DNA & Protein Synthesis

• TRANSFERRING CODED INFORMATION:

  • DNA determines the amino acid sequence coded & is located in the nucleus, but protein synthesis takes place in the cytoplasm. This is done by transcribing DNA code onto single-stranded RNA.
• TRANSFERRING CODED INFORMATION:

  • DNA= the genetic material in all living organisms. Gene= A section of DNA containing coded info for making polypeptides & RNA. Codon= sequence of 3 bases. Genome= the complete set of genes in a cell (including in mitochondria & chloroplasts). Proteome= The full range of proteins produced by the genome (complete proteome= proteins produced by a given type of cell under certain conditions). RNA= a single stranded polymer made up of repeating mononucleotide sub units
• RNA STRUCTURE:

  • RNA= a single stranded polymer made up of repeating nucleotide sub units
  • NUCLEOTIDE: = the pentose sugar ribose, one organic base (adenine, guanine, cytosine, uracil), a phosphate group
  • RNA IN PROTEIN SYNTHESIS: : Messenger RNA/ mRNA (transfers DNA code from nucleus to cytoplasm). Transfer RNA/ tRNA (reads the mRNA to transfer amino acids in the ribosome to form the right sequence of the polypeptides)
• RNA STRUCTURE:

  • MESSENGER RNA: Long strand (1000s mononucleotides) in a single helix, sequence is determined/ complementary to DNA in transcription (& different types of mRNA), once forms it leaves the nucleus (via pores in the nuclear envelope) and associates with the ribosomes (proteins that translate mRNA into amino acids & synthesises the polypeptide)  which acts as a template for protein synthesis. This is due to mRNA’s codons which determines the amino acid sequence and therefore the specific polypeptide made
• RNA STRUCTURE:

  • TRANSFER RNA: Relatively short molecule (~80 nucleotides). Single stranded chain folded into a clover shape, one end extending beyond the other- where the amino acid easily attaches. Amino acid is bound to tRNA by ATP. Opposite end of the tRNA: anticodon (sequence of 3 organic bases). Many types of tRNA for 1 type of amino acid as there are coding triplets (degenerate code).
• RNA STRUCTURE:

  • RIBOSOMAL RNA: Are part of ribosomes, the centre of protein synthesis.
  • RNA POLYMERASE: Important enzyme in transcription. Prokaryotes: it catalyses every step of transcription including the separation of DNA strands and the production of the mRNA strand. Eukaryotes: produces the mRNA strand
• OVERALL PROCESS:

  1. DNA: Provides the instructions (as long sequence of bases)
  2. TRANSCRIPTION: A complementary section of part of the sequence is made in the form pre-mRNA
  3. SPLICING: The pre-mRNA is spliced to form mRNA
  4. TRANSLATION: The mRNA is used as a template where complementary tRNA molecules attach & amino acids they carry link to form a polypeptide
• TRANSCRIPTION:

  1. An enzyme (RNA polymerase) binds to the promoter region of the target gene -> the hydrogen bonds break-> the 2 strands separate -> the nucleotide bases are exposed
• TRANSCRIPTION:

  2)Nucleotide bases (on the template strand 5-3 direction ) pairs with free complementary nucleotides (from the pool in the nucleus). Thymine is replaced by uracil.
• TRANSCRIPTION:

  3) Enzyme RNA polymerase moves along the strand and joins complementary nucleotides together -> phosphodiester bonds form between the nucleotides in a condensation reaction-> forms a pre-mRNA molecule
• TRANSCRIPTION:

  4) As the RNA polymerase adds the nucleotides 1 at a time to build the pre-mRNA, the DNA rejoins behind it (so only ~12 base pairs on the DNA are exposed at a time)
• TRANSCRIPTION:

  5) When the RNA polymerase reaches a specific base sequence (the stop triplet code) it stops separating DNA & producing mRNA as it detaches (production of pre-mRNA is complete)
• TRANSCRIPTION:

  6) The mRNA strand is separated from the template strand by RNA polymerase to leave the nucleus and enter the cytoplasm --> the hydrogen bonds between the 2 strands form again (*not via RNA polymerase) & the strands join together
• TRANSCRIPTION:

  DEFINITION: Process of making pre-mRNA using part of DNA as a template
• SPLICING:

  • DEFINITION & CONTEXT: Splicing: introns (which would prevent the synthesis of the polypeptide) are removed and the exons joined together.Prokaryotic: transcription results directly from DNA to mRNA, in eukaryotes: transcription results from DNA to pre-mRNA which is spliced to form mRNA (DNA of eukaryotes contain exos(coding) & introns(non-coding)). Introns in pre-mRNA are removed
• SPLICING:

  1. Introns removed
  2. Exons are joined together
  3. mRNA molecules are too large to diffuse out of the nucleus so once spliced they leave via nuclear pore
  4. Outside nucleus: mRNA is attached to ribosomes for translation
• TRANSLATION:

  1. A ribosome is attached to the start codon (at one end of the mRNA molecule)
• TRANSLATION:

  2) The tRNA molecule with complementary anticodon (to the start codon) moves to the ribosome and pairs with the codon on the mRNA which carries a specific amino acid (1st codon) & another tRNA molecule with complementary anticodon pairs with the next codon (also carrying a specific amino acid) (2nd codon).
• TRANSLATION:

  3) The ribosome moves along the mRNA bringing 2 tRNA molecules at a time (pairing with the complementary 2 codons on the mRNA). The 2 amino acids on the tRNA are joined by a peptide bond (using an enzyme and ATP that is hydrolysed to provide the needed energy)
• TRANSLATION:

  4) The ribosome moves onto the 3rd codon on the mRNA, linking the amino acids on the 2nd and 3rd tRNA molecules with a peptide bond, as this happens the first tRNA is released from its amino acid and is free to collect another amino acid (from the amino acid pool in the cell)
• TRANSLATION:

  5) The process continues (up to 15 amino acids added each second) until a polypeptide chain is built. Up to 50 ribosomes can pass immediately behind the first so many identical polypeptides can be assembled simultaneously 
• TRANSLATION:

  6) The synthesis of a polypeptide continues until the ribosome reaches a stop codon, where the ribosome, mRNA, and last tRNA molecule all separate (the polypeptide chain is complete/ released from the ribosome)
• TRANSLATION:

  7) Some polypeptide chains are joined to other chains or a prosthetic group is added (eg haemoglobin is made up of 4 polypeptides and each has an iron prosthetic group)
• TRANSLATION:

  • The amino acids are coded by the mRNA, so are complementary to the DNA
• TRANSCRIPTION:

  
• TRANSLATION:

  
• TRANSLATION: