gene and protein synthesis

• Genes are sequences of hundred to millions of DNA nucleotides that code for the assembly of a functional protein
• Genes

  Segments of a chromosome that consist of a specific base sequence, each coding for a polypeptide or an RNA molecule with a specific sequence and number of bases
• Genome

  The sum of all genes in a single diploid cell of an organism
• Exons

  Genes that carry information for the production of polypeptides (codes for amino acids)
• Introns

  Segments of DNA found in genes in eukaryotes that do not code for specific amino acids
• Both exons and introns are transcribed

  Only the information contained in exons is translated to form a polypeptide in eukaryotes
• Unique sequence of bases on the DNA that codes for a gene is copied from the DNA molecule by transcription to form an RNA molecule
• Non-edited or immature messenger RNA is made up of exons and introns. Introns are edited to remove them, forming a mature mRNA molecule used to form a polypeptide in translation
• During translation, the base sequence on the mRNA codes for the sequence of amino acids of a single polypeptide
• In eukaryotic cells, transcription occurs in the nucleus
• Ribosomal RNA (rRNA)

  The most abundant RNA with associated proteins that forms the ribosome, the site of protein synthesis and the site of attachment of RNA during translation
• Ribosomal RNA (rRNA) function

  It acts as an enzyme in the large ribosomal subunit by catalysing the synthesis of a protein from amino acids. The ribosome synthesises a protein using genetic information stored in mRNA
• Two subunits of the ribosome: one small ribosomal subunit reads the mRNA and the largest subunit joins amino acids to form a protein molecule
• Transfer RNA (tRNA)

  A small RNA molecule that transfers amino acids to the ribosome during translation
• Each transfer RNA consists of a single polynucleotide strand of RNA of about 80 bases long which is folded on itself to form a clover leaf arrangement. One end of the tRNA attaches to a specific amino acid and the other end has a triplet of bases called anticodon pair with complementary triplet bases or codon on the mRNA
• Transcription

  Synthesis of RNA from a DNA template. Process occurs in the nucleus of a eukaryotic cell and in the chloroplast of a prokaryotic cell. Messenger RNA (mRNA) is formed by copying information from DNA
• Location of transcription

  • Nucleus of a eukaryotic cell
  • Chloroplast of a prokaryotic cell
• messenger RNA (mRNA)

  Information in the DNA is copied as RNA
• Transcription process

  1. When gene is switched on, RNA polymerase attaches to the promoter of the region of a gene on a strand of DNA (start of the gene)

  2. RNA polymerase separates the DNA strand, moves along DNA strand and reads bases one by one to make mRNA strand

  3. mRNA strand builds up bases as it moves along the entire gene

  4. mRNA pairs with a DNA template strand: guanine to cytosine, adenine to thymine, adenine to uracil

  5. Once mRNA strand is complete, RNA polymerase detaches to form immature RNA

  6. Immature RNA is spliced or edited to remove introns and form mature RNA

  7. Mature mRNA containing exons moves out of the nucleus into the cytoplasm

  8. DNA strands rejoin and recoil to form the double helix
• Translation process

  1. The synthesis of a polypeptide from an mRNA template

  2. Translation occurs in the cytoplasm

  3. ribosomal RNA (rRNA) is produced in the nucleolus

  4. transfer RNA (tRNA) is involved in converting the coded message on the mRNA into a sequence of amino acids

  5. The codon AUG signals the start of protein synthesis and codes for methionine

  6. mRNA attaches to the ribosome in the cytoplasm

  7. Elongation phase adds amino acids by peptide bonds as specified by the mRNA sequence

  8. tRNA with complementary anticodon attaches to mRNA codon carrying an amino acid

  9. New tRNA with anticodon complementary to the next code in the chain joins the ribosome

  10. Adjacent amino acids join by a peptide bond to form a polypeptide chain

  11. Stop codons UAA, UAG, UGA signal termination of polypeptide synthesis

  12. At the stop codon, the ribosome releases the mRNA and an amino acid chain

  13. Amino acid chain folds into a three-dimensional shape called a protein

  14. The polypeptide is ready for use within the cell or to be secreted out of the cell