A: DNA, RNA and Protein Synthesis

• in prokaryotic cells DNA is short, circular and not associated with proteins
• In eukaryotic cells, DNA molecules are very long, linear and associated with proteins, called histones
• Together a DNA molecule and its associated proteins form a chromosome
• The mitochondria and chloroplasts of eukaryotic cells also contain DNA which, like the DNA of prokaryotes, is short, circular and not associated with proteins
• A gene is a base sequence of DNA that codes for:


  • the amino acid sequence of a polypeptide

  • a functional RNA (including ribosomal RNA and tRNAs)
• A gene occupies a fixed position, called a locus, on a DNA molecule.
• A sequence of three DNA bases, called a triplet, codes for a specific amino acid
• genetic code can be described as

  • universal - all species use the same bases
  • degenerate - multiple codons code for one amino acid
  • non overlapping - each nucleotide is part of only one codon
• introns are non coding DNA
• exons are coding DNA
• genes that dont code for polypeptides code for functional RNA - RNA molecules other than mRNA which preform tasks during protein synthesis eg: tRNA or rRNA
• the full range of proteins a cell is able to produce is called its proteome
• the complete set of genes in a cell is called its genome
• eukaryotic DNA contains regions of multiple repeats outside of genes that don't code for amino acids called non coding multiple repeats
• alleles are different forms of the same gene
• humans have 23 pairs of chromosomes
• pairs of matching chromosomes are called homologous pairs - they are the same size and contain the same genes although they could have different alleles
• mRNA is a single polynucleotide strand
• tRNA is a single polynucleotide strand folded into a clover shape that is held together by hydrogen bonds. they each have a specific sequence of three bases at one end called an anticodon and an amino acid binding site at the other end

  
• stages of transcription

  1. RNA polymerase attaches to the beginning of a gene and breaks the hydrogen bonds between bases the break the strand and expose some bases - one of the strands is used as a template to form the mRNA
  2. RNA polymerase lines up free RNA nucleotides along the template strand and they are joined by complimentary base pairing, forming an mRNA strand
  3. RNA polymerase moves along the strand assembling the mRNA. hydrogen bonds between the uncoiled strands of DNA re-form once the RNA polymerase has passed by
  4. when RNA polymerase reaches a stop signal it detaches from the DNA
• in eukaryotes introns and exons are copied into mRNA during transcription - this is called pre RNA - it then undergoes splicing to remove the introns and join together the exons
• there are no introns in prokaryotic DNA
• stages of translation

  1. mRNA attaches itself to ribosome
  2. tRNA molecules carry an amino acid with a codon complimentary to the anticodon on the mRNA and they join by complimentary base pairing
  3. more tRNA molecules attach in the same way and a peptide bond forms between the amino acids
  4. continues until tRNA reaches a stop signal on the mRNA
• 2 roles of ATP in protein synthesis

  • attach two amino acids together
  • attach amino acid to tRNA
• in RNA uracil replaces thymine