Information Flow in Cells

Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a biological system. It explains how DNA is transcribed into RNA and then translated into protein, which determines an organism's phenotype.

• DNA: The molecule that stores genetic information.

• Transcription: The process by which DNA is copied into messenger RNA (mRNA).

• Translation: The process by which mRNA is decoded to synthesize proteins.

• Protein: The functional product that determines traits.

• Gene Expression: The process by which information from a gene is used to synthesize a functional gene product (protein).

Transcription and Translation

Transcription occurs in the nucleus, where DNA is used as a template to produce mRNA. Translation occurs in the cytoplasm at the ribosome, where mRNA is read in codons (three-nucleotide sequences) to assemble amino acids into a polypeptide chain.

• Codon: A sequence of three nucleotides on mRNA that codes for a specific amino acid.

• tRNA: Transfer RNA brings amino acids to the ribosome during translation.

• Polypeptide: A chain of amino acids that folds into a protein.

Mutations and Their Effects

Types of Mutations

Mutations are changes in the DNA sequence. They can occur spontaneously during DNA replication or be induced by environmental factors. Mutations are the ultimate source of genetic variation.

• Point Mutation: A change in a single nucleotide.

• Missense Mutation: Changes one amino acid in the protein.

• Silent Mutation: Does not change the amino acid sequence due to redundancy in the genetic code.

• Frameshift Mutation: Shifts the reading frame, altering all subsequent codons.

• Nonsense Mutation: Changes a codon to a stop codon, truncating the protein.

Impact of Mutations

Mutations can have different effects on an organism's fitness:

• Deleterious: Decreases fitness.

• Neutral: No effect on fitness.

• Beneficial: Increases fitness.

Most point mutations are neutral, some are deleterious, and a few are beneficial.

Mutation Example: Cystic Fibrosis

Some mutations can cause diseases. For example, 70% of cystic fibrosis cases are due to the deletion of a phenylalanine codon, resulting in a nonfunctional protein that is not inserted into the membrane.

Alleles and Genetic Variation

Alleles and Their Origin

Alleles are alternative forms of a gene that arise by mutation. They can differ by as little as one nucleotide.

• Crossing Over, Independent Assortment, and Random Fertilization shuffle alleles during sexual reproduction.

• Mutation is the ultimate source of new alleles.

Allele Example: β-globin Gene

The β-globin gene has multiple alleles (A, S, C) that result in different proteins and phenotypes. Some alleles confer resistance to malaria.

Gene Expression and Phenotype

From Genotype to Phenotype

The genotype (genetic makeup) determines the phenotype (observable traits) through gene expression. A single nucleotide change can result in a different amino acid, altering protein function and phenotype.

• Example: A missense mutation in the Mc1r gene changes coat color in mice.

Mendelian Genetics

Mendel’s Laws of Inheritance

Mendel’s laws describe how traits are inherited:

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes for different traits assort independently during gamete formation.

Genotypic and Phenotypic Ratios

When two heterozygous individuals (Mm x Mm) mate, the expected genotypic ratio is 1:2:1 (MM:Mm:mm), and the phenotypic ratio is 3:1 if M is dominant.

• Dominant Allele: Expressed in the phenotype when present.

• Recessive Allele: Expressed only when two copies are present.

Pedigree Analysis

Pedigrees are diagrams that show the inheritance of traits across generations. They help determine genotypes based on observed phenotypes.

Translation and Codon Tables

Using the Codon Table

The mRNA codon table is used to determine which amino acid corresponds to each codon. RNA is read from 5' to 3'.

Allele Comparison Example: Mc1r Gene

Comparing two alleles of the Mc1r gene shows how a single nucleotide change can alter the amino acid sequence and phenotype.

Properties of Life and Information Flow

Fundamental Properties

Living organisms exhibit several fundamental properties:

• Self-replication

• Information storage and processing

• Membrane delimitation

• Energy storage and usage

• Control and response to environment

• Evolution (heritable change over time)

Hydrophobic and Hydrophilic Amino Acids

Amino Acid Classification

Amino acids can be classified based on their chemical properties, which affect protein structure and function.

• Hydrophobic: Nonpolar side chains (e.g., alanine, valine, leucine)

• Hydrophilic: Polar or charged side chains (e.g., serine, lysine, glutamic acid)

• Special: Unique properties (e.g., glycine, proline, cysteine)

Summary Table: Types of Mutations and Their Effects

Additional info: These notes integrate concepts from DNA replication, transcription, translation, mutations, alleles, Mendelian genetics, and pedigree analysis, providing a comprehensive overview for General Biology students.