Genes and Protein synthesis 9

• What is a gene?

  a section of DNA bases that codes for one polypeptide, polypeptides in turn determine the nature and development of organisms
• What is an allele?

  a different form of gene
• A gene occupies a fixed position, called locus, on a particular chromosome
• What is the only difference between DNA strands?

  the sequence of bases and length
• How many bases need to combine to form a code for the 20 types of amino acids?

  4x4x4=64

  3 bases are required, a triplet code for one amino acid
• the code in DNA is a triplet code , 3 bases code for 1 amino acid
• important features of the genetic code

  • non-overlapping: each base is part of only one triplet code. in an overlapping code, each base would be part of 3 triplets
  • degenerate: means that some of the amino acids are coded for by more than one triplet. arisen as there are 64 different triplet codes but only 20 amino acids
  • universal: a given triplet specifies the same amino acid in all organisms
• How does the gene code information?

  the specific sequence of bases on 1 strand of DNA controls the sequence of amino acids in proteins (primary structure) that are made by a cell's ribosomes and therefore tertiary structure and function of that protein
• a different base sequence leads to...

  • a different amino acid sequence which leads to
  • bonds ( H, ionic, disulfide) forming in different places
  • so there is a different tertiary structure in the protein coded for by that gene
  • if the protein is an enzyme, the active site will change shape, the substrate will not fit and so fewer/no enzyme-substrate complexes will form
  • if not an enzyme= protein changes of shape so no longer complementary
• ribosomal RNA


  rRNA

  ribosome= rRNA together with proteins

  ribosomes are the site of mRNA translation and protein synthesis

  rRNA is coded for by numerous genes in many different chromosomes
• messenger RNA

  mRNA is formed by transcription of a gene in DNA in the nucleus

  it's complementary to the DNA in its base sequence

  mRNA molecules may consist of thousands of nucleotides in single linear strand

  an amino acid is coded for by a triplet of bases on mRNA called a codon

  mRNA has unpaired bases and so is easily broken down in cytoplasm, only needs to exist temporarily until protein is manufactured
• if there are 4 possible bases coding as a triplet, how many possible codons are there?

  64
• transfer RNA

  shortest RNA molecule made up of around 80 nucleotides

  it's a single strand which folds back on itself, the tRNA molecule form hydrogen bonds within complementary sections of the molecule causing the folding back. H bonds stabilise the molecule.

  one end of the chain attaches to an amino acid
• • there are several types of tRNA, each able to carry a single specific amino acid, at the base of the tRNA molecule is a sequence of 3 bases, called the anticodon. For each amino acid carried, there is a different sequence of bases on the anticodon of tRNA
  • in RNA, however, the base thymine is always replaced by a similar base called uracil
  • so complementary base pairings are G and C, A and U
• non coding DNA

  much of the DNA in the nucleus doesn't code for the synthesis of proteins. Non coding DNA between genes, this contains multiple repeats of base sequences. These are called Variable Number Tandem Repeats (VNTRs), important in genetic fingerprinting.
• introns


  non coding sections within genes
• prokaryotes do not have introns
• exons


  coding sections
• 3 differences between prokaryote DNA and eukaryote DNA

  prokaryotes: circular, not associated with protein, shorter

  eukaryotes: linear, associated with protein, longer
• mitochondria and chloroplasts of eukaryotic cells contain DNA like that found in prokaryotes
• genome


  the complete set of genes in a cell, including those in mitochondria and/or chloroplasts
• proteome


  the full range of proteins that a cell is able to produce
• complete proteome


  the full set of proteins that can be made by an organism
• Both the pancreas and the liver cells contain the gene that codes for insulin, but only pancreas cells actually produce insulin
• Would the genome of a pancreas cell and liver cell be the same?

