DNA and protein synthesis

• Gene


  A length of DNA that codes for a polypeptide/protein
• Genome


  The complete set of genes present in a cell
• The full genome is present within every cell of an organism, but not every gene is expressed in every cell
• Proteome

  The full range of proteins that a cell is able to produce
• Reason for proteome being larger than genome

  • Large amount of post-translational modification of proteins (often in the Golgi apparatus)
  • Each gene is also capable of producing multiple different proteins via alternative splicing
• RNA (ribonucleic acid) is a polynucleotide – it is made up of many nucleotides linked together in a long chain
• the nitrogenous bases in nucleotides of RNA

  adenine (A)

  guanine (G)

  cytosine (C)

  uracil (U)
• RNA has the pentose sugar ribose
• DNA vs RNA nucleotides

  A) phosphate group

  B) nitrogenous base (A,C,G, U)

  C) pentose sugar (ribose)

  D) phosphate group

  E) nitrogenous base (A,C,G, T)

  F) pentose sugar (deoxyribose)

  
• RNA Strand

  made up of one polynucleotide strand

  made up of alternating ribose sugars and phosphate groups linked together

  sugar-phosphate bonds are covalent bonds called phosphodiester bonds

  • these form sugar- phosphate backbone of RNA polynucleotide
  • phosphodiester bonds between carbon-5 on ribose sugar molecule and phosphate group from same nucleotide, this links another phosphodiester bond to a 3-carbon of ribose sugar molecule on neighbouring nucleotide
• • An example of an RNA molecule is messenger RNA (mRNA), which is the transcript copy of a gene that encodes a specific polypeptide. Two other examples are transfer RNA (tRNA) and ribosomal RNA (rRNA)
• mRNA

  • mRNA is a single-stranded molecule
  • It is made up of a sugar-phosphate backbone and exposed unpaired bases
  • Uracil bases are present instead of thymine bases (which are found in DNA)
• tRNA

  • tRNA is a single-stranded molecule
  • It has a sugar-phosphate backbone
  • It has a folded shape
  • There are hydrogen bonds between some of the complementary bases
  • Amino acids bind to a specific region of the molecule
  • The specific anticodon found on the tRNA molecule is complementary to a specific codon on an mRNA molecule
• structures of tRNA


  shaped like a clover

  
• structure of mRNA

  short, single stranded molecule

  exposed bases

  
• • A gene is a sequence of nucleotide bases in a DNA molecule that codes for the production of a specific sequence of amino acids, that in turn make up a specific polypeptide (protein)
  • This process of protein synthesis occurs in two stages:
  • Transcription – DNA is transcribed and an mRNA molecule is produced
  • Translation – mRNA (messenger RNA) is translated and an amino acid sequence is produced
• Transcription

  Occurs in nucleus of cell
• Transcription

  1. DNA molecule unwinds by DNA helicase (breaks H bonds)

  2. Exposes gene to be transcribed

  3. Complementary copy of code from gene is made by building single-stranded nucleic acid called mRNA

  4. Free activated RNA nucleotides pair up with complementary unpaired bases on template strand

  5. Sugar-phophate groups of RNA nucleotides are bonded together by enzyme RNA polymerase forming sugar-phosphate backbone of mRNA molecule

  6. When transcribed, H nonds between mRNA and DNA strands break and double-stranded DNA molecule re-form

  7. mRNA molecule leave nucleus via pore in nuclear envelope
• picture of transcription

  
• Template strand


  The strand of the DNA molecule that is transcribed to form the mRNA molecule
• Non-template strand


  The strand of the DNA molecule that is not transcribed
• Transcription stage of protein synthesis

  1. DNA molecule where the gene is located unwinds

  2. Hydrogen bonds between complementary base pairs break

  3. Two DNA strands 'unzip'

  4. Free activated RNA nucleotides pair up with exposed bases on one DNA strand-template strand

  5. RNA polymerase binds the RNA nucleotides together to create the sugar-phosphate backbone of the mRNA molecule.

  6. The mRNA molecule will then be translated into an amino acid chain
• The mRNA molecule will then be translated into an amino acid chain
• template and non- template strands

  
• The genome within eukaryotic cells contains many non-coding sections
• Non-coding DNA can be found


  • Between genes, as non-coding multiple repeats
  • Within genes, as introns
• Transcription in eukaryotic cells


  Transcribe the whole gene (all introns and exons) to produce pre-mRNA molecules
• pre-mRNA


  Contains the introns and exons of a certain gene
• Splicing of pre-mRNA

  1. Non-coding sections are removed

  2. Coding sections are joined together

  3. resulting mRNA carries only the coding sequences (exons) of the gene

  4. this exits the nucleus before joining a ribosome for translation
• Alternative splicing

  • The exons (coding regions) of genes can be spliced in many different ways to produce different mature mRNA molecules through alternative splicing
  • This means that a single eukaryotic gene can code for more than one polypeptide chain
  • This is part of the reason why the proteome is much bigger than the genome
• Translation


  This stage of protein synthesis occurs in the cytoplasm of the cell
• Translation

  1. mRNA leaves nucleus

  2. mRNA attaches to ribosome

  3. tRNA with anticodon binds to complementary codon on mRNA

  4. Two tRNA molecules fit on ribosome, bringing amino acids side by side

  5. Peptide bond formed between amino acids, requiring energy (ATP) provided by mitochondria

  6. Process continues until stop codon on mRNA
• tRNA


  Has triplet of unpaired bases at one end (anticodon) and region where specific amino acid can attach at other
• In cytoplasm there are free molecules of tRNA
• tRNA molecules bind with specific amino acids and bring them to the mRNA molecule on the ribosome
• Triplet of bases (anticodon) on each tRNA molecule pairs with complementary triplet (codon) on mRNA molecule
• The amino acid chain forms the final polypeptide
• there are at least 20 different tRNA molecules, each with specific anticodon and specific amino acid binding site
• translation stage of protein synthesis- tRNA molecule bind with specific amino acids

  
• • The genetic code is non-overlapping
  • Each base is only read once in the codon it is part of