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The Molecular Basis of Inheritance

G. C. A. T. T. A. 1 nm. C. G. 3.4 nm. C. G. A. T. C. G. T. A. T. A. A. T. T. A. G. C. 0.34 nm. A. T. Figure 16.7a, c. (a) Key features of DNA structure. (c) Space-filling model. The Molecular Basis of Inheritance. Transcription.

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The Molecular Basis of Inheritance

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  1. G C A T T A 1 nm C G 3.4 nm C G A T C G T A T A A T T A G C 0.34 nm A T Figure 16.7a, c (a) Key features of DNA structure (c) Space-filling model The Molecular Basis of Inheritance

  2. Transcription • Transcription is the DNA-directed synthesis of RNA • RNA synthesis • Is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides • Follows the same base-pairing rules as DNA, except that in RNA, uracil substitutes for thymine

  3. Home Work Questions TATA BOX  -It is a type of promoter sequence, which specifies to other molecules where transcription begins.

  4. Home Work Questions • INTRONS: • intervening non-coding sequence in a eukaryotic gene. • EXONS: • the coding region of a eukaryotic gene. Each gene is composed of one or more exons.

  5. DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Amino acids Polypeptide tRNA with amino acid attached Ribosome Trp Phe Gly tRNA C C C G G Anticodon A A A A G G G U G U U U C Codons 5 3 mRNA *Translation* • Translation is the RNA-directed synthesis of a polypeptide • Translation involves • mRNA • Ribosomes - Ribosomal RNA • Transfer RNA • Genetic coding – codons

  6. Gene 2 DNA molecule Gene 1 Gene 3 DNA strand (template) 5 3 A C C T A A A C C G A G TRANSCRIPTION A U C G C U G G G U U U 5 mRNA 3 Codon TRANSLATION Gly Protein Phe Trp Ser Amino acid *The Genetic Code* • Genetic information is encoded as a sequence of nonoverlapping base triplets, or codons

  7. *The Genetic Code* Codons: • Basic unit of genetic code • set of 3 nucleotides in mRNA that codes for an amino acid placement on polypeptides. • 4 bases and 3 positions in each codon, there are 4 x 4 x 4 = 64 possible codons • 64 codons but only 20 amino acids, therefore most have more than 1 codon • 3 of the 64 codons are used as STOP signals which mark the end of the protein • One codon is used as a START signal: start of every protein

  8. Second mRNA base U C A G U UAU UUU UCU UGU Tyr Cys Phe UAC UUC UCC UGC C U Ser UUA UCA UAA Stop Stop UGA A Leu UAG UUG UCG Stop UGG Trp G CUU CCU U CAU CGU His CUC CCC CAC CGC C C Arg Pro Leu CUA CCA CAA CGA A Gln CUG CCG CAG CGG G Third mRNA base (3 end) First mRNA base (5 end) U AUU ACU AAU AGU Asn Ser C lle AUC ACC AAC AGC A Thr A AUA ACA AAA AGA Lys Arg Met or start G AUG ACG AAG AGG U GUU GCU GAU GGU Asp C GUC GCC GAC GGC G Val Ala Gly GUA GCA GAA GGA A Glu GUG GCG GAG GGG G *The Genetic Code* • A codon in messenger RNA is either translated into an amino acid or serves as a translational start/stop signal

  9. 3 A Amino acid attachment site C C 5 A C G C G C G U G U A A U U A U C G * G U A C A C A * A U C C * G * U G U G G * G A C C G * C A G * U G * * G A G C Hydrogen bonds (a) Two-dimensional structure. The four base-paired regions and three loops are characteristic of all tRNAs, as is the base sequence of the amino acid attachment site at the 3 end. The anticodon triplet is unique to each tRNA type. (The asterisks mark bases that have been chemically modified, a characteristic of tRNA.) G C U A G * A * A C * U A G A Anticodon *Transfer RNA* • Consists of a single RNA strand, only 80 nucleotides long • Each carries a specific amino acid on one end and has an anticodon on the other end • Enzymes pair up the proper tRNA molecules with their corresponding amino acids. • tRNA brings the amino acids to the ribosomes, The “anticodon” is the 3 RNA bases that matches the 3 bases of the codon on the mRNA molecule

  10. Amino acid attachment site 5 3 Hydrogen bonds A A G 3 5 Anticodon Anticodon (c) Symbol used in the book (b) Three-dimensional structure *Transfer RNA* • 3 dimensional tRNA molecule is roughly “L” shaped

  11. DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Exit tunnel Growing polypeptide tRNA molecules Large subunit E P A Small subunit 5 3 mRNA Computer model of functioning ribosome. This is a model of a bacterial ribosome, showing its overall shape. The eukaryotic ribosome is roughly similar. A ribosomal subunit is an aggregate of ribosomal RNA molecules and proteins. (a) *Ribosomes* • Ribosomes facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis • The 2 ribosomal subunits are constructed of proteins and RNA molecules named ribosomal RNA or rRNA

