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Protein synthesis: Transcription and Translation

Protein synthesis: Transcription and Translation. RNA. Ribonucleic acid Single stranded molecule   Found in both nucleus & cytoplasm Contains ribose sugar . Contains the nitrogen base uracil (U) instead of thymine so A (adenine) pairs with U; also has the bases guanine and cytosine

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Protein synthesis: Transcription and Translation

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  1. Protein synthesis: Transcription and Translation

  2. RNA • Ribonucleic acid • Single stranded molecule   • Found in both nucleus & cytoplasm • Contains ribose sugar

  3. Contains the nitrogen base uracil (U) instead of thymine so A (adenine) pairs with U; also has the bases guanine and cytosine • Basic unit is a nucleotide (ribose, phosphate, nitrogen base) • Three types of RNA exist (mRNA, tRNA, & rRNA)

  4. mRNA • Messenger RNA • Single, uncoiled, straight strand of nucleic acid • Found in the nucleus & cytoplasm • Copies DNA’s instructions & carries them to the ribosomes where proteins can be made • mRNA’s base sequence is translated into the amino acid sequence of a protein • Three consecutive bases on mRNA called a codon (e.g. UAA, CGC, AGU) • Reusable

  5. tRNA • Transfer RNA • Single stranded molecule • containing nucleotides in • the shape of a cloverleaf • Carries amino acids in the cytoplasm to ribosomes for protein assembly

  6. Three bases on tRNA that are complementary to a codon on mRNA are called anticodons (e.g. codon- UUA; anticodon- AAU) • Amino acid attachment site across from anticodon site on tRNA • Enters a ribosome & reads mRNA codons and links together correct sequence of amino acids to make a protein • Reusable

  7. rRNA • Ribosomal RNA • Globular shape • Helps make up the structure of the ribosomes • rRNA & protein make up the large & small subunits of ribosomes • Ribosomes are the site of translation (making polypeptides) • Aids in moving ribosomes along the mRNA strand as amino acids are linked together to make a protein

  8. Amino Acids • 20 exist • Linked together in a process called protein synthesis in the cytoplasm to make polypeptides (subunits of proteins) • Amino acids bonded together by peptide bonds • DNA contains the instructions for making proteins but is too large to leave the nucleus

  9. Three consecutive bases on DNA called a triplet (e.g. TCG, ATG, ATT) • mRNA codon table tells what 3 bases on mRNA code for each amino acid (64 combinations of 3 bases) • Methionine (AUG) on mRNA is called the start codonbecause it triggers the linking of amino acids • UAA, UGA, & UAG on mRNA signal ribosomes to stop linking amino acids together – stop codons

  10. Protein Synthesis • Consists of 2 parts --- Transcription & Translation • Begins in the nucleus with mRNA copying DNA’s instructions for proteins (transcription) • Completed in the cytoplasm when tRNA enters ribosomes to read mRNA codons and link together amino acids (translation)

  11. Steps in Transcription 1. DNA helicase (enzyme) uncoils the DNA molecule 2. RNA polymerase (enzyme) binds to a region of DNA called the promoter which has the start codon TAC to code for the amino acid methionine 3. The 2 DNA strands separate, but only one will serve as the template & be copied 4. Free nucleotides are joined to the template by RNA polymerase to form the mRNA strand

  12. 5. mRNA sequence is built until the enzyme reaches an area on DNA called the termination signal 6. RNA polymerase breaks loose from DNA and the newly made mRNA 7. Eukaryotic mRNA is modified (unneeded sections snipped out by enzymes & rejoined) before leaving the nucleus through nuclear pores

  13. Steps in Translation 1. mRNA brings the copied DNA code from the nucleus to the cytoplasm 2. mRNA attaches to one end of a ribosome 3. tRNA’sattach the correct amino acid floating in the cytoplasm to themselves

  14. 4. The tRNAanticodon “reads” & temporarily attaches to the mRNA codon in the ribosome 5. Two amino acids at a time are linked together by peptide bonds to make polypeptide -chains (protein subunits) 6. tRNA’sbreak loose from amino acid, leave the ribosome, & return to cytoplasm to pick up another amino acid

  15. 7. Ribosomes) move along the mRNA strand until they reach a stop codon (UAA, UGA, or UAG); called termination

  16. Mutations • Mutations are changes in genetic material • Gene mutations produce changes in a single gene • Point mutation is the change in one or a few nucleotides in the DNA sequence; includes both the change in one base (substitution) as well as insertion and deletion (frameshift)

  17. A substitution usually involves no more than a single amino acid • for example – the dog bit the cat could become the log bit the cat • Frameshifts causes a shift in reading frames and may change every amino acid following the change and can change a protein so much that it is unable to perform its normal function • for example – the fat cat ate the ratwith a deletion could become tefatcatatct her atand with a insertion could become the fat gca tat eth era t

  18. Most mutations are neutral – have little or no effect • Mutations that cause dramatic changes in the function of a gene are often harmful • Sometimes mutations are beneficial by giving an organism an advantage in their environment • Mutations are a source of genetic variability in a species

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