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http://www.pbs.org/wgbh/nova /education/body/rnaidiscovered.html Recipe/chef dna to protein Unit 5A Gene Expression Review • What is a gene? – A segment of DNA that codes for a specific protein • What do you think it means to say “express a gene”? – Gene expression- The process by which information in a gene is used to create the protein that it codes for. How does the cell “decode” the sequence of DNA bases of a gene? • In general: DNA The master copy of instructions RNA Protein The temporary copy of the DNA The product of the DNA’s instructions Trait Characteristic of a cell/organism RNA • A nucleic acid • Consists of one long chain of nucleotides • There are multiple types of RNA, each with a different function • One type of RNA can be used as a temporary copy of DNA in order to make protein • The DNA is precious and stays safely in the nucleus DNA vs. RNA • RNA is mostly single stranded • RNA is made of nucleotides that have a phosphate, ribose sugar and a base • Bases in RNA are A, U, C, G – U = uracil – What’s missing? Compare and Contrast DNA and RNA Structure DNA RNA RNA is used in Protein Synthesis Amino acids • Proteins are made of __________. • RNA molecules direct the assembly of Polypeptides amino acids into _____________. • Which then fold into proteins. Types of RNA • Messenger RNA (mRNA)- a copy of the instructions for making protein from the DNA Types of RNA • Ribosomal RNA (rRNA)- RNA molecules that bind with proteins to make the ribosomes (where proteins are made) Types of RNA • Transfer RNA (tRNA)- transfers the correct amino acid to the ribosome to make proteins based on the mRNA sequence The “Central Dogma” of Molecular Biology DNA The master copy of instructions RNA The temporary copy of the DNA Protein The product of the DNA’s instructions • First DNA is transcribed into mRNA • Then, the mRNA is sent out of the nucleus to a ribosome • At the ribosome, tRNAs bring together amino acids to translate the message in the mRNA into protein. DNA to RNA to Protein 2 processes are required for gene expression 1.Transcription- the process that uses base pairing to make mRNA based on the sequence of DNA – Occurs in the nucleus 2. Translation decodes the mRNA message into protein – Occurs at the ribosome (in the cytoplasm) Draw in diagram from DNA -> Secreted Protein Making RNA 1. Transcription- DNA serves as the template for making complementary RNA molecules • RNA is made in the nucleus, then moves to the cytoplasm for protein synthesis • RNA polymerase separates the DNA strands and assembles RNA by matching up base pairs Making RNA What pairs with adenine in the DNA when making the new RNA molecule? DNA to mRNA practice (transcription) • Transcribe the following DNA sequence (gene) • TACAATGGCGCTAGT • The DNA is ultimately read three letters at a time or we say in “triplets” DNA Triplet code Making a Polypeptide 2. Translation decodes the mRNA message into protein – Occurs at the ribosomes – Remember: the ribosome is made of two subunits of rRNA and proteins The Genetic Code: • The language spelled out by the sequence of bases on the mRNA (A, U, C, G) • Read 3 letters at a time • Each 3 letter “word” is called a codon • Each codon corresponds to 1 amino acid DNA Triplet code Transcription mRNA codons Translation Amino acids U- U- A - C - A - G - C - C - A How to Read Codons • There are 64 possible codons • The genetic code table 1. Start in the middle of the circle with the first letter of the mRNA codon 2. Move to the second ring to find the second letter of the codon 3. Find the third letter of the codon in the smallest set of letters 4. Read the amino acid in that sector Amino Acids • There are 20 amino acids commonly found in proteins • A string of amino acids is a polypeptide • A completed and folded polypeptide is a protein 3 letter abbreviation 1 letter abbreviation Start and Stop Codons • Genetic punctuation marks • AUG is the start codon – codes for the amino acid methionine • The mRNA is read starting at AUG • After the mRNA is fully read, a stop codon ends the production of the protein • Three stop codons: UAA, UAG, UGA – No amino acids are coded mRNA to Protein Practice • AUGUUACCGCGAUCA How does the cell read the codons? 1. 2. 3. 