Microbial Genetics

Central Dogma

Chain of events during the expression of a gene (DNA → mRNA → Protein → Function)

Genetics

The study of genes; how genes carry info, how gene info is expressed & how genes are replicated

Chromosomes

Structures containing DNA that carry hereditary info, contain genes

Genes

DNA segments that encode functional products, usually proteins

Genome

All of the genetic info in a cell

Genomics

Sequencing & molecular characterization of genomes

Genotype

The genetic makeup of an organism

Phenotype

The expression of genes (physical traits)

DNA Replication (1)

• DNA forms a double helix

• “Backbone” = deoxyribose-phosphate

• 2 nucleotide strands held by hydrogen bonds between A-T & C-G

• Antiparallel strands (opposite directions)

• Order of the nitrogen-containing bases = organismal genetic instructions

• 2 complementary strands = each strand acting as a synthesis template for a new strand

Helicase

Enzyme responsible for unwinding double-stranded DNA into single strands for replication

Topoisomerase & Gyrase

Enzymes responsible for relaxing supercoiled DNA ahead of replication fork, separates DNA circles @ the end of DNA replication

DNA Ligase

Makes covalent bonds to join DNA strands; Okazaki fragments & segments in excision repair

DNA Replication (2)

• DNA polymerase adds nucleotides to growing DNA strand

• Each DNA strand = 5’ → 3’ direction

• Initiated by an RNA primer

• Leading strand = synthesized continuously

• Lagging strand = synthesized discontinuously, creates Okazaki fragments

• DNA polymerase removes RNA primers → Okazaki fragments joined by DNA polymerase & DNA ligase

DNA Polymerases

Synthesize DNA, proofread & facilitate DNA repair

DNA Replication (3)

DNA Replication (4)

Rules of Replication

• DNA polymerase can only extend pre-existing nucleic acid

• Nucleic acid = always created in 5’ → 3’ direction of strand being made

5’ → 3’

“Five prime” & “three prime” = the carbon #s in the DNA’s sugar backbone → the 5' carbon has a phosphate group attached to it & the 3' carbon a hydroxyl (-OH) group

DNA Replication (5)

• Most bacterial DNA replication = bidirectional (2 replication forks move in opposite directions from a single replication origin)

• Replication = semiconservative (each strand in the DNA double helix acts as a synthesis template for a new/complementary strand)

DNA Transcription

• Ribonucleic acid (RNA = single strand of nucleotides, 5-carbon ribose sugar, contains uracil (U) instead of thymine (T)