Thread by @Biotech2k1

#Transcription

The activation and transcription of genes.

1/ The DNA of the cell serves as a master blue print. Every cell of our body has a complete copy of this
DNA blueprint. Depending on the cell and its functions, it will use specific parts of the blueprint found in the DNA to produce the proteins and enzymes needed by that cell.

2/ The rule is that each gene creates
one fully functional protein or enzyme.

3/ The gene itself has a few key regions that are important to be familiar with. The very first nucleotide to be copied is the start of the actual gene. The part that in copied is called the open reading frame.

4/ The gene includes all the nucleotides that make up the actual messenger RNA. About 50 to 100 nucleotides before the start site will be the
promoter region. The most basic part of the promoter is called the TATA box. Its a sequence of
Thymine and Adenine nucleotides.

5/ This is the most basic promoter region and allows for a low level of gene expression. The transcription factors will bind to the promoter region and increase the level of gene expression. That means more copies will be made of that gene.

6/ When transcription begins, the RNA polymerase will bind to the promoter and begin copying the gene.

7/ There is actually a big complex of proteins that must bind to the promoter region before the RNA polymerase. This is called the initiation complex. It starts with the TATA binding protein (TBP) and builds this complex that recruits RNA polymerase.

8/ There are other regulatory segments in the DNA that can influence gene expression. They are called
regulatory elements and are located thousands of nucleotides before the promoter.

9/ They can bind transcription factors called Repressors or Enhancers. The DNA will fold in a loop which allows the regulatory sequence to come in contact with the promoter region.

10/ This will allow the Repressor to block binding of the RNA polymerase. The Enhancer can also make contact with the RNA polymerase, but it will increase the activity of the RNA polymerase leading to increased gene expression.

11/ Once the transcription factors have bound to the promoter region, the RNA polymerase can bind to the
transcription factors and begin copying the gene.

12/ The RNA polymerase opens up the DNA itself. It
doesn't need any assistance like the DNA polymerase did. It will begin copying the DNA using the 3' to
5' strand of DNA as a template and create a RNA strand in the 5' to 3' direction.

13/ It begins with the first start nucleotide and continues until it reaches the end point. Then the polymerase will fall off and the RNA strand will be free to move into post transcriptional modification.

14/ The initial strand of the RNA is called the primary transcript. It will undergo three modifications before
it can leave the nucleus of the cell. The first is it will undergo splicing by the spliceosome complex.

15/ The vast majority of the gene is made up of non coding information called introns. The actual coding part of the gene is called the exons. The spliceosome will follow splicing marks on the primary RNA and
remove all the introns and paste the exons back together.

16/ This also leads to the ability of alternative
splicing by cells to use a gene differently. One cell might use Exons 1, 2, 4, and 5 while another might
use Exons 1, 3, 4 and 5. That is the concept of alternative splicing.

17/ Two different proteins can be made from the same gene, but by different splicing of the exons.

18/ After the splicing of the RNA is complete, there are enzymes that will add a Guanine cap to the 5' end
of the RNA. This serves to allow the RNA to exit the nucleus and assists in loading the RNA into the
Ribosome.

19/ The last process is the addition of the poly A tail. This just adds about 250 or more Adenine
nucleotides to the end of the RNA. This signifies the end of the RNA when its read by the Ribosome.

20/ The final RNA product is now called a messenger RNA (mRNA). Its built to take the DNA information
and deliver it to the Ribosome for production of a protein.