Protein Synthesis

• Protein Synthesis


  D N A serves as a blueprint for making proteins
• Gene


  D N A segment that carries a blueprint for building one protein or polypeptide chain

  
• Proteins have many functions including

  Fibrous and Globular proteins
• Fibrous proteins (structural proteins)


  building materials for cells

  
• Globular proteins (functional proteins)


  can act as enzymes (biological catalysts)

  
• D N A information is coded into

  A sequence of bases

  
• A sequence of three bases (triplet) codes for


  An amino acids

  For example, a DNA sequence of AAA specifies the amino acid phenylalanine

  
• Role of DNA


  - Most ribosomes, the manufacturing sites of proteins, are located in the cytoplasm​

  - DNA never leaves the nucleus in interphase cells​

  - DNA requires a decoder and a messenger to carry instructions to build proteins to ribosomes​

  - Both the decoder and messenger functions are carried out by RNA (ribonucleic acid)
• How does RNA differ from DNA?

  - RNA is single-stranded​

  - RNA contains ribose sugar instead of deoxyribose​

  - RNA contains uracil (U) base instead of thymine (T)
• Three varieties of RNA carry out the DNA orders for protein synthesis

  tRNA, rRNA, and mRNA
• Transfer RNA (tRNA)


  Escorts appropriate amino acids to the ribosome for building the protein

  
• Ribosomal RNA (rRNA)


  Helps form the ribosomes, where proteins are built

  
• Messenger RNA (mRNA)


  Carries the instructions for building a protein from the nucleus to the ribosome

  
• Protein synthesis involves two major phases:

  transcription and translation
• Transcription


  -Transfer of information from DNA's base sequence to the complementary base sequence of m R N A​

  - DNA is the template for transcription; mRNA is the product​

  - Each DNA triplet corresponds to an m R N A codon​

  - If DNA sequence is A A T-C G T-T C G, then the m R N A corresponding codons are U U A-G C A-A G C
• Translation


  - Base sequence of nucleic acid is translated to an amino acid sequence; amino acids are the building blocks of proteins​

  -Occurs in the cytoplasm and involves three major varieties of RNA
• Steps of Translation:


  - Step 1: covers transcription

  - Step 2: m R N A leaves nucleus and attaches to ribosome, and translation begins​

  - Step 3: incoming tRNA recognizes a complementary mRNA codon calling for its amino acid by temporarily binding its anticodon to the codon

  - Step 4: as the ribosome moves along the m R N A, a new amino acid is added to the growing protein chain​

  - Step 5: released tRNA reenters the cytoplasmic pool, ready to be recharged with a new amino acid