Overview

-       Terminology

-       Flow of genetic information

-       Structure of DNA and DNA synthesis

-       Protein production

-       Transcription and translation

-       Regulation

-       Genetic changes

-       Mutation

-       Recombination

-       Transformation

-       Conjugation

-       Transduction

-       Plasmids

-       Transposons

 

Terminology

-       Genetics: The study of what genes are, how they carry information, how information is expressed, and how genes are replicated

-       Gene: A segment of DNA that encodes a functional product, usually a protein

-       Chromosome: Structure containing DNA that physically carries hereditary information; the chromosomes contain the genes

-       Genome: All the genetic information in a cell

-       Genomics: The molecular study of genomes

-       Genotype: The genes of an organism

-       Phenotype: Expression of the genes

 

DNA Structure

-       Polymer of nucleotides: Adenine, thymine, cytosine, and guanine

-       Double helix associated with proteins

-       "Backbone" is deoxyribose-phosphate

-       Strands are held together by hydrogen bonds between AT and CG

-       Strands are antiparallel

 

DNA Synthesis

-       DNA is copied by DNA polymerase

-       In the 5' -> 3' direction

-       Initiated by an RNA primer

-       Leading strand is synthesized continuously

-       Lagging strand is synthesized discontinuously

-       Okazaki fragments

-       RNA primers are removed and Okazaki fragments joined by a DNA polymerase and DNA ligase

 

Transcription

-       DNA is transcribed to make RNA (mRNA, tRNA, and rRNA)

-       Transcription begins when RNA polymerase binds to the promoter sequence

-       Transcription proceeds in the 5' -> 3' direction

-       Transcription stops when it reaches the

terminator sequence

 

Translation

-       mRNA is translated in codons (three nucleotides)

-       Translation of mRNA begins at the start codon: AUG

-       Translation ends at nonsense codons: UAA, UAG, UGA

 

The Genetic Code

-       64 sense codons on mRNA encode the 20 amino acids

-       The genetic code is degenerate (or, redundant)

-       tRNA carries the complementary anticodon

 

Regulation

-       Constitutive genes are expressed at a fixed rate (expression is always “on”)

-       Other genes are expressed only as needed

-       Repressible genes must be turned “off” (i.e., they are normally induced, or “on”)

-       Inducible genes must be turned “on” (i.e., they are normally repressed, or “off”)

-       Catabolite repression inhibits expression (e.g., lac gene inhibition is related to inactive catabolite activator protein [CAP]; if cAMP is not present to activate CAP, which then stimulates the lac operon, lac expression is repressed)

Mutation

-       A change in the genetic material

-       Mutations may be neutral, beneficial, or harmful

-       Mutagen: Agent that causes mutations

-       Spontaneous mutations: Occur in the absence of a mutagen (as a result of sloppy DNA replication or repair)

-       Base substitution (point mutation)

-       Change in one base

-       Missense mutation

-       Result in change in amino acid

-       Nonsense mutation

-       Results in a nonsense codon, usually a stop codon in the middle of an mRNA

-       Frameshift mutation

-       Insertion or deletion of one or more nucleotide pairs, causing all downstream nucleotides to “shift” into new codon reading frames

 

The Frequency of Mutation

-       Spontaneous mutation rate = 1 in 109 replicated base pairs or 1 in 106 replicated genes

-       Mutagens increase to 10–5 or 10–3 per replicated gene

 

Radiation

-       Ionizing radiation (X rays and gamma rays) causes the formation of ions that can react with nucleotides and the deoxyribose-phosphate backbone

 

Radiation

-       UV radiation causes thymine dimers

 

Repair

-       Photolyases separate thymine dimers

-       Nucleotide excision repair

 

Selection

-       Positive (direct) selection detects mutant cells because they grow or appear different (e.g., penicillin resistant cell appears on penicillin-infused medium)

-       Negative (indirect) selection detects mutant cells because they do not grow

-       Replica plating

 

Genetic Recombination

-       Vertical gene transfer: Occurs during reproduction between generations of cells.

-       Horizontal gene transfer: The transfer of genes between cells of the same generation.

-       Exchange of genes between two DNA molecules

-       Crossing over occurs when two chromosomes break and rejoin

 

Plasmids

-       Conjugative plasmid: Carries genes for sex pili and transfer of the plasmid

-       Dissimilation plasmids: Encode enzymes for catabolism of unusual compounds

-       R factors: Encode antibiotic resistance

 

Transposons

-       Segments of DNA that can move from one region of DNA to another

-       Contain insertion sequences for cutting and resealing DNA (transposase)

-       Complex transposons carry other genes

 

Genes and Evolution

-       Mutations and recombination provide diversity

-       Fittest organisms for an environment are selected by natural selection