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 then translated into proteins.

• Replication: DNA is copied to produce identical DNA molecules.

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

• Translation: mRNA is translated to synthesize polypeptides (proteins).

Key Takeaway: Each step serves a distinct purpose in gene expression and inheritance.

DNA Replication

Replication Forks and Bubbles

DNA replication begins at specific locations called origins of replication, forming replication bubbles with two replication forks moving in opposite directions. The process involves several key enzymes and the synthesis of new DNA strands.

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

• Lagging Strand: Synthesized discontinuously as Okazaki fragments, later joined together.

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

Hint: Replication forks and bubbles refer to the same concept. Understanding strand directionality is crucial for identifying leading and lagging strands.

Transcription and RNA Processing

Transcription

Transcription is the process by which a segment of DNA is used as a template to synthesize mRNA. The template strand runs 3' to 5' from the promoter region, and transcription factors bind at the promoter to initiate transcription.

• Promoter: DNA sequence where transcription factors and RNA polymerase bind to start transcription.

• Directionality: mRNA is synthesized in the 5' to 3' direction.

RNA Splicing

Before translation, the primary mRNA transcript undergoes splicing to remove non-coding regions (introns) and join coding regions (exons). The mature mRNA, containing only exons, exits the nucleus for translation.

• Exons: Coding sequences that remain in mature mRNA ("exons exit").

• Introns: Non-coding sequences removed during splicing ("introns stay in").

Hint: The names help: exons exit the nucleus, introns stay in.

Translation

Process and Key Components

Translation is the synthesis of proteins from mRNA. It occurs in the ribosome and involves three main sites: A (aminoacyl), P (peptidyl), and E (exit).

• mRNA: Provides the codon sequence for amino acid assembly.

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

• Ribosome: Facilitates the binding of tRNA and the formation of peptide bonds.

• Direction: mRNA is read 5' to 3'.

• E site: Where tRNA is ejected after delivering its amino acid.

Example: The anticodon is found on tRNA, and tRNA is ejected from the E site.

Cell Division: Mitosis and Meiosis

Mitosis

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

• Prophase: Chromosomes condense, 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 that produces gametes (sperm and egg cells) with half the number of chromosomes. It consists of two divisions:

• Meiosis I: Homologous chromosomes separate (includes crossing over and independent assortment).

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

Key Takeaway: Most genetic variation arises during meiosis I, especially through independent assortment and crossing over.

Genetic Variation and Mendelian Genetics

Sources of Genetic Variation

• Mutation: Changes in DNA sequence.

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

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

• Random Fertilization: Random combination of gametes during fertilization.

Meiosis is the primary source of genetic variation in sexually reproducing organisms.

Mendel's Laws

• Law of Independent Assortment: Genes for different traits can segregate independently during the formation of gametes.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete carries only one allele.

Example: Random alignment of homologs in metaphase I (independent assortment); separation of homologs in anaphase I (segregation).

Probability and Test Crosses

Genetic probability problems often involve predicting the likelihood of offspring inheriting certain traits, especially in the context of recessive diseases and test crosses.

• Punnett Squares: Used to predict genotype and phenotype ratios.

• Test Cross: Crossing an individual with a dominant phenotype with a homozygous recessive individual to determine genotype.

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

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 (synthesis).

• G2 phase: Preparation for mitosis.

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

• G0 phase: Resting state where cells exit the cycle.

Hint: M phase is for cell division, G phases are for growth and preparation, and S phase is for DNA replication.

Biomolecule Application in Central Dogma

Key Biomolecules

• Promoter sequences: Composed of DNA (nucleic acids).

• Transcription factors: Proteins that regulate gene expression by binding to DNA.

Summary Table: Mitosis vs. Meiosis

Additional info: For exam preparation, practice interpreting codon tables, identifying DNA and RNA strands, and drawing diagrams of independent assortment and ploidy changes during mitosis and meiosis.