Genetics and Cell Cycle PDF

GENETICS Genetics - genes, genetic variation, and heredity in living organism CELL- fundamental unit of life NUCLEUS CHROMOSOME and DNA  GENETIC INFORMATION Gene – basic unit of genetic information. Genes determine the inherited characters. Genome – the collection o...

GENETICS Genetics - genes, genetic variation, and heredity in living organism CELL- fundamental unit of life NUCLEUS CHROMOSOME and DNA  GENETIC INFORMATION Gene – basic unit of genetic information. Genes determine the inherited characters. Genome – the collection of genetic information. Chromosomes – storage units of genes. DNA - is a nucleic acid that contains the genetic instructions specifying the biological development of all cellular forms of life  HUMAN GENOME human cells contain 46 chromosomes: 2 sex chromosomes (X,Y): XY – in males. XX – in females. 22 pairs of chromosomes named autosomes.  Gregor Johann Mendel Austrian Monk, born in what is now Czech Republic in 1822 Son of peasant farmer, studied Theology and was ordained priest Order St. Augustine. Went to the university of Vienna, where he studied botany and learned the Scientific Method Worked with pure lines of peas for eight years Prior to Mendel, heredity was regarded as a "blending" process and the offspring were essentially a "dilution"of the different parental characteristics.  Mendel’s peas Mendel looked at seven traits or characteristics of pea plants:  Mendel was the first biologist to use Mathematics – to explain his results quantitatively. Mendel predicted The concept of genes That genes occur in pairs That one gene of each pair is present in the gametes  Genetics terms you need to know: Alleles – two genes that occupy the same position on homologous chromosomes and that cover the same trait (like ‘flavors’ of a trait). Locus – a fixed location on a strand of DNA where a gene or one of its alleles is located.  Homozygous – having identical genes (one from each parent) for a particular characteristic. Heterozygous – having two different genes for a particular characteristic. Dominant – the allele of a gene that masks or suppresses the expression of an alternate allele; the trait appears in the heterozygous condition. Recessive – an allele that is masked by a dominant allele; does not appear in the heterozygous condition, only in homozygous.  Genotype – the genetic makeup of an organisms Phenotype – the physical appearance of an organism (Genotype + environment) Monohybrid cross: a genetic cross involving a single pair of genes (one trait); parents differ by a single trait. P = Parental generation F1 = First filial generation; offspring from a genetic cross. F2 = Second filial generation of a genetic cross  Monohybrid cross Parents differ by a single trait. Crossing two pea plants that differ in stem size, one tall one short T = allele for Tall t = allele for dwarf TT = homozygous tall plant t t = homozygous dwarf plant TT  tt  Monohybrid cross for stem length: P = parentals true breeding, TT  tt homozygous plants: (tall) (dwarf) F1 generation Tt is heterozygous: (all tall plants)  Punnett square A useful tool to do genetic crosses For a monohybrid cross, you need a square divided by four…. Looks like a window pane… We use the Punnett square to predict the genotypes and phenotypes of the offspring.  Using a Punnett Square STEPS: 1. determine the genotypes of the parent organisms 2. write down your "cross" (mating) 3. draw a p-square Parent genotypes: TT and t t Cross TT  tt  Punnett square 4. "split" the letters of the genotype for each parent & put them "outside" the p-square 5. determine the possible genotypes of the offspring by filling in the p-square 6. summarize results (genotypes & phenotypes of offspring) T T TT  tt Genotypes: t Tt Tt 100% T t Phenotypes: t Tt Tt 100% Tall plants  Monohybrid cross: F2 generation If you let the F1 generation self-fertilize, the next monohybrid cross would be: Tt  Tt (tall) (tall) Genotypes: 1 TT= Tall T t 2 Tt = Tall 1 tt = dwarf Genotypic ratio= 1:2:1 T TT Tt Phenotype: 3 Tall t Tt tt 1 dwarf Phenotypic ratio= 3:1  Secret of the Punnett Square Key to the Punnett Square: Determine the gametes of each parent… How? By “splitting” the genotypes of each parent: If this is your cross T T  t t The gametes are: T T t t Once you have the gametes… T T  t t t t T Tt Tt T Tt Tt  Shortcut for Punnett Square… If either parent is HOMOZYGOUS T T  t t t Genotypes: 100% T t T Tt Phenotypes: 100% Tall plants You only need one box!  Understanding the shortcut… t t t T Tt Tt = T Tt Genotypes: T Tt Tt 100% T t Phenotypes: 100% Tall plants  If you have another cross… A heterozygous with a homozygous T t  t t You can still use the shortcut! t Genotypes: 50% T t T Tt 50 % t t Phenotypes: t t t 50% Tall plants 50% Dwarf plants  Another example: Flower color For example, flower color: P = purple (dominant) p = white (recessive) If you cross a homozygous Purple (PP) with a homozygous white (pp): PP  pp Pp ALL PURPLE (Pp) Cross the F1 generation: Pp  Pp Genotypes: 1 PP P p 2 Pp 1 pp P PP Pp Phenotypes: 3 Purple p Pp pp 1 White  Mendel’s Principles 1. Principle of Dominance: One allele masked another, one allele was dominant over the other in the F1 generation. 