7.3.1 DNA transcription is carried out in a 5’→3’ direction

HL

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 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.

7.3.2 Distinguish between the sense and antisense strands of DNA

Could you identify the sense and anti-sense strand in this DNA molecule?

HL

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.

7.3.3 DNA transcription in prokaryotes HL

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 withthymine 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

1. Initiation: After the RNA polymerase binds to the promoter, the DNA strands unwindand 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

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.

7.3.3 DNA transcription in prokaryotes HL

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.

HL7.3.4 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.