DNA AND PROTEIN SYNTHESIS

• RNA

  • RNA is a single polynucleotide strand and it contains uracil (U) as a base instead of thymine. Uracil always pairs with adenine during protein synthesis. RNA isn’t all the same though - there are different types:
  • mRNA (messenger RNA)
  • tRNA (transfer RNA)
  • rRNA (ribosomal RNA) - combines with proteins to make ribosomes

  
• mRNA

  • copy of a gene
  • single stranded polymer of RNA
  • mRNA made during transcription
  • it carries the genetic code from the DNA to the ribosomes, where it’s used to make a protein during translation
  • mRNA is a single polynucleotide strand
  • in mRNA, groups of three adjacent bases are usually called codons (they’re sometimes called triplets or base triplets)

  
• tRNA

  • found in cytoplasm only
  • single stranded
  • a single polynucleotide strand
  • folded into a clover shape
  • hydrogen bonds between specific base pairs hold the molecule in this shape
  • every tRNA molecule has a specific sequence of 3 bases at one end called an anticodon
  • also has an amino acid binding at the other end
  • each tRNA is specie to one amino acid + has an anticodon thats specific to that amino acid
  • tRNA is involved in translation. It carries the amino acids that are used to make proteins to the ribosomes.

  
• STRUCTURAL COMPARISONS IN RNA

  DIFFERENCES

  • tRNA is clover leaf shaped
  • tRNA has an anticodon
  • tRNA has an amino acid binding site
  • tRNA has hydrogen bonds
  • mRNA is longer / has more nucleotides
• STRUCTURAL COMPARISONS IN RNA

  SIMILARITIES

  • uracil base
• WHAT IS PROTEIN SYNTHESIS?

  PROTEIN SYNTHESIS: the production of proteins (polypeptides) from the information contained within a cells DNA

  • the DNA is too large to leave the nucleus so a copy is made
  • it involves 2 main stages:

  1. TRANSCRIPTION: where the DNA code is copied into a molecule called mRNA
  2. TRANSLATION: where the mRNA joins a ribosome and the code it carries is used to synthesise a protein
• TRANSCRIPTION: where the DNA code is copied into a molecule called mRNA
• TRANSLATION: where the mRNA joins with a ribosome and the code it carries is used to synthesise a protein
• PROTEIN SYNTHESIS BASIC OVERVIEW

  1. DNA provides the instructions in the form of a long sequence of bases
  2. a complementary section of part of this sequence is made in the form of a molecule called pre-mRNA - a process called transcription
  3. the pre-mRNA is spliced to form mRNA
  4. the mRNA is used as a template to which complementary tRNA molecules attach and the amino acids they carry we are linked to form a polypeptide - a process called translation
• TRANSCRIPTION

  The process of making pre-mRNA using part of the DNA as a template

  1. The hydrogen bonds between DNA bases break, separating the DNA strands and exposing the bases
  2. one of the strands acts as a template + free RNA nucleotides align by complementary base pairing along the template in the 5’ to 3’ direction
  3. adenine in DNA pairs pairs with uracil in RNA rather than thymine
  4. the enzyme RNA polymerase then moves along the strand + joins the nucleotides together, forming phosphodiester bonds between them to form pre-mRNA
  5. splicing occurs to remove introns

  
• TRANSCRIPTION

  As the RNA polymerase adds the nucleotides one at a time to build a strand of pre-mRNA, the DNA strands rejoin behind it. As a result, only about 12 base pairs on the DNA are exposed at any one time. When the RNA polymerase reaches a particular sequence of bases on the DNA that it recognises as a ’stop’ triplet code, it detaches, and the production of pre-mRNA is then complete

  
• SPLICING OF PRE-mRNA

  • in eukaryotes both introns and exons are copied during transcription
  • intervening introns would prevent the synthesis of a polypeptide
  • in the pre-mRNA of eukaryotic cells the base sequences corresponding to the introns are removed and the functional exons are joined together during a process called splicing
  • this forms mRNA that is ready to leave the nucleus via a nuclear pore for translation

  
• TRANSLATION: 2

  7. tRNA released after amino acid joined to polypeptide and is now free to collect another amino acid

  8. The ribosome moves along the mRNA to form the polypeptide and the process repeats until a polypeptide chain is built up

  9. The synthesis or a polypeptide continues until a ribosome reaches a stop codon. At this point, the ribosome, mRNA and the last tRNA molecule all separate and the polypeptide chain complete.

  
• TRANSLATION: 1

  Process by Which a polypeptide chain is formed by the ribosome

  1. mRNA attaches to ribosomes or RER and finds start codon
  2. tRNA anticodons bind to complementary mRNA codons
  3. this tRNA brings a specific amino acid
  4. a tRNA molecule with a complementary anticodon pairs with the next codon on the mRNA. This tRNA molecule carries another amino acid
  5. the ribosome moves along the mRNA bringing together 2 tRNA molecules
  6. amino acids join by peptide bonds (condensation reaction) with the use of ATP which is hydrolysed to provide the energy required

  
• TRANSLATION

  Sometimes a single polypeptide chain is a functional protein. Often, a number of polypeptides are linked together to give a functional protein (quaternary structure). What happens to the polypeptide next depends upon the protein being made, but usually involves:

  • the polypeptide is coiled or folded, producing its secondary structure
  • secondary structure is folded, producing tertiary structure
  • different polypeptide chains, along with any non-protein groups, are linked to form the quaternary structure