differences between DNA & RNA
- DNA has bases GCAT, RNA has bases GCAU.
- DNA has two nucleotide strands, RNA has one nucleotide strand.
- DNA has deoxyribose sugar, RNA has ribose sugar.
- DNA has more nucleotides in chain than RNA.
- DNA is helical, RNA is either straight or clover shaped.
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Differences between mRNA & tRNA
- mRNA has no hydrogen bonds but tRNA does.
- mRNA has a bigger nucleotide chain than tRNA.
- mRNA has a straight chain. tRNA is clover shaped.
- mRNA doesn't have a binding site, tRNA has a binding site for an amino acid.
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- DNA unwinds at the point where a gene is to be used.
- Hydrogen bonds break so the strands of DNA seperate, exposing the bases.
- RNA nucleotides complementary base pair with the sense strand.
- A single chain of RNA nucleotides which are complementary to the DNA has been produced.
- RNA polymerase joins the ribose-phosphate backbone to form a molecule of RNA.
- The molecule created is mRNA.
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post transcriptional processing of mRNA
- The molecule of mRNA that has been produced is edited before it leaves the nucleus.
- non-coding intron sequences are removed.
- The coding sequences are then spliced back together.
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- mRNA travels from the nucleus to the cytoplasm.
- mRNA binds to a ribosome.
- two mRNA codons are exposed on the ribosome.
- A tRNA carrying a specific amino acid binds to the mRNA.
- this is due to complementary base pairing.
- A 2nd tRNA-amino acid complex binds to the 2nd codon on the mRNA.
- Condensation reaction takes place between the two amino acids, forming a peptide bond.
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- Ribosome moves along the mRNA to the next codon.
- 1st tRNA goes back to the cytoplasm to pick up another amino acid.
- The process continues until a stop codon is reached.
- The polypeptide terminates here.
- The polypeptide is released into the cytoplasm or the lumen of the E.R.
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The genetic code ...
- Is universal - the same for all organisms.
- Is degenerate - There are at least two possible codons for many amino acids.
- Is non-overlapping - Each base is part of one codon only.
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- A change in the base sequence carried by DNA.
- Mutagenic agents e.g uv radiation, x rays and cigarette smoke trigger mutations.
- Alterations to DNA may have major effects on the shape and function of a protein.
- A mutation may cause no change (because the code is degenerate).
- A change to one triplet ( when substitution takes place).
- A change to several triplets (addiition/deletion causing a frameshift).
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