protein synthesis

• Protein synthesis

  The making of proteins, using the information that is found in DNA

  
• Functional proteins


  • Enzymes catalyze most biochemical reactions
  • Hormones that are released into the bloodstream are involved in cell signaling and homeostasis
  • Antibodies are part of the body defense mechanism
• Structural proteins

  • Actin and myosin, collagen, tendons, ligaments, skins, and keratin
• The properties of the different types of protein are determined by the sequence of amino acids in their polypeptide chain, which in turn, is determined by the genes in DNA

  
• Proteins can be seen as the link between the genotype and phenotype of an individual or species
• Flow of Genetic Information

  1. Transcription

  2. Translation

  
• Transcription


  The information in DNA is transcribed to a single-stranded RNA molecule called messenger RNA. One of the DNA strands acts as a template to synthesize a complementary mRNA molecule.

  
• Translation


  The genetic information carried in mRNA is translated by ribosomes into a sequence of amino acids on a polypeptide chain.

  
• Protein Synthesis in Prokaryotic and Eukaryotic Cells

  • Prokaryotic cells do not have a nucleus that segregates DNA from ribosomes. Therefore, the translation of mRNA can begin immediately after the mRNA has detached itself from the DNA template.
  • In eukaryotic cells, the nuclear envelope separates the DNA from the ribosomes. Transcription occurs in the nucleus. mRNA is transported to the cytoplasm where translation occurs. Pre-mRNA is modified through RNA processing to become mature mRNA. The mature mRNA leaves the nucleus to be translated in the cytoplasm.

  
• Transcription

  1. mRNA is synthesized using a specific segment of the DNA as a template

  2. Promoter - sequence of base pairs in DNA that specifies where transcription begins

  3. mRNA-coding sequence - sequence of base pairs that contains coding information for the polypeptide chain specified by the chain

  4. Terminator - sequence of base pairs in DNA that specifies the end of the mRNA transcript

  
• Transcription - Initiation


  1. In prokaryotes, initiation of transcription occurs with the specific recognition of the promoter sequence by RNA polymerase. In eukaryotes, RNA polymerase recognizes a complex of proteins that help the binding of the enzyme to the promoter sequence.

  2. Transcription begins with the unwinding of the DNA strands by the RNA polymerase at the promoter site. One of the DNA strands acts as template for transcription.

  
• Transcription - Elongation


  1. As RNA polymerase moves along the 3'-5' DNA template, free nucleotides are added in the 5'-3' direction complementary to the DNA template.

  2. Nucleotides are added to the growing RNA strand according to the same base pairing rules as for DNA.

  
• Transcription - Termination

  1. Termination occurs when RNA polymerase encounters the terminator sequence on the DNA template.

  2. When transcription is complete, the RNA transcript is released from the DNA template, followed by the RNA polymerase.

  
• Initiation is the beginning of transcription. It occurs when the enzyme RNA polymerase binds to a region of a gene called the promoter. This signals the DNA to unwind so the enzyme can read the bases in one of the DNA strands. The enzyme is now ready to make a strand of mRNA with a complementary sequence of bases.

  
• Elongation is the addition of nucleotides to the mRNA strand. RNA polymerase reads the unwound DNA strand and builds the mRNA molecule, using complementary base pairs. There is a brief time during this process when the newly formed RNA is bound to the unwound DNA. During this process, an adenine (A) in the DNA binds to an uracil (U) in the RNA.

  
• Termination is the ending of transcription, and occurs when RNA polymerase crosses a stop (termination) sequence in the gene. The mRNA strand is complete, and it detaches from DNA.
• RNA processing in eukaryotic cells

  1. The RNA transcript is a pre-mRNA molecule which requires modification before it can leave the nucleus.

  2. Eukaryotic genes contain non-coding segments (introns) that break up the amino acid coding sequence into segments known as exons.

  3. The introns are removed from the pre-mRNA and the exons are then joined together to form a mature mRNA molecule with a continuous coding sequence. RNA processing (slicing) requires the aid of enzymes.

