Central Dogma of Molecular Biology

Overview

The Central Dogma describes the flow of genetic information within a biological system. It explains how DNA is used as a template for its own replication, for transcription into mRNA, and how mRNA is translated into proteins.

• DNA Replication: DNA serves as a template to produce identical DNA molecules.

• Transcription: DNA is transcribed into messenger RNA (mRNA).

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

Key Takeaway: Each step has a distinct purpose: replication for copying DNA, transcription for making RNA, and translation for synthesizing proteins.

Cell Division: Mitosis and Meiosis

Mitosis

Mitosis is the process by which a cell divides to produce two genetically identical daughter cells. It is essential for growth, repair, and asexual reproduction.

• Prophase: Chromosomes condense and become visible; nuclear membrane breaks down; spindle fibers form.

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

• Anaphase: Sister chromatids are pulled apart to opposite poles.

• Telophase: Nuclear membranes reform; chromosomes decondense.

• Cytokinesis: Cytoplasm divides, resulting in two identical cells.

Key Takeaway: Mitosis produces identical cells and does not contribute to genetic variation.

Meiosis

Meiosis is the process by which gametes (sperm and egg) are produced, reducing chromosome number by half and increasing genetic diversity.

• Meiosis I: Separates homologous chromosomes.

  • Prophase I: Homologous chromosomes pair and undergo crossing over; nuclear membrane breaks down.

  • Metaphase I: Homologous pairs align at the equator.

  • Anaphase I: Homologous chromosomes separate; sister chromatids remain together.

  • Telophase I + Cytokinesis: Two cells form, each with half the chromosome number.

• Meiosis II: Separates sister chromatids.

  • Prophase II: Chromosomes condense; spindle forms.

  • Metaphase II: Chromosomes align at the equator.

  • Anaphase II: Sister chromatids separate.

  • Telophase II + Cytokinesis: Four haploid cells are produced.

Key Takeaway: Meiosis generates genetic variation through independent assortment and crossing over.

DNA Replication

Replication Forks and Bubbles

DNA replication occurs at replication forks and bubbles, where the double helix unwinds and new strands are synthesized.

• 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.

  • Primase: Adds RNA primers.

  • Ligase: Joins Okazaki fragments.

Transcription and RNA Processing

Transcription

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

• Promoter Region: Site where transcription factors bind to initiate transcription.

• Template Strand: Runs 3' to 5' from the promoter; used to make mRNA.

RNA Splicing

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

• Exons: Exit the nucleus and are expressed in the protein.

• Introns: Stay in the nucleus and are removed.

Translation

Process of Translation

Translation is the synthesis of proteins from mRNA by ribosomes.

• Direction: mRNA is read from 5' to 3'.

• tRNA: Carries amino acids and contains the anticodon that pairs with mRNA codons.

• Ribosome Sites:

  • A site: tRNA enters with amino acid.

  • P site: Peptide bond forms.

  • E site: tRNA is ejected.

Genetic Variation and Mendelian Genetics

Sources of Genetic Variation

Genetic variation is essential for evolution and is generated by several mechanisms:

• Mutation: Changes in DNA sequence.

• Independent Assortment: Random alignment of homologous chromosomes during metaphase I of meiosis.

• Crossing Over: Exchange of DNA between homologous chromosomes during prophase I.

• Random Fertilization: Random combination of gametes.

Meiosis is the primary source of genetic variation, especially during meiosis I.

Mendel's Laws

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

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

Probability and Test Crosses

• Probability calculations are used to predict the likelihood of offspring inheriting certain traits.

• Punnett Squares: Visual tools for predicting genetic outcomes.

• Test Cross: Used to determine genotype by crossing with a homozygous recessive individual.

Cell Cycle

Phases of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide.

• G1 Phase: Cell growth and preparation for DNA replication.

• S Phase: DNA replication (chromosomes are duplicated).

• G2 Phase: Preparation for mitosis.

• M Phase: Cell division (mitosis and cytokinesis).

• G0 Phase: Resting phase; cells may exit the cycle.

Key Takeaway: Each phase has a specific function, and proper regulation is essential for healthy cell division.

Biomolecule Application in Central Dogma

Key Biomolecules

• Promoter Sequences: Made of DNA (nucleic acids).

• Transcription Factors: Made of proteins.

Summary Table: Mitosis vs. Meiosis

Additional info:

• Practice interpreting codon tables, identifying coding/template/mRNA/tRNA strands, and using Punnett squares for test crosses.

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