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Chapter 10 – Biology Honors DNA DNA is often called the blueprint of life. In simple terms, DNA contains the instructions for making proteins within the cell. Why do we study DNA? We study DNA for many reasons, e.g., its central importance to all life on Earth, medical benefits such as cures for diseases, better food crops. DNA by the numbers Each cell has about 2 m of DNA. The average human has 75 trillion cells. The average human has enough DNA to go from the earth to the sun more than 400 times. DNA has a diameter of only 0.000000002 m. The earth is 150 billion m or 93 million miles from the sun. Chromosomes and DNA Our genes are on our chromosomes. Chromosomes are made up of a chemical called DNA. How did we find out that it was DNA that carried information? Use of viruses; bacteriophages (phages) infecting E.coli bacteria Hershey-Chase experiments The Shape of the Molecule DNA is a very long polymer. The basic shape is like a twisted ladder or zipper. This is called a double helix. Introduction to nucleotides Nucleotides are nitrogen-containing organic substances that form the basis of the nucleic acids DNA and RNA. All nucleotides contain the following three groups: a nitrogena phosphate group containing base a pentose sugar In DNA the sugar is deoxyribose, whereas in RNA the sugar is ribose. Bases There are five bases, split into two types: adenine (A) and guanine (G) are purine bases. A thymine (T), cytosine (C) and uracil (U) are pyrimidine bases. T G C DNA contains A, G, T and C, whereas RNA contains A, G, U and C. U Identifying bases Base pairing rules Formation of nucleotides Formation of polynucleotides Determining the structure of DNA The double-stranded structure of DNA was determined in 1953 by the American biologist James Watson and the British physicist Francis Crick. X-ray diffraction studies by British biophysicist Rosalind Franklin strongly suggested that DNA was a helical structure. The Austrian chemist Erwin Chargraff had earlier showed that DNA contained a 1:1 ratio of pyrimidine:purine bases. Structure of DNA DNA Replication Before a cell can divide, in order to reproduce a new cell, it must duplicate its DNA. This ensures that each new cell will have a complete set of DNA to carry out its life functions. DNA Replication This process of making more DNA from an existing strand is known as DNA replication. Replication is carried out by a series of enzymes. 4 main enzymes - helicase, primase, DNA polymerase, ligase DNA Replication: The Process 1. 2. 3. Helicase (enzyme) separates or “unzip”the 2 strands of the double helix, by breaking hydrogen bonds. Primase insert the appropriate base pairs on each side of the template strand, starting at specific spots, then DNA polymerase extends the nucleotide sequence all the way down the strand. Ligase “proofreads” the bases inserted to make sure they were paired correctly and fills in gaps in the phosphate-sugar backbone. DNA replication animation Protein Synthesis (A completely different process than replication, involving DNA!) DNA leads the production of proteins Making proteins involves 2 complex processes: Part 1: Transcription – “To write” Part 2: Translation – “to change form and interpret” REMEMBER! DNA codes for proteins, which are made in ribosomes! The proteins are what do the work! Why proteins? The reason proteins are made is because most enzymes are proteins. Proteins (specifically enzymes) control every biochemical process in an organism. Proteins direct synthesis of lipids, carbohydrates, and nucleotides. Proteins are also responsible for cell structure and movement. DNA transcription DNA contains a set of instructions coded in letter sequences of A, T, G, C. The first step in decoding DNA is to copy part of the message into mRNA (messenger ribonucleic acid). RNA is a nucleic acid that acts as a messenger between DNA and ribosomes. Structure of RNA RNA, like DNA, consists of a long chain of nucleotides. Each nucleotide is made up of a phosphate group, 5carbon sugar (ribose), and a base. MAJOR DIFFERENCES between RNA and DNA DNA 1) Deoxyribose sugar 2) Double stranded 3) Bases A, G, C, T RNA 1) Ribose - sugar 2) Single strand 3) Bases A, G, C, U. RNA contains Uracil in place of thymine. Examples of different forms of RNA Different forms of RNA are used depending on the job. All have same chemical structure and make up. – mRNA : messenger RNA – tRNA : transfer RNA – rRNA : ribosomal RNA PROTEIN SYNTHESIS: Part 1 TRANSCRIPTION In order to get the DNA message to the ribosomes, which make the proteins, we need to copy the message onto a RNA strand. (WHY??) TRANSCRIPTION - process which a molecule of DNA is copied into a complementary strand of RNA. “writing information on an mRNA strand” Transcription Process 1) Helicase attaches to special places on DNA strand, which causes the DNA strands to “unzip”. 2) RNA polymerase pairs nucleotides with ONE side of the DNA strand, to create a single-stranded mRNA molecule. (U substitutes for T) Transcription Process (cont.) 3) 4) Special spaces on the DNA strand, called promoters, signal the RNA polymerase where to start reading and base pairing on one side of the DNA. When the RNA polymerase reaches a specific terminator sequence it stops and detaches from the DNA strand. *These are 3 letter sequences called START and STOP codons. Codons – 3 letter base segments on the mRNA 5) DNA strand “zips” back together and mRNA is now free to leave the nucleus and head towards the ribosomes in the cytoplasm to make proteins. Transcription process Protein Synthesis: Part 2 Translation Once the mRNA leaves the nucleus and arrives at the ribosomes, translation begins. Translation involves “translating” or decoding of mRNA sequence into a polypeptide (protein chain) This process involves an elaborate system of 2 types of RNA molecules, tRNA (transfer RNA) and rRNA (ribosomal RNA). Translation (continued) tRNA - single strand of RNA that loops back on itself. There are 20 different tRNA molecules for each of the 20 different amino acids. Polypeptide - many amino acids held together in a chain by peptide bonds. (PROTEIN CHAIN) In order to decode the information, mRNA is broken up into “words”or codons that can be read by the ribosomes. This is how the ribosomes know which amino acid to link to the protein chain! Translation: The Process 1. First, the mRNA begins to move through the ribosomes, getting read codon by codon. (3 letter sequence of mRNA). The start signal to begin reading is always the START CODON sequence AUG! 2. Each codon is matched up with the 3 letter sequence of tRNA called an ANTICODON. 3. The tRNA anticodon will bind to AUG (anticodon UAC) carrying the first amino acid of the polypeptide, methionine. Translation process (continued) 1. Next, the ribosome moves down the mRNA to the next codon. The next tRNA binds and attaches its amino acid to the first one, with a peptide bond. 2. This continues down the mRNA strand, until the ribosome reaches one of the STOP codons (UAA, UAG, UGA). Then, mRNA and polypeptide are released from the ribosome. TRANSLATION PROCESS ENDS! CH. 10 Animations (under Helpful websites on class website) Ribosomes Ribosomes are made of 2 subunits. Each subunit is made up of ribosomal RNA and protein. They come together during this process surrounding the mRNA strand. Amino Acid Genetic codes P. 192 in textbook (Lists codon = mRNA sequence) NOTICE! Some codons code for the same amino acid. (CUA, CUG, CUU, CUC all code for the amino acid - leucine) Specific codons – UAA, UAG, UGA are “STOP” codons. They do NOT code for an amino acid. They are like periods at the end of a sentence and signify the end of a protein chain. – AUG is always the “START” codon and will start every protein with a Methionine (Met).