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Gene Expression

Gene Expression. Chapter 17: From Gene to Protein. The Central Dogma of Biology. DNA (in genes) is a is an essential partner in this process are the links between genotype and phenotype Gene expression , the process by which DNA directs protein synthesis, includes 2 stages:.

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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 • is an essential partner in this process • are the links between genotype and phenotype • Gene expression, the process by which DNA directs protein synthesis, includes 2 stages:

  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 • Using crosses, they identified three classes of arginine-deficient mutants • Each lacked a different necessary for synthesizing arginine • They developed a • States that • 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 • Transcription is the synthesis of • Occurs in the (where the DNA is located) • Produces • Translation is the synthesis of a • 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 • In a eukaryotes, the separates transcription from translation • A primary transcript is the • Eukaryotic RNA transcripts are modified through

  6. Genetic Code • How are the instructions for assembling amino acids into proteins encoded into DNA? • There are , but there are only • The flow of information from gene to protein is based on a : a series of • 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 template strand provides a • During translation, the • Each codon specifies the addition of

  7. Codons • The genetic code is • No codon specifies • But there are that code for the same amino acid • Code has start and stop signals • All were deciphered by the mid-1960s • 61 code for • 3 are • Codons must be read in the correct reading frame (correct groupings) in order for the

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

  9. Initiation • RNA polymerase binds to the promoter • RNA polymerase is an enzyme that and hooks together the • The promoter is the • (10-20 bases at a time) • Similar to DNA replication • RNA synthesis begins at the start point on the

  10. Eukaryotic Promoters • Promoters signal the • 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 • 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 • Double helix as section is passed • This creates a • RNA synthesis follows the same base-pairing rules as DNA, except

  12. Termination • RNA polymerase reaches the • Signals the of the transcription unit • Involves a • RNA transcript is • RNA polymerase • 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 • This tells ribosome where to begin translating • The 3 end gets a • Chain of 150-200 • Helps to get transcript

  14. mRNA processing • Next the transcript must be edited by • Most eukaryotic genes and their RNA transcripts have long of nucleotides that lie between • The introns must be • 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 • Ribozymes are • 3 properties of RNA enable it to function as an enzyme • It can form a 3-D structure because of its ability • Some bases contain • It may hydrogen-bond with

  16. Alternative RNA Splicing • Some genes can encode more than one kind of polypeptide, depending on • Such variations are called alternative RNA splicing • Because of alternative splicing, the

  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 that is about nucleotides long • Bases to each other • Cloverleaf shape • 3’ end picks up the and carries it to ribosome • Amino acid is determined by 3 bases opposite the amino acid called an • Anticodons pair with

  19. tRNA • Molecules of tRNA are unique since • Accurate translation requires two steps: • 1. A correct match between a , done by the enzyme aminoacyl-tRNA synthetase • 2. A correct match between the • 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 made of RNA & protein • Two functions • 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 • A number of ribosomes can translate a single mRNA simultaneously, forming a • Polyribosomes enable a cell to make

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

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

  24. Chain Elongation • During the elongation stage, amino acids are added • 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 in the mRNA reaches the A site of the ribosome • The A site accepts a protein called a • is released • is released

  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 • Polypeptide chains are after translation • During and after synthesis, a polypeptide chain spontaneously • Remember: A protein’s shape • Some polypeptides are • Other polypeptides come together to form the

  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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