1)which meiosis division does ploidy reduction occur ?give reason for the reduction .does ploidy reduction occur in mitosis?2)what is heterospory?briefy comment on its significance.give two examples?
Ploidy reduction occurs during the first meiosis division. In Anaphase I two duplicated sets of homologous chromosomes are transported to the opposite poles of the cell. Sister chromatides remain connected to each other. Despite DNA amount in daughter cells is as in diploid cells, the reduction has happened, because daughter cells contain only one allele of each gene, albeit duplicated. It means that after I meiotic division genetic material of two daughter cells is different, since homologous chromosomes contain different alleles of the genes. During the second meiosis division sister chromatides divide between cells. Sister chromatides are copies of each other (if you do not consider the effect of homologous recombination in Prophase I). Therefore there is no reduction of ploidy when they separate between cells. Reduction is necessary for sex reproduction because it provides maintenance of ploidy during generations. It prevents doubling of chromosome set number after each fertilization. Thanks to reduction gamete gets only one allele of a pair of alleles of the gene in parent organism. There is no ploidy reduction during mitosis. Sister chromatides bear to the opposite sides of the dividing cell. Therefore genotype of the daughter cells is identical to that of the mother cells. 2) What is heterospory? Briefly comment on its significance. Give two examples. Answer: Heterospory is the production of two kindes of spores: male (smaller ones, microspores) and female (larger ones, megaspores) by sporophyte of some higher plants. Microspores develop into male gametophyte with male sex organs and megaspores form female gametophyte with female sex organs. Corresponding gametophytes produce male and female gametes. Heterosporous plants include several Lycopodiophyta such as Selaginallales and Isoetales, several Pteridophyta (Marsileales and Salviniales), all Gymnospermatophyta and Magnoliophyta.& Significance: 1. Heterospory makes possible division of labour between male and female gametophytes. Megaspores generally have supply of nutrients for gametophyte and then - embryo development. Microspores develops into small male gametophytes and have some devices for effective spread. Such division of labour facilitate and accelerate reproduction of heterosporous plants. 2. Gametophytes of heterosporous plants are frequently small and develop inside micro- and megaspores. It provides effective protection of gametophyte against adverse environmental conditions what also accelerate development. 3. Heterosporous plants are usually dioicous what promotes outcrossing.