Human Gene Expression
From WikiGenetics
A given gene is considered "expressed" when it is transcribed and turned into a protein product. Different organisms have varying ways of influencing when, how, and how often their DNA is transcribed, but there are a few major themes that most organisms share. There are regulatory regions of DNA that give directions as to how often a gene is to be expressed, and there are also quality control measures to ensure proper amounts of gene products are being made correctly.
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[edit] Transcription
Transcription is the process of encoding a sequence of DNA onto a strand of messenger RNA (mRNA). Transcription takes place in the nucleus of the cell. First a group of enzymes called transcription factors bind to the TATA box which is located in the promoter region of the coding strand. The transcription factors allow another enzyme called RNA polymerase II to bind to the TATA box. ATP is then carried to the RNA polymerase to begin the transcription process. The transcription process is broken down into 3 steps.
Initiation: In this step the RNA polymerase breaks the hydrogen bonds between the base pairs of the two DNA strands unwinds the 2 strands. The strand used to synthesize the RNA is known as the template strand, while the strand the codes for the protein is known as the coding strand. The mRNA strand synthesized is identical to the coding strand only the thymine bases are substituted with uracil bases.
Elongation: In this step the RNA polymerase travels down the anticoding DNA strand attaching new RNA nucleotides to its corresponding DNA nucleotides. The RNA polymerase travels down the DNA strand until it reaches the termination site.
Termination: Transcription ends when the RNA polymerase reaches the termination sequence which acts as a termination signal for the transcription process.
These 3 steps produce a primary transcript consisting of precursor mRNA (pmRNA). The primary transcript still contains intron sequences which need to be removed. The intron sequences are removed in a process known as RNA splicing.
[edit] RNA Splicing
The intron sequences are cut out of the strand by a protein RNA complex called a spliceosome. The spliceosome consists of RNA-protein complexes called small nuclear ribonucleoproteins or snRPS and pmRNA and various other proteins. The spliceosome cuts out the intron sequences and merges the exon sequences together. The human genome contains approximately 25,000 genes but these 25,000 genes code for over 200,000 proteins. This is because RNA can be spliced in different ways to yield different mRNA sequences. This process is known as alternative splicing.
