The Genetic Code
- There are 4 nucleotide bases in the DNA molecule.
- There are 20 amino acids synthesised from DNA.
- Only a code composed of 3 bases can incorperate all 20 amino acids.
- Such a code produces 64 combinations of bases.
- A sequence of 3 base pairs is called a codon, and codes for 1 amino acid.
- The triplet code is universal, the codons are the same in all organisms.
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The Genetic Code (cont)
- There are 64 triplet codons and only 20 amino acids.
- This is because the triplet code is degenerate. Each amino acid is coded for by more than 1 codon.
- In most cases, the multiple codons for each amino acid differ by only 1 base.
- This ensures that even if a mutation changes a base on a codon, it can still code for the same amino acid.
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- RNA polymerase unzips DNA helix (breaks H bonds).
- RNA polymerase brings together complimentary bases to one of the DNA strands, except thymine is replaced with uracil.
- These nucleotides are joined by phosphodiester bonds to form messenger RNA.
- mRNA is a copy of half the DNA molecule.
- mRNA is small enough to leave the nucleus via nuclear pores and go to ribisomes in the cytoplasm.
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- Messenger RNA is a single stranded molecule, that contains the pentose sugar ribose, and the pyrimidine base uracil instead of thymine.
- Ribosomal RNA is synthesised in the nucleolus. When joined to certain proteins it becomes a functional ribosome for amino acid synthesis.
- Transfer RNA is functional during translation. Each amino acid has its own tRNA molecule, with the anticodon to the amino acid's codon.
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- mRNA attaches to the small sub-unit of a ribosome.
- The first codon on the mRNA is read by the ribosome, and the tRNA with the complimentary anticodon brings the corresponding amino acid to the ribosome and binds to the ribosome/codon.
- This happens for the sequence of codons on the mRNA.
- Peptide bonds form between synthesised amino acids held next to eachother by the tRNA.
- When a stop codon is read, the synthesised polypeptide chain is released into the cytoplasm.
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- Mutations are changes in the arrangement of bases in an individual gene or in the structure of a chromosome.
- Both change the arrangement of the gene on a chromosome .
- Germ line mutations occur when gametes are formed, and are passed onto sexually produced offspring.
- Somatic mutations occur during mitotic division of somatic cells, and are passed onto daughter cells.
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- A point mutation is the mutation of a single gene.
- One allele becomes another because of small alterations in the sequence of nucleotides.
- Many point mutations consist of the substition of one base for another in the DNA and hence the mRNA.
- A chromosomal mutation involves a change in the position of a gene on a chromosome.
- This occurs during replication when chromosomes break and rejoin in different shapes.
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Mutations and Enzymes
- Point mutations can create a defect on the 3D structure of an enzyme.
- This is because 1 wrong amino acid in the primary structure will change its folding and bonding to form a tertiary structure.
- The active site may be of incorrect shape, and so be comprimised.
- This point mutation may have no effect due to degeneracy of the code.
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A suitable organism in a genetic experiment should:
- be easy and cheap to raise.
- have a short life cycle
- produce large numbers of offspring so that results can be statistically reliable.
- have clear, distinguished characteristics.
- have few ethical concerns.
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