nature of the genetic code and protein synthesis

nature of the genetic code:

• universal

  • all living organisms use the same DNA

• non - overlapping

  • each single base is only used in one codon

• degenerate

  • a single amino acid may be coded for by more than one codon

the degenerate code:

• if one base = one amino acid, possible amino acids = 4

• if 2 bases = one amino acid, possible amino acids = 16

• if 3 bases = one amino acid, possible amino acid = 64

but only 20 amino acids exist. therefore, the triplet code is degenerate, which means that each amino acid is coded for by more than one triplet e.g. GCA and GGC might but code for the same amino acid

What is a mutation?

• a change that occurs in our DNA sequence

• occur by:

  • mistakes when the DNA is copied

  • the result of environmental factors e.g. UV light, cigarette smoke

CTGGATGCGTAATGC

CTGAATGCGTAGTGC

• inversions

  • this is when part of the DNA code is reversed

  • Again, the DNA sequence can become meaningless

CTGATGCGTAATGC

CTGGATGCGTAATGC

• frame shift mutation

  • when one or more bases are inserted or deleted

  • this type of mutation can make the DNA meaningless and often results in a shortened protein

CTGGATGCGTAATGC

CTGGACGCGTAATGC

• point mutation

  • where one base is replaced by another

  • these can be:

    • silent = same aa

    • nonsense = stop codon

    • mis-sense = different aa

transcription and translation:

• transcription is where the DNA code is copied onto mRNA in the nucleus

  • DNA unwinds

  • DNA helicase separates the H- bonds between strands

  • Complementary mRNA nucleotides H - bond to the DNA

  • RNA polymerase forms phosphodiester bonds between mRNA nucleotides

  • messenger RNA (mRNA) complementary to the DNA template ( antisense) strand is made

• the mRNA separates from the DNA

• the mRNA leaves the nucleus through nuclear pores

• the DNA strands come together and base pair ( unless its used again)

• translation is where mRNA is used to join the correct sequence of amino acids to build a new protein

  • mRNA attaches to ribosomes ( attached to the rER)

  • tRNAs in the cytoplasm carries specific amino acids

  • tRNA triplet of bases ( anticodon) complementary base pairs with a mRNA triplet ( codon) - triplet code

  • the first codon is always a start codon that codes for methionine

  • peptide bonds between the amino acids are made in the ribosome using a condensation reaction

  • the ribosome moves along the mRNA until it reaches a STOP codon

transcription:

• DNA double helix unzips as

• hydrogen bonds between complementary bases break

• and the two polynucleotide strands separate

• one strand acts as a template ( the anti - sense strand)

• free RNA nucleotides complementary base pair to the exposed bases on this strand by forming hydrogen bonds

• RNA polymerase forms phosphodiester bonds between nucleotides

• the mRNA detaches

• the two DNA strands join together by complementary base pairing

• the DNA molecules winds back up into a helix

Translation: ( cytoplasm)

• mRNA attaches to ribosomes ( attached to rER)

• tRNAs in the cytoplasm carries specific amino acids

• tRNA triplet of bases ( anticodon) complementary base pairs with mRNA triplet ( codon) - triplet code

• the first codon is always a start codon that code for methionine

• peptide bonds between the amino acids are made in the ribosome using a condensation reaction

• the ribosome moves along the mRNA until it reaches a STOP codon