Outline and explain the process of gene expression, including transcription and translation.

Gene Expression:
Polypeptides are required at different times and in different circumstances in the cell. There are mechanisms that determine when a particular gene is expressed to produce a polypeptide. A gene is fully expressed when its polypeptide is synthesised, converted to a protein and the protein is fully functional.
How is DNA converted into a particular protein? - Transcription and Translation
The central dogma of molecular biology states that genetic information flows from DNA to RNA and this information is then converted into the structure (and therefore function) of a particular protein.
For a protein-coding gene to be expressed, it must first be transcribed. In Transcription, the code in the gene’s DNA is converted into a complementary code in an RNA molecule. The RNA molecule then participates in the second phase of gene expression: translation, the code in the RNA is converted into an amino acid sequence in a protein. Transcription and Translation are the main events of Gene Expression

The process of transcription copies the information of DNA on mRNA.
Transcription initiation occurs at a promoter – a region on the DNA, where RNA polymerase binds. RNA Polymerase attaches to the promoter and begins to unwind the DNA. AT the initiation site the polymerase begins reading the DNA template strand and building a complementary RNA strand from free nucleoside triphosphates.

Note: when bonding, paired as follows
A à U
U à A
G à C
C à G

Also note that in DNA, a T is used, however when converted to RNA a U replaces it.
The RNA strand grows by the addition of these substrate molecules to its three prima end – this being the ELONGATION phase of transcription.
NOTE: During the reaction, a pyrophosphate ion is released and broken down, releasing energy that fuels transcription.
The DNA double helix rewinds as the RNA polymerase moves through. Like the unwinding, the rewinding is an energy requiring process that is accomplished by RNA polymerase and fuelled by the breakdown of pyrophosphate ions.
When RNA polymerase reaches the termination site, the RNA transcript is released from the template, the DNA rewinds completely, and the RNA polymerase dissociates from it. The DNA and RNA polymerase can then participate in other rounds of transcription.
The RNA strand created is called messenger RNA or mRNA.
NOTE: The DNA unwinds and only one side is copied to form mRNA
The genetic information of DNA is carried from the nucleus to the cytoplasm by the mRNA. The mRNA then moves to the ribosomes where protein synthesis occurs. Protein synthesis involves a second, much smaller RNA molecule called transfer RNA or tRNA.

external image transcription.gif

*The above link about transcription offers a step by step explanation, animations and a small quiz to test your knowledge.*


The decoding of genetic instructions occurs through the process of transltion which takes place in the cytoplasm. By the end of this process, the genetic instructions carried in mRNA have been decoded and translated into a protein chain built of amino acids. The coded instruction in the mRNA is unchanged after this process and can be used multiple times. There are three stages in translation –

1. Initiation

The mRNA has a unique 3’ end, called the Poly-A tail. mRNA also contains codons, that will encode for specific amino acids. A methylated cap is found on the 5’ end. Translation initiation begins when the small subunit of the ribosome attaches to the cap, and moves to the initiation site. This is where the tRNA comes in. The tRNA contains an anti-codon which is complementary to the codons making up the mRNA. Attached to the end of the tRNA is the corresponding amino acid.

2. Elongation

Then a large ribosome subunit binds to the small ribosome subunit to create a substrate for the elongation, (of amino acids into a polypeptide), to occur.

One end of the substrate is the Pepdidyl (P) site and the other end is the Aminoacyl (A) site. The first tRNA is found at the P site and a second tRNA enters the A site. The second tRNA is the anti-codon of the second codon found on the mRNA – the two matching pairs of codons and anti-codons line up.

The amino acid found on the end of the first tRNA moves and attaches onto the amino acid found on the end of the second tRNA – like a conga line.

The first tRNA, now with no amino acid on its end, leaves the P site and ribosome substrate. The second tRNA moves into the P site and a third tRNA enters the substrate and replaces the second tRNA at the A site.

The process repeats – now the two amino acids on the end of the second tRNA move and attach onto amino acid on the end of the third tRNA. The second tRNA then exits the P site, the third tRNA moves to the P site and a fourth tRNA enters the ribosome and replaces the third ribosome at the A site.

Peptide bonding is occurring between each amino acid.

3. Termination

This process continues until the necessary polypeptide chain is synthesised and a stop codon ((UAA, UAG or UGA) is encountered at the mRNA. A molecule called the release factor enters the A site and binds with the stop codon.

The polypeptide which was attached to the last tRNA (at the P site) is released.

The protein carries out its required functions in the body while the subunits of ribosomes and mRNA split and are once again available to begin translation.

external image Translation.gif

*The above link is a simple video of how translation occurs, step by step. It can be easily followed with the above explanation.*

external image translation.gif

*The above link about translation offers a step by step explanation, animations and a small quiz to test your knowledge.*



· http://bcs.whfreeman.com/thelifewire/content/chp12/1202001.html

· http://bcs.whfreeman.com/thelifewire/content/chp12/1202003.html

· http://www.youtube.com/watch?v=5bLEDd-PSTQ

· http://edtech.clas.pdx.edu/gene_expression_tutorial/translation.html


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