Genetics 1 Reduction Division - Meiosis

Review: Cell Cycle and Mitosis Draw the cell cycle http://outreach.mcb.harvard.edu/animations/ce llcycle.swf List the steps of mitosis http://www.johnkyrk.com/mitosis.html

What stages?

Keyword Definition basic unit of heredity; codes for a specific trait the specific location of a gene on a chromosome the total hereditary endowment of DNA of a cell or organism all body cells except reproductive cells reproductive cells (i.e. sperm & eggs) elongate cellular structure composed of DNA and protein - they are the vehicles which carry DNA in cells cellular condition where each chromosome type is represented by two homologous chromosomes cellular condition where each chromosome type is represented by only one chromosome chromosome of the same size and shape which carry the same type of genes one of two duplicated chromosomes connected at the centromere region of chromosome where microtubules attach during mitosis and meiosis GENE LOCUS GENOME SOMATIC GAMETES CHROMOSOME DIPLOID (2n) HAPLOID (n) HOMOLOGOUS CHROMATID CENTROMERE

Meiosis The process by which a diploid nucleus (2n) divides to produce four haploid daughter nuclei (n) which are not genetically identical to each other or the parent nucleus. Produces gametes. Results in genetic variation.

Stages of Meiosis Follows G2 stage of interphase DNA replication has already occurred. Two distinct divisions – meiosis 1 followed by meiosis 2

Meiosis 1 and 2 Meiosis 1 Homologous pairs of chromosomes are separated Meiosis 2 Sister chromatids are separated

At end of Interphase DNA replicated but not coiled so no chromosomes visible Nuclear membrane present Two centrioles Nucleolus present

Chromosome Structure

Meiosis 1 Early Prophase 1 DNA condenses and becomes visible Two chromatids attached at a CENTROMERE Centrioles begin to move Nucleolus disappears

Meiosis 1 – Mid Prophase 1 Homologous chromosomes pair up and lie along side each other (SYNAPSIS) Each pair called a BIVALENT Centrioles move further towards poles

Meiosis 1 – Late Prophase 1 Chromatids tangle at points called CHIASMATA (chiasma sing.) Chromosomes may break and rejoin CROSSING OVER occurs Reformed chromatids called RECOMBINANTS

Meiosis 1 – Metaphase 1 Nuclear membrane breaks down Spindle forms Bivalents move to the equator and attach to the spindle by the centromere

Meiosis 1 – Anaphase 1 Contraction of spindle fibres pulls whole chromosomes towards poles Homologous chromosomes separate

Meiosis 1 – Telophase 1 Nuclear membrane reforms around each set of chromosomes Spindle fibres disappear Centrioles divide into two

Cytokinesis Division of the cytoplasm occurs. There may be a short pause

Meiosis 2 – Prophase 2 Centrioles move towards opposite poles

Meiosis 2 – Metaphase 2 Nuclear membranes disappear Spindles form at right angles to the first Chromosomes move to the equator and attach to spindle fibres by centromeres

Meiosis 2 – Anaphase 2 Centromeres divide Sister chromatids move towards opposite poles

Meiosis 2 – Telophase 2 Each chromatid now called a chromosome Nuclear membrane reforms around each group of chromosomes Spindle fibres disappear and centrioles may divide

Cytokinesis Division of the cytoplasm Four haploid daughter cells are produced Each cell genetically different

Variation Results from: Crossing over – during prophase 1 creates new combinations of genes Reduction – fertilisation of haploid gametes produces new combinations of genes and restores the chromosome number from generation to generation Independent assortment –during metaphase 1 it is random which way the chromosomes line up

Independent Assortment

Possible combinations No. of possible combinations in a gamete = 2n (where n = haploid number of the species) In humans n = 23 So any one of 8,388,608 combinations of maternal and paternal chromosomes may be found in ONE gamete. Crossing over means the variety is endless

Comparison between Mitosis and Meiosis Meiosis animation Mitosis v Meiosis List three similarities between the two types of cell division. Complete the table on the worksheet by selecting the appropriate statements