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Genetics BIO 221 Genetics Heredity and Variation: *Heredity is the branch of science explains how related organisms resemble each other. *Variation is a science discovers why related individuals differ from each other. Mendel’s work:Gregor Mendel born in 1822 and in 1843 studding natural history in Vienna University. In 1856 studied the inheritance in pea by hybridization. The choice of pea has 5 advantages which are :1- Presence of 22 varieties of pea. 2- Pea easy to cultivate. 3- Plant self pollinating because flower is closed = Pure breeding. 4- Easy artificial cross –breeding between varieties. 5- There are 7 characters (traits) as: stem length - seed shape – seed color - pod shape – pod color – flower color – flower position. Monohybrid inheritance and the principle of segregation:He work on 2 varieties one with terminal flower and the other with axial flower. He takes pollen from axial flower to terminal flower plant and vice versa. In all cases the resulted generation F1 was axial flower in the first generation so axial flower called dominant. Genetics 1 Genetics BIO 221 The F1 plants enclosed in a bags to prevent cross-pollination and left to selfpollinated to give the 2nd generation F2, he found some flowers with axial and other with terminal flowers Parents : in ratios 3:1 Axial flowers F1 × Terminal flowers All axial flowers F2 Axial flowers 3 : Terminal flowers 1 The separation of the pair of parental factors (one factor present in each gamete)is known as Mendel’s first law or the principle of segregation. This stated that: the characteristics of an organism are determined by internal factors which occur in pairs, only one of a pair of such factors can be represented in a single gamete.The initial letter of the dominant character (genotype) is capital. ► Monohybrid inheritance:-=First low of Mendel,=( low of segregation): (Members of a pair of factors segregate from each other in the formation gametes). Example:A = Axial Flower (dominant). Genetics a = Terminal flower (recessive). 2 Genetics BIO 221 Write genetic explanation of Mendel’s monohybrid crosses:Parents phenotype Pure Axial Flower × Parents genotype AA Pure Terminal Flower aa Meiosis Gametes A A a a Aa Aa Aa Aa (n) Randomefertilization (F1 genotypes 2n) ►F1 Generation were self – pollinated:F1 phenotype: Axial Flower F1 Genotype: Aa × Terminal Flower Aa (2n) Meiosis A Gametes (n) A AA Randome fertilization (F2 genotypes 2n) F2 phenotype a a aa Aa Homozygous Aa Heterozygous 3Axial Flower Homozygous 1 Terminal Flower Q: What is the 1st and 2nd Generation of the offspring of tall plant (DD) dwarf plant (dd) where Tall character is the dominant? Whereas the tall character is dominant & short character is ressive. Genetics 3 Genetics BIO 221 Tall Plant Parents phenotype Parents genotype TT × short Plant X tt Meiosis Gametes T T t t Tt Tt Tt Tt (n) Randomefertilization (F1 genotypes 2n) ►F1 Generation were self – pollinated:F1 phenotype: Tall Plant F1 Genotype: Tt × Tall Plant Tt (2n) Meiosis Gametes (n) Randome fertilization (F2 genotypes 2n) F2 phenotype T t T TT tt Tt Homozygous Tt Heterozygous 3 Tall Plant Genetics t Homozygous 1 short Plant 4 Genetics BIO 221 ► Punnet quadrat:♀ ♂ D D Genetics D d DD Dd Tall Tall Dd dd Tall dwarf 5 Genetics BIO 221 »» Glossary of common genetic terms:►Gene = Basic unit of inheritance. ►Allele = One of a number of alternative forms of the same gene. ►Homozygous = Diploid conditions with alleles are identical (AA). ►Heterozygous = Diploid conditions with alleles are different (Aa). ►Genotype = Genetic constitution with respect to alleles. ►Phenotype = Characters resulting from interaction between genotype and environment. ►Dominant = Allele controls the phenotype even in the presence of other allele (A). ►Recessive = Allele controls