Topic 7.3 Transcription

7.3.1 DNA transcription is carried out HL 7.3.1 DNA transcription is carried out in a 5’→3’ direction Like the DNA polymerases that function in DNA replication, RNA polymerases can add nucleotides only to the 3’ end of a growing polymer. Thus, a RNA molecule elongates in its 5’ →3’ direction The 2 sections involved in transcription have complementary sequences. One sequence is referred as the sense sequences and the other the anti-sense sequence. DNA template has the antisense sequence & mRNA has the same sequence than the sense strand of DNA.

7.3.2 Distinguish between the sense and antisense strands of DNA HL 7.3.2 Distinguish between the sense and antisense strands of DNA The sense strand has the same base sequence as the transcribed mRNA except that the base thymine is replaced by the base Uracil. The anti-sense strand acts as the template for the transcription of mRNA. The RNA nucleotides are polymerised along the sugar phosphate backbone by RNA polymerase. Could you identify the sense and anti-sense strand in this DNA molecule?

7.3.3 DNA transcription in prokaryotes HL 7.3.3 DNA transcription in prokaryotes Specific sequences of nucleotides along the DNA mark where transcription of a gene begins and ends. The DNA sequence where RNA polymerase attaches and initiates transcription is known as the promoter; the sequence that signals the end of transcription is called the terminator. The stretch of DNA that is transcribed into an RNA molecule is called a transcription unit. Molecular biologists refer to the direction of transcription as "downstream" and the other direction as "upstream." These terms are also used to describe the positions of nucleotide sequences within the DNA or RNA. Thus, the promoter sequence in DNA is said to be upstream from the terminator.

The Central Dogma: DNA  RNA  protein DNA  RNA: DNA – genes on antisense (or template) strand of DNA transcribed to an RNA copy (mRNA) -the sense strand (coding strand) has the same sequence as the newly transcribed RNA except with thymine in place of uracil -RNA polymerase must attach to the promotor region (which determines which DNA strand is the antisense strand) -terminator = sequence of nucleotides that signals RNA polymerase to stop & mRNA copy to detach

7.3.3 DNA transcription in prokaryotes HL 7.3.3 DNA transcription in prokaryotes Initiation: After the RNA polymerase binds to the promoter, the DNA strands unwind and the enzyme initiates RNA synthesis at the start point on the template strand. 2. Elongation: The polymerase moves downstream unwinding the DNA and elongates the RNA transcript 5’→3’. In the way of transcription, the DNA strand re-form a double helix. RNA polymerase links ribonucleoside 5'-triphosphates (ATP, GTP, CTP, and UTP, represented generically as NTPs) in an order specified by base pairing with a DNA template

7.3.3 DNA transcription in prokaryotes HL 7.3.3 DNA transcription in prokaryotes The enzyme moves along a DNA strand in the 3‘→5' direction, joining the 5'-phosphate of an incoming NTP to the 3'-OH of the previous residue. Thus, the RNA chain grows 5‘→3' during transcription, just as DNA chains do during replication. 3. Termination: Eventually, the polymerase transcribes a terminator sequence, which signal the end of the transcription unit. Shortly thereafter, the RNA transcript is released and the polymerase detaches from DNA.

7.3.4 Eukaryotic RNA needs the removal of introns to form mature mRNA HL 7.3.4 Eukaryotic RNA needs the removal of introns to form mature mRNA Pre-mRNA has been produced through transcription of the anti-sense strand as described for prokaryotic transcription. (a) The non coding introns are spliced out of the mRNA. The introns are broken down in the nucleus. (b) The remaining mRNA is called mature mRNA and is exported from the nucleus to the cytoplasm for translation into the polypeptide.