ASSET GRADE 12 REVISION 2020 MEIOSIS PART 3

ASSET GRADE 12 REVISION 2020 MEIOSIS PART 3: IMPORTANCE OF MEIOSIS, ABNORMAL MEIOSIS, MITOSIS VS MEIOSIS For each section please read the exam guidelines first before going on to the content. Learn all the content in this presentation

Exam guidelines

Importance of Meiosis (importance is the same as function or purpose or significance of Meiosis) State the importance of meiosis with regard to each of the following: Production of haploid gametes (refer to detail of gametogenesis in Human Reproduction) Halving of the chromosome number (diploid to haploid) Mechanism to keep a constant chromosome number in a species Mechanism to introduce genetic variation through: o Crossing-over o Random arrangement of chromosomes on the equator at Metaphase 1/I and Metaphase 2/II

Cell Division in humans: An Overview Male embryo 2 N Many mitotic divisions Male adult 2 N Meiosis A single set of chromosomes Sperm 1 N A double set of chromosomes Fertilization Gamete production Somatic cell production Zygote Many mitotic divisions 2 N Many mitotic divisions Embryo 2 N Somatic cell production Female embryo 2 N Many mitotic divisions Femal e adult 2 N Meiosis Eg g 1 N Adult 2 N Somatic cell production

Revise crossing over Mechanism to introduce genetic variation through: o. Crossing-over

Revise: The random arrangement of chromosomes at the equator Mechanism to introduce genetic variation through Random Arrangement

Abnormal Meiosis State what is meant by non-disjunction q Non disjunction is the failure of homologous chromosomes or sister chromatids to separate properly during anaphase in cell division q See following 2 slides which show normal separation of chromosomes during anaphase and non disjunction q There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis. State the consequences of non-disjunction q Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy).

Normal Meiosis

Non- disjunction learn the definition

Describe how non-disjunction of chromosome pair 21 in humans may lead to the formation of an abnormal gamete with an extra copy of chromosome 21. Look at the last row of cells which are the gametes

Describe how the fusion between the abnormal gamete and a normal gamete may lead to Down Syndrome

Karyotype of a person with Trisomy 21 which is the cause of Down Syndrome

Some characteristics of a person with Down Syndrome Epicanthic fold Small, oblique eyes Flattened, nasal bridge Open mouth Protruding tongue Broad neck Small underdeveloped ears set low on head

Mitosis vs Meiosis Similarities Both mitosis and meiosis are associated with cytokinesis. The end result of both are daughter cells produced from a parent cell. The fundamental sequence of events in mitosis is the same as in meiosis (in meiosis it happens twice). Both processes include the breakdown of the nuclear membrane, the separation of genetic material into two groups, followed by cell division and the reformation of the nuclear membrane in each cells DNA replication must take place in interphase before both kinds of cell division

Mitosis vs Meiosis Differences – state or tabulate

From MTG