TRANSCRIPTION AND TRANSLATION NOTES!

Section: 6.2 Transcription: Synthesizing RNA from DNA

• Transcription is can be defined as the synthesis of singles stranded RNA from a double stranded DNA template. The DNA is what stores the information needed for reproduction a survival, however, there must be a method for reading this information. To utilize this information, transcription is needed to accurately copy a smal section of DNA into RNA. RNA are simple stranded polynucleotide molecules that contain the four bases of adenine, thymine, guanine and cytosine with alternating phosphate groups and ribose sugars. 

Here's the process of transcription broken down into three stages: Initiation, Elongation, and Termination:

Initiation

• Synthesis of RNA occurs in the nucleus.
• Transcription starts with assembling the transcription machinery and locating the site to begin the transcription process. The strand being transcribed is referred to as the antitsense strand or template strand and the complementary strand that is not being transcribed is known as the sense strand or coding strand. 
• RNA polymerase are the enzymes that catalyzes the synthese of RNA from a DNA template. The RNA polymerase binds to a promoter region on the DNA which signals where the translation will begin. It is signaled by a TATA box on the coding strand of the DNA and this is where the RNA polymerase will bind. 
• Before translation starts, the DNA strand needs to be unwound. 

Elongation

• During the elongation phase, the RNA polymerase will will begin to synthese the RNA that is complementary to the DNA with uracil replacing the thymine in the direction of 5' to 3'.  
• RNA polymerase follow one another to produce many copies of the RNA.
• Since there is no proof reading stage in translation, the process is much faster than that of DNA replication. This is usually because, there is not much risk with mistakes as an error in transcription will only result in a single protein failure so the speed of RNA production weighs heavier then accuracy in this case.  

Termination 

• Find AAAAAATAAAAAAA on the coding strand for termination of transcription 
• Since mRNA is single stranded, it is weaker than DNA. It is more prone to damage so the mRNA that results from transcription is called precursor mRNA. To make it into a mature mRNA, a 5' G-cap and a 3' poly-A-tail are added for it to survive longer in the cytoplasm. 
• Also, by a process called splicing, spliceosomes remove the introns (non-coding regions) so that the RNA is only left with the remaining exons (coding regions). Lastly, the exons are rejoined together. 

Section: 6.3 Translation: Synthesizing Proteins from mRNA

• Translation is the synthesis of protein from an mRNA template. This occurs in the cytoplasm.
• mRNA contains the genetic information that determines the amino acid sequence of a protein while tRNA are responsible for delivering the amino acids to the translation machinery with great accuracy and precision. Each tRNA contains an anticodon that base pairs with a codon on the mRNA and that it how the correct amino acid is coded for. The amino acid is connect to an acceptor stem and the enzyme aminoacyl-tRNA-synthetase will attach the appropriate amino acid to the tRNA

Like transcription, translation can also be categorized into three stages which are Initiation, Elongation, and Termination 

Initiation  

• During the initiation phase of translation, the components of the ribosomal sub units, mRNA and tRNA assemble. The ribosomal sub unit will attach to the start codon (AUG).
• A reading frame will determine how the codons are read. There are three binding sites for tRNAs and they are the peptide site, the amino acid site and the exit site.
•  The first amino acid will be attached to the P-site and the while the A-site awaits the next tRNA carrying the amino acid. The amino acids will form a peptide chain and the tRNA will be uncharged as it no-longer carries any amino acids. 

Elongation

• During the elongation phase, protein synthesize and the chain becomes longer one amino acid at a time. There amino acids are linked by peptide bonds.
• The polypeptide chain will move from the tRNA on the P-site to the tRNA on the A-site. Each time this occurs, the chain is one amino acid longer.
•  tRNAs the exit from the P-site to the E-site. The tRNA from the A-site will then move to the P-site and another tRNA will occupy the A-site. The process will repeat until the chain is complete. 

Termination 

• During termination, a stop codon is reached on the mRNA and the poly peptide and the components that came together in the initiation phase disassembles. 
• The polypeptide is then released to carry out its purpose. 

Gene Expression 

• The expression of a gene refers to the synthesis of a protein that is encoded by that gene. It involves transcription to form mRNA which is used as a template for protein synthesis. Gene expression governs the development of living organisms.