the phenotype only in the presence of another identical allele (a). ►F1 generation = G produced by crossing homozygous parents ►F2 generation = G produced by crossing 2 F1 organisms Genetics 6 Genetics BIO 221 Mendel’s second law =Dihybrid inheritance = principle of independent assortment: -Dihybrid inheritance means the case of two pairs of alleles ► Mendel’s second law or the principle of independent assortment:(Members of different pairs of factors segregate independently during gametes formation). Example of dihybrid cross:He used pea seed shape and pea seed colour, he crossed plant with (round seed with yellow colour) and (wrinkled seed with green colour). F1 = Round and yellow it is dominant. R = Round seed (dominant). r = wrinkled (recessive). Y = Yellow (dominant). y = green (recessive). Parents phenotype Parents genotype Round seed and yellow × Wrinkled and green RRYY rryy RY ry Meiosis Randomefertilization (F1 genotypes 2n) Genetics All RrYy (Hetero) Round and Yellow 7 Genetics BIO 221 F1 phenotype: Round seed and yellow × Round seed and yellow RrYy F1 Genotype: RY Ry RrYy rY ry X RY Ry rY ry ► Punnet quadrat:♀ ♂ RY Ry rY ry RY Ry RRYY RRYy round round yellow yellow RRYy * RRyy round round yellow green RrYY RrYy round round yellow yellow RrYy rY ry RrYY round RrYy round yellow yellow RrYy round *Rryy round yellow green #rrYYwrinkled #rrYywrinkled yellow yellow *Rryy #rrYy @ rryy round round wrinkled wrinkled yellow green yellow green The Ratio of 2nd generation F2 in dihybrid inheritance .9:3:3:1 The Proportions of each phenotype to ratio 9:3:3:1 is known as dihybrid ratio. Allelomorphic characters = Phenotypes determined by different alleles. Genetics 8 Genetics BIO 221 ► Polyhybrid inheritance:- more than 2 characters Crossing of parents have 3 pairs of characters. The characters are: » Tall = D = Dominant » Round seed = R = Dominant » Yellow seed = Y = Dominant » Dwarf = d = Recessive » Wrinkled seed = r = Recessive » Green seed = y = Recessive DDRRYY × Tall Round yellow DRY ddrryy Dwarf wrinkled green × Dry Parents Parents Gametes DdRrYy F1 All Tall round yellow 1st generation Crossing of this generation Genetics 9 Genetics BIO 221 Formation of Gametes:- R D r d R Y DRY y DRy Y DrY y Dry Y dRY y dRy Y DrY y dry r ♂ DRY ♀ DRY DRy DDRRYY DD RR Yy DrY DD Rr YY Dry DD Rr Yy dRY Dd RR YY dRy Dd RR Yy drY Dd Ry YY dry Dd Ry Yy DRy DrY Dry dRY dRy Genetics 10 Genetics BIO 221 drY dry 27 Tall round yellow. 9 Tall round green. 9 Tall wrinkled yellow. 3 Dwarf wrinkled yellow. 9 Dwarf round yellow. 1 Dwarf wrinkled green. 3 Tall wrinkled green. Genetics 11 Genetics BIO 221 Gene interaction: Variation of Mendelian ratios,: A) Incomplete dominance:-F2 (1:2:1 ). 1) Flower color in snapdragon:When cross Red flower × white flower we found the F1 all pink but F2 has Red : Pink : White (1:2:1 ). Red = W Parents White = × ww (white) WW ( Red ) F1 w Ww (Pink) Crossing between F1 Ww × WW (Red) Ww (Pink) Ww Ww ww (Pink) (white) B) Co dominance:-F2 (1:2:1 ). 1) Feather color in hens:White Feather W1W1 F1 F2 × Black Feather W2W2 W1W2 All Blue Feather W1W1 White F2 in Co dominance has W1W2 Blue W1W2 Blue W2W2 Black White : Blue : Black (1:2:1 ). We found that blue color in Co dominance condition due to presence of both black and White in the same feather. Genetics 12 Genetics BIO 221 C) Lethal genes:- F2 in Lethal Genes =2 : 1 Lethal genes change the Mendel’s percentages from 3:1 to 2:1 F2 in Lethal Genes =2 : 1 Example: Yellow color in Rates:- In some living rats yellow color never occur in pure homozygous because Rats carrying YY are die. In F1 Crossing yellow × Black give ½ yellow and ½ black parents Yy (yellow) × Gametes Y yy (black) y y y F1: Yy( yellow) : Yy ( yellow) : yy (Black) : yy (Black) : Crossing between F1 YY : Pure yellow Die Genetics Yy × Yy Yy : Yy : yy yellow : black 2 : 1 13 Genetics