  yes as they contain identical genes due to mitosis
• overview of transcription

  takes place in the nucleus of the cell and involves the formation of a precursor RNA called pre-mRNA. This has a complementary sequence of bases to the DNA. In eukaryotes, it's the process of making pre-mRNA using part of the DNA as a template
• RNA processing

  takes place in nucleus so that non-functioning base sequences are spliced from the pre-mRNA to form mRNA. The mRNA leaves the nucleus and attaches to a ribosome
• Translation

  occurs on ribosomes, involves the translation of the mRNA message into a specific sequence of amino acids in the polypeptide
• Summary of transcription


  • the hydrogen bonds between DNA bases are broken which separates the two strands of DNA
  • one strand of the DNA acts as the template strand upon which pre-mRNA is built
  • free RNA nucleotides are found in the nucleoplasm. They are attracted to the exposed DNA bases on the DNA template strand and align by complementary base pairing
  • e.g Uracil and Adenine
  • RNA polymerase joins RNA nucleotides together to make an RNA polynucleotide chain via phosphodiester bonds
  • introns are removed from pre-mRNA and exons are spliced back togthether
• splicing of pre-mRNA (eukaryotes)

  • DNA is made up of sections called exons that code for the amino acid sequence of polypeptides
  • sections called introns do not code for amino acid sequences.
  • exons are sections of DNA that are expressed to produce proteins
  • in the pre-mRNA the introns are removed by enzymes before the mRNA moves out of the cytoplasm
  • the remaining exons are then joined together-splicing
  • Following splicing, mRNA molecules leave the nucleus through the nuclear pores
• Protein synthesis in prokaryotes

  Prokaryotic genes do not contain non coding sections, therefore they don't produce pre-mRNA that requires splicing.

  Transcription takes place in the cytoplasm and involves the formation of a functional mRNA that is a complementary sequence of bases to the DNA. Translation occurs on ribosomes, involving the translation of the pre-mRNA message into a specific sequence of amino acids to form a polypeptide
• Translation which occurs on ribosomes, involves the translation of the mRNA message into a specific sequence of amino acids to form a polypeptide
• Genetic code features


  • Apply to mRNA codons as well
  • Often only the first two bases of the triplet are specific for a particular amino acid, any third base will do
  • This reduces the chance that a change in the bases will alter function of polypeptide
• Stop codons


  • 3 stop codons
  • Indicating the end of a section of mRNA
  • After which point translation stops
• Start code


  • The code for methionine, AUG
  • Used as a start code
  • Polypeptides normally start with a methionine group when they're freshly translated
  • Often removed in the processing stage that converts the polypeptide into functional protein
• Describe how translation happens

  • mRNA attaches to ribosomes
  • tRNA anticodons bind to mRNA codons by complementary base pairing
  • each tRNA brings a specific amino acid
  • 2 tRNA molecules (with their amino acids) are held together at a ribosome at any one time
  • a peptide bond forms between adjacent amino acids, requires ATP
  • tRNA molecules are released after their amino acids have been joined to the growing polypeptide chain
  • the ribosome moves along the mRNA forming the polypeptide until a stop codon is reached, at which point the ribosome and mRNA dissociate
• Why are there always 2 codons on the ribosome?

  so a peptide bond can form between them
• what is the anticodon to the mRNA codon AAG?

  UUC
• Compare DNA, mRNA and tRNA

  DNA- double polynucleotide chain. tRNA, mRNA- single polynucleotide chain

  DNA- longest of the 3, mRNA- shorter than DNA, longer than tRNA, tRNA- shortest of the 3

  DNA-double helix shape, mRNA-single linear strand, tRNA-clover leaf shaped

  DNA- deoxyribose, mRNA and tRNA- ribose

  DNA- ATCG, mRNA and tRNA- AUCG

  DNA-found in mostly the nucleus, mRNA and tRNA- made in nucleus but found throughout the cell

  DNA-chemically stable, mRNA-least stable usually broken down within a few days, tRNA- more stable than mRNA but less stable than DNA
• What is the advantage of mRNA being broken down relatively quickly?

  mRNA is used in protein production.

  A protein only needs to be produced when it is needed

  mRNA breaks down once used and is made again when protein production is required.