Transcript 7.3 Transcription (AHL)

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
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7.3.2 Distinguish between the sense and
antisense strands of DNA
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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
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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.
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
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1. 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
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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
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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.