2. Principle of Segregation: When gametes are formed, the pairs of hereditary factors (genes) become separated, so that each sex cell (egg/sperm) receives only one kind of gene.  Human case: CF Mendel’s Principles of Heredity apply universally to all organisms. Cystic Fibrosis: a lethal genetic disease affecting Caucasians. Caused by mutant recessive gene carried by 1 in 20 people of European descent (12M) One in 400 Caucasian couples will be both carriers of CF – 1 in 4 children will have it. CF disease affects transport , in tissues – mucus is accumulated in lungs, causing infections. Autosomal recessive  Inheritance pattern of CF IF two parents carry the recessive gene of Cystic Fibrosis (c), that is, they are heterozygous (C c), one in four of their children is expected to be homozygous for cf and have the disease: C c C C = normal C c = carrier, no symptoms C CC Cc c c = has cystic fibrosis c Cc cc  Probabilities… Of course, the 1 in 4 probability of getting the disease is just an expectation, and in reality, any two carriers may have normal children. However, the greatest probability is for 1 in 4 children to be affected. Important factor when prospective parents are concerned about their chances of having affected children. Blood group inheritance from parents to offspring. The 4 pairs of alleles in smaller font are the genotypes based on the parental alleles. Note the only way the result will be type O with no other possibility is when both parents are homozygous recessive for type O. Otherwise, A or B dominate or are codominant as type AB  Dihybrid crosses Matings that involve parents that differ in two genes (two independent traits) For example, flower color: P = purple (dominant) p = white (recessive) and stem length: T = tall t = short  Dihybrid cross: flower color and stem length TT PP  tt pp (tall, purple) (short, white) tp tp tp tp Possible Gametes for parents TP TtPp TtPp TtPp TtPp TP and t p TP TtPp TtPp TtPp TtPp TP TtPp TtPp TtPp TtPp F1 Generation: TP All tall, purple flowers (Tt Pp) TtPp TtPp TtPp TtPp  Dihybrid cross F2 If F1 generation is allowed to self pollinate, Mendel observed 4 phenotypes: Tt Pp  Tt Pp (tall, purple) (tall, purple) Possible gametes: TP Tp tP TP Tp tP tp tp TP TTPP TTPp TtPP TtPp Four phenotypes observed Tall, purple (9); Tall, white (3); Tp TTPp TTpp TtPp Ttpp Short, purple (3); Short white (1) tP TtPP TtPp ttPP ttPp tp TtPp Ttpp ttPp ttpp  Dihybrid cross 9 Tall purple TP Tp tP tp TP TTPP TTPp TtPP TtPp Tp 3 Tall white tP TTPp TTpp TtPp Ttpp tp TtPP TtPp ttPP ttPp TtPp Ttpp ttPp ttpp 3 Short purple Phenotype Ratio = 9:3:3:1 1 Short white  Dihybrid cross: 9 genotypes Genotype ratios (9): Four Phenotypes: 1 TTPP 2 TTPp Tall, purple (9) 2 TtPP 4 TtPp Tall, white (3) 1 TTpp 2 Ttpp Short, purple (3) 1 ttPP Short, white (1) 2 ttPp 1 ttpp Principle of Independent Assortment Based on these results, Mendel postulated the 3. Principle of Independent Assortment: “Members of one gene pair segregate independently from other gene pairs during gamete formation” Genes get shuffled – these many combinations are one of the advantages of sexual reproduction  Incomplete Dominance Snapdragon flowers come in many colors. If you cross a red snapdragon (RR) with a white snapdragon (rr) You get PINK flowers (Rr)! RR  rr Genes show incomplete dominance when the heterozygous phenotype Rr is intermediate.  Incomplete dominance When F1 generation (all pink flowers) is self pollinated, the F2 generation is 1:2:1 red, pink, white Incomplete Dominance R r R R R Rr r Rr rr  Recessive Inheritance Unaffected Unaffected ‘Carrier’ ‘Carrier’ Father Mother R r R r R R r R R r r r ‘Carrier’ Affected 1 in 4 Unaffected 1 in 4 ‘Carrier’ Unaffected 1 chance chance Unaffected 1 in in 4 chance 4 chance R = A dominant genetic feature Unaffected ‘Carrier’ Unaffected r = a recessive genetic feature Affected  X-linked Inheritance Unaffected Usually Father Unaffected ‘Carrier’ Mother X Y X’ X X X X Y X X’ X’ Y Unaffected Unaffected ‘Carrier’ Affected Unaffected SON DAUGHTER 1 DAUGHTER 1 SON 1 1 in 4 chance in 4 chance in 4 chance in 4 chance X’ =A genetic feature carried Unaffected ‘Carrier’ on the X chromosome Unaffected Affected THE CELL CYCLE AND MITOSIS MEOSIS  CELL REPRODUCTION Cell Division: process by which a cell divides to form two new cells (daughter cells) Three types of cell division, or cell reproduction – Prokaryotes (bacteria) Binary fission divides forming two new identical cells – Eukaryotes Mitosis – Cell or organism growth – Replacement or repair of damaged cells Meiosis – formation of sex cells, or gametes  PROKARYOTIC CELL DIVISION Binary fission – 3 main steps: 1: DNA Replication—DNA is copied, resulting in 2 identical chromosomes 2: Chromosome Segregation—2 chromosomes separate, move towards ends (poles) of cell 3: Cytokinesis—cytoplasm divides, forming 2 cells – Each new daughter cell is genetically identical to parent