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Gene Expression

Gene Expression. Chapter 17: From Gene to Protein. The Central Dogma of Biology. DNA (in genes) is a recipe book for polypeptides (proteins) RNA is an essential partner in this process Proteins are the links between genotype and phenotype

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Gene Expression

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  1. Gene Expression Chapter 17: From Gene to Protein

  2. The Central Dogma of Biology • DNA (in genes) is a recipe book for polypeptides (proteins) • RNA is an essential partner in this process • Proteins are the links between genotype and phenotype • , the process by which DNA directs protein synthesis, includes 2 stages: transcription and translation

  3. Relationship between genes and proteins • George Beadle and Edward Tatum exposed • This created mutants that were unable to survive on minimal medium as a result of inability to synthesize certain molecules • Using crosses, they identified three classes of arginine-deficient mutants • Each lacked a different enzyme necessary for synthesizing arginine • They developed a • States that each gene dictates production of a specific enzyme • Some proteins aren’t enzymes, so researchers later revised the hypothesis to the

  4. Protein synthesis • RNA is the intermediate between genes and the proteins for which they code • is the synthesis of RNA under the direction of DNA • Occurs in the (where the DNA is located) • Produces • is the synthesis of a polypeptide, which occurs under the direction of mRNA • Occurs on the (cytoplasm) • mRNA transcript language is changed to protein language

  5. Prokaryotes vs Eukaryotes • In prokaryotes, mRNA produced by transcription is immediately translated without more processing • In a eukaryotes, the nuclear envelope separates transcription from translation • A is the initial RNA transcript from any gene (pre-mRNA) • Eukaryotic RNA transcripts are modified through to yield finished mRNA

  6. Genetic Code • How are the instructions for assembling amino acids into proteins encoded into DNA? • There are 20 amino acids, but there are only four nucleotide bases in DNA • The flow of information from gene to protein is based on a : a series of three-nucleotide words called • Example: AGT on a DNA strand results in the placement of the amino acid serine at the corresponding position of the polypeptide • During transcription, one of the two DNA strands called the provides a template for ordering the sequence of nucleotides in an RNA transcript • During translation, the mRNA codons are read in the 5 to 3 direction • Each codon specifies the addition of one of 20 amino acids

  7. Codons • The genetic code is but not • No codon specifies more than one amino acid • But there are many codons that code for the same amino acid • Code has start and stop signals • All 64 codons were deciphered by the mid-1960s • 61 code for amino acids • 3 are “stop” signals to end translation • Codons must be read in the correct (correct groupings) in order for the specified polypeptide to be produced

  8. Transcription • The stretch of DNA that is transcribed is called a • The three stages of transcription: • Initiation • Elongation • Termination

  9. Initiation • RNA polymerase is an enzyme that pries the DNA strands apart and hooks together the RNA nucleotides • The promoter is the DNA at the beginning of a gene sequence • DNA helix unwinds (10-20 bases at a time) • Similar to • RNA synthesis begins at the start point on the template strand

  10. Eukaryotic Promoters • Promoters signal the initiation of RNA synthesis • mediate the binding of RNA polymerase and the initiation of transcription • The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a • A promoter called a is crucial in forming the initiation complex in eukaryotes • Transcription factors bind here and help mediate the binding of RNA polymerase • A gene can be transcribed simultaneously by several RNA polymerases

  11. Elongation • RNA polymerase moves • It continues to unwind the DNA and elongate the transcript in the 5’ to 3’ direction (40 nucleotides per second) • Double helix reforms as section is passed • This creates a copy of the “recipe” • RNA synthesis follows the same base-pairing rules as DNA, except uracil substitutes for thymine

  12. Termination • RNA polymerase reaches the • Signals the end of the transcription unit • Involves a • RNA transcript is released • RNA polymerase detaches from DNA • Transcription Animation

  13. mRNA processing • Enzymes in the eukaryote modify pre-mRNA before the messages are sent to the cytoplasm • Each end of a pre-mRNA molecule is modified in a particular way: • The 5 end receives a modified • This tells ribosome where to begin translating • The 3 end gets a • Chain of 150-200 adenines • Helps to get transcript out of nucleus • Protects mRNA from degradation by enzymes

  14. mRNA processing • Next the transcript must be edited by • Most eukaryotic genes and their RNA transcripts have long noncoding regions of nucleotides that lie between coding, or expressed regions • The introns must be cut out to make a mature mRNA transcript • In some cases, RNA splicing is carried out by • A variety of proteins and several small nuclear ribonucleoproteins (snRNPs) that recognize the splice sites

