DNA, RNA and Protein Synthesis

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DNA storage

  • Structure of DNA is the same in all organisms but stored differently.
  • In eukaryotic cells the DNA is non linear and exists as chromosomes.The DNA is long and is wound around histoes which help to support the DNA which is then coiled up tightly to make a compact chromosome.
  • The mitochondria and chloroplasts have their own DNA that is similar to prokaryotic DNA as its not wrapped around histones and is circular and shorter than the DNA in the nucleus.
  • In prokaryotic cells DNA is also carried as chromosomes but its shorter and isnt wound around histones, instead its supercoiled.
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Genes

  • Sequence of DNA bases that code for either a polypeptide or functional RNA.The sequence of amino acids in a polypeptide forms the primary structure of a protein.Different polypeptides have a different number and order of amino acids in a particular polypeptide.Each amino acid is coded for by a sequence of 3 bases in a gene called a triplet code or codon.Genes that dont code for polypeptides code for functional RNA instead, this is RNA molecules other than mRNA.
  • Nucleic acids carry the genetic code that determines the order of amino acids in a protein.Genetic material stores information, can be replicated and undergoes mutations.
  • The complete set of genes in a cell is known as a genome.The full range of proteins that the cell is able to produce is known as a proteome.Alleles are when genes exist in more than one form, the order of bases is slightly different, so they code for different versions of the same polypeptide.
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Non-coding DNA

  • These sections of DNA are called introns. There can be several in a gene. 
  • They are removed during protein synthesis so they dont affect the amino acid order.
  • Prokaryotic DNA has no introns.
  • The parts that do code for amino acids are called exons.
  • Non-coding multiple repeats are also found in eukaryotic DNA, they are sections that repeat over and over. eg ATTTATTTATTTATTT. 
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Homologous chromosomes

  • Pairs of matching chromosomes are called homologous pairs.
  • In a homologous pair both chromosomes have the same size and have the same genes, but may have different alleles. 
  • Alleles are found in the same locus on each chromosome in a homologous pair.
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mRNA and tRNA

  • mRNA made during transcription. It carries the genetic code from the DNA to ribosomes and is used to make a protein during translation. Its a single polynucleotide strand. Groups of adjacent bases are usually called codons.
  • tRNA is used in translation and carries the amino acids that are used to make proteins to the ribosomes. Its a single polynucleotide strand thats folded into a clover shape and has hydrogen bonds between specific base pairs to hold the molecule in shape. Every tRNA molecule has a specific sequence of 3 bases at one end called an anticodon and an amino binding site at the other end.
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Editing mRNA

  • Transcription produces different products in eukaryotes and prokaryotes.
  • In eukaryotes the introns and exons are both copied into mRNA during transcription, mRNA strands that contain introns and exons are called pre-mRNA.
  • Splicing occurs in the nucleus where introns are removed and exons are joined together which forms mRNA strands.
  • In prokaryotes mRNA is produced directly from DNA without splicing happening because there are no introns.
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Comparison of DNA, mRNA, tRNA

Image result for mrna trna dna (http://slideplayer.com/1631325/6/images/22/Comparison+of+DNA%2C+mRNA+and+tRNA.jpg)

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Transcription

  • RNA polymerase attaches to the DNA double helix at the start of a gene. The hydrogen bonds between the two strands break and strands separate. The DNA molecule uncoils and some bases are exposed. One strand acts as a template to make an mRNA copy.
  • RNA polymerase lines up the free RNA nucleotides alongside the exposed free bases on the DNA template strand. Complementary base paring happens. mRNA strand ends up being a complementary copy of the DNA template strand. Once the nucleotides have paired up with their specific bases theyre joined together by RNA polymerase and form an mRNA strand.
  • RNA polymerase moves along the DNA assembling the strand and the hydrogen bonds are reformed once RNA polymerase has moved past.When RNA polymerase reaches a stop signal it stops making mRNA and detaches from the DNA. The mRNA moves out the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm.
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Translation

  • Occurs in the cytoplasm at the ribosomes.Amino acids joined together to make a polypeptide chain.
  • mRNA attaches itself to a ribosome and tRNA carries amino acids to it. ATP provides the energy needed for thebond between the amino acid and the tRNA moleucle to form.
  • tRNA molecule with an antiocodon thats complementary to the first codon on the mRNA attaches to the mRNA by complementary base pairing. A second tRNA molecule then attaches to the next codon in the same way.
  • The amino acids attached to the tRNA molecule are joined by a peptide bond. The first tRNA molecule moves away leaving an amino acid behind.This process continues producing a chain of linked amino acids until theres a stop signal eg UAG on the mRNA molecule. The polypeptide chain then moves away from the ribosome.
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Genetic code

  • Sequence of base triplets in mRNA which code for specific amino acids. 
  • Each triplet is read in a sequence seperate from the triplet before and after it, non-overapping.
  • It is also degenerate because a single amino acid may be coded for by more than one codon.
  • Some triplets dont code for amino acids, eg some code for stop signals.
  • The genetic code is universal as the same triplet codes for the same amino acids in all living things
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