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 to create proteins, which are essential for cellular function.

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

• Transcription: DNA is used as a template to synthesize messenger RNA (mRNA).

• Translation: mRNA is used as a template to assemble amino acids into a polypeptide (protein).

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

Cell Division: Mitosis and Meiosis

Mitosis

Mitosis is the process by which a eukaryotic 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: New nuclear membranes form; chromosomes decondense.

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

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

Meiosis

Meiosis is a two-part cell division process that produces gametes (sperm and eggs) with half the number of chromosomes of the original cell. It is crucial for sexual reproduction and genetic diversity.

Meiosis I (Separation of Homologous Chromosomes)

• Prophase I: Homologous chromosomes pair up and exchange segments (crossing over); nuclear membrane breaks down.

• Metaphase I: Homologous pairs align at the cell's equator.

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

• Telophase I & Cytokinesis: Two cells form, each with half the original chromosome number (still duplicated).

Meiosis II (Separation of 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 result, each genetically unique.

Key Point: Most genetic variation in meiosis arises from events in meiosis I, especially independent assortment and crossing over.

DNA Replication and Replication Forks

Replication Fork Structure and Function

During DNA replication, the double helix unwinds to form a replication fork. Each strand serves as a template for the synthesis of a new complementary strand.

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

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

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

Hint: Replication forks and bubbles refer to the same concept; know how to identify leading and lagging strands based on directionality.

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, pre-mRNA 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").

Translation

Process of Translation

Translation is the process by which ribosomes synthesize proteins using mRNA as a template. Transfer RNA (tRNA) molecules bring amino acids to the ribosome, matching their anticodons to mRNA codons.

• Direction: mRNA is read 5' to 3'.

• Sites: A (aminoacyl), P (peptidyl), and E (exit) sites on the ribosome.

• tRNA: Contains the anticodon and brings the correct amino acid.

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

Genetics: Mendelian Laws and Genetic Variation

Mendel's Laws

• Law of Independent Assortment: Homologous chromosomes align randomly during metaphase I of meiosis, leading to genetic variation.

• Law of Segregation: Homologous chromosomes separate during anaphase I, ensuring each gamete receives only one allele of each gene.

Sources of Genetic Variation

• Mutation: Changes in DNA sequence.

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

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

• Random Fertilization: Random combination of gametes during fertilization.

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

Cell Cycle

Phases of the Cell Cycle

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 (chromosome duplication).

• G2 phase: Preparation for mitosis.

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

• G0 phase: Resting state where cells exit the cycle and do not divide.

Hint: Know the purpose of each phase, especially that S phase is for DNA replication and M phase is for cell division.

Practice and Application

Genetic Crosses and Probability

• Practice Punnett squares and test crosses to determine the probability of offspring genotypes and phenotypes.

• Understand how to interpret codon tables and identify coding, template, mRNA, and tRNA strands.

• Be able to draw and interpret diagrams showing independent assortment and ploidy changes during mitosis and meiosis.

Example: If two heterozygous (carrier) parents have a child with a recessive disease, the probability that their next two children will also be affected is .

Summary Table: Key Differences Between Mitosis and Meiosis