Protein Synthesis

What are the transcription steps in protein synthesis?

Transcription is the first step of protein synthesis, and it happens inside the nucleus, where our DNA is stored. It describes the stage in which we make pre-messenger RNA (pre-mRNA), which is a short single-strand of RNA complementary to a gene found on our DNA. The term 'complementary' describes the strand as having a sequence that is opposite to the DNA sequence (ie, if the DNA sequence is ATTGAC, the complementary RNA sequence would be UAACUG).

Pre-mRNA applies to eukaryotic cells, as these contain both introns (non-coding regions of DNA) and exons (coding regions). Prokaryotic cells make mRNA directly, as they do not contain introns.

But why do we need to make another polynucleotide when we already have DNA? Simply put, DNA is far too large a molecule! Nuclear pores mediate what comes in and out of the nucleus, and DNA is too large to pass through and reach the ribosomes, which is the next location for protein synthesis. That is why mRNA is made instead, as it is small enough to exit into the cytoplasm.

Read and understand these important points first before reading the steps of transcription. It'll be easier to understand.

• The sense strand, also known as the coding strand, is the DNA strand containing the code for the protein. This runs from 5 'to 3'.
• The antisense strand, also known as the template strand, is the DNA strand that does not contain the code for the protein and is simply complementary to the sense strand. This runs 3 'to 5'.

• The DNA containing your gene unwinds, meaning the hydrogen bonds between the DNA strands are broken. This is catalyzed by DNA helicase.
• Free RNA nucleotides in the nucleus pair with their complementary nucleotides on the template strand, catalysed by RNA polymerase. This enzyme forms phosphodiester bonds between adjacent nucleotides (this bond forms between the phosphate group of one nucleotide and the OH group at the 3 'carbon of another nucleotide). This means the pre-mRNA strand being synthesized contains the same sequence as the sense strand.
• The pre-mRNA detaches once the RNA polymerase reaches a stop codon.

Fig. 1 - A detailed look into RNA transcription

Enzymes involved in transcription

DNA helicase is the enzyme responsible for the early step of unwinding and unzipping. This enzyme catalyses the breaking of the hydrogen bonds found between complementary base pairs and allows the template strand to be exposed for the next enzyme, RNA polymerase.

RNA polymerase travels along the strand and catalyses the formation of phosphodiester bonds between adjacent RNA nucleotides. Adenine pairs with uracil, while cytosine pairs with guanine.

What is mRNA splicing?

Eukaryotic cells contain introns and exons. But we only need the exons, as these are the coding regions. mRNA splicing describes the process of removing introns, so we have an mRNA strand containing just exons. Specialized enzymes called spliceosomes catalyse this process.

Fig. 2 - mRNA splicing

Once splicing is complete, the mRNA can diffuse out from the nuclear pore and towards the ribosome for translation.

What are the translation steps in protein synthesis?

Ribosomes are organelles responsible for the translation of mRNA, a term that describes the 'reading' of the genetic code. These organelles, which are made of ribosomal RNA and proteins, hold the mRNA in place throughout this step. The 'reading' of the mRNA begins when the start codon, AUG, is detected.

First, we'll need to know about transfer RNA (tRNA). These clover-shaped polynucleotides contain two important features:

• An anticodon, which will bind to its complementary codon on the mRNA.
• An attachment site for an amino acid.

Ribosomes can harbour a maximum of two tRNA molecules at a time. Think of tRNAs as the vehicles delivering the correct amino acids to the ribosomes.

Below are the steps for translation:

• The mRNA binds to the small subunit of a ribosome at the start codon, AUG.
• A tRNA with a complementary anticodon, UAC, binds to the mRNA codon, carrying with it the corresponding amino acid, methionine.
• Another tRNA with a complementary anticodon for the next mRNA codon binds. This allows the two amino acids to come close.
• The enzyme, peptidyl transferase, catalyses the formation of a peptide bond between these two amino acids. This uses ATP.
• The ribosome travels along the mRNA and releases the first bound tRNA.
• This process repeats until a stop codon is reached. At this point, the polypeptide will be complete.

Fig. 3 - Ribosome mRNA translation

Translation is a very quick process because up to 50 ribosomes can bind behind the first so that the same polypeptide can be made simultaneously.

Enzymes involved in translation

Translation features one main enzyme, peptidyl transferase, which is a component of the ribosome itself. This important enzyme uses ATP to form a peptide bond between adjacent amino acids. This helps form the polypeptide chain.

What happens after translation?

Now you have a completed polypeptide chain. But we are not done yet. Although these chains can be functional by themselves, the majority undergo further steps to become functional proteins. This includes polypeptides folding into secondary and tertiary structures and Golgi body modifications.