Genetics: Genes, Genotype and Phenotype

• Genetics is the study of genes and how they carry, express, replicate, and propagate information.
• A gene is a DNA segment encoding a functional product, usually a protein.
• The genotype includes all genes present in an organism.
• An organism's phenotype consists of its physical characteristics, based on gene expression.
• The genome is all of the genetic material in a cell including both coding and non-coding regions.

Role of Genetic Material

• Genetic material stores information for the structural components of a cell.
• Genetic material replicated, allows it to pass information down to the next generation of cells.
• Genetic variation, provides diversity within the population.

Genetic Material

• DNA stores genetic material for all organisms and some viruses
• RNA stores genetic material for some viruses and comprises viroids.
• DNA has the nitrogenous bases adenine, guanine, cytosine, and thymine.
• RNA has the nitrogenous bases adenine, guanine, cytosine, and uracil.
• DNA uses deoxyribose as its pentose sugar
• RNA uses ribose as its pentose sugar.
• DNA has two strands twisted to form a double helix, held together by base pairing.
• RNA only has one strand.

DNA Replication

• Because adenine always pairs with thymine, and cytosine always pairs with guanine, each strand of the double helix serves as a template for the production of its partner.
• DNA is copied by DNA polymerase, always in the 5' to 3' direction.
• DNA replication is initiated by an RNA primer.
• Helicase unwinds double-stranded DNA.
• DNA gyrase removes supercoils as DNA strands unwind.
• Single-stranded binding protein binds to the single DNA strands and prevents them from reattaching.
• Primase synthesizes an RNA primer
• DNA polymerase III polymerizes nucleotides to synthesize DNA and proofreads for mistakes.
• DNA polymerase I removes RNA primers and replaces them with nucleotides.
• DNA ligase links Okazaki fragments together.
• The leading strand is synthesized continuously
• The lagging strand is synthesized discontinuously in Okazaki fragments.
• DNA replication in many bacteria occurs bi-directionally, resulting in two replication forks.

Gene Expression

• Genetic information is used within a cell to produce the proteins needed for the cell to function.
• Transcription, in molecular biology, is the process of making an RNA copy of a gene sequence. This copy, called a messenger RNA (mRNA) molecule, leaves the cell nucleus and enters the cytoplasm, where it directs the synthesis of the protein, which it encodes.
• Translation is the process of making proteins using RNA

Transcription

• RNA is made from a DNA template.
• The process begins when RNA polymerase binds to the promoter sequence.
• Transcription proceeds only in the 5' to 3' direction and stops when the terminator sequence is reached.
• Transcription produces messenger RNA.
• Messenger RNA requires additional processing in eukaryotes to remove introns.

Translation

• Translation is the process of converting the information stored in RNA into proteins.
• Messenger RNA is translated in codons (groups of three nucleotides).
• Translation begins at the start codon, AUG, and ends at a stop codon (UAA, UAG, UGA).
• There are 64 possible codons, but just 20 different amino acids.
• Because of the disparity, the code is described as degenerate: several codons encode the same amino acid.
• There are three types of RNA molecule involved in translation: Messenger RNA, Transfer RNA and Ribosomal RNA.
• Translation is linked to transcription in bacteria.

Regulation

• Genes, which produce protein all the time at a fixed rate, are constitutively expressed.
• Genes, which are expressed only as needed, are either repressible or inducible.
• Genes encoding the enzyme components of a metabolic pathway are often organized together into an operon

Bacterial Gene Organisation

• The structure of the operon consists of the promoter and operator sites, and structural genes that code for the protein.
• The operon is regulated by the product of the regulatory gene.

Inducible gene expression

• In the lac operon system, a repressor protein is always constitutively made and attaches to the promoter region, preventing transcription of the genes needed for lactose.
• If lactose is present, it binds to and inactivates the repressor, allowing transcription to proceed.

Repressible gene expression

• In the tryptophan biosynthesis operon one of the main processes is repressible gene expression. Tryptophan binds to the repressor allowing it to bind with the operator, which in turn prevents the transcription process from the bacterial operon

Genetic Recombination

• Genetic recombination is the process by which genetic material is transferred between cells of the same generation.
• Bacteria reproduce asexually and produce variation by recombination.
• There are 3 principal mechanisms: Transformation, Conjugation and Transduction.

Transformation

• Transformation is when DNA is released from dead bacteria and is taken up by living bacteria.
• Similarity in the DNA sequences is required for the bacteria to integrate into the bacterial chromosome
• The bacterium then passes this new genome onto the next generation
• Transformation results in cells of donor DNA

Conjugation

• Conjugation involves the copying and passing of a plasmid from a donor (F+) to a recipient (F-) cell.
• Conjugative plasmids contain genes encoding the pilus.
• Plasmids often contain genes for antibiotic resistance, resulting in antibiotic resistance cells

Transduction

• Transduction is the transfer of genes between cells by viruses.
• Viruses sometimes take a piece of bacterial genome with it.
• This gets packaged into a virus particle and injected into the next cell the virus infects.

Mutation Rates

• Any change in the normal sequence of nucleotides in a piece of DNA is called a mutation.
• There are different types of mutation, including missense, nonsense, and frame-shift mutations.
• With Missense mutation the change in the nucleotide alters the triplet codon creating a different amino acid to be inserted into the protein. Where as Nonsense mutation alters the triplet codon into a stop codon which truncates the protein. Frame-shift mutation, is caused by insertion or deletion of a nucleotide.
• The frequency at which mutations naturally occur is called the spontaneous mutation rate.
• Agents which increase the mutation rate are called mutagens.
• Examples of mutagens include ultraviolet light, radiation, and some chemicals.
• These are sometimes used to kill microbes and sterilize laboratory and medical equipment.