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From DNA to Proteins

This chapter delves into the fundamental steps of protein synthesis, highlighting the key processes of transcription and translation from DNA to proteins. Understand the roles of DNA, RNA, and the three classes of RNAs in protein production. Explore the differences between DNA and RNA, nucleotide subunits, genetic coding, tRNA structure, and the stages of translation. Delve into gene mutations, their types, causes and the impact on genetic information. Gain insights into the fascinating world of molecular biology with this comprehensive guide.

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From DNA to Proteins

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  1. From DNA to Proteins Chapter 13

  2. Central Dogma • DNA • RNA • Protein

  3. Steps from DNA to Proteins Same two steps produce all proteins: 1) DNA is transcribed to form RNA • Occurs in the nucleus • RNA moves into cytoplasm 2) RNA is translated to form polypeptide chains, which fold to form proteins

  4. Three Classes of RNAs • Messenger RNA (mRNA) • Carries protein-building instruction • Ribosomal RNA (rRNA) • Major component of ribosomes • Transfer RNA (tRNA) • Delivers amino acids to ribosomes

  5. DNA vs RNA • 1) Sugar (ribose) differs • DNA sugar is deoxyribose (missing O) • RNA sugar is ribose • 2) Possible bases differ • DNA uses bases A, C, G, T • RNA uses bases A, C, G, U • 3) DNA stable as double-stranded structure RNA stable as single-stranded structure

  6. A Nucleotide Subunit of RNA

  7. Base Pairing during Transcription • A new RNA strand can be put together on a DNA region according to base-pairing rules • As in DNA, C-G, but A-T does not exist • Uracil (U) pairs with adenine (A)

  8. Transcription & DNA Replication • Like DNA replication • Nucleotides added to growing RNA strand in the 5’ to 3’ direction • Unlike DNA replication • Small in length compared to DNA • RNA polymerase makes RNA from DNA • Product is a single strand of RNA

  9. Base Pairing Compared

  10. Promoter - place where RNA polymerase binds • A base sequence in the DNA that signals the start of a gene • For transcription to occur, RNA polymerase must first bind to a promoter promoter region

  11. Gene Transcription newly forming RNA transcript DNA template at selected transcription site DNA template winding up DNA template unwinding

  12. Adding Nucleotides direction of transcription 3´ 5´ 3´ 5´ growing RNA transcript

  13. Transcript Modification

  14. Genetic Code • Set of 64 base triplets • Codons • Nucleotide bases read in blocks of three • 61 specify amino acids • 3 stop translation

  15. Code Is Redundant • Twenty (20) kinds of amino acids are specified by 61 codons • Most amino acids can be specified by more than one codon • i.e. six codons specify leucine • UUA, UUG, CUU, CUC, CUA, CUG

  16. Near-Universal Genetic Code AUG AAA CGA UGA

  17. tRNA Structure - brings 20 different amino acids to ribosome There are at least 20 different populations of tRNAs inside the cytoplasm of the cell.

  18. Ribosomes - structures where proteins are made

  19. Three Stages of Translation Initiation Elongation Termination

  20. Initiation • Initiator tRNA binds to small ribosomal subunit • Small subunit/tRNA complex attaches to mRNA and moves along it to an AUG “start” codon • Large ribosomal subunit joins complex

  21. Binding Sites on Large Subunit binding site for mRNA A (second binding site for tRNA) P (first binding site for tRNA)

  22. Elongation • mRNA passes through ribosomal subunits • tRNAs deliver amino acids to the ribosomal binding site in the order specified by the mRNA • Peptide bonds form between the amino acids and the polypeptide chain grows

  23. Elongation

  24. Termination A STOP codon moves into the area where the chain is being built. It is the signal to release the mRNA transcript from the ribosome. The new polypeptide chain is released from the ribosome. It is free to join the pool of proteins in the cytoplasm or to enter rough ER of the endomembrane system. The two ribosomal subunits now separate also.

  25. Polysome - a lot of ribosomes • A cluster of many ribosomes translating one mRNA transcript • Transcript threads through the multiple ribosomes like the thread of bead necklace • Why? - Allows rapid synthesis of proteins

  26. What Happens to the New Polypeptides (proteins)? • Some just enter the cytoplasm • Many enter the endoplasmic reticulum and move through the cytomembrane system where they are modified

  27. Overview

  28. Gene Mutations Base-Pair Substitutions (small) Insertions (small or large) Deletions (small or large)

  29. Frameshift Mutations • Insertion • Extra base added into gene region • Deletion • Base removed from gene region • Both shift the reading frame • Result in many wrong amino acids

  30. Mutations in Genes - example of substitution & deletion

  31. Transposons - naughty things! • DNA segments that move spontaneously about the genome • When they insert into a gene region, they usually inactivate that gene

  32. Mutation Rates • Each gene has a characteristic mutation rate • Average rate for eukaryotes is between 10-4 and 10-6 per gene per generation • Only mutations that arise in germ cells can be passed on to next generation

  33. Causes of Mutations • Exposure to harmful radiation and chemicals in the environment can cause DNA mutations • Genetic defects to genes that repair mutations leave mutations behind in large numbers.

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