Molecular Genetics

Complementary Base Pairing

Complementary base pairing is a fundamental principle in molecular genetics, describing how nitrogenous bases in DNA and RNA pair with each other through hydrogen bonds.

• DNA base pairs: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).

• RNA base pairs: Adenine (A) pairs with Uracil (U), and Guanine (G) pairs with Cytosine (C).

• Hydrogen bonds: Hold the complementary bases together; A-T pairs have 2 hydrogen bonds, G-C pairs have 3 hydrogen bonds.

• Example: If a DNA strand has the sequence 5'-ATGC-3', the complementary strand will be 3'-TACG-5'.

Nucleotides and Nucleic Acids

Nucleotides are the building blocks of nucleic acids (DNA and RNA). Each nucleotide consists of three components:

• Phosphate group

• Pentose sugar: Deoxyribose in DNA, ribose in RNA

• Nitrogenous base: Adenine (A), Thymine (T), Guanine (G), Cytosine (C) in DNA; Uracil (U) replaces Thymine in RNA

Genetic Code: Codons and Anticodons

The genetic code is composed of codons, which are sequences of three nucleotides that correspond to specific amino acids or stop signals during protein synthesis.

• DNA codons: Triplets of DNA bases (e.g., ATG, CGT)

• RNA codons: Triplets of RNA bases (e.g., AUG, CGU)

• Anticodons: Triplets on tRNA that are complementary to mRNA codons

• Start codon: AUG (codes for Methionine, initiates translation)

• Stop codons: UAA, UAG, UGA (signal termination of translation)

• Example: The mRNA codon AUG pairs with the tRNA anticodon UAC.

DNA Replication, Transcription, and Translation

Genetic information flows from DNA to RNA to protein through three main processes: replication, transcription, and translation.

• DNA replication: The process by which DNA makes a copy of itself. Key enzymes include DNA polymerase, helicase, and ligase.

• RNA transcription: The synthesis of RNA from a DNA template, catalyzed by RNA polymerase.

• Protein translation: The process by which mRNA is decoded to build a polypeptide (protein) at the ribosome.

Key enzymes:

• DNA polymerase: Synthesizes new DNA strands

• Helicase: Unwinds the DNA double helix

• Ligase: Joins Okazaki fragments on the lagging strand

• RNA polymerase: Synthesizes RNA from DNA template

Equations:

• DNA replication:

• Transcription:

• Translation:

Roles of mRNA, rRNA, and tRNA

Three types of RNA play distinct roles in protein synthesis:

• mRNA (messenger RNA): Carries genetic information from DNA to ribosomes for protein synthesis

• rRNA (ribosomal RNA): Forms the core of ribosome structure and catalyzes peptide bond formation

• tRNA (transfer RNA): Brings amino acids to the ribosome and matches them to the mRNA codon via its anticodon

Mutations

Mutations are changes in the DNA sequence that can affect gene function and phenotype.

• Types: Substitution, insertion, deletion, frameshift, duplication, translocation

• Effects: Can be silent, missense, nonsense, or cause genetic disorders

• Example: Sickle cell anemia is caused by a single base substitution in the hemoglobin gene.

Genetics and Inheritance

Key Terms in Genetics

Understanding genetics requires familiarity with several key terms:

• Chromosome: Structure made of DNA and proteins that contains genetic information

• Gene: Segment of DNA that codes for a specific protein or trait

• Allele: Different forms of a gene

• Homozygous: Having two identical alleles for a trait

• Heterozygous: Having two different alleles for a trait

• Genotype: Genetic makeup of an organism

• Phenotype: Observable traits of an organism

Mendelian Genetics

Gregor Mendel's laws describe how traits are inherited through generations.

• 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.

• Punnett Square: A diagram used to predict the outcome of a genetic cross.

• Example: Crossing a heterozygous (Aa) with a homozygous recessive (aa) individual yields a 1:1 ratio of Aa:aa offspring.

Patterns of Inheritance

Traits can be inherited in various patterns beyond simple dominance.

• Incomplete dominance: Heterozygotes show an intermediate phenotype (e.g., pink flowers from red and white parents).

• Codominance: Both alleles are fully expressed (e.g., AB blood type).

• Polygenic inheritance: Traits controlled by multiple genes (e.g., skin color, height).

• X-linked inheritance: Traits carried on the X chromosome; males are more likely to express recessive X-linked traits.

Genetic Disorders and Chromosomal Changes

Genetic disorders can result from mutations or chromosomal abnormalities.

• Inherited genetic disorders: Cystic fibrosis, sickle cell anemia, Tay-Sachs disease (all involve recessive alleles)

• Chromosomal changes: Nondisjunction (failure of chromosomes to separate), deletions, translocations

• Example: Down syndrome is caused by trisomy 21 due to nondisjunction.

Predicting results: Chromosomal changes during meiosis can lead to genetic disorders or altered inheritance patterns.

Application: Using Codon Tables and Predicting Outcomes

Codon tables are used to translate mRNA sequences into amino acids, predicting the sequence of newly formed proteins.

• Given a DNA sequence: Find the complementary strand, transcribe to mRNA, and use the codon table to determine the amino acid sequence.

• Example: DNA: 5'-ATG-GCC-TAA-3' → mRNA: 5'-AUG-GCC-UAA-3' → Protein: Methionine-Alanine-Stop

Summary Table: DNA vs. RNA

Additional info: Some explanations and examples have been expanded for clarity and completeness, including the summary tables and stepwise processes for replication, transcription, and translation.