4. The ribosome attaches to a mRNA molecule that has entered the cytoplasm As each codon on the mRNA passes through the ribosome, tRNAs bring the proper amino acids into the ribosome The ribosome attaches these amino acids to each other with peptide bonds creating a polypeptide This occurs until the stop codon enters the ribosome -at that point, the polypeptide is complete and is released from the ribosome 5. After the polypeptide is released, it finishes folding up properly into a fully functioning protein Video Video How does tRNA know which amino acids to bring? • Each tRNA carries only 1 kind of amino acid • Each tRNA has 3 unpaired bases that match up with the codons on the mRNA – These are called anticodons anticodon The Genetic Code is Nearly Universal!! • Almost all organisms have the same three letter codes in DNA leading to the same amino acids brought in to make the proteins. • From bacteria to humans, the triplet code is the same. DNA (gene): TGA CGA TTT CTC ACT ACA CGC GCC CTT Transcription↓ mRNA Translation↓ Polypeptide An different way to model protein synthesis Stanford’s take on Protein Synthesis Mutations Mutations • Mistakes in copied DNA that cause variations in genetic material • These mistakes are passed down to offspring if they occur in sex cells Overactive feather blocking gene creates a bald neck Mistake in the “bithorax” gene causes 2 sets of wings 2 Basic Categories 1.Gene mutations- changes in a single gene 2.Chromosomal mutations- changes in a whole chromosome 1. Gene Mutations • Are point mutations- changes at a single point in the sequence • Occur during replication • Are passed down to daughter cells • 3 types: – Substitutions – Insertions – Deletions Substitutions • One base is changed to a different base • Affects a single amino acid • Sometimes have no effect at all – How could this happen? – If the substitution creates a codon that leads to the same amino acid as before Insertions • One extra base is inserted into the DNA sequence • Every codon after the insertion will change! • Considered a “frameshift” mutation because they shift the reading frame of the genetic message • The amino acids chosen will change • Will the resulting protein work? – Very unlikely! Deletions • One base is deleted from the DNA sequence • Every codon after will change • Also a “frameshift” mutation • Changes the amino acid sequence and resulting protein Analogy Read the following sentence (remember, it’s read in triplets): The fat cat ate the rat. Insertion: • Add a b in front of the f in fat. Shift everything to the right. • _____ _____ _____ _____ _____ _____ Deletion: • Delete the f in fat. Shift everything to the left. • _____ _____ _____ _____ _____ _____ Substitution: • Substitute the letter b for the f in the word fat and read the sentence again. • _____ _____ _____ _____ _____ _____ • Which change does the least damage to the sentence structure?____________________ • What does this tell you about the mutation types? 2. Chromosomal Mutations • Changes in the number of chromosomes or the structure of chromosomes • Can change the location of genes on a chromosomes • Can change the number of copies of a gene 4 Types of Chromosomal Mutations • Deletion- loss of part of chromosome • Duplication- extra copy made of part of chromosome • Inversion- reverses direction of part of chromosome • Translocation- part of one attaches to another chromosome Causes of Mutations • Mistakes in replication • Mutagens- chemical or physical environmental agents – Pesticides – Tobacco smoke – X-rays – UV rays Mutations are Harmful and Helpful • Harmful- if the change results in a nonfunctional protein • Ex: a point mutation in the hemoglobin gene results in sickle cell anemia Sickle Cell Anemia • A nucleotide substitution causes the hemoglobin protein to change shape and no longer carry oxygen efficiently • This causes the rigid, sickle shape in the red blood cells • These cells get caught in capillaries and cause pain and other health problems Sickle Cell Anemia Mutations are Harmful and Helpful • Helpful- produces a better functioning protein for a certain environment • Mutations can lead to evolution • Example: There is a human mutation that makes people have increased resistance to HIV Cellular Specialization • When replication takes place, is all of the DNA in a cell copied? • Is the DNA in different cells of your body exactly the same? • If the DNA in every cell is the same, how do cells become different from each other? • The light switch analogy: – Cells “turn on,” or transcribe and translate only those genes that code for the proteins they need. Other genes are “turned off” because the proteins they code for are not needed by the cell. Cellular Specialization Example The following is a short sequence of DNA. Four different imaginary genes are shown. The protein for each gene is listed under the code. Given the following cells, list the genes above that would be “switched on” or expressed (active) in the cell and list those that would be “switched off” or not expressed (inactive)? WBC Skin Cell Muscle Cell Nerve Cell ON__________ ON___________ ON___________ ON_____________ OFF_________ OFF___________ OFF___________ OFF_________ Coding vs. Non-coding DNA • Coding DNA: areas of DNA that code for genes proteins, also known as ___________. – ~2% of the genome • Non-coding DNA: stretches of DNA that do not code for protein – What does the other ~98% of the DNA do then? – Some areas regulate/control the genes, other areas are genes that no longer function (i.e. the genes for a tail), etc. – The function of much of the non-coding DNA is still unknown Is Complexity Dictated by the NonCoding DNA?