DNA and Protein Synthesis

• Define Genome

  = The complete set of genes in the cell.
• Define Proteome

  = The entire range of proteins that a cell is able to produce
• Structure of mRNA

  1. Single polynucleotide chain.

  
  

  2. Smaller than DNA but > tRNA.

  
  

  3. Single helix molecule (except in some viruses).

  
  

  4. Pentose sugar = ribose.

  
  

  5. A--U and C--G.

  
  

  6. Manufactured in nucleus but found throughout cell.

  
  

  7. Less stable than DNA and tRNA ---> molecules usually broken down in cells within a few days.

  
• Structure of tRNA

  1. Single polynucleotide chain.

  
  

  2. Smaller than mRNA and DNA.

  
  

  3. Clover-shaped molecule.

  
  

  4. Pentose sugar = ribose.

  
  

  5. A--U & C---G.

  
  

  6. Manufactured in nucleus but found throughout cell.

  
  

  7. Stability:

  
  

  DNA > tRNA > mRNA.

  
• Outline the process of transcription

  = process of making pre-mRNA using part of the DNA as a template.

  
  

  1. DNA-helicase enzyme acts on specific region of the DNA molecule, causing H-bonds to break

  
  

  2. Two strands separate, exposing the nucleotide bases in that region

  
  

  3. Nucleotide bases on template strand (one of 2 DNA strands) pair with complimentary free RNA nucleotides in nucleus.

  
  

  3. RNA-polymerase moves along strand, forming phosphodiester bonds between nucleotides, forming the phosphate-sugar backbone.

  => forms pre-mRNA molecule.

  
  

  4. H-bonds reform after RNA-polymerase has built the relevant part of the strand of pre-mRNA ---> double helix reforms.

  
  

  5. When the RNA-polymerase reaches a particular stop codon/triplet code, it detaches ---> completing production of pre-mRNA.
• Describe the role of RNA polymerase in transcription

  RNA polymerase allows the formation of phosphodiester bonds between the nucleotide bases, forming the phosphate-sugar backbone. This forms a pre-mRNA molecule.
• Explain how in prokaryotes and eukaryotes the production of mRNA from DNA differs

  Prokaryotes: transcription results directly in the production of mRNA from DNA.

  
  

  Eukaryotes: transcription results in the production of pre-mRNA; this is then spliced to form mRNA.
• Outline the process of splicing

  - Eukaryotes only - most prokaryotes don't have introns in their DNA.

  
  

  - Exons code for proteins, introns do not.

  
  

  - If introns were not removed, it would prevent polypeptide synthesis.

  
  

  1. Intron base sequences removed and the functional enzymes are joined together.

  
  

  2. mRNA molecules are too large to diffuse out of the nucleus ---> after splicing, they leave via nuclear pores ---> outside nucleus, mRNA is attracted to ribosomes which it attaches to for translation.
• What is translation?

  = The production of polypeptides from the sequence of codons carried by mRNA
• Outline the process of translation

  1. Ribosome attaches to start codon at one end of the mRNA molecule.

  
  

  2. tRNA molecule with complementary anticodon sequence moves to ribosome and pairs with the codon on mRNA - this tRNA carries a specific amino acid.

  
  

  3. tRNA molecule with complementary anticodon pairs with the next codon on the mRNA ---> tRNA molecule carries another (next in sequence) amino acid.

  
  

  4. Ribosome moves along mRNA, bringing 2 tRNAs together, forming a peptide bond.

  
  

  5. Ribosome moves to the 3rd codon, linking 2nd and 3rd amino acids. tRNA molecules released where they go to cytoplasm and join with complementary amino acids.

  
  

  6. Process continues with up to 15 amino acids added per second, until the desired polypeptide chain produced.

  
  

  7. Up to 50 ribosomes can pass immediately behind 1st ---> identical polypeptides can be made simultaneously.

  
  

  8. Polypeptide synthesis continues until ribosome reaches a stop codon ---> ribosome, mRNA and final tRNA molecule all separate and polypeptide chain is now complete.
• Describe the role of ATP in translation

  To provide energy needed for the bond between the amino acid and the tRNA molecule
• How are some proteins assembled?

  1. Polypeptide coiled/folded to produce secondary structure.

  
  

  2. Secondary structure coiled/folded further to produce tertiary structure.

  
  

  3. Different polypeptide chains along with any prosthetic (non-protein) groups are linked to form the quaternary structure.