  4. The mature mRNA then leaves the nucleus to be translated by ribosomes in the cytoplasm.

  
• Translation


  The actual process of protein synthesis where all three types of RNA are involved. It is where the genetic information carried in mRNA is translated by ribosomes into a sequence of amino acids on a polypeptide chain.

  
• Components involved in Translation

  • mRNA - carries the genetic information transcribed from DNA in the nucleus into the cytoplasm, and then attaches to the ribosome which translates the triplet codes into a sequence of amino acids in a polypeptide chain.
  • rRNA - mRNA site, P site (peptidyl-tRNA binding site), A site (aminoacyl-tRNA binding site)
  • tRNA - transports a specific amino acid from the cytoplasm to the ribosomes

  
• Translation - Initiation


  1. Initiation occurs when a ribosome recognizes a specific sequence, the 5' cap, on the mRNA and binds to that site.

  2. mRNA consist of 5' end, 3' end and sequences of nucleotides within it with a special name called codon. Codons are triplets of nucleotides. 64 permutations of codon, 61 codons that codes for an amino acids, 3 codons for termination.

  3. Anticodon: three bases on tRNA that are complimentary to mRNA codon.

  
• In theory, any mRNA can be translated in any of three different "reading frames" depending on the nucleotide sequence at which translation starts.

  
• Translation - Initiation


  1. The large ribosomal subunit attaches to the small subunit such that the first codon is aligned at the P binding site.

  2. A tRNA carrying the amino acid methionine attaches to the start codon (AUG) on the messenger RNA. The tRNA that carries methionine enters the P site.

  
• Translation - Elongation

  1. The next tRNA carrying the required amino acid enters the A site.

  2. The tRNA and its amino acid attaches to the A binding site.

  3. The methionine amino acid at the P site is bonded to the amino acid at the A site.

  4. The tRNA at the P site is now free of amino acids.

  5. Ribosome shifts over one codon (three bases). The A binding site is now open and a vacant tRNA is in the E binding site.

  6. A new tRNA attached to an amino acid enters the vacant A site on the ribosome and the process repeats again.

  
• Translation - Termination

  1. The ribosome will eventually reach a stop codon. The three stop codons are UGA, UAG, UAA.

  2. The stop codons do not code for an amino acid. Hence, there are no corresponding tRNAs.
• Polyribosome (polysome)


  A cluster of ribosomes bound to an mRNA strand. Enable the synthesis of more than one molecule of a protein from a single strand of mRNA simultaneously.

  
• The Genetic Code

  The four bases of RNA form a language with just four nucleotide bases: adenine (A), cytosine (C), guanine (G), and uracil (U). The genetic code is read in three-base words called codons. Each codon corresponds to a single amino acid (or signals the starting and stopping points of a sequence).

  
• Types of Amino Acids


  • Ala (A) = Alanine
  • Arg (R) = Arginine
  • Asp (D) = Aspartic acid
  • Asp (N) = Asparagine
  • Cys (C) = Cysteine
  • Gln (E) = Glutamine
  • Glu (Q) = Glutamic acid
  • Gly (G) = Glycine
  • His (H) = Histidine
  • Ileu (I) = Isoleucine
  • Leu (L) = Leucine
  • Lys (K) = Lysine
  • Met (M) = Methionine
  • Phe (F) = Phenylalanine
  • Pro (P) = Proline
  • Ser (S) = Serine
  • Thr (T) = Threonine
  • Trp (W) = Tryptophan
  • Tyr (Y) = Tyrosine
  • Val (V) = Valine
• DNA, mRNA, Protein


  • DNA: TAC CGA TCG TGA ACT
  • mRNA: AUG GCU AGC ACU UGA
  • Protein: Met-Ala-Ser-Thr-Stop
• Transcription


  The process of making mRNA from DNA
• Translation


  The process of making proteins from mRNA