ka3: gene expression

• Gene expression involves the transcription and translation of DNA sequences.

  • Only a fraction of the genes in a cell are expressed.
• Transcription and translation involves three types of RNA (mRNA, tRNA and rRNA).

  • RNA is single-stranded and is composed of nucleotides containing ribose sugar, phosphate and one of four bases: cytosine, guanine, adenine and uracil.
• Messenger RNA (mRNA) carries a copy of the DNA code from the nucleus to the ribosome.

  • 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.
• Transfer RNA (tRNA) folds due to complementary base pairing. Each tRNA molecule carries its specific amino acid to the ribosome. Ribosomal RNA (rRNA) and proteins form the ribosome.

  • A tRNA molecule has an anticodon (an exposed triplet of bases) at one end and an attachment site for a specific amino acid at the other end.
• • RNA polymerase moves along DNA unwinding the double helix and breaking the hydrogen bonds between the bases. RNA polymerase synthesises a primary transcript of mRNA from RNA nucleotides by complementary base pairing.
  • Uracil in RNA is complementary to adenine.
• RNA splicing forms a mature mRNA transcript.
• The introns of the primary transcript are non-coding regions and are removed
• The exons are coding regions and are joined together to form the mature transcript.

  • The order of the exons is unchanged during splicing
• tRNA is involved in the translation of mRNA into a polypeptide at a ribosome. Translation begins at a start codon and ends at a stop codon. Anticodons bond to codons by complementary base pairing, translating the genetic code into a sequence of amino acids. Peptide bonds join the amino acids together. Each tRNA then leaves the ribosome as the polypeptide is formed.
• Different proteins can be expressed from one gene, as a result of alternative RNA splicing. Different mature mRNA transcripts are produced from the same primary transcript depending on which exons are retained.
• Amino acids are linked by peptide bonds to form polypeptides. Polypeptide chains fold to form the three-dimensional shape of a protein, held together by hydrogen bonds and other interactions between individual amino acids. Proteins have a large variety of shapes which determines their functions.

  • Details of other interactions and levels of protein structure are not required.
• Phenotype is determined by the proteins produced as the result of gene expression.

  • Environmental factors also influence phenotype.
• RNA is a type of nucleic acid called ribonucleic acid.

   

  RNA is different from DNA in that it:

  • Is single stranded.
  • Has a sugar called ribose.
  • Has uracil instead of thymine

  
• Messenger RNA (mRNA):

  • Is formed in the nucleus of a cell.
  • Makes a copy base sequence of a section of DNA in a process called transcription.
  • Carries a copy of the DNA code for building a protein from the nucleus to the ribosome.

  
• Transfer RNA (tRNA):

  • Is found in the cytoplasm.
  • Folds due to complementary base pairing to form a triplet anticodon site and attachment site for a specific amino acid.

  
• Ribosomal RNA (rRNA)

  • rRNA and proteins form the ribosome.
• The genetic code determines which amino acids are joined together to make a protein. The code is read as a series of three bases called triplets.
• Each amino acid in a protein is coded for by a specific triplet on DNA and RNA.

   

  • DNA:       a triplet of bases is called a codon.
  • mRNA:    a triplet of bases is called a codon.
  • tRNA:      a triplet of bases is called an anticodon.

  
• Gene expression involves the transcription and translation of DNA sequences to produce proteins.
• A gene is a section of DNA which carries the code for the production of one protein.  Only a fraction of the genes in a cell are expressed.
• Transcription is the synthesis of mRNA from a section of DNA.
• mRNA is transcribed from DNA in the nucleus and translated into proteins by ribosomes in the cytoplasm.
• RNA polymerase (enzyme responsible for transcription) moves along DNA unwinding the double helix and breaking the hydrogen bonds. RNA polymerase then synthesises a primary transcript of RNA from RNA nucleotides by complementary base pairing.

  
• Not all the regions in a eukaryotic gene are required to produce the final protein. There are non-coding regions called introns and coding regions called exons.
• After the mRNA has been transcribed the introns are removed.  The remaining exons are joined together to form a continuous sequence. The sequence of the exons is not changed in splicing. This is called the mature transcript.
• The mature transcript then leaves the nucleus to travel to the cytoplasm.
• RNA splicing

  
• Translation is the synthesis of protein following the code with in the mature mRNA transcript.
• The mRNA is made of sequences of three nucleotides (a triplet of bases) called codons.  Each codon is code for one amino acid.
• Each tRNA has an attachment site for a specific amino acid and a triplet of bases known as an anticodon.
• Translation

  
• Many different types of tRNA are present in a cell, one or more for each type of amino acid.
• The tRNA picks up the appropriate amino acid and takes it to the ribosome. Complementary base pairing then occurs between the anticodon of the tRNA and the codon of the mRNA.

  
• Translation in the ribosome+

  1. The ribosome binds to the 5’ end of the mRNA so that the start codon (AUG) is in place.

   2. Next a tRNA carrying an amino acid (methionine) becomes attached to the ribosome.

   3. The mRNA codon binds to the complementary anticodon on the appropriate tRNA bearing the correct amino acid.

   4. A peptide bond then forms between these two amino acids.

   5. The ribosome then moves along one codon.

   6. The tRNA exits the ribosome as the polypeptide is formed.

   7. Steps 3-6 then repeat until it reaches a stop codon.

  
• The same gene can be used to make several different proteins by alternative RNA splicing.

   

  Different mRNA molecules are produced from the same primary transcript depending on which RNA segments are treated as exons and introns.  Therefore one gene can produce several different mature mRNA transcripts and subsequently several different proteins.

  
• Proteins are made of long chains of amino acids. There are 20 different amino acids.  Amino acids are linked by peptide bonds to form polypeptides.

   

  Proteins have a large variety of shapes which determines their functions.

   

  Polypeptide chains fold to form the three-dimensional shape of a protein, held together by hydrogen bonds and other interactions between individual amino acids

  
• The phenotype is determined by the proteins produced as the result of gene expression. Phenotype can also be influenced by environmental factors.