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RNA and Protein Synthesis: Central Dogma of Molecular Biology

Explore the process of gene expression through transcription and translation, detailing RNA types and genetic coding, with emphasis on the Central Dogma from DNA to RNA to protein synthesis. Discover the role of RNA splicing and post-transcriptional modifications in eukaryotic cells.

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RNA and Protein Synthesis: Central Dogma of Molecular Biology

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  1. Chapter 17AP Biology From Gene to Protein

  2. Central Dogma of Molecular Biology DNA  RNA Protein

  3. What is RNA? • Contains the bases A, C, G, and U instead of T • single-stranded (often folds onto itself) • Three types of RNA: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA)

  4. Protein Synthesis 1. Transcription - DNA message is transcribed into mRNA and sent to ribosomes. 2. Translation - mRNA is translated into a Protein by a ribosome. DNA  RNA  Protein

  5. Overview – Eukaryotic Cell Notice that transcription and translation occur in different places in a eukaryotic cell.

  6. Overview – Prokaryotic Cell

  7. The Triplet Code Note that the bases of mRNA are complementary to the template strand of DNA The bases of mRNA are read in groups of three – each group is called a codon

  8. From mRNA to Amino Acids • The mRNA base triplets are called codons • mRNA is written in the 5’ to 3’ direction • The codons code for each of the 20 amino acids • The genetic code is redundant • Different codons code for the same amino acid

  9. The Genetic Code The three bases of an mRNA codon code for the 20 amino acids that are the subunits of proteins The amino acids are designated by three letters

  10. Universal Nature of the Genetic Code

  11. RNA Processing • In eukaryotes, transcription results in pre-mRNA that needs processing before it leaves the nucleus.

  12. Processing of Pre-mRNA • Addition of a 5’ cap • A modified form of a guanine nucleotide • Addition of a poly-A tail • 50-250 adenine (A) nucleotides are added • This is referred to as the poly-A tail • RNA splicing • Editing of the initial strand of mRNA (a cut and paste job)

  13. Addition of 5’ cap and poly-A tail

  14. 5’ cap and poly-A tail • Both facilitate export of mRNA from the nucleus • Both help protect mRNA from degradation by enzymes • Both facilitate the attachment of mRNA to the ribosome

  15. RNA Splicing • Only takes place in eukaryotic cells • Large sections are spliced out; these are called introns • The sections that remain are called exons • “exons EXIT the nucleus” • These exons are spliced together to form the final mRNA that will be translated

  16. RNA Splicing

  17. Alternative RNA Splicing • Allows for different combinations of exons • This results in more than one protein per gene • This explains why we have fewer genes in our genome than what was expected • The human genome contains about 21,000 protein-encoding genes, but the total number of proteins in human cells is estimated to be between 250,000 to one million.

  18. Transcription Animation • https://www.youtube.com/watch?v=SMtWvDbfHLo

  19. 17.4 Translation • tRNA functions in transferring amino acids from the cytoplasm to a ribosome • rRNA complexes with proteins to form the two subunits that make up ribosomes

  20. tRNA • Each type of tRNA is specific for a particular amino acid • At one end tRNA loosely binds the amino acid and at the other end it has a nucleotide triplet called the anticodon • The anticodon allows it to pair specifically with a complementary codon on mRNA

  21. Structure of tRNA

  22. Basic Concept of Translation Codons are the triplet nucleotides on mRNA Anticodons are the triplet nucleotides on tRNA

  23. Three Stages of Translation • Initiation • Begins with the start codon AUG (always!) • Elongation • Codon recognition • Peptide bond formation (between 2 a.a.) • Termination • A stop codon is reached and translation stops

  24. Initiation

  25. Elongation

  26. Termination

  27. Translation Animation • https://www.youtube.com/watch?v=TfYf_rPWUdY

  28. Folding of the Polypeptide • Following release from the ribosome, the polypeptide then folds to its specific conformation (3d shape) • Chaperonins are the proteins that help with this folding process • The first 20 amino acids of the polypeptide serve as a signal peptide and act as a cellular zip code, directing the polypeptide to its final destination

  29. Targeting Proteins to the ER

  30. 17.5 Point Mutations • Mutations are alterations in the genetic material of the cell caused by mutagens • Point mutations are alterations of just 1 base pair in a gene • Base-pair substitution • Silent mutations – have no effect on the encoded protein • Missense mutations – change one amino acid to another; might still code for the correct amino acid • Nonsense mutations – change a regular amino acid codon into a stop codon • Insertions & deletions • Frameshift mutation – codons are read in the wrong “frame”

  31. Base-Pair Substitution - Silent

  32. Base-Pair Substitution - Missense

  33. Base-Pair Substitution - Nonsense

  34. Frameshift Mutation- Insertion

  35. Frameshift Mutation - Deletion

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