Sunday, 7 June 2015

Transcription as the production of mRNA from DNA. The role of RNA polymerase. The splicing of pre-mRNA to form mRNA in eukaryotic cells. Translation as the production of polypeptides from the sequence of codons carried by mRNA. The role of ribosomes and tRNA.The genetic code as base triplets in mRNA which code for specific amino acids. The genetic code is universal, non-overlapping and degenerate. The structure of molecules of messenger RNA (mRNA) and transfer RNA (tRNA). Candidates should be able to compare the structure and composition of DNA, mRNA and tRNA.

DNA is has the information needed to make proteins in its code, however proteins are made using ribosomes which are in the cytoplasm and DNA is in the nucleus and is too big to get out.

To solve this problem, sections of DNA that are needed to create a protein are translated into mRNA which is single stranded and so can fit through the nuclear pores.

The two strands of DNA are separated by DNA helicase breaking the hydrogen bonds between bases. RNA polymerase then moves along one of the strands, the template strand, attaching complimentary bases (so a Guanine where there is a Thymine and visa versa, a Adenine where there is a Thymine and a Uracil where there is a Adenine). The resulting molecule is called pre-mRNA. This process is called transcription.

This pre-mRNA contains exons (parts of the genetic code) and introns (a bit of the base sequence which doesn't code for anything). The introns are removed (and exons may be rearranged) in a process called splicing to create mRNA (happens in eukaryotic cells). The sequence of bases on mRNA is the genetic code.

mRNA then leaves the nucleus and is attracted to a ribosome.

tRNA is a molecule with one end that is complimentary to a codon on mRNA (the anticodon) and one end that is attached to the amino acid that that part of mRNA codes for.

The ribosome brings together the right piece of tRNA to the mRNA in a process called translation. The first tRNA is attached, then the second: the amino acids they are carrying then become attached to each other using an enzyme and ATP. The third tRNA then does the same thing, as it does the first bit of tRNA detaches from the mRNA but leaves its amino acid attached to the second amino acid. The process continues like this until the ribosome reads the stop codon which makes the ribosome, mRNA and tRNA all detach leaving behind a polypeptide chain.

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