MCB 181 1st Edition Lecture 22Outline of Previous Lecture I. Control of the Cell Cycle (Revisited)II. Cancer: Out of Control Cell DivisionOutline of Current Lecture I. Meiosis - The Race to Create LifeII. Process of MeiosisIII. Clicker QuestionsCurrent LectureI. Meiosis - The Race to Create Lifea. Inheritance of genesi. Genes are the units of heredity, and are made up of segments of DNAii. Genes are passed to the next generation through gametes (sperm and eggs)iii. Each gene has a specific location called a locus on a certain chromosomeiv. One set of chromosomes is inherited from each parentb. Sets of chromosomes in human cellsi. Human somatic cells (any cell other than a gamete) have 23 pairs of chromosomesii. The two chromosomes in each pair are called homologous chromosomes, or homologsiii. Chromosomes in a homologous pair are the same length and carry genes controlling the same inherited characteristicsiv. Sex chromosomes determine the sex of the individualv. All other chromosomes are called autosomesc. Ploidy – the combination of the number of sets of n in the celld. Haploid – n stands for the number of distinct types of chromosomes in a giveni. Is called the haploid numbere. Diploid – two chromosomes for each distinct type of chromosomef. Karyotype – the number and types of chromosomes present in an organismII. Process of Meiosisa. Meiosis reduces the chromosome number by halfb. In fertilization, the diploid number of chromosomes is restored c. What happens before meiosis begins?i. Just before meiosis begins, each chromosome in the diploid 2n parent cell is replicatedii. When replication is completed, each chromosome consists of two identical sister chromatids attached at the centromered. Meiosis 1i. The homologous chromosomes of each chromosome pair separate and go to different daughter cellsii. Although the daughter cells are haploid 1n, each chromosome has two identical sister chromatids still attached at the centromeree. Meiosis IIi. The sister chromatids of each chromosome separate and go to each of two daughter cellsii. At the end of meiosis II, there are four haploid gametes each containing one copy of each chromosomef. What are the end results of meiosis I?i. At the end of meiosis I, each chromosome consists of two sister chromatidsii. Because only one member of each homologous pair of chromosomes is present the cell is haploidg. The phases of meiosis IIi. At the start of meiosis II, each daughter cell is haploid; but each replicated chromosome is composed of two identical sister chromatidsii. At the end of meiosis II, each cell divides and four haploid daughter cells daughter cells are producedh. A closer look at key events in prophase of meiosis Ii. During synapsis in prophase I, the two pairs of sister chromatids are held tightly together, and both sets of non-sister chromatids may undergo crossingover and exchange segmentsii. Synapsis – the physical pairing of two homologous chromosomes during prophase I of meiosis1. Crossing over occurs during synapsisiii. Chiasma – the X-shape structure formed during meiosis by crossing over between two non sister chromatids in a pair of homologous chromosomesi. Crossing Overi. Crossing over produces new combinations of alleles on the same chromosome1. These new combination didn’t in either parentii. The phenomenon is known as recombinationiii. Genetic recombination is any change in the combination of alleles on a given chromosomeIII. Clicker QuestionsFact: Drosophila melanogaster has a ploidi of 4Assertion ReasonThe somatic cells of Drosophila contain 4 chromosomes BecauseThe germ cells of drosophila contains 8 chromosomesAssertion is false; Reason is false. Germ cells are haploid where as somatic cells, in thisorganism, are diploid so it should be 8 and 4 chromosomes respectively.Fact: In mice, certain mutations in the msh-4 gene prevent crossing-over from occurring duringmeiosisAssertion ReasonThe gametes of the msh-4 mutant mice will produce lessgenetically diverse gametesBecauseCrossing over increases the genetic diversity of the gametesAssertion is true; Reason is true; Reason is the correct