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 Takeaway: Each step is essential for the accurate expression of genetic information.

DNA Replication

Replication Forks and Bubbles

DNA replication occurs at specific sites called replication forks and bubbles, where the double helix is unwound and new strands are synthesized. Understanding strand directionality and the roles of key enzymes is crucial.

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

• Lagging Strand: Synthesized discontinuously as Okazaki fragments, also in the 5' to 3' direction.

• Key Enzymes: DNA polymerase (synthesizes new DNA), primase (lays down RNA primers), ligase (joins Okazaki fragments).

Replication forks and bubbles refer to the same concept: regions where DNA is actively being copied.

RNA Splicing

Processing of Pre-mRNA

Before translation, eukaryotic pre-mRNA undergoes splicing to remove non-coding regions (introns) and join coding regions (exons). This process produces mature mRNA that exits the nucleus for translation.

• Exons: Coding sequences that are retained and expressed.

• Introns: Non-coding sequences that are removed during splicing.

• Splicing: The process by which introns are excised and exons are joined together.

Key Point: Exons "exit" the nucleus; introns "stay in".

Transcription

Initiation and Directionality

Transcription begins at the promoter region of DNA, where transcription factors bind and recruit RNA polymerase. The template strand runs 3' to 5' from the promoter, and mRNA is synthesized in the 5' to 3' direction.

• Promoter: DNA sequence where transcription starts.

• Template Strand: The DNA strand used to synthesize mRNA; runs 3' to 5'.

• Coding Strand: The DNA strand with the same sequence as mRNA (except T/U).

Key Point: Transcription factors are proteins; promoter sequences are DNA.

Translation

Ribosome Function and Directionality

Translation is the process by which ribosomes synthesize proteins using mRNA as a template. The ribosome reads mRNA from 5' to 3', and tRNAs bring amino acids to the ribosome.

• A Site: Entry site for tRNA carrying an amino acid.

• P Site: Holds the tRNA with the growing polypeptide chain.

• E Site: Exit site where tRNA is released after donating its amino acid.

• Anticodon: Found on tRNA; pairs with mRNA codon.

Key Point: tRNA is ejected from the E site; anticodon is on tRNA.

Mitosis

Phases of Cell Division

Mitosis is the process by which a cell divides to produce two genetically identical daughter cells. It consists of several distinct phases:

1. Prophase: Chromosomes condense; nuclear membrane breaks down; spindle fibers form.

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

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

4. Telophase: Nuclear membranes reform; chromosomes decondense.

5. Cytokinesis: Cell splits into two identical cells.

Key Takeaway: Do not confuse mitosis with meiosis; synthesis of DNA is not part of mitosis.

Meiosis

Generation of Genetic Diversity

Meiosis is a specialized cell division that produces gametes with half the chromosome number of the original cell. It consists of two rounds:

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

• Meiosis II: Sister chromatids separate, similar to mitosis.

Key Point: Most genetic variation arises during meiosis I (independent assortment and crossing over).

Genetic Variation

Sources and Mechanisms

Genetic variation is essential for evolution and adaptation. Four main sources include:

• Mutation: Changes in DNA sequence.

• Independent Assortment: Random distribution of chromosomes during meiosis I.

• Crossing Over: Exchange of DNA between homologous chromosomes.

• Random Fertilization: Random combination of gametes.

Meiosis is key for genetic variation.

Mendel's Laws

Principles of Inheritance

Mendel's laws explain how traits are inherited:

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

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

Cell Cycle

Phases and Functions

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (G1, S, G2) and the M phase (mitosis and cytokinesis).

• 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.

Probability and Test Crosses

Genetic Probability Calculations

Probability calculations are used to predict the likelihood of genetic outcomes, such as the chance of offspring inheriting a recessive disease.

• Punnett Squares: Visual tools for predicting genotype and phenotype ratios.

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

• Pedigree Analysis: Used to track inheritance patterns across generations.

Example: If two carriers (heterozygotes) have a child, the probability that the next two children will be affected by a recessive disease is .

Biomolecule Application in Central Dogma

Roles of DNA and Proteins

• Promoter Sequences: Composed of DNA; serve as binding sites for transcription factors.

• Transcription Factors: Proteins that regulate gene expression by binding to promoter regions.

Summary Table: Mitosis vs. Meiosis

Comparison of Cell Division Types

Additional info:

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

• Drawing diagrams of independent assortment and ploidy changes in mitosis vs. meiosis can reinforce understanding.