Transcription

Transcription is the process by which single mRNA is coded from double stranded DNA. This process is highly regulated and controlled to ensure the right amount of a specific gene is coded for at a specific time.

Proteins are synthesised in the cytosol, however, DNA does not leave the nucleus, therefore a copy of the gene coding for the desired protein is sent as a messenger to the cytosol from the nucleus. This is called messenger RNA 'mRNA', which is a single stranded molecule that is a complementary copy of the DNA strand it was synthesised from.

DNA is double stranded as apposed to mRNA which is single stranded, therefore only one strand of DNA is copied. The copied strand is called the 'template strand', the other strand is called the 'non-template strand'. mRNA is synthesised by the enzyme 'RNA polymerase', however, in order for the RNA to synthesise mRNA it must bind to a single strand of DNA. The DNA must be unwound and unzipped, which is done via an enzyme called 'DNA helicase', which unwinds and unzipps the double stranded DNA at the loci of the gene to be transcribed, causing an area of single stranded DNA to be accessible to the RNA polymerase.

RNA polymerase must recognise and bind to a region upstream of the gene being transcribed called the 'promotor region'. This region is a sequnce of bases that determines the strength of the binding of RNA polymerase to the DNA strand and therefore determining the efficiency of trancription of the gene it is accossiated with. If the promotor is a strong promotor, then RNA polymerase binds strongly to the DNA strand. If the promotor is a weak promotor, then the RNA polymerase can become hindered and can even unbind from the DNA strand. The promotor region strength is determined by how promotor sequence compares to other promotors on separate genes. When different promotors are compared, a sequence of bases can be determined that are most common in all the promotor sequences of that type, this is called a 'consensus sequence'. The closer the promotor sequence is to the consensus sequence, the stronger the promotor and the stronger the binding of the RNA polymerase.

Sigma factors

RNA polymerase cannot bind to the promoter region unless a sigma factor is present. Sigma factors ensure that the RNA polymerase binds to the correct promoter region, this is another method in which transcription is regulated. The sigma factor binds to the RNA polymerase via specific binding sites on its structure and forms a ‘holoenzyme’.

Initiation
Once the sigma factor has bound to the RNA polymerase, the RNA can bind to the promoter region upstream of the gene on the single stranded DNA. The RNA is then free to transcribe the gene. Free ribose nucleotides bind to the DNA sequence via complimentary base pairing. Instead of the base Thymine found in DNA, the base Uracil is used in RNA. The RNA polymerase joins the nucleotides together via strong covalent phosphodiester bonds, this forms the single strand of mRNA. This process is called initiation.