DNA Replication, Gene Expression, and Phenotype Factors
DNA Replication Mechanism
DNA replication is the biochemical process that allows the duplication of DNA. For this to happen, the double helix must unwind, exposing the sequence of bases.
Semiconservative DNA Duplication Evidence
- The Watson and Crick model of semiconservative DNA duplication is widely accepted.
- In the nucleus, at a specific region called the origin of replication, the two DNA strands separate.
- Each separated strand serves as a template for synthesizing a new strand.
- The new strand is synthesized as new nucleotides are added one by one, pairing with the nucleotides of the parental strand according to base-pairing rules.
- After a new nucleotide is paired, a phosphodiester bond forms, linking it to the preceding nucleotide in the growing chain.
- The synthesis of the new chain proceeds in a direction antiparallel to the parental strand and is primarily directed by enzymes called DNA polymerases.
- At the end of the process, two DNA molecules, identical to the original parental DNA molecule, are synthesized.
One Gene, One Enzyme Hypothesis
- This hypothesis proposed that each gene contains the genetic information necessary for the synthesis of a specific enzyme.
- It is now understood more broadly: each gene typically encodes the synthesis of a polypeptide chain, many of which function as proteins. Enzymes are a specific category of proteins.
DNA Transcription and Translation Processes
Transcription: During gene transcription, DNA sequences are copied into RNA molecules by an enzyme called RNA polymerase. The resulting mRNA (messenger RNA) transcript is the first step in protein synthesis. DNA transcription can also be referred to as mRNA synthesis.
Translation: This is the process of constructing an amino acid sequence (a polypeptide) using the information encoded in the mRNA molecule. The genetic information carried by mRNA is translated in the cytoplasm by ribosomes (cellular structures composed of ribosomal RNA (rRNA) and proteins), which act as protein synthesis factories.
The Genetic Code Explained
The process of peptide synthesis demonstrates the existence of a genetic code, which dictates the sequence of amino acids in a polypeptide chain or protein based on the nucleotide sequence in mRNA.
Genetic Code Characteristics
- Universal: It is largely the same across nearly all species.
- Degenerate (Redundant): Most amino acids are specified by more than one three-nucleotide sequence (codon). These different codons specifying the same amino acid are called synonymous codons.
- Unambiguous: Each specific codon corresponds to only one specific amino acid (or a stop signal).
- Non-overlapping and Continuous: The code is read in continuous triplets without gaps or overlaps between codons.
Mutations: Definition and Types
A mutation is an alteration in the sequence of nitrogenous bases within DNA. Mutations can occur in somatic (body) cells or in germline cells (gametes); the latter are inheritable.
Codon Definition
A codon is a sequence of three consecutive nucleotides in DNA or RNA that specifies a particular amino acid or signals the termination of translation (stop codon). It is also referred to as a triplet.
Inheritance and Environment Interaction
Wilhelm Johannsen’s Bean Experiment
In 1909, Wilhelm Johannsen conducted experiments using bean seeds. He separated them into groups based on weight (“light” and “heavy”). He planted seeds from each group and allowed them to self-pollinate. His results showed minimal variation within each group but significant differences between the groups. Johannsen inferred that intra-group differences were primarily due to environmental factors, as individuals within a group shared a similar genetic makeup (pure lines). Conversely, the inter-group differences were attributed to variations in their genetic information (genotype).
Potential Phenotype
The potential phenotype represents the full range of possible physical expressions of a genotype if the organism could develop across all possible environmental conditions.
Real Phenotype
The real phenotype is the actual observable characteristic or trait expressed by an individual. It results from the complex interaction between its genotype and the specific environment in which it develops.
Environmental Factors Affecting Phenotype
When individuals with similar genotypes develop under different environmental conditions (e.g., variations in diet, light exposure, temperature), they often express different phenotypes.
Endocrine Factors and Phenotype
Hormonal (endocrine) factors can significantly influence phenotype. Examples include conditions like pituitary dwarfism and gigantism.
Gene Penetrance Explained
Penetrance refers to the proportion (percentage) of individuals in a population carrying a specific genotype who actually express the corresponding phenotype. A gene shows complete penetrance if all individuals with the genotype express the phenotype.
Gene Expressivity Explained
Expressivity describes the degree or intensity to which a particular genotype is expressed as a phenotype in an individual. Even with complete penetrance, the severity or manifestation of the trait can vary among individuals (variable expressivity).