DNA, genes and protein synthesis

• what are the main steps in translation?

  1. mRNA is in the cytoplasm
  2. ribosomes attach to mRNA at the start codon.
  3. tRNA with its complementary anticodons line up with mRNA's codons.
  4. tRNA brings amino acids to the ribosome
  5. process continues at next codon
  6. peptide bonds form between adjacent amino acids
  7. tRNA dissociates with first codon
  8. ribosomes continues down the mRNA strand until it reaches a stop codon.
  9. tRNA detaches meaning a polypeptide chain can exit the nucleus.
• The genome

  • The genome is the complete set of genes in an individual's DNA.
  • The genes in the genome encode the different proteins a cell needs.
  • The proteome is the full range of proteins an individual can produce.
• Base sequence

  • DNA is made from a combination of four different bases (A, T, C, G).
  • A specific sequence of three bases (called a codon) encodes a specific amino acid.
  • The order of the bases on the DNA tells us the order for combining amino acids to create particular proteins.
• types of RNA:

  mRNA

  • Messenger RNA (mRNA) is transcribed from DNA in the nucleus to produce a single-stranded RNA.
  • The mRNA strand is complementary to the DNA base sequence.
  • mRNA travels from the nucleus to the ribosomes in the cytoplasm.
  • Ribosomes are proteins that translate mRNA into amino acids and synthesise the polypeptide.
• types of RNA:

  tRNA

  • Transfer RNA (tRNA) forms a clover-like shape and is vital for translation.
  • tRNA reads the mRNA codons and brings the corresponding amino acid into the ribosomes.
• Accuracy of tRNA

  • It is important that the correct amino acid is brought to the ribosomes by tRNA so that the protein is assembled correctly.
  • Each tRNA molecule has an anticodon which is complementary to a specific codon on mRNA.
  • The amino acid that corresponds to a specific anticodon binds to a specific attachment site on the tRNA molecule.
• DNA → amino acid

  • There are many processes in producing an amino acid from the original DNA sequence.
  • If the original DNA sequence has the codon GCC, the codon in each step is:
  • DNA codon: GCC.
  • mRNA complementary codon: CGG.
  • tRNA anticodon: GCC.
  • Amino acid: Arginine.
• transcription:

  1. Binding of RNA polymerase

  • RNA polymerase is the enzyme that allows transcription to take place.
  • RNA polymerase binds to the locus of the gene to be transcribed (the target gene).
• transcription:

  2) Separation of DNA strands

  • When RNA polymerase binds to DNA, the hydrogen bonds that bind the two strands together break. This is different to DNA replication, where DNA helicase separates the strands.
  • The DNA strands separate.
  • The bases of the target gene are exposed
• transcription:

  3) Binding to template strand

  • RNA polymerase binds free-floating RNA nucleotides to the template strand.
  • The template strand is the DNA strand that is complementary to the base sequence of the target gene.
  • The RNA nucleotides form a strand of mRNA that is complementary to the template strand.
  • The template strand is complementary to the gene so this means mRNA is a copy of the gene.
• transcription:

  4) Joining the nucleotides

  • The free-floating nucleotides are joined together by RNA polymerase.
  • Phosphodiester bonds form between the nucleotides in a condensation reaction to form the completed strand of mRNA.
• transcription:

  5) STOP codon

  • RNA polymerase eventually reaches the triplet of bases that signal 'stop'.
  • E.g. UAG encodes a STOP codon.
  • The RNA polymerase stops separating the DNA and producing mRNA.
• transcription

  6) Removal of the mRNA

  • The mRNA strand is separated from the template strand by RNA polymerase.
  • The hydrogen bonds between the two strands of DNA form again and the strands join together.
• transcription:

  7) mRNA leaves the nucleus

  • The completed mRNA strand leaves the nucleus and enters the cytoplasm.
  • mRNA is used in translation, the next step in protein synthesis.
• 1)Binding of RNA polymerase

  2) Separation of DNA strands

  3) Binding to template strand

  4) Joining the nucleotides

  5) STOP codon

  6) Removal of the mRNA

  7) mRNA leaves the nucleus
• translation

  1. Attachment to the ribosome

  • mRNA that has been produced during transcription binds to a ribosome in the cytoplasm.
  • The ribosome is the site of protein synthesis.
• translation

  2) Binding of tRNA

  • Six bases (two codons) can fit inside the ribosome at one time.
  • One molecule of tRNA binds to the first codon in the ribosome.
  • The tRNA molecule has an anticodon that is complementary to a specific codon.
  • The anticodon allows the correct tRNA molecule to bind to the correct codon.
• translation

  3) Bringing in amino acids

  • Each tRNA molecule carries a specific amino acid into the ribosome.
  • The amino acid is bound to tRNA using ATP.
• translation

  4) Binding of the second tRNA

  • A tRNA molecule binds to the second codon in the ribosome.
  • When a tRNA molecule binds to mRNA, the corresponding amino acid is brought into the ribosome.
  • The two amino acids in the ribosome form a peptide bond
• translation

  5) Movement of the ribosome

  • When the two amino acids bind together, the ribosome moves along the mRNA strand so that a new codon enters the ribosome.
  • A complementary tRNA molecule binds to the new codon.
  • A new amino acid is brought into the ribosome.
  • A peptide bond forms between the new amino acid and the existing chain of amino acids (a polypeptide chain).
• translation

  6) STOP codon

  • When the ribosome reaches a STOP codon (e.g. UAG) there is no corresponding tRNA molecule.
  • The polypeptide chain is released from the ribosome.
• translation

  7) Completion of the polypeptide

  • The polypeptide chain has been formed and is ready to complete its function (e.g. as a protein channel).
  • Some polypeptide chains are joined to other chains or a prosthetic group is added.
  • E.g. Each haemoglobin molecule is made up of four polypeptide chains and each polypeptide has an iron prosthetic group.
• Pre-mRNA

  • mRNA is produced during transcription in both prokaryotes and eukaryotes.
  • The mRNA produced in eukaryotes is called pre-mRNA.
  • Pre-mRNA is spliced before it enters the ribosome for translation.
  • Splicing is a process that removes sections of non-coding DNA called introns from the mRNA molecule.
• Direct production of mRNA

  • The mRNA produced in prokaryotes is the completed form of mRNA.
  • No splicing takes place.
• RNA polymerase

  • RNA polymerase is an important enzyme in transcription.
  • In prokaryotes, RNA polymerase catalyses every step of transcription including the separation of the DNA strands and the production of the mRNA strand.
  • In eukaryotes, RNA polymerase produces the mRNA strand.
• Transcription

  • Prokaryotes vs eukaryotes
  • Prokaryotes
  • mRNA is in the completed form
  • RNA polymerase
  • Eukaryotes
  • Pre-mRNA is spliced to produce mRNA
  • RNA polymerase & DNA helicase