Promoters:
Two Promoter regions:
CAAT box - located 60-80 bases upstream of 5' end of the coding region
TATA (or Hogness) box - located 25 bases upstream
Gene transcription begins when RNA polymerase II attaches at one of these promoter sites in a process facilitated by numerous general transcription factors (In eukaryotes, RNA polymerase II alone is unable to recognize the TATA box).
Enhancers:
In contrast to promoters, enhancers increase the rate of transcription initiation
through protein binding and interactions with transcription factors bound to promoter
sequences.
Enhancers or repressors can be located anywhere upstream, downstream or even within the transcribed gene.
Enzymes
| Enzyme | Function |
|---|---|
| DNA helicase | Unwinds the DNA double helix at the replication fork |
| DNA topoisomerase | Prevents supercoiling during replication Fluoroquinolones inhibit topoisomeras II (DNA gyrase) and topoisomerase IV |
| DNA polymerase III | Only found in prokaryotes. Elongates the leading strand of DNA in a 5'-3' direction The 3'-5' exonuclease activity of DNA polymerase III proofreads the newly synthesised DNA strand |
| DNA polymerase I | Only found in prokaryotes. Removes RNA primers (5' to 3' exonuclease activity) and replaces them with DNA |
| DNA ligase | Seals up the fragments |
Differences between DNA replication occurs in prokaryotes and eukaryotes
| Prokaryotes | Eukaryotes | |
|---|---|---|
| Location of DNA replication | Cytoplasm | Nucleus |
| Origin of replication | One | Multiple |
Multiple origins of replication make eukaryotic DNA synthesis quick and effective despite the large size of the genome compared to that of prokaryotic organisms.
Telomerase
a reverse transcriptase enzyme (RNA-dependent DNA polymerase) that adds TTAGGG repeats to the 3' end of DNA strands at the terminal end of chromosomes, the telomere region.