Biology — Gene expression

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mRNA

‘messenger’ RNA — role is to produce copy of the genetic base sequence of the template strand of DNA, by transcription, and transport this from the nucleus to the ribosomes

tRNA

‘transfer’ RNA — carries amino acids to the ribosome. tRNA’s anticodon matches with the correct mRNA codon and adds the correct amino acid to the growing polypeptide chain.

Ribosome

A structure that ‘reads’ the mRNA using the tRNA and adds amino acids that correspond to each codon on the mRNA until a stop codon is reached

Polypeptide chain

chain of amino acids produced during translation, which will make a functional protein by folding into a specific shape

Transcription process

1. Transcription occurs in the nucleus and is when a template strand of mRNA is produced from the DNA

2. The DNA unwinds into two single strands

3. Enzyme RNA polymerase then binds to the promoter region of the gene and moves along the DNA, adding complementary RNA bases until a terminating region is reached

4. The complementary mRNA strand then leaves the nucleus through the nucleic pores and enters the cytoplasm to the ribosomes to be translated

Why transcription is required

Transcription must occur before translation as DNA cannot leave the nucleus due to its size, and risk of damage if it were to leave the nucleus. Transcription is therefore required to make the transportable mRNA, so the instructions for making a polypeptide chain can reach the ribosomes to be translated.

Translation process

1. Occurs in the ribosomes, which are located in the cytoplasm

2. mRNA is read by the ribosomes and an amino acid chain is created based on the codons on the mRNA

3. The ribosome moves along the mRNA reading sets of three bases

4. tRNA has sets of three complementary bases to the mRNA codons, called anticodons, which then code for a specific amino acid

5. tRNA then enters the ribosome and drops off its amino acid

6. The next matching tRNA then enters the ribosome and drops off its amino acid

7. These amino acids are joined together by the ribosome by hydrogen bonds to form a polypeptide chain

8. This process continues until the ribosome encounters a stop codon

Translation is essential to protein synthesis as it translates the genetic code on the mRNA into a polypeptide chain.

However, translation is only the second part of protein synthesis, and could not occur without transcription occuring first.

Importance of translation

Translation is required as it is the process wherein the information on the mRNA stand is translated into a protein

Importance of base pairing rules

Bases on the DNA code for a particular protein to be made. For a protein to be functional, the amino acids must be in the correct order in the polypeptide chain. This means they can undergo protein folding in a very specific way to enable the protein shape to be correct in order to be functional. If this shape is altered the protein will not be able to function. CBP ensure the DNA code is copied exactly as DNA triplets are copied into mRNA codons—e.g. triplet=ATG codon=ACG anticodon=UGC amino acid=TYR This is significant because if the code on DNA was not translated exactly, the wrong amino acid could be brought into the polypeptide chain, which would change the folding and eventually the shape of the protein meaning it would be non-functional.

Substitution mutation

One base on the DNA is swapped with another

Addition mutation

One base in DNA is inserted into the DNA

Deletion mutation

One base on DNA is removed from the DNA

Effect of substitution on gene + protein

A substitution mutation is where a single nucleotide in the DNA is swapped—this will change a single triplet of the DNA, where the mutation occurs, and therefore change the corresponding codon on the mRNA produced during transcription and the corresponding tRNA anticodon and amino acid in translation.

This may result in a missense mutation, where the new codon codes for a new amino acid and can affect the function of the protein. If the substitution mutation produces a STOP codon then the result will be a nonsense mutation, where translation will terminate at this point and the polypeptide chain will be shorter. This will result in a non-functioning protein as it cannot fold into its correct shape to carry out its function.

Comparison of substitution and deletion/addition mutation harm

Substitution may form a codon that codes for the same amino acid e.g. amino acid proline has 4 possible codons CCU, CCC, CCA, CCG. If a substitution mutation occurs and produces a codon for the same amino acid, then the resulting polypeptide chain will be able to fold correctly into in bts functional shape and the mutation will be classed as same sense.

When there are multiple codons for one amino acid this is called degeneracy of the genetic code, because some codon codes are not needed. It usually arises from the redundancy of the third base in a codon as can be seen in the example above, the first two bases are the same, only the third base is changed each time. This serves to reduce the effect of mutations on the resulting proteins. If this happens then the right protein shape will form and the individual will not be affected negatively.

In contrast, addition or deletion mutations cause frameshift in the DNA code. This means that every triplet will be different from the point mutation on this changing every codon on the mRNA and every tRNA and then every amino acid in the resulting polypeptide chain. This means that the protein will not fold correctly at all and will not be able to function. This will cause more severe affects on the individual.

Metabolic pathway

An enzyme controlled reaction where the product of one reaction is the substrate for the next

Enzyme

Biological catalyst that converts a specific substrate into a specific product. It has a particular shaped active site that will only bind to a substrate with a complementary shape

Gene

A short section of DNA that codes for a specific protein made, such as an enzyme.

How genes + enzyme control metabolic pathway

Explain metabolic pathway in question—e.g. As can be seen in the example genes A codes for the production of enzyme A which will only catalyse choline into trimethylamine (product). The FMO3 gene then codes for another enzyme, FMO3 enzyme which uses the product of the first reaction as a substrate, trimethylamine and catalyses it into trimethylamine N oxide product

How metabolic pathway is affected by mutation

Answer with context from question—e.g. A mutation in one of the genes, e.g. FMO3 gene will result in a non functioning enzyme, FMO3 enzyme as it will not be the correct shaped active sight which means that the enzyme cannot cannot catalyse the substrate i.e TM into the non odorous product TM-N oxide. This then prevents the pathway from progressing to the end and can result in a metabolic disorder. When this happens, there might be too much of an intermediate product i.e. building up causing the odour in the phenotype