Meiosis

Meiosis

1. The significance of meiosis in life cycles

   • ~sexual reproduction incr genetic variation, as it involves the combining of genetic material from 2 (usually) unrelated individuals of the same species, by the process of fertilisation. ->Genetic variation within a pop incr its chances of survival when the environment changes, as some individuals will have the characteristics that enable them to be better adapted to change.
     • ->in many organisms, the body cells are DIPLOID. ->for sexual reproduction to occur they must produce HAPLOID GAMETES, so that when 2 gamete nuclei fuse during fertilisation, a diploid ZYGOTE is produced & the normal chromosome number is maintained through the generations.
       • ~Meiosis means 'reduction' & it occurs in diploid germ cells to produce haploid gametes. ->the diploid cells undergoing meiosis are in specialised organs celled gonads -ovaries & testes (have been in interphase before the enter meiosis

2. Homologous chromosomes

   • ~in your body there are 46 chromosomes ->23 from mother in egg nucleus & 23 from father in sperm nucleus =can form matching pairs, 1 maternal, 1 paternal chromosome containing the same genes at the same places on the chromosome. ->matching pairs are called HOMOLOGOUS CHROMOSOMES ->have same genes but may contain different alleles (variants) for the genes

3. Main stages of meiosis

   • ~before meiosis, during S phases of interphases, each chromosome was duplicated as its DNA replicated, after which each chromosome consist of 2 SISTER CHROMATIDS. ->in meiosis, the chromosomes pair up in their homologous pairs
   • *are 2 divisions in meiosis & in each there are 4 stages
     • *first meiotic division =4 stages called: prophase 1, metaphase 1, anaphase 1, & telephone 1
       • *cell may enter a shot interphase before embarking on second meiotic divisions that also has 4 stages =prophase 2, metaphase 2, anaphase 2, & telephase 2. ->takes place in a plane at RIGHT ANGLES to that of meiosis 1
         • *at end of second division =cytokinesis may occur

4. How meiosis produced genetic variation

   • *crossing over during prophase 1 shuffles alleles
     • *independent assortment of chromosomes in anaphase 1 =random distribution of maternal & paternal chromosomes of each pair
       • *independent assortment of chromatic in anaphase 2 =further ransom distribution of genetic material
         • *Haploid gametes are produced, which can undergo ransom fusion with gametes derived from another organism of the same species
     • (when homologies chromosomes pair up & twist around each other creating tension which causes a break ->corresponding fragments may get swapped over ->new genetic combinations are produced & variation in the gametes is increased.)