Protein Synthesis

• mRNA (messenger) – copies the code from the DNA molecule and carries it from the nucleus to the ribosomes.
• rRNA (ribosomal) – forms a complex with protein

  molecules to make the ribosome.
• tRNA (transfer) – found in the cytoplasm where it binds to specific amino acids and transports them to the ribosome. tRNA folds due to complementary base pairing.
• • Protein synthesis has two stages:

  1. Transcription

  2. Translation
• • The first stage of protein synthesis takes place in the nucleus.

  • mRNA is transcribed from DNA in the nucleus and translated into proteins by ribosomes in the cytoplasm.

  • Each triplet of bases on the mRNA molecule is called a codon and codes for a specific amino acid.
• The role of RNA Polymerase in Transcription

  
  

  • RNA polymerase moves along the DNA unwinding the double helix and breaking the hydrogen bonds between the bases.

  • RNA polymerase then synthesises a primary transcript of mRNA from RNA nucleotides by complementary base pairing.

  • Free mRNA nucleotides in the nucleus form complementary base pairs with the coding strand of the DNA

  • RNA polymerase synthesises an mRNA

  primary transcript.
• RNA Splicing

  • There are long stretches of the newly formed primary mRNA (transcribed from the DNA) that do not play a part in the coding to make proteins.

  • Non-coding regions are called introns.

  • The introns are removed in a process called RNA splicing.

  • The exons are coding regions and are joined (spliced) together to form mature transcript. The order of the exons is unchanged during splicing.
• After the primary transcript has been produced, the introns must be cut out.

  
• The exons join together to produce a mature mRNA strand.

  
• Translation

  • The second stage of protein synthesis takes place at the ribosome.

  • Translation is the synthesis of protein as a polypeptide chain made up of amino acids under the direction of mRNA.

  • The process requires energy, in the form of ATP.
• transfer RNA (tRNA)

  • Found in the cytoplasm.

  • Although RNA is single-stranded tRNA folds into a 3D structure due to complementary base pairing.

  • Each tRNA molecule carries its specific amino acid to the ribosome.
• • Each molecule of tRNA has only one particular triplet (3!) of bases exposed.

  • This triplet is called an anticodon.

  • Each anticodon is complementary to an mRNA codon.

  • Each tRNA molecule carries a specific amino acid at its attachment site.
• • There are many different types of tRNA in one cell.

  • Each tRNA picks up its specific amino acid at it’s site of attachment and carries it to the ribosome.

  • The amino acid is added to the growing end of the polypeptide chain.
• Ribosome

  • Ribosomes contain ribosomal RNA (rRNA) and proteins.

  • The function of a ribosome is to bring tRNA molecules in contact with mRNA.
• • Translation begins at a start codon (AUG) on mRNA.

  • Anticodons on the tRNA bond to codons by complementary base pairing, translating the genetic code into a sequence of amino acids.

  • When the first two amino acid molecules are adjacent, they becomes joined by a peptide bond.

  • Eventually, a stop codon on the mRNA is

  reached. This ends translation.
• Modification After Transcription

  • The primary RNA transcript can be modified by Alternative RNA splicing

  • This means that different mature mRNA transcripts are produced from the same primary transcript.
• Alternative RNA splicing

  Different mature mRNA transcripts can be produced from the

  same primary transcript depending on which exons are included

  in the mature RNA transcript.
• RNA is single stranded. It is composed of nucleotides containing ribose (Sugar), phosphate and base.
• The four bases are adenine, cytosine, guanine and uracil.
• Groups of three nucleotides form codons in mRNA and anticodons in tRNA.
• Start and stop codons exist.
• tRNA folds due to base pairing and has an attachment site for a specific amino acid.
• rRNA and protein forms a ribosome.
• Transcription occurs in the nucleus.
• RNA polymerase forms mRNA / unwinds and unzips DNA.
• Complementary base pairing occurs - adenine with uracil, guanine with cytosine.
• The primary transcript mRNA contains introns and exons.
• Introns are removed and exons remain after RNA splicing.
• Splicing forms the mature transcript, or mRNA.