Central Dogma of Molecular Biology

Overview of the Central Dogma

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.

• 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 Point: Each step has a distinct purpose: replication for inheritance, transcription for gene expression, and translation for protein synthesis.

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 separated and pulled to opposite poles.

• Telophase: Nuclear membranes reform; chromosomes decondense.

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

Key Point: Mitosis produces genetically identical cells; do not confuse with meiosis.

Meiosis

Meiosis is the process by which gametes (sperm and egg) are produced, resulting in cells with half the chromosome number of the original cell. It is crucial for sexual reproduction and genetic diversity.

• Meiosis I: Homologous chromosomes pair and may undergo crossing over (exchange of genetic material). Chromosome pairs are separated, but sister chromatids remain together.

• Meiosis II: Sister chromatids are separated, similar to mitosis, resulting in four haploid cells.

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

DNA Replication

Replication Forks and Bubbles

DNA replication occurs at replication forks, where the double helix is unwound 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 DNA), primase (adds RNA primers), ligase (joins Okazaki fragments).

Key Point: Understanding strand directionality and enzyme functions is essential for identifying replication mechanisms.

Transcription and RNA Processing

Transcription

Transcription is the process by which a segment of DNA is copied into mRNA.

• Template Strand: The DNA strand running 3' to 5' from the promoter is used as the template.

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

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

RNA Splicing

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

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

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

Key Point: Only exons are present in mature mRNA; introns are removed during splicing.

Translation

Mechanism of Translation

Translation is the process by which ribosomes synthesize proteins using mRNA as a template.

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

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

• Ribosome Sites: A site (aminoacyl), P site (peptidyl), E site (exit). tRNA is ejected from the E site.

Key Point: The anticodon is found on tRNA; translation occurs in the ribosome.

Genetics: Mendel's Laws and Genetic Variation

Mendel's Laws

Mendel's laws explain the inheritance of traits:

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

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

Sources of Genetic Variation

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

• Mutation: Changes in DNA sequence.

• Independent Assortment: Random chromosome combinations during meiosis I.

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

• Random Fertilization: Random combination of gametes.

Key Point: Meiosis, especially meiosis I, is the primary source of genetic variation.

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 Point: S phase is for DNA replication; M phase is for cell division.

Genetic Probability and Test Crosses

Probability in Genetics

Probability calculations are used to predict the likelihood of offspring inheriting certain traits, especially in cases involving recessive diseases.

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

• Punnett Squares: Visual representation of genetic crosses and probabilities.

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

Summary Table: Mitosis vs Meiosis

Comparison of Cell Division Processes

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 can help visualize concepts.