DNA, genes and protein synthesis

• DNA in prokaryotic cells are shorter, circular and not associated with histone proteins.
• DNA in eukaryotic cells are longer, linear and associated with histone proteins. DNA and histone coil up tightly to form chromosomes.
• DNA in mitochondria and chloroplasts are shorter, circular and not associated with proteins.
• A gene is a sequence of DNA bases that codes for:

  • the amino acid sequence of polypeptides
  • a functional RNA
• Each sequence of 3 DNA bases (codon) code for one amino acid.
• Each gene is located on a fixed position (locus) on a DNA molecule.
• The genetic code is…

  • universal: the same codons code for the same amino acids in all living things
  • non-overlapping: codons don’t share bases
  • degenerate: multiple codons code for the same amino acid (20 amino acids but 64 possible combinations of bases)
• Non-coding DNA doesn‘t code for amino acids.

  • between genes: multiple repeats
  • within genes: introns that separate exons (removed during splicing)
• • Genome: the entire set of genes in an organism
  • Proteome: the full range of different proteins that can be produced by a cell
• mRNA:

  • carries the genetic code from the nucleus to the ribosomes
  • a single polynucleotide strand
• tRNA:

  • carries amino acids to ribosomes
  • a single polynucleotide strand, folded into a clover shape
  • hydrogen bonds between complementary bases
  • one end slightly longer -> amino acid binding site
  • anticodon complementary to mRNA codon
• Protein Synthesis - Transcription:

  1. DNA helicase unwinds DNA double helix by breaking hydrogen bonds, creating a template strand.
  2. Free RNA nucleotides move towards complementary bases on the template strand, hydrogen bonds form between them.
  3. RNA polymerase joins adjacent RNA nucleotides together by forming phosphodiester bonds.
  4. Hydrogen bonds between mRNA and DNA break, DNA strands coil back into a double helix.
  5. Splicing: introns are removed, exons are joined together
  6. mRNA leaves the nucleus through a nuclear pore.
• Protein Synthesis - Translation:

  1. mRNA attaches to a ribosome in the cytoplasm.
  2. tRNA anticodon binds to a complementary mRNA codon.
  3. tRNA brings a specific amino acid, attached at its amino acid binding site.
  4. when 2 tRNA are binded, a peptide bond forms between the 2 amino acids, energy provided by ATP
  5. ribosome moves along the mRNA, releasing the first tRNA.
  6. The process repeats until a stop codon is reached. A polypeptide chain is produced and released.
• tRNA vs mRNA

  • anticodon vs codon
  • clover leaf shape vs linear
  • yes binding site vs no
  • yes hydrogen bonds vs no
  • have same lengths vs have different lengths
  • less nucleotides vs more nucleotides
• Homologous pair of chromosomes: 2 chromosomes that carry the same genes
• Eukaryotic vs Prokaryotic DNA

  • linear vs circular
  • longer vs shorter
  • histones vs none
  • introns vs none
  • none vs plasmids
• Similarities between eukaryotic and prokaryotic DNA

  • nucleotide structure is identical
  • nucleotides are joined by phosphodiester bonds
• Exons: base sequences of DNA coding for the amino acid sequence of a polypeptide
• Role of ribosomes in protein synthesis

  • mRNA binds to ribosomes
  • provides binding sites for 2 tRNA molecules
  • catalyses formation of peptide bonds between amino acids
  • moves along mRNA to next codon