The main goal of transcription is to make mRNA so that it could go into the second step of protein synthesis. In transcription RNA polymerase is in charge of separating the two DNA strands that link the nucleotides. The only difference between DNA polymerase and RNA polymerase is that instead of adding a thymine whenever an adenine is present, RNA polymerase adds a uracil.
The first step of transcription is initiation. During this process the promoter--which consists of nucleotide pairs known as the TATA Box since most of its nucleotide pairings are thymine and adenine--binds to the RNA polymerase, determining the side of DNA that will become the template to make RNA. Transcription Initiation Complex starts initiation allowing the separation of DNA's nucleotides. This process only happens when a certain gene needs to be made.
As the DNA begins to unwind, the RNA polymerase starts to move down the DNA strand, adding the matching nucleotides to create the mRNA that will contain the code to make the protein. The RNA polymerase moves down the DNA strand from the 3' to the 5' end. As the polymerase is moving down its path, the DNA strands behind it begin to wind back together. The addition of nucleotides move until transcription termination occurs.
Just like transcription starts with a certain set of nucleotides, termination happens when the RNA polymerase comes across a certain pattern of base pairs that tell the polymerase to stop enlongating the mRNA strand.
In eukaryotic cells, pre mRNA is formed at the end of transcription meaning that it has to under go some changes before the strand can be sent to the ribosomes. Two modifications that take place are changing the beginning and the end of the mRNA. The beginning is changed by adding a 5' guanine cap in order to protect the RNA and serve as a place for ribosomes to attach to. The other one is adding a Poly-A-Tail in order to protect the mRNA and transfer it to the ribosomes.
Another type of modifying the mRNA is by removing sections of pre-mRNA called introns. This is done by a group of small nuclear RNA proteins (snRNPs) and sliceosomes cutting out the introns from the pre-mRNA. If the introns are not removed, the protein would not be able to function because it would have more stuff that it needs.
Translation is the second step to make a protein. During this process the mRNA is arranged into groups of three called codons. These codons translate into certain amino acids necessary for building the correct protein. Every mRNA strand has a start and stop codon that signal where to start and stop building a protein.
Initiation is where the 5' cap attaches to a ribosome and the start codon sends a signal for translation to start. This combines mRNA, tRNA--"bridges that connect codons to their amino acids--and ribosomal subunits.
- Amino acid chain gets longer since a hydrogen bond forms between the codon and anticodon
- The existing amino acid chain bond to the amino acids of the tRNA
- The polypeptide chain shifts by one amino acid, leaving a new codon to be read
Termination during transcription is similar to termination during transcription. The translation of codons continues until a stop is read, causing the end of translation. At the very end a release factor adds water to the end of the polypeptide in order to stop translation.
A mutation is caused by a change in DNA so when the mRNA is copying it, the protein being coded for might not be the one that should be produced. The two main types of mutations are point mutations--substitutions-- and frameshift mutations. A substitution mutation happens when a nucleotide replaces another in a place where it doesn't belong. An example of this could be when a G in a basepair of GC is replaced by an A so the basepair becomes AG. This type of mutation can have no effect, can code for a different protein, or it could create a stop codon where it isn't supposed to. The second type of mutation is a frameshift or a deletion/addition. In the case of a deletion, one of the nucleotides is removed, causing the way the chain is read to be altered. This type of mutation can cause serious problems since it produces a protein that does not function correctly.