DNA Replication

Key Enzymes in DNA Replication

DNA replication is a fundamental process that ensures genetic information is accurately passed from cell to cell. Several enzymes are involved in this process:

• Helicase: Unwinds the DNA double helix, separating the two strands to provide single-stranded templates for replication.

• DNA Polymerase: Synthesizes new DNA strands by adding nucleotides complementary to the template strand.

• Additional info: Other essential enzymes include primase (synthesizes RNA primers) and ligase (joins Okazaki fragments on the laggi ng strand).

Properties of Life: Genetic Information and the Central Dogma

The Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a biological system:

• DNA stores genetic instructions in its nucleotide sequence.

• Transcription: DNA is transcribed into RNA.

• Translation: RNA is translated into protein.

Gene: A segment of DNA whose nucleotide sequence determines the sequence of nucleotides in mRNA, which in turn determines the sequence of amino acids in a protein.

• Changes in DNA sequence (mutations) can alter protein function.

Gene Expression: The process by which instructions in DNA are converted into a functional product, such as a protein. Different cell types express different genes, leading to cell specialization.

Translation: Initiation, Elongation, and Termination

Initiation of Translation

Translation is the process by which mRNA is decoded to synthesize proteins. It occurs in several steps:

• Step 1: The initiation complex (initiation factors + small ribosomal subunit + tRNA) forms at the mRNA cap and scans for the AUG start codon.

• Step 2: At AUG, the large ribosomal subunit joins. The P site holds the Met-tRNA, and initiation factors are released. A tRNA complementary to the next codon binds to the A site.

• Step 3: A reaction transfers the Met to the amino acid on the tRNA in the A site, forming a peptide bond.

• Step 4 (Elongation): The ribosome shifts one codon to the right.

Termination: Occurs when a stop codon (UAA, UAG, UGA) enters the A site. A release factor binds, breaking the bond between the polypeptide and tRNA, releasing the protein.

Ribosome Sites: The A site is where incoming tRNAs bind; the P site holds the growing polypeptide; the E site is where tRNAs exit.

mRNA Reading: mRNA is read and synthesized in the 5' to 3' direction.

Membrane Protein Processing and Amphipathic Molecules

Secretory Pathway

Membrane and secreted proteins are synthesized on the rough endoplasmic reticulum (RER):

• Proteins surrounded by membranes or inserted into membranes are processed through the ER.

• mRNA is read and synthesized 5' to 3'.

Amphipathic Molecules: Molecules with both hydrophilic (water-loving) and hydrophobic (water-fearing) regions, such as phospholipids in cell membranes.

Cell Cycle and Mitosis

Overview of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. DNA replication is essential for growth and reproduction.

• Genome: One copy of all genetic information; humans inherit one genome from each parent (diploid, 2n).

• Ploidy (n): Number of unique chromosomes; most somatic cells are diploid (2n = 46 chromosomes).

• Homologous chromosomes are the same size and shape, with the same genes in the same locations.

Stages of Mitosis

• Prophase: DNA condenses, spindle forms, centrosomes migrate.

• Prometaphase: Chromosomes move, nuclear envelope breaks down, spindles attach to centromeres.

• Metaphase: Chromosomes align in single file along the metaphase plate.

• Anaphase: Sister chromatids separate and are pulled to opposite poles.

• Telophase: Nuclear envelope re-forms, chromosomes decondense.

• Cytokinesis: Division of cytoplasm, resulting in two cells.

Additional info: Mitosis does not change ploidy; daughter cells remain diploid.

Meiosis and Evolutionary Consequences of Sex

Meiosis and Genetic Diversity

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

• Homologous Recombination (Crossing Over): Exchange of chromatid segments during prophase I creates new gene combinations.

• Independent Assortment: Random orientation of chromosome pairs during metaphase I leads to varied combinations.

• Random Fertilization: Any combination of gametes from parents can combine, resulting in over 70 trillion possible combinations.

• Mutation: Changes in DNA sequence further increase diversity.

Mendelian Genetics

Mendel's Laws

• Law of Segregation: Homologous chromosomes separate during anaphase I, so each gamete receives one allele (A or a).

• Law of Independent Assortment: Genes on different chromosomes assort independently during metaphase I.

• Additional info: Mendel's laws apply only to genes on different chromosomes.

Genotype and Phenotype

• Alleles: Alternative forms of a gene that lead to different traits.

• Genotype: Genetic composition (e.g., AA, Bb, dd).

• Phenotype: Observable trait or expression.

• Dominance: The dominant allele determines the phenotype in a heterozygote.

Types of Point Mutations

Mutation Classifications

Point mutations are changes in a single nucleotide in DNA:

• Missense Mutation: Changes an amino acid in the protein.

• Silent Mutation: Does not change the amino acid sequence due to redundancy in the genetic code.

• Frameshift Mutation: Alters the reading frame, changing all subsequent codons.

• Nonsense Mutation: Changes a codon specifying an amino acid into a stop codon, truncating the protein.

Summary Table: Mitosis vs. Meiosis

Key Equations and Concepts

• Central Dogma:

• Genetic Variation (Independent Assortment):

• possible combinations, where is the number of chromosome pairs