Gene Expression

The vast majority of genes are expressed in the proteins that they encode. This occurs in two main steps: Transcription of DNA into RNA and then Translation of RNA into polypeptide chains that fold up into proteins.

Transcription

Initiation: The segment of DNA to be transcribed is split apart by RNA polymerase. RNA polymerase needs to first attach to a promoter code sequence, which is a sort of indicator sequence that shows the polymerase where to start transcription and provides a secure foothold

Note: Proteins called activators and repressor regulate the transcription speed

Promoter Clearance: After forming the first RNA bonds, the polymerase must clear the promoter and make the RNA transcript long enough to not slip before elongation can occur

Elongation: RNA polymerase extends the RNA strand by matching RNA nucleotides with complementary DNA nucleotides. RNA nucleotides are the same as DNA nucleotides, except that Uracil (U) is used to pair with Adenine (A) instead of Thymine (T). RNA polymerase also forms the RNA sugar-phosphate backbone.

Termination: The hydrogen bonds between the RNA and DNA nucleotides break, allowing the single stranded RNA molecule to break free.

RNA Processing: Initial mRNA gets processed into mature mRNA

Transportation: The mRNA exits through the nucleus into the cytoplasm of the cell
RNA processing

Immediately after transcription, the resulting mRNA molecule is not ready yet to be used in translation to create a protein. It has exons, which actually code for the protein, and introns, which do not code for protein and need to be removed.

But first, the mRNA needs to be "capped" to protect the ends of the strand from degradation.

On the 5' end: This end is capped with a 7-methylguanosine
On the 3' end: A string of about 200 adenines is added to create a poly(A) tail

Before the mRNA is ready, it must first get rid of its introns, which do not actually code for the amino acids used to construct a protein.

Introns excised - Introns are removed from the RNA strand by a large protein complex called spliceosome, which includes enzymes and nuclear RNA strands that can recognized the appropriate splice sites

Exons spliced together - The remaining exons are joined together.

Important: Initial mRNA strands can be spliced in many different ways so that the finished mRNA codes for different proteins. Thus, a limited amount of DNA can be used to create a vast number of different proteins.

The result of these processes is a matured mRNA strand that can then be used in translation.

Translation

The final step in gene expression following RNA processing.

Ribosomes - Complexes of proteins and RNA with a large subunit and a small subunit

The small subunit binds to the mRNA strand and the large subunit binds to he tRNA that carries amino acids into the chain

Steps involved:

Activation: Amino acids are connected to the corresponding tRNA molecules by ester bonds. The tRNA molecules will then carry the amino acids into the ribosome to be connected to each other. Though not technically a step in translation, it is necessary for translation to occur.

Initiation: A ribosome binds to the start codon (AUG) of an mRNA strand recognized by the initiator tRNA at the 5' end. Translation goes in the 5' to 3' direction of the mRNA strand.

Elongation: The initial tRNA slides from the A site to the P site and a new tRNA carrying a new amino acid enters the A site. The amino acids carried by the two tRNA molecules attach to each other. The ester bond between the tRNA and amino acid in the P site break, and the tRNA in the P site detaches from the ribosome as the tRNA holding the amino acid chain in the A site slides over to the P site. A new tRNa carrying the next amino acids enters the A site and the process repeats.

Termination: The A site of the ribosome gets to a stop codon (UAG, UGA, UAA), activating a release factor that disassembles the ribosome complex


Article Source: Luke T Liu