BIO 221 Gene interaction 1- Linkage Linkage means some genes are situated on the same chromosome & pass into the same gamete and inherited together). Linked genes do not conform to Mendel's principle of independent assortment and do not give the ratio 9:3:3:1 in dihybrid inheritance In Drosophila genes of color and wing length are allelomorphs. * Grey and long wing dominant G and L * Black and short wing recessive g and l In case of linked genes = on the same chromosome. (We will not found gametes containing G l or gL because GL are Linked.) Grey and long G g L l Black and short × g g l l heterozygous G g L l g × l G g g g L l l l 1 Grey and long : 1 Black and short We were depicts the Genetics 14 Genetics BIO 221 following Gg Ll × ggll ♀ GL Gl gL gl GgLl Ggll ggLl ggll Grey Grey Black Black long short long short ♂ gl Crossing-over At( synapsis) is an exchange of genetic material between nonsister chromatids of a bivalent during meiosis I. Crossing over: Means the formation of chiasmata during prophase I of meiosis . the breakage and recombination of homologous chromosomes. Meiosis G g g g L l l l Grey Black long short G G g g L L l showing crossing-over G l G g g Gametes × L Genetics g l L l l 15 Genetics BIO 221 g G Offspring g g g g G l frequency % l L g Recombination L l l l l (Crossing over Value (CoV) = X/Y ×100 Grey long grey short blank long blank short Gene mapping Gene mapping meaning relative positions of genes on chromosomes Calculating (CoV) crossing over value help us to produce maps for gene position on the chromosomes, by converting CoV this value into hypothetical distances along the chromosome. Ex: a (CoV) of 4% between genes A and B means that those genes are situated 4 units apart on the same chromosome. A 4 B Sex Determination In drosophila we found one pair of chromosomes always shows difference between sexes called sex chromosomes. Sex chromosomes called heterosomes all other non sex chromosomes called autosomes. Drosophila has 4 pairs of chromosomes 3 identical in both sexes and one not which known as X and Y Genetics 16 Genetics BIO 221 In Human ♀female =XX In some birds and butterflies , ♂ male = XY ♀ =XY as human. ♂ = XX. , Genetic explanation of the sex ratio in humans. ♀ =XX ♂ = XY × Meiosis Gametes X X X Y Offspring XX XY XX XY ♀ ♂ ♀ ♂ Ratio 1♂ : 1♀ Multiple alleles =A character controlled by a one gene but this gene appeared in many forms alleles ex: Inheritance of blood groups O, B, A …etc Gene of blood group is I symbol and has 3 alleles A, B & O , A and B equally dominant but O recessive. Presence of any dominant alleles make blood produce antibody called agglutinin. Ex: I A Io form agglutinogen A on membrane but plasma will contain agglutinin anti B. Genetics 17 Genetics BIO 221 Genotype Blood group (Phenotype) I A IA A I A Io A I B IB B Human Blood IBIo IAIB IoIo Genetics B group AB O 18 Genetics BIO 221 The Cell Cycle The cell cycle is a set of stages between the time a cell divides and the time the resulting daughter cells also divide. The events that occuresWhen a cell is going to divide: 1-Cell grows larger, 2- Number of organelles doubles, 3- Amount of DNA doubles by DNA replication. Cell cycle consists of two portions 1- interphase, 3 stages (G1 + S + G2) 2- Mitotic stage(M stage) includes mitosis & cytokinesis. 1- Interphase. (Preparations for mitosis). Interphase consists of 3 stages= G1, S, & G2. G1 Stage G1 is a stage before DNA replication, Events occurs During G1 stage: 1-cell increases in size, 2- Cell doubles its organelles 3- Cell accumulates materials of DNA synthesis. {Nerves do not complete the cell cycle & entered a G0.} S Stage Stage of DNA synthesis or replication. Events occur During S stage: 1-chromosome will duplicate & will consist of two sister chromatids instead of one chromatid. G2 Stage G2 is the stage from the end of DNA replication to the beginning of mitosis. Events occurs During G2 stage: 1- Cell synthesizes proteins . 