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GeneticsHeredity and Meiosis IB Biology Campbell- Chapter 13 Orange Book- Ch. 11 Chromosomes • Condensed DNA attached to proteins • Can only be seen when a cell is actively undergoing mitosis. • Typical humans form 46 chromosomes vs. other organisms which varies significantly. • Our 46 chromosomes are thought to contain anywhere from 25,000 to 100,000 genes. Sister chromatids Centromere Terms to Know: Gene- a heritable factor that controls a specific characteristic. Allele- One specific form of a gene. Genome- the whole genetic information of an organism. Heredity and Sexual Reproduction • Heredity: the transmission of traits from one generation to the next • Sexual reproduction = new individual by a combination of two haploid sex cells (gametes). • Fertilization = combination of genetic information from two separate cells that have one half the original genetic information Sexual Reproduction • Female- produces an egg • Male produces sperm • Both gametes are haploid (n) or a single set of chromosomes • new individual called a zygote, with two sets of chromosomes (diploid or 2n). • Meiosis is a process to convert a diploid cell to a haploid gamete, and cause a change in the genetic information to increase diversity in the offspring. Ploidy: Number of sets of chromosomes in a cell • Haploid (n)-- one set chromosomes • Eggs and sperm are haploid (n) • Diploid (2n)-- two set of chromosomes. Most plant and animal cells (somatic cells) are diploid (2n) Chromosomes in a Diploid Cell • Summary of chromosome characteristics • Diploid set for humans; 2n = 46 • Autosomes; homologous chromosomes, one from each parent (humans = 22 sets of 2), plus: • Sex chromosomes (humans have 1 set of 2) – Female-sex chromosomes are homologous (XX) – Male-sex chromosomes are non-homologous (XY) The Human Life Cycle Reduction Division • Living organisms which reproduce sexually halve their chromosome number in preparation for fusion of gametes. At fertilization, a sperm fuses with an egg, forming a diploid zygote – Repeated mitotic divisions lead to the development of a mature adult – The adult makes haploid gametes by meiosis – All of these processes make up the sexual life cycle of organisms Meiosis • Chromosome replicate • 2 Cell divisions occur (Meiosis I & Meiosis II) • 4 daughter cells are made all are (n): haploid • Homologous Chrom’s separate in meiosis I • Meiosis II = Mitosis (chromatids separate) Homologous chromosomes carry different versions of genes • The differences between homologous chromosomes are based on the fact that they can carry different versions of a gene (alleles) at corresponding loci Meiosis divided into meiosis I and meiosis II • Prophase I – pairing of homologous chromosomes • Called synapsis • Animations (unique features ) Prophase 1 • Chromosomes pair up as homologous chromosomes • Nuclear membrane disappears • Spindle fibers grow from each pole Prophase 1 Homologs find their partner, becoming intimately associated, forming a synaptonemal complex ,a lattice of proteins This is called a tetrad (basically- 2 pairs of chromosome pairs that are attached) Prophase 1 • With this relationship of homologs, crossing over occurs. • Crossing over is the exchange of corresponding segments between two homologous chromosomes • Genetic recombination results from crossing over during prophase I of meiosis • The site of crossing over is called chiasmata. Prophase 1 Tetrad Chiasma Centromere Metaphase I • Terminal chiasmata hold homologs together. • Chromosomes line up at the center. • Spindle attaches to only one side, which will pull sister together. Anaphase I • Microtubules shorten and chromosomes separate. • Break chiasmata • At finish each pole has a full set of chromosomes which have been independently assorted. Telophase I • Chromosomes cluster • Nuclear membrane forms • Cell may or may not split into 2 (depends on the species) Meiosis II* • Division time variable but typically a short Interphase • Cytokinesis may or may not occur • No synthesis (chromosomes do not double) Prophase II • Breakdown nuclear membrane • Centrioles form and move toward the poles • New spindles form Metaphase II • Microtubules grow from the centrioles and attach to the centromeres • The sister chromatids line up along the cell equator Anaphase II • The centromeres break and sister chromatids separate • Cytokinesis begins Telophase II • The chromosomes may decondense (depends on species) • Cytokinesis reaches completion, creating four haploid daughter cells Origins of Genetic Variation (1) Independent assortment: How they line up during metaphase I Matters!!! Homologous pairs of chromosomes position and orient themselves Randomly. (random positioning) Different combinations are possible when gametes are produced. POSSIBILITY 1 POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 Origins of Genetic Variation (2) Crossing over (prophase I): -the reciprocal exchange of genetic material between nonsister chromatids during synapsis of meiosis I (recombinant chromosomes) (3) Random fertilization: 1 sperm (1 of 8 million possible chromosome combinations) x 1 ovum (1 of 8 million different possibilities) = 64 trillion diploid combinations! Meiosis vs. Mitosis http://www.pbs.org/wgbh/nova/baby/divi_flash.html • Synapsis/tetrad/chiasmata (prophase I) • Homologous vs. individual chromosomes (metaphase I) • Sister chromatids do not separate (anaphase I) • Meiosis I separates homologous pairs of chromosomes, not sister chromatids of individual chromosomes. Mitosis Meiosis Results in Identical cells Cells not identical Cells are diploid Cells are haploid In most all cells In only the sex cells No crossing over Crossing over/chiasmata Sister chromatids pair up Tetrads pair up One set of phases 2 sets of phases Ends after first set Meiosis II- Chromosomes do not double Let’s watch • Meiosis Review • Meiosis • meiosis = double cell division The Human Karyotype • There are 44 autosomes and 2 sex chromosomes in the human genome, for a total of 46. • Karyotypes are pictures of homologous chromosomes lined up together during Metaphase I of meiosis. • The chromosome micrographs are then arranged by size and pasted onto a sheet. Human female karyotype Human Male Karyotype Karyotypes • Used for pre-natal diagnosis of chromosome abnormalities and to positively identify gender. • Performed using cells collected by amniocentesis or chorionic villi sampling (the removal of a small piece of placenta tissue from the uterus during early pregnancy). Chorionic Villus sampling Ethical issues • Side effects (possible miscarriage) • Aborting a fetus based on sex or possible genetic diseases • Who should make the decisions- parents, health-care professionals, or both?