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Transcription Translation DNA RNA Protein From DNA to Protein • The sole function of DNA in each of our cells is to synthesize protein. • Today we’re going to find out just how DNA accomplishes this.
Why do our cells need to make protein? • Muscle contraction • Transporting O2 in the bloodstream Hemoglobin • Providing immunity Antibodies • Getting reactions going Enzymes • Determine structure and function Cytoskeleton The proteins that your DNA codes for help determine:
The color and texture of your hair The color of your eyes
How does it happen? • Where is most of the DNA in our cells? Inside the nucleus • Where in our cells does protein synthesis occur? On the ribosomes • Since DNA never leaves the nucleus to perform protein synthesis, its code must find a way to get out of the nucleus. • Another nucleic acid known as RNA helps out.
The Code Think of DNA as the “keeper of the code”, or the “recipe” for the organism that contains it. These banding patterns on chromosomes represent “genes”. Genes are regions on chromosomes that code for specific proteins. While many parts of that code are important parts of the “recipe”, some parts are simply “filler”, and are unnecessary, so far as we understand. These unnecessary sections are called introns, and they are sequences of DNA, within genes, which have no apparent purpose. Within the cells of multicellular animals and plants almost every gene has introns, but they’re fairly rare in prokaryotes, and unicellular eukaryotes. Exons are the working or expressed segments of genes. Both terms, "introns" and "exons," were coined by Harvard biologist Walter Gilbert in 1978
Three Classes of RNA • The transcription of most genes produces a messenger RNA (mRNA) • The translation of mRNA requires the work of the transfer RNA (tRNA) • Ribosomal RNA is the rRNA that the ribosomes are composed of, and the actual site of protein synthesis As we already learned, RNA is almost, but not exactly like DNA. • RNA contains the pyrmidine uracil, instead of thymine • RNA is single stranded instead of a double helix • RNA contains Ribose instead of deoxyribose • RNA contains a hydroxyl group on the 2’ carbon, not a hydrogen like DNA
The first step...transcription In the nucleus, transcription occurs when a part of the DNA molecule unzips, as if replication was going to occur. The protein that is needed by the cell is coded for by a specific sequence of nucleotides. (a gene) • a deoxyribose sugar • A phosphate group • A nitrogenous base It is that specific section that unwinds when transcription occurs. (instead of the whole strand)
Transcription is initiated at a promoter. This promoter signals the start of a gene. The section of DNA that gets transcribed is called a transcription unit. 5’ 3’ 5’ 3’ An enzyme called RNA polymerase catalyzes nucleotide additions to the growing RNA strand complementing the DNA strand. This enzyme moves along the DNA strand, reading it in a 3’ to 5’ direction, joining complementary RNA nucleotides When the protein coding section ends at the terminator, the new RNA molecule is released as a free transcript.
Not quite mRNA yet! At this point the newly formed RNA is a “Pre-mRNA”, and must be modified before its protein-building instructions can be put to use. A cap binding protein complex (CBC) forms at the 5’ end which is needed when the mRNA is exported from the nucleus. To the 3’ end, a poly-tail of about 100 to 300 nucleotides is found, which will help the newly formed mRNA bind to a location on the ribosome.
This pre-mRNA still contains non-coding sections. Some human genes can have 50 introns in a single gene! Typically there are four introns within five exons, but the introns are generally much larger than the exons. Introns must be spliced out from between the exons before the mRNA can leave the nucleus. Spliceosomes are composed of five small nuclear RNA proteins, called snRNPs, (pronounced "snurps")
Deciphering the mRNA code The protein building “words” encrypted within the mRNA’s message are read three at a time. Each base triplet is known as a codon. Think of codons as being the words that need to be translated by the tRNA. Things to notice about chart: • there are 64 possible codon combinations • there are only 20 amino acids • most amino acids correspond to more than one codon code Methionine (AUG) is the start codon which signals the beginning of translation, and there are several “stop” codons.
Translation • The mature mRNA (messenger RNA) finds its way to the ribosome outside the nucleus, and continues to help in the process of protein synthesis. • The next step in the process involves an interpretation, or “translation” of the mRNA’s message by a molecule called tRNA (or transfer RNA) • This message is translated as a three letter code, or triplet code. Every three bases in sequence codes for a different word in the protein puzzle. • The three letter word (codon) brought to the ribosome by the mRNA, is translated by the anticodon of the tRNA.
Stages of Translation Activation:Occurs when an amino acid is covalently bonded to its corresponding anticodon on the tRNA. Initiation: Occurs when the mRNA is loaded onto the ribosome, and the tRNA begins reading the code, looking for the start codon. tRNA attaches to the 5’ end of the mRNA. Methionine Elongation: Refers to the time during translation when the polypeptide or amino acid chain is being built or “elongated”. A series of enzyme building and breaking occurs. Termination: A stop codon moves into the area where the polypeptide chain is being built. It is the signal to release the mRNA transcript from the ribosome. At the same time, the polypeptide chain is also released from the ribosome
The Wobble Effect Just to make things a bit more difficult, there is a “loosening” up of the base-pairing rule in translation. For codon-anticodon interactions, the third base of the codon isn’t read as “strictly” as the third base in the DNA/mRNA sequence during transcription or DNA replication. This is called the wobble effect. CUU; CUC; CUA Look at your codon chart and find an example of different codons that code for the same amino acid. As long as the first two bases are the same, the last base doesn’t matter all that much. Each of these mRNA codon sequences will bond with a single type of tRNA that carries leucine.
Reading the code Mom fed the dog for Tad who was out all day. Creating a protein is much like decoding a sentence in a written language. • Each three-base sequence codes for a different amino acid. (each letter goes together to make a different three letter word) • The sequence of amino acids determines the type of protein that is synthesized. (each word goes into a sequence to make a certain sentence) • Different sequences create different proteins. (The sequence of words can make very different sentences) Tad who was out for all day fed Mom the dog
Codon Chart Amino Acids
Decode the message... Using the codon chart, determine the sequence of the amino acid chain that will be created. 1. AUG = methionine 7. ACC = threonine leucine 2. CUG = 8. CAG = glutamine 9. GCA = alanine 3. AAA = lysine 4. CGC= 10. GGU = glycine arginine 5. CCU = proline 11. AGU = serine 6. GAA = Glutamic acid 12. UAA = Stop codon