2- Formation of spindles. Mitotic Stage ( M stage): Genetics 19 Genetics BIO 221 This stage includes mitosis & cytokinesis. 1-mitosis means nuclear division. 2-cytokinesis means division of the cytoplasm. Control of the Cell Cycle The cells arrested in G0 & finish the cell cycle if stimulated to do so by growth factors. Significance of mitosis 1- Genetic stability: 2- Growth: Increasing the cell number 3- Asexual replication, regeneration and cell replacement. Genetics 20 Genetics BIO 221 Meiosis (Meio = to reduce) Meiosis is a nuclear division involving a reduction from diploid number (2n) of chromosomes to the haploid number (n). Meiosis occurs during the formation of sperm and ova in animals (gametogenesis) and formation of spores in plants. The behaviour of chromosomes during meiosis is shown in Fig. which shows a nucleus containing 2 chromosomes (pairs of chromosomes.) chromosome from maternal n 2n 1st meiotic Meiosis I chromosome from paternal n Meiosis II Haploid gametes Genetics 21 Genetics BIO 221 Biochemistry of heredity: In principle: Genes are made of nucleic acids. The structure of Deoxyribonucleic Acid (DNA). The Watson-Crick structure for Deoxyribonucleic acid (DNA) (1953) a double- stranded helix 1-Sugar-phosphate backbone outside. 2-Nitrogenous bases (A, C, G, T) inside. 3-Nitrogenous bases held together by hydrogen bonds. 4-Nitrogenous bases pair occurs according to specific rules: A with T G with C Building blocks of nucleic acids (DNA and RNA). I-Nitrogenous bases 1-Pyrimidines (single ring) are: Cytosine (C) thymine (T) & {uracil in RNA (U)} " Pyramids were CUT from stone" The pyrimidines in DNA are C and T; but in RNA, U replaces T. [ thymine = 5- methyl-uracil] Genetics 22 Genetics BIO 221 2- Purines (double ring) are: Adenine (A) & guanine (G) " AGs are PURe" II- The Nucleoside = Nitrogenous base + sugar Types of Sugars: 1- Deoxyribose sugar occurs in DNA (-H on 2,-C) 2- Ribose sugar occurs in RNA (-OH on 2,-C) The base is connected to the sugar through the 1,- carbon of the sugar The differences between Ribose sugar & Deoxyribose sugar ? Deoxy-ribose sugar characterized by the absence of the (-OH) group on the C.2 3- Nucleotide = nucleoside + phosophate [PO4] The molecule shown as a nucleoside monophosphate. Note that the phosphate group is attached to the 5, -carbon of the sugar. 4-Polynucleotide = nucleotide + nucleotide + nucleotide + etc Nucleotides are linked by 3, Genetics 5, phosphodiester bonds 23 Genetics BIO 221 Types of structures of DNA 1-A-DNA : not groovy, 2-B-DNA: Right -handed helix 3-Z-DNA : left-handed helix Structure of B-DNA 1- Two twisted right-handed polynucleotide 2- Helices antiparallel in 5, 3, orientation. 3- Strands held by hydrogen (H-) bonds between bases. H-bonds form according to specific base-pairing rules. A pairs with T by two H-bonds A=T G pairs with C by three H-bonds G -=C 4- Base pairs interval = 3.4 A 5- Period of helix is 10 pb (base pairs) =34 A 6- 3-D structure of B DNA. Genetics 24 Genetics BIO 221 Characters of RNA (Ribonucleic Acid): 1-uracil ( U ) Substitute thymine ( T ) 2- single-stranded 3- The sugar is Ribose Types of RNAs 1-mRNA (messenger RNA): long, single-strand. 2-rRNA (ribosomal RNA): medium-sized, complex "stem and loop" 3- t RNA (transfer RNA): small, "cloverleaf" structure. Genetics 25 Genetics BIO 221 DNA Replication and Transcription Replication means : Duplication of a double- DNA (An exact "copy" of the existing DNA molecule) Transcription means : Synthesis of a new single- DNA )"copy" of an existing strand ( Types of DNA replication: 1- conservative 2- semi-conservative 3- dispersive In principle: DNA replication is semi-conservative DNA is not the " Genetic Code" for proteins information, DNA must first be transcribed into RNA messenger RNA transcript is base-complementary to template strand of DNA and therefore homologous with sense strand of DNA. 5„- A T G C A T G C - „3 3„- T A C G T A C G - „5 5„- A U G C A U G C - „3 Genetics Sense DNA (Partner) Template DNA (Antisense) Messenger RNA 26 Genetics BIO 221 DNA Synthesis in prokaryotes: 1- Formation of replication fork Provides two single-stranded DNA template (ssDNA) 2- Synthesis of RNA primer. 