Genetics Chapter 1

Study Notes

Nucleic acids are large molecules that store and transmit genetic information
Composed of deoxyribonucleotides (DNA) or ribonucleotides (RNA) linked together
Each nucleotide consists of a phosphate group, a sugar (deoxyribose or ribose), and a nitrogenous base (A, G, C, T, or U)
The sugar and phosphate molecules make up the backbone of the DNA or RNA molecule
The nitrogenous bases project inward from the backbone and pair with each other in a complementary manner

Adenine (A) always pairs with Thymine (T) with two hydrogen bonds
Guanine (G) always pairs with Cytosine (C) with three hydrogen bonds
In RNA, Uracil (U) replaces Thymine (T) and pairs with Adenine (A) with two hydrogen bonds

The complete set of genetic information in an organism
Comprises all the genes, regulatory elements, and other sequences that make up the DNA of an organism
The human genome consists of 3 billion base pairs of DNA and is divided into 23 pairs of chromosomes

Typically consist of a single circular chromosome
Found in bacteria and archaea
Can also have plasmids, which are small circular DNA molecules that replicate independently of the chromosome

Divided into multiple linear chromosomes
Found in plants, animals, fungi, and protists
Can also have plasmids and mitochondrial and chloroplast DNA
Eukaryotic chromosomes have telomeres (repetitive DNA sequences) at the ends, centromeres (specialized regions) in the middle, and origins of replication

The process of creating two identical copies of DNA
Occurs before cell division
Initiated at specific regions called origins of replication
Involves the unwinding of the double helix, synthesis of new DNA strands, and proofreading and editing

Mitosis: the process of cell division that results in two daughter cells with the same number of chromosomes as the parent cell
Meiosis: the process of cell division that results in four daughter cells with half the number of chromosomes as the parent cell

The process by which the information encoded in DNA is converted into a functional product (protein or RNA)
Involves transcription (the synthesis of RNA from DNA) and translation (the synthesis of protein from RNA)

The process of creating a complementary RNA copy from a DNA template
Occurs in the nucleus of eukaryotic cells
Involves the binding of RNA polymerase to the DNA template, the initiation of transcription, and the elongation of the RNA transcript

The process of creating a protein from an RNA template
Occurs in the cytoplasm of eukaryotic cells
Involves the binding of ribosomes to the RNA template, the initiation of translation, and the elongation of the protein chain### Translation
In prokaryotes, termination may occur during transcription
Key steps in termination:
- Release factors recognize stop codons and modify the ribosomal subunit to sever the polypeptide from the final tRNA
- The ribosome dissociates into its subunits
Differences in eukaryotic translation:
- First amino acid is methionine rather than formylmethionine
- Ribosomes attached to ER cavity synthesize polypeptides into the ER
- Archaeal translation is more similar to eukaryotes but lacks ER

General regulation:
- Some genes are constantly expressed for essential functions
- Other genes are regulated to conserve energy, producing polypeptides only in response to environmental changes
- Regulation methods include controlling transcription initiation/termination or translation
RNA molecules in regulation:
- MicroRNA (miRNA): binds to mRNA to block translation or cut mRNA, regulating gene expression
- Small interfering RNA (siRNA): double-stranded RNA that binds to and cuts target nucleic acid
Prokaryotic operons:
- Inducible operons: usually inactive, requiring inducers to activate
- Repressible operons: typically active until deactivated by repressors
- Example: lactose (lac) operon in E. coli

Types of mutations:
- Point mutations: single nucleotide changes (substitutions, insertions, deletions)
- Frameshift mutations: insertions or deletions causing shifts in the reading frame
- Gross mutations: inversions, duplications, or transpositions
Effects of point mutations:
- Silent mutations: no change in the amino acid sequence due to code redundancy
- Missense mutations: change in one amino acid, potentially altering protein function
- Nonsense mutations: creation of a stop codon, leading to truncated proteins
Mutagens:
- Radiation: ionizing (X-rays, gamma rays) and nonionizing (UV light)
- Chemical mutagens: nucleotide analogs, nucleotide-altering chemicals, and frameshift mutagens
DNA repair mechanisms:
- Direct repair
- Base-excision repair
- Light repair
- Single-strand repair
- Nucleotide-excision repair
- Mismatch repair

Genetic recombination:
- Exchange of nucleotide sequences between DNA molecules via homologous sequences, resulting in recombinant DNA
Horizontal gene transfer:
- Transformation: uptake of environmental DNA by a recipient cell
- Transduction: DNA transfer via bacteriophages
- Bacterial conjugation: DNA transfer through physical contact between donor and recipient cells

Positive selection: selecting mutants by eliminating wild-type cells
Negative selection: identifying auxotrophic mutants by comparing growth on media with and without specific nutrients
Ames test: screens for mutagens using histidine auxotrophic Salmonella