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General Biology • Overview: Hereditary Similarity and Variation • Living organisms Course No: BNG2003" Credits: 3.00 " – are distinguished by their ability to reproduce their own kind " " 9. Meiosis and Life Cycles Prof. Dr. Klaus Heese • Genetics • Heredity – is the transmission of traits from one generation to the next – is the scientific study of heredity and hereditary variation • Offspring acquire genes from parents by inheriting chromosomes Inheritance of Genes • Variation – shows that offspring differ somewhat in appearance from parents and siblings • Genes – are the units of heredity – are segments of DNA • Each gene in an organism’s DNA has a specific locus on a certain chromosome • We inherit one set of chromosomes from our mother and one set from our father Comparison of Asexual and Sexual Reproduction • In asexual reproduction – one parent produces genetically identical offspring by mitosis • In sexual reproduction – Two parents give rise to offspring that have unique combinations of genes inherited from the two parents • Fertilization and meiosis alternate in sexual life cycles • A life cycle – is the generation-to-generation sequence of stages in the reproductive history of an organism Sets of Chromosomes in Human Cells Parent Bud • in humans – each somatic cell has 46 chromosomes, made up of two sets 0.5 mm – one set of chromosomes comes from each parent 1 What is an Allele? Homologous Chromosomes • Alleles are alternative forms of a gene • The two chromosomes, one from each of your parents, which carry the information for similar genes • One allele for a gene carried on one chromosome – Chromosome can have many, many genes • Example: • We get 23 from mom, 23 from dad – If you have type AB blood, you got the A allele from one parent, the B allele from another Diploid vs. Haploid Trick Question • Most human somatic (body) cells have 46 chromosomes • We get 23 chromosomes from our mom, and 23 from our dad. Which 23 does each of us pass on to our children? – This is called the diploid condition – 2n = 46 • Gametes (sperms and eggs) are an exception: they only have 23 chromosomes – This is the haploid condition – n = 23 Answer • The 23 that we pass on to our offspring, are each a medley of the 23 pairs of chromosomes that we have. • Hence, we pass on parts of both! • Meiosis allows us to do this • In a cell in which DNA synthesis has occurred – all the chromosomes are duplicated and thus each consists of two identical sister chromatids Key 2n = 6 Maternal set of chromosomes (n = 3) Paternal set of chromosomes (n = 3) Two sister chromatids of one replicated chromosome Two nonsister chromatids in a homologous pair Centromere Pair of homologous chromosomes (one from each set) 2 • A Karyotype Karyotyping – is an ordered, visual representation of the chromosomes in a cell • literally, a picture of your chromosomes Pair of homologous chromosomes 5 µm Centromere • can be used to detect some genetic abnormalities – too many, or too few chromosomes Sister chromatids • Homologous chromosomes Trisomy 21: Down’s syndrome – are the two chromosomes composing a pair • 3 copies of chromosome #21 present – have the same characteristics • Many symptoms – may also be called autosomes – Round face • Sex chromosomes – Flattened nose – are distinct from each other in their characteristics – Small, irregular teeth – are represented as X and Y – Short stature – determine the sex of the individual, XX being female, XY being male • A diploid cell – has two sets of each of its chromosomes – in a human has 46 chromosomes (2n = 46) – Heart defects – Respiratory infection – Leukemia – Alzheimer’s disease (APP on chr21) / similar symptoms XYY Metafemale • Extra Y chromosome • XXX • Often taller • Reduced fertility • Produces an otherwise normal male • Otherwise normal female 3 • Unlike somatic cells • The human life cycle – gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes Key • At sexual maturity – the ovaries and testes produce haploid gametes by meiosis Haploid gametes (n = 23) Haploid (n) Diploid (2n) Behavior of Chromosome Sets in the Human Life Cycle Ovum (n) during meiosis meiocytes produce haploid cells Sperm Cell (n) FERTILIZATION MEIOSIS • During fertilization Ovary – these gametes, sperm and ovum, fuse, forming a diploid zygote Diploid zygote (2n = 46) Testis Mitosis and development • the zygote – develops into an adult organism Multicellular diploid adults (2n = 46) The Variety of Sexual Life Cycles • The three main types of sexual life cycles during meiosis meiocytes produce haploid cells – differ in the timing of meiosis and fertilization • In animals – Meiosis occurs during gamete formation Key – Gametes are the only haploid cells Haploid Diploid n during meiosis meiocytes produce haploid cells n MEIOSIS FERTILIZATION Zygote 2n Diploid multicellular organism • Plants and some algae n Gametes 2n Mitosis (a) Animals • In most fungi and some protists – Exhibit an alternation of generations – The life cycle includes both diploid and haploid multicellular stages Haploid multicellular organism (gametophyte) – Meiosis produces haploid cells that give rise to a haploid multicellular adult organism – The haploid adult carries out mitosis, producing cells that will become gametes Haploid multicellular organism n Mitosis n Mitosis n n n n n Gametes FERTILIZATION MEIOSIS Diploid multicellular organism (sporophyte) 2n (b) Plants and some algae Mitosis n Gametes MEIOSIS n Mitosis Spores 2n Mitosis Zygote n FERTILIZATION 2n Zygote (c) Most fungi and some protists 4 Meiosis Basic Concepts Two Parts of Meiosis • Two things achieved: • Meiosis I – Recombines the chromosomes your parents gave you • Meiosis II – Produces haploid gamete cells, having only 23 chromosomes • Both are similar to, but also different from mitosis (---> see cell cycle, cell proliferation) Meiosis II Meiosis I • Starts with two haploid cells • This step reduces chromosomes • Produces four haploid cells – Starts with diploid (2n = 46) cell – These are 4 sperms or 4 eggs – Produces two haploid (n = 23) cells • Also is where recombination occurs • Meiosis reduces the number of chromosome sets from diploid to haploid • Meiosis takes place in two sets • Interphase and meiosis I Interphase Homologous pair of chromosomes in diploid parent cell MEIOSIS I: Separates homologous chromosomes INTERPHASE of divisions, PROPHASE I meiosis I and meiosis II METAPHASE I ANAPHASE I Chromosomes replicate Homologous pair of replicated chromosomes Centrosomes (with centriole pairs) The Stages of Meiosis Sister chromatids Diploid cell with replicated chromosomes • An overview of meiosis Sister chromatids Chiasmata Sister chromatids remain attached Centromere (with kinetochore) Spindle Metaphase plate Meiosis I • Meiosis I reduces the number of chromosomes from diploid to haploid 1 Homologous chromosomes separate Haploid cells with replicated chromosomes Meiosis II • Meiosis II produces four haploid daughter cells 2 Sister chromatids separate Nuclear Homologous Microtubule envelope chromosomes Tetrad attached to Chromatin separate replicated (homologous) kinetochore Pairs of homologous Chromosomes duplicate Tetrads line up chromosomes chromosomes split up Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example Haploid cells with unreplicated chromosomes 5 A Comparison of Mitosis and Meiosis • Telophase I, cytokinesis, and meiosis II • Meiosis and mitosis can be distinguished from mitosis – by three events in Meiosis l MEIOSIS II: Separates sister chromatids TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS • Synapsis and crossing over – Homologous chromosomes physically connect and exchange genetic information • Tetrads on the metaphase plate Cleavage furrow Two haploid cells form; chromosomes are still double Sister chromatids separate Haploid daughter cells forming – at metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates • Separation of homologues: at anaphase I of meiosis, homologous pairs move toward opposite poles of the cell; in anaphase II of meiosis, the sister chromatids separate During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes replicated chromosomes • A comparison of mitosis and meiosis MITOSIS MEIOSIS Chiasma (site of crossing over) Parent cell (before chromosome replication) MEIOSIS I Prophase I Prophase Chromosome replication Duplicated chromosome (two sister chromatids) Chromosome replication Tetrad formed by synapsis of homologous chromosomes 2n = 6 diploid cell Chromosomes positioned at the metaphase plate Metaphase Sister chromatids separate during anaphase Anaphase Telophase 2n Tetrads positioned at the metaphase plate Metaphase I Homologues separate during anaphase I; sister chromatids remain together Anaphase I Telophase I Haploid n=3 haploid cells with replicated chr Daughter cells of meiosis I 2n MEIOSIS II Daughter cells of mitosis n diploid cells n n Daughter cells of meiosis II n n=3 haploid cells Sister chromatids separate during anaphase II Independent Assortment of Chromosomes • Homologous pairs of chromosomes – orient randomly at metaphase I of meiosis • In independent assortment – each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs Key Maternal set of chromosomes Paternal set of chromosomes Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4 6 Crossing Over Random Fertilization • Crossing over • the fusion of gametes will produce a zygote with any of about 64 trillion diploid combinations – Produces recombinant chromosomes that carry genes derived from two different parents Prophase I of meiosis Nonsister chromatids Evolutionary Significance of Genetic Variation Within Populations • Genetic variation Tetrad Chiasma, site of crossing over Metaphase I – is the raw material for evolution by natural selection • Mutations – are the original source of genetic variation Metaphase II • Sexual reproduction – produces new combinations of variant genes, adding more genetic diversity Daughter cells Recombinant chromosomes • Genetic variation produced in sexual life cycles contributes to ‘evolution’ • Reshuffling of genetic material in meiosis – produces genetic variation Origins of Genetic Variation Among Offspring • In species that produce sexually – the behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises each generation Taking home message: Epigenetics In biology, and specifically genetics, epigenetics is the study of inherited changes in phenotype (appearance) or gene expression caused by mechanisms other than changes in the underlying DNA sequence, hence the name epi- (Greek: επίover, above) -genetics. These changes may remain through cell divisions for the remainder of the cell's life and may also last for multiple generations. However, there is no change in the underlying DNA sequence of the organism; instead, nongenetic factors cause the organism's genes to behave or express themselves differently. The hypermethylation of CpG islands in the promoter regions of tumor suppressor genes is a common hallmark of human cancer, and it is associated with inactivation of these genes. p60TRP is particularly interesting due to its location on the X chromosome (Xq23), one copy of which is randomly inactivated by DNA methylation in females. 7