2.3b - meiosis

• meiosis is a type of cell division that produces haploid gametes from diploid gamete mother cells
• homologous chromosomes are pairs of chromosomes in a gamete that are the same size, same centromere position and with the same genes at the same loci
• the variation in gametes produced depends on interactions and movements of homologous chromosomes
• meiosis 1 involves pairing of homologous chromosomes followed by their separation into two cells
• in meiosis 2 the sister chromatids are separated and each new chromosome segregates into one of the four haploid cells produced
• the chromosomes that have replicated prior to meiosis 1, each consist of two genetically idenatical sister chromatids attached at the centromere
• when homologous pairs separate during meiosis 1, they do so independently and irrespective of their maternal and paternal origin
• the process of independent assortment leads to variation in the alleles reaching each gamete
• homologous chromosomes undergo random crossing over at points called chiasmata, resulting in exchange of DNA and the recombination of alleles of linked genes, to produce varied combinations in the gametes
• meiosis 1

  chromosomes condense, the homologous chromosomes pair up and chiasmata form at points of contact between the non-sister chromatids of a homologous pair and sections of DNA are exchanged
• linked genes

  those on the same chromosome and crossing over can result in new combinations of alleles of the same gene
• crossing over of DNA is random and produces genetically different chromosomes through recombination
• spindle fibres attach to the homologous pairs and line them up at the equator of the spindle
• the orientation of the pairs of homologous at the equator is random, and each pair of homologous chromosomes is positioned independently of the other pairs, irrespective of their maternal and paternal origin
• the chromosomes of each homologous pair are separated and move towards opposite poles
• cytokinesis occurs and two daughter cells form
• each of the two cells in meiosis 1 undergoes a further division called meiosis 2 during which the sister chromatids of each chromosome are separated
• a total of four haploid cells are produced as a result of meiosis
• the sex of birds, mammals and some insects is determined by the presence of sex chromosomes
• in most mammals the SRY gene on the y chromosome encodes tested-determining factor (TDF) and determines the development of mak=le characteristics
• heterogametic (XY) males lack most of the alleles corresponding to those on the X chromosome on their shorter Y chromosome
• the presence of the short Y chromosome can result in sex-linked patterns of inheritance as seen with carrier females and affected males
• in homogametic females (XX) one of the two X chromosomes present in each cell is randomly inactivated at an early stage of development
• x chromosome inactivation prevents a double dose of gene products, which could be harmful to cells
• carriers are less likely to be affected by deleterious mutations on one of their X chromosomes because of random inactivation
• as the X chromosome inactivated in each cell is random, half of the cells in any tissue will have a working copy of the gene in question
• hermaphrodites are species that have functioning male and female reproductive organs in each individual
• hermaphrodites produce both male and female gametes, and usually need a pertner with to which they exchange gametes
• the benefit to the individual hermaphrodite is that if the chance of encountering a partner is an uncommon event, theres no requirement for that partner to be of the opposite sex
• for some species environmental rather than genetic factors determine sex and sex ratio
• environmental sex determination in reptiles is controlled by environmental temperature of egg incubation
• sex can change within individuals of some species as a result of size, competition or parasitic infection
• in some species the sex ratio of offspring can be adjusted in response to resource availability