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Central Dogma; Big Idea 3 , Essential Knowledge 3.A.1 b-c and 3.B.1 CH. 17 DNA TO PROTEIN (TRANSCRIPTION AND TRANSLATION) OVERVIEW AND BACKGROUND INFO DNA - Double stranded molecule - Contains thymine - Contains deoxyribose sugar - Found only in nucleus RNA - Made of nucleotides - Contain adenine, guanine, and cytosine - Single stranded molecule - Contains uracil - Contains ribose sugar - Found in nucleus and cytoplasm Does this diagram represent DNA or RNA? …how can you tell? Central Dogma- DNA to protein DNA serves as a genetic code for the synthesis (creation) of proteins We eat food, and that food is reassembled to make US (you are made of proteins, which are made of amino acids) DNA codes for RNA, which guides the synthesis of proteins (basically in order to read and express genes, it goes from DNA to RNA to protein) Types of RNA The 3 main types of RNA are: Messenger RNA (mRNA) – these travel from the nucleus to the ribosome to direct the synthesis of a specific protein Ribosomal RNA (rRNA) – help form ribosomes in the cytoplasm (remember, ribosomes help with protein synthesis); reads and decodes RNA Transfer RNA (tRNA) – transport amino acids (building blocks of protein) to the ribsomes Central Dogma - Protein synthesis Protein synthesis is the assembly of amino acids (by RNA) into proteins Involves two steps: 1. Transcription – copying DNA code into mRNA; occurs in nucleus 2. Translation – reading the mRNA code and assembling amino acids into a polypeptide chain (protein); occurs in cytoplasm Transcription overview Performed in nucleus by mRNA mRNA “reads” single DNA strand and forms the complementary copy (replacing thyamine with uracil) Chinese characters transcribed to English alphabet: Summary of Transcription The three stages of transcription: Initiation (RNA polymerase attaches to the DNA) Elongation (mRNA is created) Termination (DNA zips back together because of the hydrogen bonds holding the base pairs together) Vocab The DNA sequence where RNA polymerase attaches is called the promoter; in prokaryotes, the sequence signaling the end of transcription is called the terminator The stretch of DNA that is transcribed is called a transcription unit Transcription First step of the central dogma involves the creating of mRNA from DNA Occurs in nucleus 1. Initiation – The DNA is unzipped in the nucleus (by the enzyme helicase) RNA polymerase (another enzyme) binds to a specific section where an mRNA will be synthesiszed. Transcription factors mediate the binding of RNA polymerase and initiation of transcription Transcription cont. 2. Elongation - Uracil is used instead of thymines when the bases pair up and mRNA is made The mRNA then gets a 5’ cap on one end, a poly A tail on the other 3’ end (which is just a bunch of adenines in a row) We get rid of introns, which are areas that don’t code for a gene (exons are areas that do code for a gene) and then we splice together the mRNA (using something called slicesomes) 3. Termination - The mRNA takes the copied code into the cytoplasm for protein synthesis Practice Please create a strand of mRNA from this template strand of DNA (write this in your notes): TTAACGCATGCATAC Translation Translation occurs in ribosomes (in cytoplasm) All three types of RNA work together during translation to produce proteins Transcribed Chinese words translated to English words: Translation Once the mRNA is made, it moves out into the cytoplasm and attaches to a ribosome (can be located on the E.R.) When it connects to the ribosome, the code is read and makes a protein through a process called translation tRNA act as interpreters of the mRNA Molecules of tRNA are not identical: Each carries a specific amino acid on one end Each has an anticodon on the other end; the anticodon basepairs with a complementary codon on mRNA Steps of Translation 1. mRNA leaves the nucleus 2. mRNA attaches to a ribosome (between the 2 subunits, which are made of protein and rRNA) 3. tRNA molecules bring amino acids (building blocks of protein) to the ribosome 4. Every 3 letters in the mRNA code for a single amino acid – 3 bases form a “codon” The tRNA has a 3 letter message that matches the codon on the mRNA, called the ANTICODON 5. Amino acids get linked together in a “polypeptide chain”, which form a protein 6. The chain folds into a 3-D protein (looks kind of like