Gene Expression and Protein Structure

• Genotype

  • The genotype of an organism is determined by the sequence of DNA bases in its genes.

  • Some species have a genetic code that is quite similar to another species
• Phenotype

  • The phenotype of an organism is its physical and chemical appearance.

  • This depends on the proteins that are synthesised in that cell.

  • Protein synthesis depends on gene expression.
• Production of phenotype/protein can be affected by environmental factors acting inside (intra-cellular) or outside (extra-cellular) of the cell.
• Gene Expression

  • The nucleus of every cell in an organism contains the same genetic information

  • Genes are switched ‘on’ or ‘off’ in cells to make a specific protein. This is expression.
• Structure of Protein

  Elements: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen

  (N) and often Sulphur (S)
• Amino acids are linked by peptide bonds to form polypeptides.

  Hydrogen bonds form between amino acids causing coiling or folding of chain

  Further interactions occur between individual amino acids in the polypeptides. Polypeptide chains fold to form the final

  three- dimensional shape of the protein.
• Structure and Function Proteins

  Phenotype is determined by proteins produced as a result of gene expression.

  Proteins have a large variety of shapes which determines their functions.
• RNA is single stranded and is composed of RNA nucleotides.

  RNA nucleotides consist of a phosphate group, ribose sugar and one of three nitrogenous bases. Adenine, Uracil, Guanine and Cytosine.

  (In RNA Adenine complementary base pairs with Uracil as it does not contain Thymine)

  
  

  A-U

  G-C

  
  

  The three types of RNA are:

  • mRNA
  • tRNA
  • rRNA
• Transfer RNA (tRNA) folds due to complementary base pairing. tRNA also has a triplet of bases exposed known as an anticodon at one end of the tRNA molecule. At the other end of the tRNA molecule is the specific amino acid attachment site. The tRNA molecule carries its specific amino acid to the ribosome.
• rRNA or ribosomal RNA, together with proteins, form the ribosome.
• RNA polymerase moves along DNA unwinding the double helix and breaking the hydrogen bonds between bases. RNA polymerase synthesises a primary transcript of mRNA from RNA nucleotides by complementary base paring.

  A-U

  G-C

  RNA polymerase can only add nucleotides onto the 3’ end.

  The mRNA produced is known as the primary transcript.

  Point to note: - unlike DNA replication, RNA polymerase only unwinds the DNA at one specific gene. As this gene carries the code for the production of a specific protein.
• During RNA splicing regions known as introns, or non-coding regions, are removed from the primary transcript and the exons or coding regions remain and are spliced together.

  The exons when spliced together form a mature (mRNA) transcript. It is important to remember that during the process of splicing the order of the exons remains unchanged.
• The mRNA mature transcript leaves the nucleus to the ribosome for the process of translation into a polypeptide.

  (A polypeptide chain is a sequence of amino acids that will eventually become part of a functioning protein).

  Translation starts at a START codon and ends at a STOP codon. This is very important to ensure the entire polypeptide chain is produced without any amino acids missing.
• The ribosome exposes one codon on the mRNA allowing tRNA to bring a specific amino acid to the ribosome. The correct anticodon on the tRNA will complementary base pair with the codon on the mRNA, bringing the correct specific amino acid with it. The ribosome exposes the next codon where another tRNA and its amino acid will complementary base pair with that codon.

  Aligning amino acids are joined together by a peptide bond, allowing the tRNA to leave the ribosome as the polypeptide is formed. This happens throughout the length of the mRNA until it reaches a STOP codon.
• 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 bonded together by peptide bonds to form polypeptide chains.

  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.

  Remember that a phenotype is a physical expression of a gene/characteristic. This is determined by the protein produced as a result of gene expression.

  Environmental factors can also influence phenotype.