  12. Growing polypeptide Amino end Next amino acid to be added to polypeptide chain tRNA 3 mRNA Codons 5 (c) Schematic model with mRNA and tRNA. A tRNA fits into a binding site when its anticodon base-pairs with an mRNA codon. The P site holds the tRNA attached to the growing polypeptide. The A site holds the tRNA carrying the next amino acid to be added to the polypeptide chain. Discharged tRNA leaves via the E site. Building a Polypeptide

  13. *Building a Polypeptide* • We can divide translation into three stages • Initiation • Elongation • Termination • The AUG start codon is recognized by methionyl-tRNA or Met • Once the start codon has been identified, the ribosome incorporates amino acids into a polypeptide chain • RNA is decoded by tRNA (transfer RNA) molecules, which each transport specific amino acids to the growing chain • Translation ends when a stop codon (UAA, UAG, UGA) is reached

  14. Large ribosomal subunit P site 5 3 U C A Met Met 3 A 5 G U Initiator tRNA GDP GTP E A mRNA 5 5 3 3 Start codon mRNA binding site Small ribosomal subunit Translation initiation complex 2 1 The arrival of a large ribosomal subunit completes the initiation complex. Proteins called initiation factors (not shown) are required to bring all the translation components together. GTP provides the energy for the assembly. The initiator tRNA is in the P site; the A site is available to the tRNA bearing the next amino acid. A small ribosomal subunit binds to a molecule of mRNA. In a prokaryotic cell, the mRNA binding site on this subunit recognizes a specific nucleotide sequence on the mRNA just upstream of the start codon. An initiator tRNA, with the anticodon UAC, base-pairs with the start codon, AUG. This tRNA carries the amino acid methionine (Met). Initiation of Translation • The initiation stage of translation brings together mRNA, tRNA bearing the first amino acid of the polypeptide, and two subunits of a ribosome

  15. 1 Codon recognition. The anticodon of an incoming aminoacyl tRNA base-pairs with the complementary mRNA codon in the A site. Hydrolysis of GTP increases the accuracy and efficiency of this step. Amino end of polypeptide DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide E mRNA 3 Ribosome ready for next aminoacyl tRNA P A site site 5 GTP 2 GDP 2 E E P A P A 2 Peptide bond formation. An rRNA molecule of the large subunit catalyzes the formation of a peptide bond between the new amino acid in the A site and the carboxyl end of the growing polypeptide in the P site. This step attaches the polypeptide to the tRNA in the A site. GDP Translocation. The ribosome translocates the tRNA in the A site to the P site. The empty tRNA in the P site is moved to the E site, where it is released. The mRNA moves along with its bound tRNAs, bringing the next codon to be translated into the A site. 3 GTP E P A Elongation of the Polypeptide Chain • In the elongation stage, amino acids are added one by one to the preceding amino acid

  16. Release factor Free polypeptide 5 3 3 3 5 5 Stop codon (UAG, UAA, or UGA) The release factor hydrolyzes the bond between the tRNA in the P site and the last amino acid of the polypeptide chain. The polypeptide is thus freed from the ribosome. When a ribosome reaches a stop codon on mRNA, the A site of the ribosome accepts a protein called a release factor instead of tRNA. The two ribosomal subunits and the other components of the assembly dissociate. 2 1 3 Termination of Translation • The final stage is termination when the ribosome reaches a stop codon in the mRNA

  17. *Termination of Translation* • The final step in translation is termination. • ribosome reaches a STOP codon, there is no corresponding transfer RNA. • A small protein called a “release factor” attaches to the stop codon. • The release factor causes the whole complex to fall apart: messenger RNA, the two ribosome subunits, the new polypeptide. • The messenger RNA can be translated many times, to produce many protein copies.

  18. DNA TRANSCRIPTION 1 3 4 2 5 RNA is transcribed from a DNA template. 3 Poly-A RNA transcript RNA polymerase 5 Exon RNA PROCESSING In eukaryotes, the RNA transcript (pre- mRNA) is spliced and modified to produce mRNA, which moves from the nucleus to the cytoplasm. RNA transcript (pre-mRNA) Intron Aminoacyl-tRNA synthetase Cap NUCLEUS Amino acid FORMATION OF INITIATION COMPLEX AMINO ACID ACTIVATION tRNA CYTOPLASM After leaving the nucleus, mRNA attaches to the ribosome. Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP. Growing polypeptide mRNA Activated amino acid Poly-A Poly-A Ribosomal subunits Cap 5 TRANSLATION C C A U A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome one codon at a time. (When completed, the polypeptide is released from the ribosome.) A E A C Anticodon A A A U G U G G U U U A Codon Ribosome A summary of transcription and translation in a eukaryotic cell Figure 17.26

  19. Post-translation • The new polypeptide is now floating loose in the cytoplasm if translated by a free ribosme. • It might also be inserted into a membrane, if translated by a ribosome bound to the endoplasmic reticulum. • ***Polypeptides fold spontaneously into their 4 levels of structure (quaternary) , and they spontaneously join with other polypeptides to form the final proteins.*** • Sometimes other molecules are also attached to the polypeptides: sugars, lipids, phosphates, etc. All of these have special purposes for protein function and GENE EXPRESSION

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