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Competency 5 Heredity The Structure of DNA DNA is a very long polymer. The basic shape is a double helix. A double helix looks like a twisted ladder. In the double-helix model of DNA, the two strands twist around each other like spiral staircases. Chromosomes Chromosomes are a single strand of DNA tightly coiled around a protein called a histone. A section of DNA that carries the information to make one protein is called a gene. The Structure of DNA DNA is a nucleic acid made up of nucleotides (monomers) joined into long strands, or chains, by covalent bonds. Each nucleotide is composed of 3 parts: Sugar – DEOXYRIBOSE Phosphate group Nitrogen bases Adenine (A) Thymine (T) Guanine (G) Cytosine (C) The Structure of DNA DNA has four kinds of nitrogenous bases: adenine (A), guanine (G), cytosine (C), and thymine (T) Hydrogen bonds form between A and T base pairs as well as between C and G base pairs. The nitrogenous bases stick out sideways from the nucleotide chain. The nucleotides can be joined together in any order, meaning that any sequence of the bases is possible. The Structure of DNA Erwin Chargaff analyzed the amount of adenine, guanine, thymine, and cytosine in the DNA of various species. He found that the amount of guanine nearly equals the amount of cytosine, and the amount of adenine nearly equals the amount off thymine within a species. Adenine and guanine are purine bases Cytosine and thymine are pyrimidine bases. Base-Pairings Purines only pair with pyrimidines. Three hydrogen bonds are required to bond guanine and cytosine. Two hydrogen bonds are required to bond adenine and thymine. Practice #1 DNA complementary base-pairing practice – Fill in the blanks to create complementary strands of DNA Strand 1 A __ T G A __ __ G Strand 2 __ T __ __ __ T C __ Answers Strand 1 Strand 2 A T A T T A G C A T A T G C G C Practice #2 Percentage of Bases in Four Organisms Source of DNA A T G C Streptococcus 29.8 31.6 20.5 18.0 Yeast 31.3 32.9 18.7 17.1 Herring 27.8 27.5 22.2 22.6 Human 30.9 29.4 19.9 19.8 What does the data suggest to you? Answer The data suggests that the percentages of adenine and thymine are almost equal in any sample of DNA. The same thing is true for the percentages of cytosine and guanine. DNA Replication Before a cell divides, it duplicates its DNA in a copying process called replication. DNA replication is a process that ensures that each resulting cell has the same complete set of DNA molecules. During replication, the DNA molecule separates into two strands and then produces two new complementary strands following the rules of base pairing. Each strand of the double helix of DNA serves as a template, or model, for the new strand. Replication Fork Original Strand New Strands DNA Polymerase Transcription Comparing RNA and DNA RNA DNA Sugar – Ribose Sugar: Deoxyribose Single-stranded Double-stranded Uracil (A,G,C,U) Thymine (A,G,C,T) Types of RNA Messenger RNA (mRNA) carries the genetic message of DNA from the nucleus to other parts of the cell. Ribosomal RNA (rRNA) forms parts of ribosomes, the cell’s protein factories. Transfer RNA (tRNA) transfers each amino acid to the ribosome as it is specified by the coded messages in mRNA. Three Main Types of RNA messenger RNA (mRNA) carries copies of instructions for assembling amino acids into proteins. ribosomal RNA (rRNA) Ribosomes are made up of proteins and ribosomal rRNA. transfer RNA(tRNA) transfers each amino acid to the ribosome. Protein Synthesis Proteins are made in a two step process: -Part One: Transcription in the nucleus - Part Two: Translation happens at the ribosome Transcription Genes contain coded DNA instructions that tell cells how to build proteins. The first step is decoding the genetic instructions is to copy part of the base sequence from DNA to RNA. Transcription: Since DNA cannot leave the nucleus; free nucleotides use a strand of DNA to make mRNA (messenger RNA) inside the nucleus. The mRNA strand then leaves the nucleus and travels into the cytoplasm. mRNA snakes out of the nucleus into the cytoplasm and uses the base sequence copied from DNA to direct the production of proteins. Transcription Codon These bases form a “language,” or genetic code, with just four letters: A, C, G, and U. Each three-letter “word” in mRNA is known as a codon. A codon consists of three consecutive bases that specify a single amino acid to be added to the protein chain. Anticodons Anticodons are the three unpaired bases of tRNA. These bases are complementary to mRNA codons. Translation Translation occurs at the ribosome. A three letter anti-codons on tRNA codes for specific amino acids at the top of tRNA. There are 64 different combinations or codons, but only 20 amino acids. When the amino acids link together in a peptide bond, they produce proteins. Genes directly control the synthesis of proteins. https://www.youtube.com/watch ?v=41_Ne5mS2ls Reading Codons Suppose you wanted to determine which amino acid is encoded by the CAU codon. Find the first base “C” in the left column. Find the second base “A” in the top row, Find the box where these two letters intersect. Find the third base “U” in the right column. Find where all three intersect. CAU codes for His (histadine) Codon Wheel: Read 3 letters at a time, inside to outside. Gel Electrophoresis and DNA Fingerprinting Small DNA samples can be obtained from blood, hair, skin, or semen Technology that separates DNA fragments so they can be analyzed into a DNA fingerprint It is used to find the similarities and differences in genomes of different organisms, identify parents, catch