The genetic code

An enzyme then finally links each mRNA nucleotides together.

Only one of the exposed DNA strand is replicated in cells during transcription.

The chain that is replicated is called the 'sense' stand and the strand that is not copied is the anti-sense. Yet it is debatable as maybe the "mRNA is actually synthesized using the antisense strand of DNA (3' to 5') as the template"2

It is now known that long sequences of DNA are non-coding, these are called introns. These non-coding sequence are of no use yet they are still replicate during transcription into the mRNA and then removed from the molecule before they are used to construct proteins by the ribosome.

Translation is the transfer between mRNA and to proteins.

A strand of mRNA moves out of the nucleus and into the cytoplasm. The mRNA is smaller than the DNA so it can pass through the nuclear pores.

A ribosome becomes attached to its start codon. Once the ribosome has lactated onto the start codon it continues reading the code moving along the mRNA three bases at a time. This is the process of translation. Appropriate or different, specific tRNA carries corresponding amino acids."1

"The other "end" of the tRNA molecule has an "acceptor" site where the tRNA's specific amino acid will bind."4

Energy is needed to attach amino acids to the tRNA (ATP).

So protein synthesis doesn't rely on the diffusion of amino acids, but does rely on transfer RNA (tRNA).

Each tRNA molecule has three unpaired bases; these as a collective are called an 'anticodon'. The tRNA pairs with the mRNA the same way as before, matching with the codon in the ribosome. The tRNA bases pair with the mRNA bases A=U, C=G, G=C and U=A.

The ribosome then removes the amino acids for the tRNA molecule and bonds it to the others.

As the ribosome move along the mRNA three bases at a time, the ribosome will continue linking the amino acids brought there by the tRNA to form a protein molecule. This process stops when the ribosome reads a stop codon. The protein will have the right amino acid sequence because the code has been read correctly. The genetic code has the ability to create "practically infinite"3 numbers of proteins present in the body with only 20 amino acids. It's all to do with the shape and the shape can be different by length and variation in amino acids.

Once a tRNA has placed the correct amino acid in the ribosome, it becomes free, detaches and will go of and collect another amino acid.

Mutation is one reason that DNA it's self doesn't venture out of the nucleus. "If the DNA were to venture out into the cytoplasm where the ribosome is in order to give the instructions for which proteins were to be made, then it would be more vulnerable to damage from:

Chemicals, UV light , or other agents"2

Mutation itself can have a large impact on us or not it depends on where and what the mutation is.

It is nucleic acids that carry the genetic code. Cells can only produce the protein that they have the correct code for. A code for a particular protein may change and the cell might lose its ability to synthesis the protein. This change is called a mutation. A successful mutation, a mutation that improves functions etc. May result in a small evolutionary step. An unsuccessful mutation may result in defects and other problems.

Over time, changes from the original sequence of letters have built up in our non-coding DNA and have been passed down through the generations. These changes of course have no impact on us as they are in the non-coding DNA. So everyone, except identical twins, has a unique genetic code.

Generally these differences do not make any impact on our health and are called polymorphism (poly means many; morphisms means forms). They are quite common. So while the genetic code may be slightly changed by having a polymorphism, the change has not significantly altered the gene message: the information is still understood by the cell.

However, some changes to the genetic code in genes cause the message to be changed so that it is no longer understood by the cell: the gene is broken or faulty. Changes that make the genes faulty are called mutations. If the message to the cell comes from a faulty gene, the cell will not make the correct copy of the product, will make it in reduced amounts or will not make it at all. Faults in the genes may cause a problem with the development and functioning of different body systems or organs and result in a genetic disorder.

Mutations that may be beneficial are that of immunity to a disease or a useful family trait.

Bibliography

. AS - AQA Biology specification A - A New Introduction to Biology.

2. http://www.genetics.com.au/Genetics2003/FactSheets/2a.asp

3. http://www.mun.ca/biology/scarr/2250_Genetic_Code.htm

4. http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookPROTSYn.html

5. http://www.ncc.gmu.edu/dna/genetic.htm

Elizabeth Morden 12DR 1 The genetic code.