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Mitosis Quiz List the five stages of Mitosis in order (You Have 4 Minutes) Cancer Article Assignment 1) Working in groups of two I want you to share what your article was about (You have 3 minutes) 2. Find another partner and again share your articles with each other. (You have 3 minutes) 3. Thinking about the three articles decide which one that you found the most interesting and on the back of your paper write three reasons why. (You have 5 minutes) Meiosis – A Source of Distinction Why do you share some but not all characters of each parent? What are the rules of this sharing game? At one level, the answers lie in meiosis. Overview: Variations on a Theme • Living organisms are distinguished by their ability to reproduce their own kind • Genetics is the scientific study of heredity and variation • Heredity is the transmission of traits from one generation to the next • Variation is demonstrated by the differences in appearance that offspring show from parents and siblings • Genes are the units of heredity, and are made up of segments of DNA Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs) • Each gene has a specific location called a locus on a certain chromosome • In asexual reproduction, one parent produces genetically identical offspring by mitosis • A clone is a group of genetically identical individuals from the same parent • In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents • Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomes • A karyotype is an ordered display of the pairs of chromosomes from a cell • The two chromosomes in each pair are called homologous chromosomes, or homologs • Sister Chromatids are two copies of a duplicated chromosome attached to each other at the centromere. -The sex chromosomes are called X and Y • Males XY Females XX -The 22 pairs of chromosomes that do not determine sex are called autosomes -A diploid cell (2n) has two sets of chromosomes -A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) • A gamete (sperm or egg) contains a single set of chromosomes, and is haploid (n) • For humans, the haploid number is 23 (n = 23) • Each set of 23 consists of 22 autosomes and a single sex chromosome • In an unfertilized egg (ovum), the sex chromosome is X • In a sperm cell, the sex chromosome may be either X or Y Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 13-3b TECHNIQUE 5 µm Pair of homologous replicated chromosomes Centromere Sister chromatids Metaphase chromosome Meiosis does two things 1) Meiosis takes a cell with two copies of every chromosome (diploid) and makes cells with a single copy of every chromosome (haploid). This is a good idea if you’re going to combine two cells to make a new organism. This trick is accomplished by halving chromosome number. In meiosis, one diploid cells produces four haploid cells. Why do we need meiosis? • Meiosis is necessary to halve the number of chromosomes going into the sex cells Why halve the chromosomes in gametes? • At fertilization the male and female sex cells will provide ½ of the chromosomes each – so the offspring has genes from both parents 2) Meiosis scrambles the specific forms of each gene that each sex cell (egg or sperm) receives. This makes for a lot of genetic diversity. This trick is accomplished through independent assortment and crossing-over. Genetic diversity is important for the evolution of populations and species. Meiosis Parent cell – chromosome pair Chromosomes copied 1st division - pairs split 2nd division – produces 4 gamete cells with ½ the original no. of chromosomes Meiosis – mouse testes Parent cell 1st division 2nd division 4 gametes Fig. 13-8b Prophase I Metaphase I Centrosome (with centriole pair) Sister chromatids Chiasmata Spindle Centromere (with kinetochore) Metaphase plate Homologous chromosomes Fragments of nuclear envelope Microtubule attached to kinetochore The Stages of Meiosis: • aka: Reduction Division Meiosis I : Separates Homologous Chromosomes • Interphase – Each of the chromosomes replicate – The result is two genetically identical sister chromatids which remain attached at their centromeres Prophase I • This is a crucial phase for mitosis. • During this phase each pair of chromatids don’t move to the equator alone, they match up with their homologous pair and fasten together (synapsis) in a group of four called a tetrad. • Extremely IMPORTANT!!! It is during this phase that crossing over can occur. • Crossing Over is the exchange of segments during synapsis. • Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred Metaphase I • The chromosomes line up at the equator attached by their centromeres to spindle fibers from centrioles. – Still in homologous pairs Anaphase I • The spindle guides the movement of the chromosomes toward the poles – Sister chromatids remain attached – Move as a unit towards the same pole • The homologous chromosome moves toward the opposite pole – Contrasts mitosis – chromosomes appear as individuals instead of pairs (meiosis) Telophase I • This is the end of the first meiotic cell division. • The cytoplasm divides, forming two new daughter cells. • Each of the newly formed cells has half the number of the parent cell’s chromosomes, but each chromosome is already replicated ready for the second meiotic cell division Cytokinesis • Occurs simultaneously with telophase I – Forms 2 daughter cells • Plant cells – cell plate • Animal cells – cleavage furrows • NO FURTHER REPLICATION OF GENETIC MATERIAL PRIOR TO THE SECOND DIVISION OF MEIOSIS Figure 13.7 The stages of meiotic cell division: Meiosis I Meiosis II : Separates sister chromatids • Proceeds similar to mitosis • THERE IS NO INTERPHASE II ! Prophase II • Each of the daughter cells forms a spindle, and the double stranded chromosomes move toward the equator Metaphase II • The chromosomes are positioned on the metaphase plate in a mitosislike fashion Anaphase II • The centromeres of sister chromatids finally separate • The sister chromatids of each pair move toward opposite poles – Now individual chromosomes Telophase II and Cytokinesis • Nuclei form at opposite poles of the cell and cytokinesis occurs • After completion of cytokinesis there are four daughter cells – All are haploid (n) Figure 13.7 The stages of meiotic cell division: Meiosis II One Way Meiosis Makes Lots of Different Sex Cells (Gametes) – Independent Assortment Independent assortment produces 2n distinct gametes, where n = the number of unique chromosomes. In humans, n = 23 and 223 = 6,000,0000. That’s a lot of diversity by this mechanism alone. Another Way Meiosis Makes Lots of Different Sex Cells – Crossing-Over Crossing-over multiplies the already huge number of different gamete types produced by independent assortment. The Key Difference Between Mitosis and Meiosis is the Way Chromosomes Uniquely Pair and Align in Meiosis Mitosis The first (and distinguishing) division of meiosis Boy or Girl? The Y Chromosome “Decides” Y chromosome X chromosome Boy or Girl? The Y Chromosome “Decides” Meiosis – division error Chromosome pair Meiosis error - fertilization Should the gamete with the chromosome pair be fertilized then the offspring will not be ‘normal’. In humans this often occurs with the 21st pair – producing a child with Downs Syndrome 21 trisomy – Downs Syndrome Can you see the extra 21st chromosome? Is this person male or female?