Comparison of DNA and RNA
- DNA contains deoxyribose sugar, RNA contains ribose
- DNA contains the base thymine, RNA contains uracil
- DNA is double-stranded, RNA is single-stranded
- Both contain bases cytosine, guanine and adenine
- Both contain a phosphate group
- Both contain pentose sugar
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Comparison of mRNA and tRNA
- mRNA is a single helix linear molecule, tRNA is a clover-leaved shape molecule
- mRNA is chemically unstable, tRNA is chemically more stable
- Both have the same bases
- Both contain the pentose sugar ribose
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- Helicase breaks the hydrogen bonds causing the two DNA strands to seperate
- Free mRNA nucleotides complementary base pair with expose DNA nucleotides (on the template strand)
- RNA polymerase seals the new backbone of mRNA
- When RNA polymerase reaches a 'stop' codon it detaches
- DNA strands rejoin
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- mRNA copies whole section of DNA molecule including non-coding regions
- Introns are removed from the mRNA
- Functional exons are joined together
- Mature mRNA molecules leave nucleus via a nuclear pore
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- Ribosome attaches to mRNA at start codon AUG
- tRNA is activated in cytoplasm by binding to a specific amino acid
- This process requires ATP
- tRNA with complementary anticodon sequence base pairs with mRNA codon
- Ribosome moves along the mRNA bringing together two tRNA's at any one time
- Two amino acids on the tRNA are joined by a peptide bond, this also requires ATP
- tRNA is released from the amino acid and is recycled
- Synthesis continues until the ribosome reaches a 'stop' codon
- Polypeptide chain detaches and is folded into a functional protein
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Effect of gene mutation
- Changes the sequence of DNA
- Changes the sequence of codons
- Changes the sequence of amino acids
- May change the tertiary structure of the protein
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Substitution - Nonsense mutation
- Occurs if base change results in formation of one of the three stop codons that mark the end of a polypeptide chain.
- E.g. If G is replaced by A, the GTC will become ATC. Transcribed to UAG in mRNA.
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Substitution - mis-sense mutation
- When the base change results in a different amino acid being coded for.
- E.g. If C is replaced by G, then GTC becomes GTG.
- Important that final protein is tertiary protein, the replacement of amino acid may not form the same bonds. Therefore, protein may become a different shape and may not function properly.
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Substitution - Silent mutation
- Occurs when substituted base, although different still codes for same amino acid as before.
- Due to degenerate nature of genetic code, in which most amino acids have more than one codon.
- E.g. If third base in replaced by T, then GTC becomes GTT. However, as both DNA triplets code for glutamine, there is no change in polypeptide produced and so mutation has no effect.
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Deletion of bases
- Arises when a nucleotide is lost from the normal DNA sequence.
- Usually the amino acid sequence of a polypeptide is different.
- One deleted nucleotide creates a 'frame-shift'. The gene is read wrong and genetic message is altered.
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Causes of mutation
- Arises spontaneously during DNA replication
- Mutations occur with a set frequency
Basic mutations increased by mutagenic agents. These include:
- High-energy radiation that can disrupt DNA molecule
- Chemicals that alter DNA structure or interfere with transcription
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Role of proto-oncogenes
- Normal role is to stimulate cell division
- Mutation can cause formation of an oncogene
- Can permnanently activate the membrane receptor protein so cell division is switched on even in absence of growth factors
- Or the oncogene may code for a growth factor, which is produced in excessive amounts
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Role of tumour suppressor genes
- Normal role is to inhibit cell division
- Mutation can cause inactivation
- Cell division is no longer inhibited and becomes uncontrollable
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