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DNA • An organism’s DNA ultimately controls that organism’s traits. – All living things contain proteins – Most chemical reactions that construe metabolism are controlled by enzymes – a type of protein – Proteins can only be made if the correct instructions are present. – DNA contains the instructions to create every protein that an organism will require DNA • A DNA molecule is a polymer made up of many smaller units called nucleotides, linked together. • A nucleotide is made up of three parts: – Simple sugar (in the case of DNA, the sugar is deoxyribose) – Phosphate group – Nitrogen base • Every nucleotide is exactly like every other nucleotide, except for the nitrogen base. DNA • There are 4 possible nitrogen bases in DNA –Guanine (G) –Cytosine (C) –Adenosine (A) –Thymine (T) DNA • The structure of DNA is similar to a twisted ladder – The sides of the ladder are the phosphate group and simple sugar – The ‘rungs’ consist of two, complimentary bases, held together by weak hydrogen bonds – The bases only match up in one way…. A with T C with G Hydrogen bond 1 nucleotide Sugar A T C G C D G Phosphate G C T A T A A T N A Nitrogen Base DNA • The only difference between one nucleotide and another is the nitrogen base it contains. • There are only four nitrogen bases. • Every protein is made up of combinations of these four bases. • So, the sequence of the nucleotides (or more specifically, of the nitrogen bases) is what determines the protein. DNA Replication • When a cell makes a copy of itself (either thru mitosis or meiosis), it must make a copy of the DNA or the new cell won’t have the instructions it needs. DNA replication is the process of copying DNA so that you have two identical strands. • One strand will move on to the new cell, and one will remain with the original cell. DNA Replication • First, the double helix has to unwind. DNA helicases (enzymes) open the double helix by breaking the weak hydrogen bonds, splitting it into two separated strands. – Additional proteins attach to each strand keeping them from rejoining. • Then enzymes called DNA polymerases move along the strands, adding free nucleotides to the exposed bases, following the base-pairing rules, thus forming two identical double helixes. • Once all of the DNA has been copied, the DNA polymerases detach, and you have two identical strands of DNA – each is made up of one original strand and one new strand. BIO – DNA Replication • DNA polymerases have a proofreading role – they can only move on to the next nucleotide to the growing strand if the previous nucleotide is paired correctly. • If a mistake was made, the DNA polymerase can backtrack and fix it. • Thus, errors in DNA replication are about 1 per billion nucleotides. • It takes about 8 hours to completely replicate a human chromosome (each is made up of about 100 sections of 100,000 nucleotides each) DNA Replication • Given the DNA strand G-A-T-T-A-C-G-C-C-A, what would be the complimentary copy made DNA Replication? Protein Synthesis • DNA is the instructions to make proteins….. • A protein is a polymer, made up of many amino acids….there are only about 20 different amino acids, so the DNA gives the correct sequence for these amino acids to link up to create each different protein. Protein Synthesis • What organelle makes proteins? • And where are the instructions? • Somehow, the instructions from the DNA have to get out of the nucleus, to the ribosomes, so that the proteins can be made. Protein Synthesis • The processes involved in creating a new protein from the instructions on the DNA is called protein synthesis. • Nucleic acids called RNA are required for protein synthesis. – RNA is structurally like DNA with a few differences: • The simple sugar is called ribose • It is made up of only one strand • RNA does not have thymine, but rather has uracil (U), which matches up to adenosine. (A) Protein Synthesis • Protein synthesis begins by making a copy of the DNA, a process called trancription. – The DNA strand uncoils like it did for replication – mRNA (messenger RNA) links to the nucleotides link to the open strand, making a complimentary copy of the DNA. • Always read from the 5’ toward the 3’ end of the DNA strand. • The DNA strand is read in a series of 3, called triplets. • The complimentary mRNA strand is called a codon. DNA triplet mRNA codon T-A-C A-T-T G-A-T A-A-T A-U-G U-A-A C-U-A U-U-A Protein Synthesis • The mRNA leave the nucleus and moves to a ribosome where it binds to a rRNA (ribosomal RNA), reads the instructions and ensures that the correct amino acids are brought and assembled. • The process of reading the mRNA to assemble the correct amino acids in order is called translation. *****You read the codon to determine***** which amino acid you need. Protein Synthesis • tRNA brings the correct amino acid to the mRNA codon…..the tRNA anticodon is the complimentary sequence to the mRNA codon. – – – – DNA triplet T-A-C A-T-T mRNA codon A-U-G U-A-A Amino Acid: tRNA antiocodon: G-A-T C-U-A A-A-T U-U-A Codons: AUG GGC AGC UUA GUA GCC AUC AAC UAA Codons: AUG GAU ACC CCU CAA GAC UGA Protein Synthesis • There is a ‘start’ codon sequence, that is recognized as the beginning of a new set of instructions, just like a capital letter often indicates a new sentence. – AUG is the ‘start’ codon, and codes for methionine. – So, all proteins will have methionine as the first protein in their sequence. – There are also various ‘stop’ codons that tell you that the sequence is ended. Protein Synthesis Summary • Transcription: – Where? – What? In the nucleus DNA triplet copied onto a mRNA codon • Translation: – Where? At the ribosomes – What? rRNA reads the mRNA codon and the tRNA anticodons bring the appropriate amino acid BIO – Protein Synthesis • With few exceptions, the genetic code is the same in all organisms – GUC codes for the amino acid valine in bacteria, eagles, plants and people…..therefore it is often described as almost universal, giving rise to the idea that all life-forms have a common evolutionary ancestor. • Exceptions include the way cell organelles with their own DNA (mitochondria, chloroplasts) and how some microscopic protists read the ‘stop’ codons. Chromosomal Problems • Mutations can also happen when the chromosomal structure breaks during meiosis. There are five types: – Deletion Mutation: Piece of chromosome breaks off completely – often fatal Chromosomal Problems – Insertion Mutation: A chromosome piece breaks off from one chromosome, and inserts itself into another one. Chromosomal Problems – Duplication Mutation: Chromosome fragment attaches to its homologous chromosome, so that it now carries two copies of a certain set of genes. Chromosomal Problems – Inversion Mutation: A chromosome piece reattaches to the original chromosome, but in reverse. Chromosomal Problems – Translocation Mutation: A chromosome piece attaches to a nonhomologous chromosome…..similar to crossing over, except that it happens between two nonhomologous chromosomes. Repairing DNA • Mutagens are agents that may cause a change in DNA. – Include exposure to radiation, chemicals, high temperatures, etc. • Radiation and perhaps high temps, cause havoc because of the high amounts of energy. Radiation that passes thru your cells acts like little bullets, cutting the DNA in places. • Some chemicals that are highly reactive, may react with the DNA molecule, causing changes. – The damage may kill the cell which is preferable. – The cell may live but the if the damage is be in one of the introns, then no harm is done – The cell may live, and if the damage is to one or more sequences that code for proteins, then either needed proteins are not made or proteins you do want are – resulting in problems! Repairing DNA • There are fail-safes in the cells to avoid mutations. – During replication, there are enzymes that act as a spellcheck to ensure that the DNA strand is copied correctly. – When DNA is damaged by mutagens, those same enzymes and others act to again proofread the strand and replace incorrect sequences with free nucleotides. • The greater the exposure to the mutagen, the more likely the chance that a mistake will sneak by.