D1.2 - Protein Synthesis

• A rough sketch of a ribosome during translation

  
• II is the location where the mRNA binds to iniciate translation

  
• Ribosomes are the cellular structures responsible for protein synthesis.
• Translation is the process by which the information in mRNA is used to synthesize a protein.
• Transcription is the process by which the information in a strand of DNA is copied into a new molecule of messenger RNA (mRNA).
• Ribosomes are composed of ribosomal RNA (rRNA) and proteins.
• I is the location where the tRNA binds to initiate translation

  
• IV contains peptide bonds

  
• 1. Transcription – the DNA code is transcribed to mRNA code.
• Translation – the mRNA code is translated to polypeptides (proteins) by ribosomes
• Transcription

  • The enzyme RNA polymerase attaches to the promoter region for a gene on the antisense strand. 
  • RNA polymerase breaks the hydrogen bonds between nitrogenous bases of DNA.
  • A mRNA molecule is produced on the antisense DNA strand using the enzyme RNA polymerase to link RNA nucleotides into a single polynucleotide (by condensation reactions).
  • The nucleotides are organised using complementary base pairing between DNA bases and RNA bases.
  • RNA polymerase reaches the terminator region and the RNA polymerase and new mRNA molecule separate from the DNA.
• Translation

  • Translation is the synthesis of polypeptides (proteins) on ribosomes using the mRNA code.
  • The amino acid sequence of polypeptides is determined by mRNA according to the genetic code.
  • Each codon of three bases on mRNA corresponds to one amino acid in a polypeptide.
• Types of RNA

  • mRNA (messenger RNA) – contains the genetic code (in the form of codons) which is translated by ribosomes into polypeptides
  • tRNA (transfer RNA) – transfers the amino acids from the cytoplasm to the ribosome. tRNA contains anticodons which complementary base pair with codons on mRNA. 
• Ribosomes consist of 2 subunits known as:

  • Large ribosomal subunit
  • Small ribosomal subunit
• • The small ribosomal subunit binds to mRNA in the cytoplasm and moves along the mRNA until it reaches a start codon (AUG).
  • An anticodon on a tRNA molecule aligns opposite the complementary codons (according to complementary base pairing )

  (e.g. DNA Codon AUG ⇄ RNA anticodon UAC)

  • The large ribosomal subunit attaches to the small ribosomal subunit.

  
• The process of Translation

  
• Polypeptides are chains of amino acids joined together by condensation reactions
• During translation, condons on mRNA undergo hydrogen bonding with tRNA.
• The process of Translation (simplified)

  
• Four amino acids can be coded for by this piece of mRNA
• As more than one codon may code for an amino acid the code is known as degenerate
• In Sickle Cell Disease the structure of haemoglobin is altered. This occurs because point mutation altering the amino acid sequence in haemoglobin