Central Dogma of Molecular Biology

Overview of Information Flow

The central dogma describes the directional flow of genetic information within a cell, from DNA to RNA to protein. This process is fundamental to all living organisms and underpins gene expression and regulation.

• DNA Replication: DNA serves as a template for its own duplication, ensuring genetic continuity.

• Transcription: DNA is transcribed into messenger RNA (mRNA), which carries genetic instructions from the nucleus to the cytoplasm.

• Translation: mRNA is translated into a polypeptide (protein) by ribosomes.

Key Terms:

• Template strand: The DNA strand used for mRNA synthesis.

• Promoter region: DNA sequence where transcription factors bind to initiate transcription.

DNA Replication

Replication Forks and Bubbles

DNA replication occurs at structures called replication forks and bubbles, where the double helix is unwound and new strands are synthesized. Understanding strand directionality and enzyme function is crucial.

• Leading strand: Synthesized continuously in the 5' to 3' direction.

• Lagging strand: Synthesized discontinuously as Okazaki fragments.

• Key enzymes:

  • DNA polymerase: Synthesizes new DNA strands.

  • Helicase: Unwinds the DNA helix.

  • Primase: Synthesizes RNA primers.

  • Ligase: Joins Okazaki fragments.

Example: The replication fork diagram shows the origin, directionality, and placement of RNA primers and Okazaki fragments.

Transcription and RNA Processing

Transcription

Transcription is the process by which a DNA template strand is used to synthesize mRNA.

• Template strand: Runs 3' to 5' from the promoter region.

• Transcription factors: Proteins that bind to the promoter to initiate transcription.

RNA Splicing

Before translation, eukaryotic mRNA undergoes splicing to remove non-coding regions (introns) and join coding regions (exons).

• Exons: Coding sequences that exit the nucleus as part of mature mRNA.

• Introns: Non-coding sequences that remain in the nucleus.

• Splicing: The process of removing introns and joining exons.

Example: The RNA splicing diagram illustrates the removal of introns and formation of mature mRNA.

Translation

Mechanism of Protein Synthesis

Translation occurs at the ribosome, where mRNA is decoded to synthesize proteins.

• mRNA direction: Moves 5' to 3' through the ribosome.

• tRNA: Carries amino acids and contains the anticodon complementary to mRNA codons.

• Ribosome sites:

  • A site: Entry point for tRNA.

  • P site: Holds tRNA with growing polypeptide.

  • E site: Exit site for tRNA after amino acid transfer.

Example: The translation diagram shows the ribosome, tRNA, and mRNA interaction.

Mitosis and Meiosis

Mitosis

Mitosis is the process of cell division that produces two genetically identical daughter cells.

• Prophase: Chromosomes condense, spindle fibers form, nuclear membrane breaks down.

• Metaphase: Chromosomes align at the cell's equator.

• Anaphase: Sister chromatids are pulled apart.

• Telophase: Nuclear membranes reform, chromosomes decondense.

• Cytokinesis: Cell splits into two.

Meiosis

Meiosis produces gametes with half the chromosome number, increasing genetic variation.

• Meiosis I: Homologous chromosomes pair, cross over, and separate.

• Meiosis II: Sister chromatids separate.

• Key events for variation:

  • Independent assortment: Random alignment of homologs in metaphase I.

  • Crossing over: Exchange of DNA between homologs in prophase I.

Cell Cycle

Phases and Functions

The cell cycle consists of distinct phases that regulate cell growth, DNA replication, and division.

• G1 phase: Cell growth and preparation for DNA synthesis.

• S phase: DNA replication (chromosome duplication).

• G2 phase: Preparation for mitosis.

• M phase: Mitosis and cytokinesis (cell division).

• G0 phase: Resting state (cells not actively dividing).

Example: The cell cycle diagram illustrates the sequence and purpose of each phase.

Mendelian Genetics and Probability

Mendel's Laws

Mendel's principles explain inheritance patterns in sexually reproducing organisms.

• Law of Independent Assortment: Homologous chromosomes align randomly during metaphase I.

• Law of Segregation: Homologous chromosomes separate during anaphase I.

Genetic Variation

Four main sources contribute to genetic diversity:

• Mutation: Changes in DNA sequence.

• Independent assortment: Random chromosome combinations.

• Crossing over: Exchange of DNA between homologs.

• Random fertilization: Random combination of gametes.

Probability and Test Crosses

Probability calculations and test crosses are used to predict genetic outcomes.

• Punnett squares: Visualize allele combinations.

• Pedigree analysis: Track inheritance in families.

• Example: If two carriers have a child with a recessive disease, the probability their next two children will be affected is .

Summary Table: Mitosis vs. Meiosis

Comparison of Cell Division Processes

Additional info:

• Promoter sequences are DNA; transcription factors are proteins.

• Most genetic variation in meiosis arises from events in meiosis I.

• Practice interpreting codon tables, Punnett squares, and ploidy diagrams for exams.