D1.2 Protein Synthesis

• RNA - has a single strand, the sugar group is ribose and nitrogen bases are Uracil, Adenine, Guanine and Cytosine

  DNA - has two strands, the sugar group is deoxyribose and nitrogen bases are Thymine, Adenine, Guanine and Cytosine
• Transcription is the synthesis of RNA using DNA as a template. the enzyme RNA polymerase transcribes DNA to RNA.
• Transcription works by RNA polymerase attaching to the beginning of a gene on DNA, it separates the DNA strands, and an mRNA molecule is produced on the template DNA strand using the enzyme (RNA polymerase) to link RNA nucleotides into a single polynucleotide (this linking happens through condensation reactions)
• RNA polymerase during transcription in protein synthesis uses complementary base pairing to pair DNA and RNA nucleotides

  when RNA polymerase reaches the end of the gene the mRNA molecule is released
• the RNA to DNA base pairings are:

  Uracil to Adenine

  Guanine to Cytosine

  Cytosine to Guanine

  Adenine to Thymine
• DNA is a stable molecule due to the strong covalent bonds between the nucleotides.
• Single DNA strands can be used as a template for transcribing a base sequence without the DNA base sequence changing. In somatic cells these sequences must be conserved throughout the life of a cell
• somatic cells are non-reproductive cells in an organism
• transcription is the first stage of gene expression
• protein synthesis consists of two stages; transcription and translation
• translation is the synthesis of polypeptides (proteins) by ribosomes using the genetic code of mRNA
• gene expression is a process by which genetic information in a gene is used to synthesize a functional polypeptide
• in gene expression, a gene must first be transcribed to mRNA before mRNA can be translated by a ribosome
• gene expression can be switched on and off by regulating transcription of genes
• in translation ribosomes require mRNA (messenger RNA that brings the genetic code to the ribosome) and tRNA (transfer RNA that brings amino acids to the ribosome) to synthesize polypeptides.
• ribosomes are composed of two subunits; small and large
• mRNA can bind to the small subunit of ribosomes and two tRNA can bind to the large subunit of a ribosome simultaneously
• mRNA contains the genetic code in the form of codons, it binds to the small subunit to initiate translation (ribosomes use the sequence of codons on mRNA to determine the sequence of amino acids in a polypeptide)
• codons are a sequence of three nucleotides that code for an amino acid
• tRNA transfers amino acids from the cytoplasm to the ribosome.

  these tRNA molecules have an anticodon which binds to the mRNA codons through complementary base pairings (this ensures that the correct sequence of amino acids is synthesized by the ribosome)
• codons and anticodons form hydrogen bonds between complementary nucleotides (both tRNA and mRNA contain uracil instead of thymine and so the complementary base pairings are A-U & C-G)
• There are 20 amino acids in all living organisms
• there are 64 different codons (3 nucleotides in each codon) that make up the genetic code for 20 amino acids
• the genetic code is universal and degenerate
• the ribosomes move along the mRNA from codon to codon, at each codon a tRNA enters the ribosome. when 2 tRNA are present in the ribosome a peptide bond forms between their amino acids
• the small ribosomal subunit binds to mRNA at the start codon (in translation)
• the large ribosomal subunit attaches to form a ribose (in translation)
• a mutation is a change in the DNA sequence of an organism
• mutations to a single nucleotide in the gene for hemoglobin can cause the disease sickle cell anemia
• the enzyme RNA polymerase can only add nucleotides in the 5' to 3' direction during transcription
• ribosomes move from 5' to 3' during translation, the 5' end of mRNA enters the ribosome and moves along carbon 5 to 3
• transcription involved 3 stages; initiation, elongation and termination
• non-coding DNA are sections of DNA that do not code for any functional peptides
• sections of non-coding DNA function as:

  controlling gene expression, some base sequences like promotor regions are sites where regulatory proteins bind (they can promote or inhibit the transcription of genes)

  genes for tRNA and rRNA, transcription of these genes produces tRNA and ribosomal RNA
• Telomeres are repetitive base sequences at the ends of chromosomes
• when DNA is replicated in eukaryotes, the ends of the molecule cannot be copied so a short section of the base sequence is lost, telomeres ensure that important genes are not lost
• genes consist of introns and exons
• introns are sections of RNA or DNA that do not code for a protein and interrupt the gene sequence
• exons are sections of DNA that code for proteins
• introns are removed from mRNA after transcription