REDUCTION DIVISION The number of chromosomes is characteristic of an organism (e.g humans have 46 chromosomes). Replication makes the second identical chromatid. 44 autosomes and 2 sex chromosomes (gonosomes) Normal MaleNormal Female Where does meiosis occur WHERE WILL MEIOSIS OCCUR IN PLANTS? Forms sex cells (n) differ from somatic/body cells (2n) Sperm and Ovum REVIEW OF MITOSIS How meiosis takes place Two stages can be found in the process of meiotic cell division Meiosis 1 : is a reduction division which results in two cells being with half the number of chromosomes (haploid) Meiosis 2 : is a copying division which involves the two haploid cells formed each dividing again by mitosis, to form 4 haploid cells. PHASES OF MEIOSIS Note, meiosis is a continuous process and is divided into phases for convenience. INTERPHASE DNA replication takes place Single stranded chromosomes become double-stranded Each chromosome will now consist of two chromatids joined by a centromere PROPHASE 1 Centrosome splits into centrioles which move to opposite poles forming spindle threads. Nuclear membrane and nucleus start to disappear. Chromosomes become distinct. Crossing over takes Crossing over takes place as follows Homologous chromosomes lie next to each other Chromatids of homologous chromosomes overlap Point of overlap is called the chiasmata A break occurs at the chiasmata Chromatid segments (genetic material) has now been exchanged METAPHASE 1 Chromosomes align at equator in a random fashion and in homologous pairs, attached to the spindle threads. RANDOM ASSORTMENT Chromosomes are arranged randomly at the equator. Different combinations of maternal and paternal chromosomes can occur. ANAPHASE 1 Spindle fibres contract. Homologous chromosomes move apart towards the poles. The cytoplasm starts to divide TELOPHASE 1 2 new cells have been formed. Each new cell has half of the chromosome complement of the original cell (it is haploid). The cells are not identical. (NB Meiosis 2 is similar to mitosis) PROPHASE 2 Same as prophase 1 only no crossing over. METAPHASE 2 Chromosomes aligned at the equator singly ANAPHASE 2 Spindle fibres contract The centromeres separate Two chromatids of each chromosome move to opposite poles ( The separated chromatids are now called chromosomes) TELOPHASE 2 four daughter cells (gametes, in animals), each with a haploid set of chromosomes are produced. Importance/significance of meiosis Production of gametes Halving of the chromosome number (diploid to haploid) Mechanism to introduce genetic variation (Crossing-over and the random arrangement of chromosomes at the equator) Homework questions 1. List similarities between meiosis and mitosis 2. Tabulate 4 differences between meiosis and mitosis 3. Draw a labelled and annotated diagram showing the phases of meiosis ABNORMAL MEIOSIS NON-DISJUNCTION When a chromosome/chromatid fails to separate during anaphase I/II ABNORMAL MEIOSIS CONSEQUENCES OF NON-DISJUNCTION Some gametes will have extra chromosomes while others will have less. Non-disjunction in chromosome 21 Normally occurs in human egg cell One or more abnormal eggs formed (n = 24) Fertilised by normal sperm (n = 23) Formation of abnormal zygote (2n = 47) Downs Syndrome Downs syndrome Down's syndrome is caused by the presence of three copies of the 21st chromosome. This chromosomal defect is known as Trisomy-21. Down's syndrome almost always results in mental retardation, though the severity of the retardation varies. Characteristics of Down syndrome Small, oblique eyes Flattened, nasal bridge Open mouth Protruding tongue Broad neck Small underdeveloped ears set low on head Epicanthic fold Incurved finger Single palmar (simian) crease Short broad hands Wide gap between first and second toe