BIOL 243 Theme 2A

Overview of Central Dogma of Molecular Biology

Central Dogma - Eukaryotes vs Prokaryotes vs Viruses

Central dogma of molecular biology: universal information flow from DNA → RNA → protein in order to convert genotype to phenotype

Transcription and translation:

* prokaryotes: occurs in cytoplasm; both processes simultaneous
* eukaryotes: transcription and processing of precursor mRNA molecules in nucleus; translation in cytoplasm
* viruses: retroviruses genetic material in form of RNA. (RNA → DNA → host cell → RNA → proteins)
* Upon entering into the host cells converts RNA to DNA by the enzyme reverse transcriptase enzyme
* Then that DNA moves to the host cell genome to get integrated inside it.
* The integrated viral DNA is again further transcribed to RNA and to proteins to synthesize more viral molecules.

Genetic Code

the instructions contained in the 4 nucleotide bases of a gene that tell a cell how to make a specific protein by providing the amino acid sequence

* part of central dogma because it is involved in translation process: an mRNA sequence is read using the genetic code which then codes for the amino acids in polypeptides

One Gene-One Enzyme Hypothesis

* Garrod studied hereditary disease alkaptonuria; patients inherit a mutated gene coding for a defective enzyme → abnormal cellular metabolism
* Beadle + Tatum hypothesized that genes encode enzymes that function at each step of a biochemical pathway needed to make an essential nutrient
* mutating a gene encoding an enzyme would cause a block in the metabolic pathway and the organism can no longer synthesize the needed nutrient (auxotroph)

Beadle and Tatum Experiment

* Garrord suggested that genes were connected to enzymes
* Beadle and Tatum confirmed Garrod's hypothesis using genetic and biochemical studies of the bread mold *Neurospora*
* Beadle and Tatum identified bread mold mutants that were unable to make specific amino acids. In each one, a mutation had "broken" an enzyme needed to build a certain amino acid.
* Wild-type strain: makes nutrient for itself from minimal medium raw materials
* Mutant strain: only grows if researchers supplied the nutrient; used to discover which specific nutrient mutant needed to grow thus which gene defect it had
* Assembly line process; different enzyme catalyzing each step
* Deduced pathway from precursor to given aa (arginine) and showed which gene encoded the enzyme that carried out each step

Template and Coding Strand

* for every gene, an RNA molecule is only produced (transcription) from one of the DNA strands (template/- sense strand)
* nontemplate/coding/+ sense strand - other DNA strand, has same 5'-3' orientation and sequence as mRNA (except “U" is "T")
* template strand always read from 3’-5' (by the RNA polymerase but it makes mRNA in 5’-3’) \n

Orientation of Molecules

* template 3’-5’ (read in this direction to direct synthesis of RNA in 5’-3’ direction)
* coding 5’-3’ (antiparallel to template)
* transcription of mRNA 5’-3’ (antiparallel to template - transcription occurs off template DNA strand)
* translation N terminal - C terminal

Life cycle of RNA viruses

1. Viral particle enters cell.
2. Viral **reverse** **transcriptase** that entered cell in viral capsid makes double-stranded DNA copy of viral RNA genome.
3. Viral-encoded **integrase** enzyme splices the DNA copy of viral genome into the host DNA. The inserted viral DNA is the provirus. Provirus is dormant and is replicated and passed on as the cell divides. In this state, the virus is hidden from the immune system.
4. When the provirus ceases to be dormant, it is transcribed to produce viral RNA genomes and to produce viral mRNAs, which are translated to produce viral proteins.
5. Viral RNAs and proteins assemble into new infective viral particles, which are released from the cell by budding


1. **Protease** is a protein-based enzyme that breaks the polyprotein into functional proteins before it’s released by budding

Origin of the information system

* likely RNA came first because it can store genetic info (code for amino acids like DNA) and can also catalyze reactions (like an enzyme)
* ribozymes (ribonucleic acid enzymes) can catalyze their own synthesis and cleave RNA molecules (2 structure)
* DNA developed later with advantages over RNA such as more stable and double stranded which allows complementary strand to be used as a template to repair the damaged strand