B8 DNA, genes + protein synthesis

• a gene is a section of DNA that codes for a polypeptide or functional RNA molecule
• DNA has 4 bases:

  • adenine
  • cytosine
  • guanine
  • thymine
• a triplet is a combination of 3 bases which codes for one amino acid
• the genetic code has the following features:

  • degenerate
  • non-overlapping
  • universal
• the genetic code being degenerate means most amino acids are coded for by more than one triplet
• the genetic code being non-overlapping means each base in the sequence is read only once as part of a triplet
• the genetic code being universal means each triplet codes for the same amino acid in all organisms
• prokaryotic cells have shorter, circular DNA which is not associated with proteins
• eukaryotic cells have longer, linear DNA which is associated with proteins called histones
• the mitochondria and chloroplasts in a eukaryotic cell contain DNA which is very similar to prokaryotic DNA, short, circular, not associated with proteins
• chromosomes are made up of two sister chromatids which are connected at the middle by a centromere, the DNA is associated with histones
• homologous chromosomes are two chromosomes which carry the same genes at the same loci but not necessarily the same alleles of the genes
• an allele is a version of a gene, there can be two or more different alleles of any gene
• the genome is the complete set of genes in a cell
• the proteome is the full range of proteins able to be produced by a cell
• RNA is a polymer made of nucleotide monomers, forming a single strand
• an RNA nucleotide is made up of:

  • ribose
  • adenine/cytosine/guanine/uracil
  • phosphate group
• mRNA is messenger ribonucleic acid
• the shape of mRNA is a long strand arranged in a single helix
• the function of mRNA is to act as a template for protein synthesis by leaving the nucleus through pores and attaching to ribosomes in the cytoplasm
• tRNA is transfer ribonucleic acid
• the shape of tRNA is short single strand folded into a clover-leaf
• the function of tRNA is to bind to a specific amino acid using the end chain which is complementary, and bind to an mRNA codon using its complementary anticodon
• in prokaryotic cells, transcription produces mRNA directly because there are no introns to remove
• in eukaryotic cells, transcription produces pre-mRNA which needs to be spliced to remove the introns and keep the exons
• transcription is the process of making mRNA or pre-mRNA using part of the DNA as a template
• transcription follows the process:

  • DNA helicase breaks hydrogen bonds between complementary bases
  • one DNA strand is used as a template
  • free nucleotides line up by complementary base pairing
  • RNA polymerase catalyses the formation of phosphodiester bonds between adjacent nucleotides
  • the process stops when a STOP codon is reached
  • after RNA polymerase moves away, DNA is joined together again, exposing only 12 bases at a time
  • mRNA leaves the nucleus through a pore and attaches to a ribosome, pre-mRNA is spliced
• splicing is the process of removing non-coding sections of pre-mRNA to make mRNA
• splicing follows the process:

  • introns are removed
  • exons are kept
  • a strand of mRNA is produced
  • mRNA leaves the nucleus through a pore and attaches to a ribosome
• introns are non-coding, exons are coding
• once the mRNA attaches to a ribosome, a polypeptide begins to be synthesised
• translation is the process of producing a polypeptide from a strand of mRNA
• translation follows the following process:

  • mRNA attaches to the START codon of a ribosome
  • tRNA brings amino acids from the cytoplasm to the ribosome by binding its end chain to them
  • the tRNA molecule with the complementary anticodon binds to the mRNA codon
  • the next tRNA molecule with the next complementary anticodon binds to the next mRNA codon, and so on
  • the ribosome moves along the mRNA pairing up more tRNA molecules
  • adjacent amino acids on tRNA molecules are joined by peptide bonds
  • tRNA molecules are released and can collect more amino acids
  • the process stops at a STOP codon
• exons are sections of the DNA base sequence which code for the sequence of amino acids in a polypeptide
• introns are sections of the DNA base sequence which are non-coding, they are positioned between genes and at the end of chromosomes
• DNA nucleotides are made up of deoxyribose, a phosphate group and adenine/cytosine/guanine/thymine
• in DNA nucleotides there are phosphodiester bonds between the deoxyribose and the phosphate group, these are formed by a condensation reaction catalysed by DNA polymerase
• structure of tRNA:

  A) anticodon

  B) amino acid binding site

  
• not all mutations in the nucleotide sequence of a gene cause changes to polypeptide structure because:

  • the genetic code is degenerate, most amino acids are coded for by multiple triplets so the mutated triplet could still code for the correct amino acid
  • the mutation could occur in the introns so would be non-coding
• differences between eukaryotic and prokaryotic DNA:

  • eukaryotic is long, prokaryotic is short
  • eukaryotic is linear, prokaryotic is circular
  • eukaryotic has introns, prokaryotic does not have introns
  • eukaryotic is associated with histones, prokaryotic is not associated with histones