  15. RNA as an enzyme • are RNA molecules that function as enzymes (like splicesome) • 3 properties of RNA enable it to function as an enzyme • It can form a 3-D structure because of its ability to base pair with itself • Some bases contain functional groups • It may hydrogen-bond with other nucleic acid molecules

  16. Alternative RNA Splicing • Some genes can encode more than one kind of polypeptide, depending on which segments are treated as exons during RNA splicing • Such variations are called • Because of alternative splicing, the number of different proteins an organism can produce is much greater than its number of genes

  17. Components of Translation • There are 3 different forms of RNA, each with a specific purpose during translation • is a copy of the DNA recipe, created in transcription • together with proteins forms ribosomes where proteins are made • brings an amino acid to the ribosome to help create the polypeptide

  18. tRNA • A tRNA molecule consists of a single RNA strand that is about • Bases hydrogen bond to each other • Cloverleaf shape • 3’ end picks up the amino acid and carries it to ribosome • Amino acid is determined by 3 bases opposite the amino acid called an • Anticodons pair with complementary codons in mRNA at a ribosome

  19. tRNA • Molecules of tRNA are unique since each carries a specific amino acid • Accurate translation requires two steps: • 1. A correct match between a tRNA and an amino acid, done by the enzyme • 2. A correct match between the tRNA anticodon and an mRNA codon • Flexible pairing at the third base of a codon is called and allows some tRNAs to bind to more than one codon

  20. rRNA • Ribosomes consists of two subunits (large and small) made of RNA & protein • Two functions • Pair complementary codons with anticodons • Bond amino acids together to form the polypeptide • A ribosome has three binding sites for tRNA: • The holds the tRNA that carries the next amino acid to be added to the chain • The holds the tRNA that carries the growing polypeptide chain • The is where discharged tRNAs leave the ribosome

  21. rRNA • Ribosome moves along mRNA until it reaches termination sequence • A number of ribosomes can translate a single mRNA simultaneously, forming a • Polyribosomes enable a cell to make many copies of a polypeptide very quickly

  22. Steps of translation • There are 3 stages of translation: • Chain initiation • Chain elongation • Chain termination • All three stages require protein “factors” that aid in the translation process

  23. Chain Initiation • First, a small ribosomal subunit binds with mRNA and a special • Then the small subunit moves along the mRNA until it reaches the start codon (AUG) in the P site • Anticodon of the initiator tRNA will complementary base pair with first codon, which always codes for methionine • Proteins called initiation factors bring in the large subunit that completes the

  24. Chain Elongation • During the elongation stage, amino acids are added one by one to the preceding amino acid • Each addition involves proteins called elongation factors and occurs in 3 steps: • (complementary tRNA comes into the A site) • (amino acid from the tRNA at the P site is attached to the amino acid on the tRNA at the A site) • (mRNA moves the tRNA at the A site to the P site so the next tRNA can go into the P site)

  25. Chain Termination • Occurs when a stop codon in the mRNA reaches the A site of the ribosome • The A site accepts a protein called a and the parts are dissociated • is released • is released • Ribosomal subunits disjoin

  26. Translation Animations • Animation 1 • http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/translation.swf • Animation 2 • http://carbon.cudenver.edu/~bstith/transla.MOV • Practice • http://gslc.genetics.utah.edu/units/basics/transcribe/

  27. Polypeptides vs Proteins • Often translation is not sufficient to make a functional protein • Polypeptide chains are after translation • During and after synthesis, a polypeptide chain spontaneously coils and folds into its • Remember: A protein’s shape determines its • Some polypeptides are activated by that cleave them • Other polypeptides come together to form the subunits of a protein

  28. Review Questions Explain the central dogma of biology. Define gene expression, including its 2 main parts. Explain how Beadle & Tatum’s experiment related genes to proteins. Differentiate between transcription and translation. Explain how gene expression differs in prokaryotes and eukaryotes. Define codons and their importance to gene expression. Name and describe the 3 stages of transcription. Define the roles of the promoter sequence, RNA polymerase, transcription factors, and the terminator sequence in transcription. Describe the 3 main events that occur in mRNA processing. Differentiate between introns and exons. Define ribozymes. Describe alternative RNA splicing. Differentiate between mRNA, rRNA, and tRNA. Explain the importance of an anticodon to gene expression. Name and describe the importance of the 3 sites of the ribosome. Name and describe the 3 steps of translation. Name 2 main events that occur during chain initiation of translation. Explain the 3 steps of translational elongation. Name 3 events that happen during chain termination of translation. Differentiate between polypeptides and proteins.

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