Protein Synthesis Also Known As … Decoding the Central Dogma

Why the Central Dogma? The process of protein synthesis is summarized by the central dogma of modern biology. (DNA  RNA  Protein) Why this strategy? –DNA must stay within the nucleus. Why? –The environment of the cytoplasm is very harsh in comparison to the nucleus. You can’t risk the DNA being damaged. –If DNA could exit nucleus to meet with ribosomes, it would need an exiting and reentry method for passing through the nuclear membrane. –How could the DNA make preparations for a second type of protein if it were already preoccupied with a protein already? To solve all of these problems, the process uses an intermediary – RNA – which is capable of passing from the nucleus to the cytoplasm with losing its form or function.

Transcription (DNA  RNA) INITIATION RNA Polymerase will enter the area of the gene and bind to the DNA upstream of the gene. This area upstream of the gene is called the promoter. This is an area that has many A’s and T’s so there are fewer H- bonds to split apart in order to access the gene – energy conservation.

Transcription (DNA  RNA) ELONGATION Once the DNA is opened up the RNA polymerase begins to build the mRNA molecule in a 5' to 3' direction. There is no primer required to get the RNA polymerase to go (unlike DNA replication). The polymerase will only make an mRNA copy of one of the strands – this is the template strand. The other strand is known as the coding strand. The promoter is not transcribed in this process – only the area within the gene.

Transcription (DNA  RNA) TERMINATION RNA polymerase moves down the template strand until the end of the gene is reached. At the end of the gene is a terminator sequence which tells the RNA polymerase to stop and get off the DNA. The mRNA is now free to go while the RNA polymerase may move on to another gene’s promoter and make more mRNA.

Transcription (DNA  RNA)

Post-Transcriptional Mods Once the mRNA is released from the DNA it is modified in order to perform its role in the cytoplasm successfully. A 5' Cap is added to the start of the primary transcript (made 5' to 3'). This “cap” is an inverted, tri-phosphate guanine nucleotide. The 5' cap help protect the mRNA strand from enzymes that would digest it. The 5' cap also aids in the initiation of translation as it can signal the start point for the ribosome. A 3' Poly-A tail is added to the 3' end of the mRNA. The 3' poly-A tail helps the mRNA against degradation by other molecules. Lastly, the non-coding regions of the mRNA – known as the introns – are spliced out of the strand much like an editing room floor of a movie studio. This cutting and removing is carried out by spliceosomes. The remaining, useful parts of the mRNA (called exons) are joined together to make the finished product – a complete and functional mRNA transcript.

Translation (RNA  Pn) INITIATION Translation is performed by the ribosome – the protein builder of the cell. The ribosome consists of two smaller parts – the 60S and the 40S subunits. ( The number refers to the size and the S is for the “sedimentation rate” of the molecule when placed in a centrifuge. ) The ribosome recognizes the 5' cap of the mRNA transcript and begins the process of translation at this end of the mRNA. The ribosome moves along the mRNA transcript in a 5' to 3' direction. The ribosome reads the mRNA in three nucleotide segments at a time – these segments are called codons on the mRNA. The role of the initiator sequence becomes very important so that the codons are read correctly in order to make the protein according to specificity standards.

Translation (RNA  Pn) The Role of Transfer RNA (tRNA) The tRNA molecules deliver the correct amino acid to the ribosome to be bonded together. tRNA is a single stranded molecule of RNA that has an anticodon loop that consists of three nucleotides that are complementary to the mRNA codons. Each tRNA has its own specific anticodon that matches up with a specific amino acid. There are three mRNA codons that do not have a matched tRNA anticodon and so these are called terminator sequences.

Translation (RNA  Pn) ELONGATION The ribosome has two compartments – an A-site (acceptor) and a P-site (peptide). The first tRNA with its amino acid (MET) is brought into the P-Site and the anticodon and codon verify the match/sequence. The next tRNA and its amino acid enter the A-site according to sequence. A peptide bond is formed between the two amino acids by the ribosome. The ribosome then slides one codon length (three nucleotides) down the mRNA strand. The tRNA that was in the P-site is now ejected from the ribosome and the tRNA that was in the A-site is now in the P-site. The amino acids are connected with the peptide bond. The ejected tRNA will let go of its amino acid and can be used again to pick up an amino acid and bring it back to the ribosome if the sequence is required. This continues until the ribosome hits the terminator codon on the mRNA – there is no matching tRNA (or amino acid) for the terminator sequences.

Translation (RNA  Pn) TERMINATION The ribosome hits the stop codon on the mRNA (no matching tRNA or amino acid) and just stops. A protein called a release factor sees the stalled ribosome and helps separate the ribosome and the polypeptide chain. The two subunits of the ribosome will let go. They can be used again. The polypeptide chain will begin to assume its 3-D conformation/shape.

Translation (RNA  Pn)

Polysomes The cell often needs many copies of the protein being made so to maximize product, with minimal energy cost, the mRNA strand may be translated by many ribosomes at the same time in order to get many proteins from one mRNA molecule. The mRNA molecule can’t last forever because of digestive enzymes that will eventually degrade it and break it up so you have to make the most of its brief existence.

Polysomes

The Big Picture DNA  RNA  Pn

That’s All I Got Genetics Fans!!!