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Molecular Genetics Introduction The topic ofMolecular Genetics deals with the DNA oflbe ceD and the process that is used to decode its genetic code and use the information to make proteins. Genes are made ofDNA. The expression ofDNA is protein. The term given for making a protein is called "protein synthesis." This requires DNA to provide the coded genetic infonnabOD, the three types of RNA, and the amino acids that are the componen1s of the protein. Protein synthesis is similar in some ways to manufacturing a car. The car is made up ofdifferent parts that are brought together on the assembly line. Deoxyribonucleic acid is DNA DNA is a long polymer consisting ofphosphate groups altematiDg with sugars. Nucleotides are the subunits ofnucleic acids. A nucleotide consists of a base, a sugar and a phosphate. The sugar in DNA is called deoryri~se. Each sugar bas a base attached to it. The bases are made of carbon and nitrogen and are called nitroge1lOflS bases. There are two kinds ofnitrogenous bases kDown as purine bases and pyrimidine bases. The purine baSes found in DNA are adenine (A) and guanine (a). The pyrimidine bases fOlind in DNA are cylosine (C) and thymine (1"). Ribonucleic acid is RNA RNA is also a long polymer consisting ofphosphate groups alternating with sugars. The sugar in RNA is called ribose. Each sugar bas a base attached to it. The purine bases found in RNA are adenine (A) and guanine (0).. The pyrimidine bases found in RNA are cytosine (C) and lD'acil (U). Thus, three bases are the same as in DNA. . Both molecules are similar. There are two basic di1ferences between them. DNA bas deoxyribose and thymine. RNA bas ribose and uracil. .Structure of DNA DNA bas two strands, each with the sugars altema1iDs with the phosphates, IDd with a base attached to each sugar. The bases pair between the DNA strands. Adenine always pairs with Thymine. If there is an Adenine OD the first strand ofONA, there will be a Thymine opposite it. Also, a Thymine on the first stnmd will be matched by an Adenine on the other. . . . Similarly, Guanine pairs with Cytosine. A Guanine on the first strand will be paired with a Cytosine on the other stJ'IlDd. Also, a Cytosine on the first strand will be paired with a Guanine on the other. 20-1 20-2 The base pairs ofDNA are held together by weak attractions known as hydrogen bonds. Hydrogen bonds are weaker than the covalent bonds that hold two carbon atoms together. Howevert there are a lot ofbydrogen bonds holding a DNA molecule together. These bonds serve to hold the strands together under normal temperatun: conditions. Replication of DNA The two DNA strand unzip at the hydrogen bonds and each acts as a template. The template is a pattern that will be replicated by the enzymes synthesizing the new DNA strands. After the DNA strands are unzipped, the enzyme DNA-dependent DNA polymerQ&e comes and makes a new strand matching each base with its correct partner. This enzyme is called DNA polymerase because it produces a DNA polymer as its product. It is described as DNA -depemJelll because it relies on the pe-existiDg strand of DNA to tell it what the pattern is. Wherever the template strand has an At the new strand will receive a T; and wherever there is a T, the new strand will receive an A. SimilarlYt wherever the template strand has a G, the new strand will receive a C; and wherever there is a C, the new strand will re<:eive a G. As a result, the new strand will be an exact copy of the original complementary strand. This process is called semiconservative replication. GENE EXPRESSION Protein synthesis The DNA causes a protein to be produced as a result of a series of steps. These steps are known as transcription and translation. Transcription . The DNA template is used to make messenger RNA (mRNA). The mRNA is the transcribed copy of the DNA molecule and so it contains the genetic message encoded in . the DNA. The mRNA travels to the endoplasmic reticulum where ribosomes attach to it The n"bosomes decode the coded genetic message and translate it to make a protein molecule. The code that the mRNA contains was broken in the 19605. It is shown in Figure 20-1. .The genetic code is read from the mRNA molecule in units of three bases known as Cod01lS. In order to use the DNA code, lookup the first base, then the second base, then the third base. For example if the codon is UUU, the amino acid is phe (phenylalBDine). .. , "; 20-3 Second base of codon e u u UUU) ph. uue UUA) leu UUG UCU uee UeG UAU)tyr UAC UAA··stoP UAG··stoP CUU ceo cee CAU)h1S CAC i cuc '; CUA CUG le u •• • A I.u Sir UCA CCA CCG } pro CAA)gln CAG AUt' ) . ACU) AUC II., ACC 1ft .. A thr ..I::a u:: AUA AUG-=" GOO) GUC val G GUA GUG ACA ACG GCU) GCe GCA GCG AAU)asn AAC :::)11$ GAU) asp GAC ala GAA)gIu . GAG G UGU )cys u C UGC UGA -stop A G UGG -try eGU CGC eGA eGG U arg AGU>s.r AGe AGA) AGG ara GGC GOO) GGA GGG c: 0 C "a A u0 G -0 • • • U ..:. C "! A ~ G U gIy C A G Figure 20-1. Decoding messenger RNA codoDs. Translation . Translation of the coded message involves the n1Josomes. Ribosomes arc structures made ofribosomal RNA (rRNA) and protein molecules. Each noosome contains two components. The large component serves IS the worksp8ce where the protein is synthesized while the smaJl component serves IS the mechanism that helps to join the ammo acids together. Bringing the amino acids of tile' protein to the noosome is the job ofttansfer RNA (tRNA). Each tRNA molecule cames a specific amino acid. The system knows which one to use because the tRNA molecule has 8 specific anticodon that exaetJy matches the codon of the mRNA. So, ifthe mRNA codon was UUU, the an1icodon for the tRNA carrying phe would be AAA. Adenine BDd Uracil 8I"e complementary so the AAA anticodoD would exactly match the UUU codoD. , . 20-4 As its tRNA brings each amino acid to the ribosome, the chain of amino acids grows in length by one amino acid. Enzymes on the ribosome remove the incoming amino acid from its tRNA and add it 10 1he growing polypeptide chain using a condensatioD mlC'tion.