criminals, because no two people, except identical twins, have the exact same DNA Who is guilty? Who is guilty? Who are the parents? PCR: Polymerase Chain Reaction The polymerase chain reaction (PCR) is a biomedical technology in molecular biology used to amplify a single copy or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence. https://www.youtube.com/watch?v=0HCWmD7Mv 8U Selective Breeding Selective breeding allows only those organisms with desired characteristics to produce the next generation. Humans use selective breeding to pass desired traits on to the next generation of organisms. Nearly all domestic animals and most crop plants have been produced by selective breeding. Recombinant DNA Most DNA molecules are too large to be analyzed, so biologists cut them into smaller fragments using restriction enzymes. Recombinant DNA is used in producing insulin and producing growth hormones from different sources. examples: chickens that grow faster and make human insulin Other Important Terms Genetic Engineering- making changes in an organism’s DNA Transformation- the process used to place recombinant DNA back into a living organism Transgenic- term used to describe an organism that contains the DNA from a different organism using recombinant example: disease resistant plants DNA Reproductive cloning – process of creating an organism that is genetically identical to a donor cell Genetic Terminology Traits are any characteristic that can be passed from parent to offspring. Heredity is the passing of traits from parent to offspring. Genetics is the study of heredity. Monohybrid cross is a cross involving a single trait (ex: flower color). Dihybrid cross is a cross involving two traits (ex: flower color and plant height). Alleles are two forms of a gene (two letter combination of recessive and dominant). Genetic Terminology Genotype is a gene combination for a trait (ex: RR, Rr, rr). Phenotype is the physical feature resulting from a genotype (ex: red, yellow) Genotypes and Phenotypes in flowers: Genotype of alleles: R = red flower r = yellow flower All genes occur in pairs, so 2 alleles affect a characteristic. Possible combinations are: Genotypes RR Phenotypes Red Rr Red rr yellow Genetic Terminology Dominant is the stronger of the two genes expressed in the hybrid; represented by a capital “R.” Recessive is a gene that shows up less often in the cross; represented by a lower “r.” Homozygous genotype is a gene combination involving 2 dominant or 2 recessive genes (ex: RR or rr); also called pure. Heterozygous genotype is a gene combination of one dominant and one recessive allele (ex: Rr); also called hybrid. Punnett Squares Punnett squares use mathematical probability to help predict the genotype and phenotype combinations in genetic crosses. One of the best ways to predict the outcome of a genetic cross is by drawing a simple diagram known as a Punnett square. Punnett squares allow you to predict the genotype and phenotype combinations in genetic crosses using mathematical probability. How to Make a Punnett Square 1st: Make a grid with four boxes. Place the alleles for Parent 1 along the top of the grid using one letter for each box. Place the alleles for Parent 2 along the left hand side using one letter for each box. 2nd: Fill in the grid. Combine the parent alleles inside the boxes (capital letter always goes in the front). 3rd: Fill in the offspring. Use the Law of Dominance to determine the phenotype ratio of the offspring. Monohybrid Cross Trait: Plant Height Alleles T- tall ; t –short Cross: Tt X Tt Genotypes: TT, Tt, tt Genotypic Ratio: 1:2:1 (TT=25% Tt = 50%, tt=25%) Phenotype: 3:1 (Tall = 75% Short = 25%) Incomplete Dominance In incomplete dominance neither allele is not completely dominant over another. In incomplete dominance, the heterozygous phenotype is an intermediate (blended) phenotype between the two homozygous phenotypes. Codominance In codominance, an organism that has both alleles of agene displays both phenotypes at the same time (co-exists). Multiple Alleles Multiple alleles are gene with more than two alleles. Many genes have multiple alleles, including genes for blood type. Mother Father Sex-linked Genes Sex-linked genes are located on the X chromosome. Traits determined by sex-linked genes are called sex- linked traits. c= colorblind, C =normal) Ex .Color blindness female Xc Xc male Xc Y A carrier is a person that has the trait on only one chromosome and does not express the trait. Carriers of sex-linked traits are always women ( XC Xc). Practice #3 Eye color is a sex-linked trait in fruit flies. Complete the chart below. Character Trait Alleles Eye Color Red Eye R Eyes Color White eye r Genotypes Phenotypes XRXR XRXr _______ ___________ ___________ White-eyed female XRY ________ ____________ White-eyed male Answers Eye color is a sex-linked trait in fruit flies. Complete the chart below. Character Trait Alleles Eye Color Red Eye R Eyes Color White eye r Genotypes Phenotypes XR XR XR Xr Xr Xr Red-eyed female Red-eyed female White-eyed female XR Y Xr Y Red-eyed male White-eyed male Karyotype A karyotype is a visual representation of an individual’s chromosomes. A karyotype shows the complete diploid set of chromosomes grouped together in pairs, which are arranged in order of decreasing size. Karyotype Practice #4 Is this a male or a female? Answer It is a female because it has two X chromosomes for the 23rd pair. Pedigrees A pedigree is a diagram that traces the inheritance of a particular trait through several generations. A pedigree uses symbols to illustrate the inheritance of the trait. Common Pedigree Symbols