Properties of DNA Polymerases 

• Enzymes can only use deoxynucleotide 5 prime triphosphates while copying the template.
• Addition of nucleotides to a primer is required.
• Polymerase I has 5 prime to 3 prime and 3 prime to 5 prime exonuclease activity while polymerase III has 3 prime to 5 prime activity.
• Polymerase III is very complex and contains > 10 subunits.

Polymerase II does not play an essential role in replication. 

Basic Elements of DNA Replication

• Replication begins at a specific site called the origin of replication.
• Replication is semiconservative and proceeds bidirectionally.
• A primer is required before each template is replicated.
• Boths strands serve as templates concurrently.
• Synthesis is discontinuous (Okazaki fragments) for the lagging strand and continuous for the leading strand. 

Transcription

• The first step in expression of the genetic material.
• Properties of RNA:
• •Usually single stranded
• •Ribose instead of deoxyribose
• •Uracil substitutes for thymine
• Products of transcription include mRNA, tRNA, and rRNA  

Basic Features of RNA Synthesis

• Precursors in synthesis are the 4 ribonucleoside 5 prime triphosphates, ATP, GTP, CTP and UTP.
• 3 prime OH of one nucleotide reacts with 5 prime triphosphate of second nucleotide.
• Sequence of RNA determined by the sequence of DNA.
• Only 1 DNA strand used at one time.
• RNA chains grow 5 prime to 3 prime.
• Divided into initiation, elongation and termination phases. 

Differences in Prokaryotic and Eukaryotic Transcription

• Eukaryotes contain 3 classes of RNA polymerases
• mRNA is very long lived in eukaryotes.
• Eukaryotic mRNA modified at the 5 prime and 3 prime ends
• Eukaryotic mRNA is monocistronic
• Transcription and translation is coupled in bacteria.

Initiation of Translation in Bacteria

• Requires energy, 3 initiation factors, the 30S and 50S ribosomal subunits, mRNA and tRNA.
• Ribosomes contain over 50 proteins and 3 different RNAs.

Gene Regulation in Prokaryotes 

• Gene Regulation is primarily at the level of transcription in prokaryotic cells although translational control can operate.
• This ensures a rapid response to changes in the environment.
• mRNAs are very unstable
• Transcription and Translation are coupled.
• Genes are grouped into operons

Regulation of Gene Function: Terms

• OPERON -  A unit of DNA that is coordinately regulated and transcribed into a single polycistronic mRNA.
• INDUCIBLE - A substance needs to be added to induce gene expression.
• REPRESSIBLE - A substance needs to be added to repress gene expression.
• NEGATIVE CONTROL - When the regulatory protein is bound to DNA the operon is OFF.
• POSITIVE CONTROL - When the regulatory protein is bound the operon is ON

Example of Control Mechanisms