Protein Synthesis: Transcription Miss Richardson SBI4U

Transcription Purpose: to produce a copy of a segment of genomic DNA Location: Nucleus Stages: Initiation Elongation Termination Transcription

Only one DNA strand is transcribed – the antisense/template strand The other strand – the sense/coding strand has the same sequence as the mRNA (with T instead of U) Initiation

Transcription factors guide the binding of the enzyme RNA polymerase to the promoter region and the helix separates A sequence of approximately 40 bases, upstream from the gene Contains high concentration of adenine and thymine bases Initiation

RNA polymerase is like DNA polymerase but doesn’t need primers It moves along unwinding & unzipping the template DNA strand, and assembles mRNA in the 5’ to 3’ direction Ribonucleotides form H-bonds with complementary bases Phosphodiester bonds form between the ribonucleotides The resulting mRNA is the same as the coding DNA strand (with U replacing T) 100’s of mRNA molecules can be made from one gene at a time Elongation

Elongation

RNA polymerase recognizes the termination sequence and new mRNA strand is released

Eukaryotic mRNA is modified with the aide of enzymes prior to translation 5’ end: 7 methylated guanosine cap the transcript to prevent digestion 3’ end: poly-A tail (200 adenines) added to stabilize molecule Splicing: introns (non-coding regions) are removed by enzymes (spliceosomes) exons (coding regions) are joined to form mature mRNA mRNA Modifications