1.3 5

• mRNA


  Messenger RNA formed in the nucleus that carries the genetic code out into the cytoplasm
• Antisense (template) strand


  The DNA strand that codes for proteins
• DNA-directed RNA polymerase (RNA polymerase)


  The enzyme that polymerises nucleotide units to form RNA in a sequence determined by the antisense strand of DNA
• tRNA


  Small units of RNA that pick up particular amino acids from the cytoplasm and transport them to the surface of the ribosome to align with the mRNA
• Anticodon


  A sequence of three bases on tRNA that are complementary to the bases in the mRNA codon
• rRNA


  RNA that makes up about 50% of the structure of the ribosome
• Polysomes


  Groups of ribosomes, joined by a thread of mRNA, that can produce large quantities of a particular protein
• Sense strand


  The DNA strand with the same base sequence as the mRNA transcribed from the antisense strand
• Transcription in the nucleus

  DNA message transcribed onto messenger RNA
• Translation at the ribosome


  mRNA message translated into polypeptide chain
• In eukaryotes, the DNA that codes for the individual proteins is in the nucleus of the cell. The ribosomes where proteins are synthesised are in the cytoplasm. DNA from the nucleus has never been detected in the cytoplasm, so the message cannot be carried directly. RNAs (ribonucleic acids) carry the information from the nuclear DNA to the ribosomes.
• RNA is closely related to DNA. However it contains a different sugar (ribose) and a different base (uracil instead of thymine). It consists of a single helix and does not form enormous and complex molecules like DNA.
• The sequence of bases along a strand of RNA relates to the sequence of bases on a small part of the DNA in the nucleus. RNA enables DNA to act as the genetic material.
• The three main functions of RNA

  • Carries the instructions for a polypeptide from the DNA in the nucleus to the ribosomes where proteins are made
  • Picks up specific amino acids from the protoplasm and carries them to the surface of the ribosomes
  • Makes up the bulk of the ribosomes themselves
• Messenger RNA (mRNA)


  Formed in the nucleus, usually has instructions for one polypeptide, codes for a polypeptide
• Sense strand of DNA

  The coding strand of DNA, mRNA formed on this strand would be nonsense and would not code for a protein
• Antisense (template) strand of DNA


  The strand of DNA that mRNA is formed on
• Transcription of mRNA


  1. Parts of the DNA molecule unravel and are transcribed onto strands of mRNA by the enzyme DNA-directed RNA polymerase

  2. Complementary RNA nucleotides line up alongside the exposed DNA

  3. DNA-directed RNA polymerase catalyses the formation of phosphodiester bonds to form a strand of mRNA

  4. Hydrogen bonds maintain the helical structure of the RNA molecule
• Codon


  A triplet of bases in mRNA that codes for a specific amino acid
• Transfer RNA (tRNA)


  Found in the cytoplasm, has a complex 'clover leaf' shape, carries a specific amino acid to the ribosome
• Anticodon


  The sequence of three bases on tRNA that matches the genetic code of the DNA and corresponds to one particular amino acid
• Ribosomal RNA (rRNA)


  Makes up about 50% of the structure of the ribosome, made in the nucleus and moves out into the cytoplasm
• Protein synthesis


  1. Genetic code of DNA transcribed onto mRNA in the nucleus

  2. mRNA moves out of the nucleus into the cytoplasm and attaches to a ribosome

  3. tRNA molecules carry individual amino acids to the ribosome

  4. Anticodons of tRNA line up with complementary codons in mRNA, held in place by hydrogen bonds

  5. Enzymes link the amino acids together to form a polypeptide chain

  6. Ribosome moves along the mRNA until it reaches the end, leaving a completed polypeptide chain
• Polysomes


  Groups of ribosomes joined by a thread of mRNA, a form of mass production of particular proteins