1.3

• Chromosomes


  The structure made of DNA that codes for all the characteristics of an organism
• Complementary base pair


  Refers to the specific way bases pair with each other (in DNA: A-T, G-C or DNA-RNA/RNA-RNA: A-U, G-C)
• Exons


  The coding regions of a gene
• Gene


  A section of DNA which codes for a protein
• Gene expression


  The process by which specific genes are activated to produce a required protein
• Introns


  The intervening non-coding regions
• Proteins


  Made of a chain of amino acids joined together in a unique sequence. It is these folded into a unique shape to allow it to carry out its function (e.g. enzyme, hormone, structural, antibody)
• Protein synthesis


  The production of proteins from amino acids, which happens in the ribosomes of the cell. Protein synthesis occurs in two stages - transcription and translation
• Ribosomes


  The organelle found in the cytoplasm of a cell involved in protein synthesis
• RNA (ribonucleic acid)


  A single-stranded molecule composed of nucleotides containing a ribose sugar, phosphate and one of four bases: cytosine, guanine, adenine and uracil
• RNA polymerase


  An enzyme that is responsible for copying a DNA sequence into an mRNA sequence during transcription
• RNA splicing


  The process in which the genes to be expressed by a cell need to be copied and the non-coding parts edited out before they can be turned into a protein
• Transcription


  The process by which the base sequence of DNA is copied into mRNA in the nucleus
• Translation


  The process by which mRNA is translated into a polypeptide at a ribosome
• Every cell in a multicellular organism contains a complete set of chromosomes. In humans, this would be 46 chromosomes per cell
• These chromosomes have sections called genes which code for every protein required by an organism
• In any particular cell, only a very small fraction of these genes is ever expressed. This is because cells are specialised to carry out a certain function and only need to express certain genes
• Gene expression involves the transcription and translation of DNA sequences
• Transcription


  1. RNA polymerase moves along the DNA unwinding the strand in the nucleus

  2. Hydrogen bonds between base pairs break which allows the unzipping of the double helix

  3. As RNA polymerase breaks the bonds, it synthesises a primary transcript of mRNA using RNA nucleotides. These form hydrogen bonds with the exposed DNA strand by complementary base pairing

  4. The primary transcript of mRNA is processed to produce a mature transcript of mRNA by RNA splicing. RNA splicing removes the introns of the primary transcript and joins the exons together. The order of the exons is unchanged during RNA splicing

  5. The mature mRNA transcript is now ready to leave the nucleus and travel to the ribosome
• Translation


  1. The mRNA molecule travels through the cytoplasm and attaches to the ribosome

  2. tRNA molecules transport specific amino acids to the ribosome

  3. Each mRNA codon codes for a specific amino acid

  4. The first codon of an mRNA molecule is a start codon. This signals the beginning of translation

  5. The anti-codons and codons match up and form complementary base pairs

  6. Peptide bonds form between the adjacent amino acids to form the polypeptide (protein)

  7. Used tRNA molecules exit the ribosome and collect another specific amino acid

  8. The last codon of an mRNA molecule is a stop codon which signals the end of translation
• 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
• Protein structure


  • After translation, the polypeptide is finally folded into the correct shape and becomes a protein. Peptide bonds form between the adjacent amino acids to finalise the structure
  • 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 (enzymes, hormones, antibodies etc.)
  • The polypeptide chain can be changed by cutting and combining polypeptide chains or by adding phosphate or carbohydrate groups to the protein
• Phenotype is determined by proteins produced as the result of gene expression. Environmental factors also influence phenotype