres
- the bivalents are arranged randomly (random assortment) with each member of the homologous pair facing opposite poles
Describe what happens during meiosis 1 (anaphase 1)
- spindle fibres contract and the bivalent is pulled apart
- so chromosomes are pulled to opposite poles
- the chiasmata separate and the lengths of chromatid that have been crossed over remain with the chromatid to which they have become newly attached
Describe what happens during meiosis 1 (telophase 1)
ANIMAL CELLS ONLY
• a nuclear envelope forms around each group of chromosomes and the cytoplasm divides (cytokinesis) so there are now two haploid daughter cells
• there is a grief interphase and chromosomes uncoil
Describe what happens during meiosis 2 (prophase 2)
- chromosomes condense
- centrioles migrate to opposite poles
- spindle forms
- nuclear envelope breaks down
Describe what happens during meiosis 2 (metaphase 2)
- chromosomes align along the equator
- they attach to spindle fibre at centromere
- chromatids or each chromosome arranged in a random assortment
Describe what happens during meiosis 2 (anaphase 2)
- spindle contracts
- centromeres divide and sister chromatids are pulled to opposite poles by spindle fibres
- the chromatids randomly segregate
Describe what happens during meiosis 2 (telophase 2)
- nuclear envelope reforms around the haploid nuclei
- in animals the two cells divide by cytokinesis to give 4 daughter cells
- in plants a tetrad of 4 haploid cells is formed
Define the term allele
- alternative/different version of a gene
* has a difference in the DNA base sequence that is expressed as a slightly different polypeptide
Define the term locus
The position of a gene on a chromosome
Define the term crossing over
When non-sister chromatids exchange alleles during prophase 1 of meiosis
What factors can lead to meiosis and fertilisation leading to genetic variation?
- crossing over
- reassortment of chromosomes in meiosis 1
- reassortment of chromatids in meiosis 2
- fertilisation
- mutation
What happens during crossing over?
- during prophase 1 homologous chromosomes pair up and come together to form bivalents
- in each pair one chromosome is maternal and one is paternal
- the non-sister chromatids wrap around each other tightly and attach at points called a chiasmata
- the chromosomes can break at these points and the broken ends rejoin to the ends of non-sister chromatids in the same bivalent but they have been swapped
- the chromosomes now have different combinations of alleles
- this means that each of the 4 daughter cells contains chromatids with a different combinations of alleles
What happens during the reassortment of chromosomes in meiosis 1?
- each gamete has a random assortment of maternal and paternal chromosomes
- during meiosis 1 different combinations of maternal and paternal chromosomes go into each cell so each cell ends up with a different combination of alleles
