Why does Benzene not react with bromine unlike Alkenes?

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In benzene the Pi electrons are delcoised whereas in an alkene the Pi electrons are loated inbetween 2 carbon so it has a higher electon density, this means bromie more attracted to alekenes



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  • takes place in the nucleus
  • DNA double helix unzip/ unwind strands are broken this reaction is catalysed by the enzymes DNA HELICASE
  • The exposed DNA Nucleotides complenary base pairs with free activated RNA nucletioides in the nucleus’s
  • Enzyme RNA POLYMERASE joins RNA nucleotide together in a condensation reaction.
  • m-RNA detaches from the DNA
  • m-RNA Protected is completed to the DNA coding strand
  • m-RNA leaves the nucleus though the pore in the nucleus envelope and attaches to the ribosome in the cytoplasm.

Steps of Transcription

Transcription takes place in steps: initiation, elongation, and termination.

  1. Initiation is the beginning it when the enzymes catalyst RNA polymerase  which makes the DNA to unwind
  2. This helps m-RNA
  3. Elongation is the when you add more nucleotides to the mRNA strand.
  4. Termination is the ending of transcription, its when the rna polymerase meets a STOP CODON.

Nitrogen bases

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  • Purines – 2 double rings
  •   -ADENINE 
  •  – GUANINE
  • Pyrimidines – 1 ring


        – cytosine

  •  Order of bases contains the ‘code’ to make proteins
  • In a DNA molecule it must be made of a purines and a pyrimdines which makes it an exact length g if it was 2 purines it would be too long and if it was 2 pyrimidines it would be too short!

How to calculate the Empirical Formula

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for gcse but very descriptive 🙂


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theres are 2 types Longutiudal and transeves!