Cell Division - Meiosis Flashcards
Meiosis
DNA replicates forming 2 copies of each chromosome (chromatids
DNA condenses to form two sister chromatids joined by centromere
First division = chromosomes arranged into homologous pairs
Homologous pairs then separated, halving chromosome number
Second division - centromere divided and sister chromatids that make up each chromosome are separated
Four haploid cells (genetically different from each other) are formed
Crossing Over
occurs during Meiosis I
homologous pairs of chromosomes formed
chromatids twist around each other, causing parts to swap over.
chromatids still contain same genes but have different combinations of alleles
Independent segregation of chromosomes
homologous pairs are separated randomly at Meiosis I
TF four daughter cells produced by Meiosis have different combinations of maternal and paternal chromosomes
called independent segregation of chromosomes
this ‘shuffling’ is what leads to genetic variation in potential offspring
Random fertilisation
produces zygotes with different combinations of chromosomes to both parents
mixing of genetic material in sexual reproduction increases genetic diversity within a species
Gametes
normal body cells have diploid number (2n - in humans n=23) - one set of chromosomes from each parent
Gametes have haploid (n) number - only one copy of each chromosome
at fertilisation, haploid sperm fuses with haploid egg forming zygote with normal diploid number.
The sperm can fertilise any egg!!!
Chromosome mutations
caused by errors in meiosis
cells produced contain variations in numbers of whole/partial chromosomes
lead to inherited conditions as errors are present in the gametes or HEREDITARY CELLS
Non disjunction mutation
failure of chromosomes to separate properly
i.e. non disjunction of chromosome 21 during meiosis can lead to Down Syndrome (caused by an extra copy/ part of chromosome 21)
gamete with extra copy then fuses with another gamete to form a zygote with 3 copies of chromosome 21 as opposed to normal 2 copies
