L3 - protein synthesis

• The triplet code:

  -sequence of DNA nucleotide bases found within a gene is determined by a triplet (three letter) code

  -each sequence of three bases in a gene codes for one amino acid

  -these triplets code for different amino acids (there are 20 different amino acids that cells use to make up different proteins)

  -for example; CAG codes for the amino acid valine, TTC codes for lysine, and more
• The triplet code:

  -some of these triplets of bases code for start and stop signals

  -these start and stop signals tell the cell where individual genes start and stop

  -as a result, the cell reads the DNA correctly and produces the correct sequences of amino acids that it requires to function properly
• The genetic code is non-overlapping (each base is only read once)
• The triplet code:

  -there are four bases, so there are 64 different codons (triplets) possible , yet there are only 20 amino acids that commonly occur in biological proteins

  -this is why the code is said to be degenerate; multiple codons can code for the same amino acids

  -the degenerate nature of the genetic cod can limit the effect of mutations
• The triplet code:

  -the genetic code is also universal, meaning that almost every organism uses the same code (there are a few rare and minor exceptions)

  -the same triplet codes code for the same amino acids in all living things (genetic information is transferable between species)

  -the universal nature of the genetic code is why genetic engineering (transfer of genes from one species to another) is possible
• Protein synthesis involves 2 major processes: transcription and translation
• Transcription: DNA is transcribed and an mRNA molecule is produced
• Translation: mRNA is translated and an amino acid sequence s produced
• Transcription:

  -this stage of protein synthesis occurs in the cell nucleus

  -part of a DNA molecule unwinds (the hydrogen bonds between the complementary base pairs break)

  -the exposed gene can then be transcribed (the gene from which a particular polypeptide will be produced)

  -a complimentary copy of the code from the gene is made by building a single stranded nuclei acid molecule known as mRNA
• Transcription:

  -free RNA nucleotides pair up (hydrogen bonds) with their complementary (now exposed) bases on one strand of the unzipped DNA molecule

  -the sugar phosphate groups of these RNA nucleotides are then bonded together with phosphodiester bonds by the enzyme RNA polymerase to form the sugar phosphate backbone of the mRNA molecule
• Transcription:

  -when the gene has been transcribed (when mRNA is complete), the hydrogen bonds between the mRNA and DNA strands break and the double stranded DNA reforms

  -the mRNA molecule then leaves the nucleus via a pore in the nuclear envelope

  -mRNA is synthesised in the 5’ to 3’ direction
• Transcription:

  -each chromosome in a human cell nucleus contains one very long DNA molecule

  -this molecule is made of thousands of specific nucleotide sequences

  -even though these genes are all found within the same molecule and are all linked up, the cells knows where each gene stops and starts

  -this ensures the cell reads the DNA correctly and can produce the correct protein molecules that it requires to function properly
• Transcription: image

  
• Translation:

  -occurs in the cytoplasm of the cell

  -mRNA attaches to a ribosome (with ribosomal RNA, rRNA is a stable molecule that allows mRNA to attach to the ribosomes)

  -in the cytoplasm, there are free molecules of tRNA (transfer RNA)

  -tRNA has triplet code of unpaired bases at one end (the anticodon) and a region for specific amino acids to attach at the other (about 20 different tRNAs, each with specific anticodon and binding site)
• Translation:

  -tRNA molecules bind with their specific amino acids (also in the cytoplasm) and bring them to the mRNA molecule on the ribosome

  -the triplet of bases (anticodon) on each tRNA molecule pairs with a complementary triplet (codon) on each of the mRNA molecule
• Translation:

  -two tRNA molecules fit onto the ribosome at any one time, bringing the amino acid they are each carrying side by side

  -a peptide bond is then formed (via a condensation reaction) between the two amino acids

  -this reaction is catalysed by the rRNA subunits of the ribosome
• Translation:

  -this process continues instil a ‘stop’ codon on the mRNA molecule is reached (eg. UAG) there is no corresponding tRNA molecule

  -this acts as a signal for translation to stop and at this point the amino acid chain coded for by the mRNA molecule is complete

  -the amino acid chain is released then forms the final polypeptide
• Translation image:

  
• Not all bases in the DNA code for amino acids so the mRNA just transcribed contains non-coding regions known as introns
• Introns are removed by enzymes before the mRNA leaves the nucleus. This just leaves exons or coding regions of mRNA