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Starter – recap from GCSE List the differences between meiosis and mitosis? This may help! Mitosis Daughter cells are genetically identical 2 daughter cells produced Daughter cells are diploid One division Normal body cells form Occurs in all areas of the body Produces genetic clones Meiosis Answers Mitosis Meiosis Daughter cells are genetically identical Daughter cells are genetically different 2 daughter cells produced Four daughter cells produced Daughter cells are diploid Daughter cells are haploid One division Two divisions Normal body cells form Gametes/Sex Cells form Occurs in all areas of the body Occurs in reproductive organs Produces genetic clones Produces genetic variation in cells Meiosis Learning objectives Understand the process and purpose of meiosis Explain how meiosis can lead to genetic variation Success criteria Describe, with the aid of diagrams and photographs, the behaviour of chromosomes during meiosis, and the associated behaviour of the nuclear envelope, cell membrane and centrioles. (Names of the main stages are expected, but not the subdivisions of prophase) Explain the terms allele, locus and crossing-over Explain how meiosis and fertilisation can lead to variation through the independent assortment of alleles Diploid and Haploid Normal body cells have the normal number of chromosomes 46 You might see this explained elsewhere as ‘2n’. We call these cells DIPLOID. It means that each body cell contains two of each chromosome – one from mum and one from dad. Gametes though, have half the normal number of chromosomes And this might be explained as just ‘n’. 23 We call these cells HAPLOID. There’s only one copy of each chromosome. At fertilisation, a haploid sperm will fuse with a haploid egg.... which makes a cell with the normal, diploid number of chromosomes Key definitions Meiosis – this is a reduction division resulting in the daughter chromosomes having half the original number of chromosomes. They are haploid. Allele – a version of a gene Locus – the position of a gene on a chromosome Key Terms: Put on as many labels as you know Key Terms: Answers Sister Chromatids Bivalents/ Homologous Pairs Chromatid Centromere Gene Gene Meiosis Review the notes you made in preparation for this lesson on the processes in meiosis as we whiz through the next powerpoint and animation Meiosis whiz thru Meiosis animation Micrographs: Put in order and name that stage! Stages: Answers Prophase I Metaphase II Metaphase I Anaphase II Anaphase I Telophase II Telophase I Genetic variation Crossing over – lengths of DNA are swapped form one chromosome to another Genetic reassortment (independent assortment/segregation) Of chromosomes - Random distribution of maternal and paternal chromosomes at metaphase 1 Of chromatids – Random distribution of sister chromatids at metaphase 11 Fertilisation – any egg can fuse with any sperm Mutation – may occur during interphase when DNA replicates Plenary – part 1 Event Division I or II 1. The chromosomes, each in the form of a pair of chromatids, have arrived at the poles of the spindle and have started to uncoil. 2. Homologous chromosomes form bivalents with chiasmata. 3. Sister chromatids move towards opposite poles of the spindle. 4. Pairs of homologous chromosomes line up at the equator of the spindle. I Phase (anaphase, metaphase, prophase or telophase) Telophase I Prophase II Anaphase I Metaphase Plenary - part 2 The diagram shows a cell undergoing cell division. Identify the type and stage of cell division shown. Give evidence from the diagram to support your answer. (3) (meiosis) anaphase I; chromosomes are moving apart; chromosomes still double structures; Homework In preparation for the next lesson write definitions for the following words: Gene Linkage Dominant Homozygous Haploid Allele Phenotype Recessive Heterozygous Diploid Locus Genotype Codominant Differences between mitosis and meiosis MITOSIS MEIOSIS One division Two divisions The number of chromosomes remains the same The number of chromosomes is halved Homologous chromosomes do not pair up Homologous chromosomes pair up to form bivalents Chiasmata do not form and crossing over never occurs Chiasmata form and crossing over occurs Daughter cells are genetically identical Daughter cells are genetically different from parent cells Two daughter cells are formed Four daughter cells are formed