cell  THE CELL CYCLE G1 phase M phase S phase G2 phase  CELL CYCLE-INTERPHASE Interphase: period of growth and DNA replication between cell divisions Three phases: – G1 Phase cell increases in size – S Phase Replication of chromosomes – Now two strands called sister chromatids joined by a centromere – G2 Phase organelles double new cytoplasm forms All other structures needed for mitosis form  DNA containing cell’s genetic code Each chromosome has a matching pair -- Homologous Pair During interphase, each chromosome copies itself  EUKARYOTIC CELL DIVISION DNA found on chromosomes located in nucleus of cell Cell cycle continuous process – Cells grow – DNA replicated – Organelles duplicated – Divide to form daughter cells – 2 Main steps: 1: Mitosis (4 steps—Prophase, Metaphase, Anaphase, Telophase) Nucleus divides 2: Cytokinesis—Cytoplasm divide, forming 2 cells Each new daughter cell is genetically identical to parent cell  MITOSIS Process that divides cell nucleus to produce two new nuclei each with a complete set of chromosomes Continuous process Four phases (PMAT) – Prophase – Metaphase – Anaphase – Telophase  Prophase 1. chromosomes visible (sister chromatids) 2. centrioles migrate to the poles (only in animals) 3. nuclear membrane disappears 4. spindle forms 1. chromosomes line up on the equator of the cell 2. spindles attach to centromeres Equator 1. sister chromatids separate 2. centromeres divide 3. sister chromatids move to opposite poles 1. chromosomes uncoil now chromatin 2. nuclear membranes reform 3. spindle disappears -Occurs at end of Mitosis --division of the cytoplasm to form 2 new daughter cells --organelles are divided -Daughter cells are genetically identical Cells return to interphase  Control of the Cell Cycle Regulatory proteins called cyclins control the cell cycle at checkpoints: G1 Checkpoint—decides whether or not cell will divide S Checkpoint—determines if DNA has been properly replicated Mitotic Spindle Checkpoint—ensures chromosomes are aligned at mitotic plate  CANCER CELLS Result of uncontrolled cell division of cells that have lost ability to regulate cell cycle Reproduce more rapidly than normal cells Masses formed called ‘tumors’  Overview of Meiosis Meiosis is a form of cell division that leads to the production of gametes. gametes: egg cells and sperm cells – contain half the number of chromosomes of an adult body cell – Adult body cells (somatic cells) are diploid, containing 2 sets of chromosomes. – Gametes are haploid, containing only 1 set of chromosomes. Sexual reproduction includes the fusion of gametes (fertilization) to produce a diploid zygote. Life cycles of sexually reproducing organisms involve the alternation of haploid and diploid stages. Some life cycles include longer diploid phases, some include longer haploid phases. 65  Features of Meiosis Meiosis includes two rounds of division – meiosis I and meiosis II. During meiosis I, homologous chromosomes (homologues) become closely associated with each other. This is synapsis. Proteins between the homologues hold them in a synaptonemal complex. Crossing over: genetic recombination between non-sister chromatids Physical exchange of regions of the chromatids chiasmata: sites of crossing over The homologues are separated from each other in anaphase I. Meiosis involves two successive cell divisions with no replication of genetic material between them. Meiosis II resembles mitosis Results in a reduction of the chromosome number from diploid to haploid. 66  The Process of Meiosis Prophase I: -chromosomes coil tighter -nuclear envelope dissolves -homologues become closely associated in synapsis -crossing over occurs between non-sister chromatids 67 Metaphase I: -terminal chiasmata hold homologues together following crossing over -microtubules from opposite poles attach to each homologue, not each sister chromatid -homologues are aligned at the metaphase plate side-by-side -the orientation of each pair of homologues on the spindle is random Anaphase I: -microtubules of the spindle shorten -homologues are separated from each other -sister chromatids remain attached to each other at their centromeres 68 Telophase I: -nuclear envelopes form around each set of chromosomes -each new nucleus is now haploid -sister chromatids are no longer identical because of crossing over Meiosis II resembles a mitotic division: -prophase II: nuclear envelopes dissolve and spindle apparatus forms -metaphase II: chromosomes align on metaphase plate -anaphase II: sister chromatids are separated from each other -telophase II: nuclear envelope re-forms; cytokinesis follows 69  Meiosis vs. Mitosis 1. Synapsis and crossing over 2. Sister chromatids remain joined at their centromeres throughout meiosis I 3. Kinetochores of sister chromatids attach to the same pole in meiosis I Meiosis produces haploid cells that are not identical to each other. Genetic differences in these cells arise from: - crossing over - random alignment of homologues in metaphase I (independent assortment) Mitosis produces 2 cells identical to each other. 70