a 3 leaf clover) *DNA – mRNA – ribosome - amino acids are brought by tRNA – polypeptide chain – protein Targeting Polypeptides to Specific Locations Two types of ribosomes are in the cell: free ribsomes (in the cytosol) bound ribosomes (attached to the ER) Free ribosomes mostly synthesize proteins that function in the cytosol Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell Ribosomes are identical and can switch from free to bound Ribosomes cont. A ribosome has three binding sites for tRNA: The A site holds the tRNA that carries the next amino acid to be added to the chain The P site holds the tRNA that carries the growing polypeptide chain The E site is the exit site, where discharged tRNAs leave the ribosome Termination of Translation Termination occurs when a stop codon (see the stop codons in the picture) in the mRNA reaches the A site of the ribosome The A site accepts a protein called a release factor The release factor causes the addition of a water molecule instead of an amino acid This reaction releases the polypeptide, and the translation assembly then comes apart The Genetic Code DNA is a three base code (eg. ATC). Three bases form a “codon”. DNA codons are converted into mRNA codons and are then interpreted by the gentic code. DNA->mRNA->Amino Acids->Protein Genetic Code cont. There are 20 amino acids, but only 4 different nucleotide bases they can combine in so many different ways, that they can create over 10,000 different proteins Practice The genetic code is a set of rules (see the chart) used to specify which amino acid is used during protein synthesis Here is a chart of the genetic code -> DNA codon: TAC mRNA: Amino Acid More Practice DNA: TACGGGTCTGGCATT mRNA: Amino Acid Sequence: Evolution of the Genetic Code The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals Genes can be transcribed and translated after being transplanted from one species to another Concept 17.6: Comparing gene expression in prokaryotes and eukaryotes reveals key differences Prokaryotic cells lack a nuclear envelope, allowing translation to begin while transcription progresses In a eukaryotic cell: The nuclear envelope separates transcription from translation Extensive RNA processing occurs in the nucleus Mutations – a permanent change in a cell’s DNA Point mutations involve a chemical change in just one base pair and can be enough to cause a genetic disorder (a point mutation where one base is exchanged for another is called a substitution) The change of a single nucleotide in a DNA template strand can lead to production of an abnormal protein Base-pair substitution can cause missense or nonsense mutations Missense mutations still code for an amino acid, but not necessarily the right amino acid Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein Missense mutations are more common *Ex – can cause some cancers, attributes to Tay-Sachs symptoms, color blindness Mutation cont. Frameshift mutations – change the multiples of three from the point of insertion or deletion and change the “frame” of the amino acid sequence Insertions are additions of a nucleotide to the DNA sequence Deletion – the loss of a nucleotide These mutations have a disastrous effect on the resulting protein more often than substitutions do Ex – can be a cause of cystic fibrosis, colorectal cancer, Crohn’s disease (inflammatory bowel disease) Insertions and Deletions Insertions and deletions are additions or losses of nucleotide pairs in a gene These mutations have a disastrous effect on the resulting protein more often than substitutions do Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation Mutations Can be caused spontaneously or by mutagens (certain chemicals or radiation) Can be found in somatic cells or gametes with different results Somatic cells will pass the mutation to all its daughter cells – can be a cause of cancer in the body Gametes don’t necessarily affect the function of the organism, but may drastically affect their offspring Can be good, bad, or neutral (may not even know you have it until there is a change in the environment) Mutagens Spontaneous mutations can occur during DNA replication, recombination, or repair Mutagens are physical or chemical agents that can cause mutations Ex – ultraviolet light, radiation, benzene (an industrial solvent found in synthetic rubber and some dyes), virus’s, sodium azide (found in car air bags)