3- Addition of nucleotides (dNTPs by DNAPol III) at 3‘ end forming the continuous synthesis known as leading strand. 4- Discontinuous synthesis on lagging strand forming Okazaki fragments 5- Excision of RNA primer by DNAPol I 6-Ligation (connection) of fragment ends at gaps by DNA ligase. Genetics 27 Genetics BIO 221 Protein Synthesis Transcription: synthesis of messenger RNA (mRNA): RNA transcribed from DNA by RNA Polymerase (RNA Pol I) Steps of transcription: 1- Recognition of transcriptional unit:- " Gene " by promoters (sequences that regulate transcription) 2- Initiation and Elongation mRNA synthesized 3„ 5„ from DNA template strand mRNA sequences therefore homologous to DNA sense strand. 3- Termination. Regulation of transcription: In prokaryotes, transcription and translation may occur simultancously. In eukaryotes, transcription occurs in nucleus translation occur in cytoplasm, RNA must cross nuclear membrane: Primary RNA transcript is processed from heterogeneous nuclear RNA (hnRNA) into mRNA. Processing of heterogeneous nuclear RNA (hnRNA) means (intron DNA sequences removed from hnRNA, exon DNA sequences represented in mRNA: “expressed" in protein) Genetics 28 Genetics BIO 221 The “Central Dogma ”in prokaryotic cells The “Central Dogma ”in prokaryotic cells In prokaryotes, in the absence of nuclear membrane, DNA Transcription and RNA Translation are not physically separated, the RNA Transcript can directly . Genetics 29 Genetics BIO 221 Protein synthesis needs 2 steps:1-Transcription. 2- Translation. 1- Cell gets message to make protein. 2- Strands of DNA unwinds exposing the gene of this portion. 3- Formation of template DNA followed by the formation of mRNA Sense: Template: mRNA: 4- Many mRNA leave nucleus and enter cytoplasm from pore. mRNA 5- mRNA binds with Ribosome which read each 3bp at each time (Triplet code or codon) Genetics 30 Genetics BIO 221 6- Each codon is specific for one amino acid. 7- tRNA has 2 ends end of binding amino acid (binding site) end of anticodon which base pair ………………………………………with codon on mRNA. 8- tRNA bind with amino acid and carries it to Ribosome, its anticodon bases with codon on mRNA. 9- A second tRNA brings its amino acid to Ribosome. 10- The 1st tRNA go and bind its amino acid to the second amino acid. Genetics 31 Genetics BIO 221 11- The process repeats until a entire message read forming poly peptide chain = protein. -*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*- Genetics 32 Genetics BIO 221 The central dogma Of Protein synthesis in prokaryotes Replication DNA Transcription mRNA Translation Protein. The central dogma Of Protein synthesis in Eukaryotes heterogenous nuclear RNA DNA Transcription hnRNA Processing mature mRNA Replication Transport ……………………………………………… to …………………………………………………………………………cytoplasm In nucleus mRNA Amino acid chain Translation Protein In Cytoplasm Genetics 33 Genetics BIO 221 DNA packaging 1- DNA is a double stranded helical. 2- DNA is complex with histones to form nucleosomes . 3- Nucleosomes core = 8 histone = 2{H2A + H2B + H3 + H4}. 4- chromatosome = Nucleosome + H1 histone 5- Each 6 nucleosomes fold up forming a fiber or solenoid. Solenoid = 6 nucleosomes Genetics 34 Genetics BIO 221 6- Fibers forming loops. 7- Fibers loops compressed forming wide fiber 8- Coiling of wide fiber forming a chromatide of Chromosome. Genetics 35 Genetics BIO 221 Genetics 36