Protein Synthesis – ‘Transcription’

Protein synthesis.
We know our DNA encodes proteins but how exactly are these made from DNA molecules? What proteins or cell processes take part into making a chemical code into a functional protein? What exactly is involved in the process from start to finish?

It starts with RNA polymerase, an enzyme which reads DNA(=deoxyribonucleic acid) and makes RNA(=ribonucleic acid). How it does this is by ripping open a small part of the DNA to create two single strands. RNA polymerase ‘reads’ the DNA sequence and constructs a mirror image. This RNA is made to protect the DNA which is the ‘original copy’ of all the proteins in the cell.

RNA Polymerase holding DNA strands open while making a complementary strand RNA.

RNA polymerase holding DNA strands open while making a complementary strand RNA.

The RNA mirror image created in prokaryotes (=cell without a core) is called pre-mRNA (=messenger RNA). This is processed by other proteins that cut out segments of RNA that do not code for anything, called ‘introns’. With the introns cut out you are left with the remaining ‘exons’. In addition to the ‘intron-splicing’ there are proteins that put a protective ‘cap’ at the ends of the pre-mRNA.  After this processing this is normal mRNA. This is all done in the nucleus and the mRNA is subsequently escorted out into the cytoplasm(=the cell’s internal suspension) or endoplasmic reticulum (ER). In prokaryotes (=cell without a core) mRNA is made in the cytoplasm and does not need processing.

After floating into the cytoplasm or ER, the ribosomes take the mRNA strand and ‘reads’ it. This is done three nucleotides at a time, each of which encode one amino acid. Since there are only 21 amino acids used for human proteins and you have combinations of three out of four different nucleotides (see blogpost “Starter Genetics!”) this will result in 64 combinations, so some combinations must result in the same amino acid. For example all combinations of GUx result in the same amino acid, this means that if there is a change in DNA which replaces the last nucleotide  of GUA into GUU it will result in the same amino acid and it will not affect the protein that is made. This change of nucleotide can be caused by UV-light, carcinogenic chemicals or other external factors.

The sets of three nucleotides that are read by the ribosomes are called a triplet or codon. The ribosomes place the according amino acids in a long chain(=polypeptide, the bindings between amino acids are called peptide bonds). All amino acids can be bound to tRNA (=transfer RNA) molecules which the ribosomes uses. Every tRNA also has a 3-nucleotide sequence (=anti-codon) that is complementary to the mRNA being read. So if the mRNA says GCU only the tRNA with CGA will be able to bind. The chain will always start with methioline (MET) and end with a stop codon (UAA, UAG, UGA). At which point the ribosomes release the chain.

The codon-chart. All possible combinations and their respective 3-letter-coded amino-acid.

The codon-chart. All possible combinations and their respective 3-letter-coded amino-acid.

This chain, a primary polypeptide, is now ready to coil and form sheets or in other words to ‘fold’. This is usually done by chaperon proteins who help shape the chain. Secondary structure refers to stable arrangements of amino acids which takes shapes like helices and sheets. Tertiary structure describes all aspects of the three-dimensional folding of a polypeptide. When a protein needs two or more sub units  the arrangement between multiple tertiary proteins is called the quarternary structure. Proteins can only perform tasks when in the tertiary or quarternary structure. Our red blood cells, for example, which are composed of haemoglobin, are 4 haemoglobine proteins bound together, with an iron atom in the middle of these four. Thus it is a quarternary structure protein.

The four different structures.

The four different structures.

Many mistakes in this protein folding can result in allergies, severe neurological conditions and many more diseases and conditions.

Extra links!

http://upload.wikimedia.org/wikipedia/commons/9/9b/MRNA.svg  – A very informative picture about the making of mRNA

References;

Edublogs – 3.5.2 Transcription – http://kemccalllum.edublogs.org/files/2011/06/dna-transcription1-24v38bf.jpg

DNA, RNA, and Snorks Teacher Guide – DNA, RNA, and Snorks – http://www.biologycorner.com/resources/codon2.gif

Molecular Experssions – Cell biology and microscopy Structure and Function of Cells & Viruses – http://micro.magnet.fsu.edu/cells/ribosomes/images/ribosomesfigure1.jpg

Lehninger Principles of Biochemistry. David L Nelson, Michael M Cox 5th edition. p 92-93

Biology by Campbell, Reece 8th edition. p 330-340

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2 responses to “Protein Synthesis – ‘Transcription’

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