Study Guide: Molecular Genetics, DNA Replication, Gene Expression, and Cell Division

Discovery and Structure of Genetic Material

Historical Experiments and the Identification of DNA as Genetic Material

The search for the hereditary material led to a series of landmark experiments in the 20th century. Early hypotheses considered both proteins and DNA as candidates, but evidence gradually established DNA as the molecule of inheritance.

• Phoebus Levene discovered that DNA is composed of four units (nucleotides), each containing:

  • Phosphate group

  • Deoxyribose sugar

  • One of four nitrogenous bases: adenine (A), cytosine (C), guanine (G), thymine (T)

• Griffith's Transformation Experiment demonstrated that a 'transforming principle' could transfer genetic information between bacteria.

• Avery, McCarty, and MacLeod identified DNA as the 'transforming principle' by showing that only DNA could transfer genetic traits between bacteria.

• Hershey-Chase Experiment used radioactive labeling to show that DNA, not protein, is the hereditary material in viruses.

Example: The Hershey-Chase experiment used bacteriophages labeled with radioactive phosphorus (for DNA) and sulfur (for protein) to infect bacteria, demonstrating that only DNA entered the cells and directed viral replication.

Structure of DNA

The structure of DNA was elucidated through the combined efforts of several scientists, most notably Rosalind Franklin, James Watson, and Francis Crick.

• DNA is a double helix composed of two antiparallel strands.

• Base pairing follows Chargaff's rules: %A = %T and %C = %G.

• Hydrogen bonds connect complementary bases (A-T, C-G).

Example: Franklin's X-ray crystallography images provided critical evidence for the helical structure of DNA.

DNA Replication

Meselson-Stahl Experiment

This experiment demonstrated that DNA replication is semi-conservative, meaning each new DNA molecule consists of one old and one new strand.

• Used isotopes of nitrogen (N-15 and N-14) to label DNA and distinguish old from new strands.

• Tested three models: conservative, semi-conservative, and dispersive.

Semi-Conservative DNA Replication

DNA replication involves a series of enzymes and steps to ensure accurate copying of genetic material.

• Prokaryotes have a single circular genome and one origin of replication.

• Eukaryotes have multiple linear chromosomes and multiple origins of replication.

• Key enzymes and proteins:

  • Helicase: Unwinds the DNA double helix.

  • Topoisomerase: Relieves supercoiling ahead of the replication fork.

  • Single-strand DNA binding proteins: Stabilize unwound DNA.

  • Primase: Synthesizes RNA primers.

  • DNA polymerase III: Main enzyme for DNA synthesis (prokaryotes).

  • DNA polymerase I: Removes RNA primers and fills in gaps.

  • Ligase: Seals nicks in the sugar-phosphate backbone.

• Replication is continuous on the leading strand and discontinuous on the lagging strand, forming Okazaki fragments.

Equation:

$\text{DNA polymerase}: 5' \rightarrow 3'$ synthesis$

Telomeres and Polymerase Chain Reaction (PCR)

• Telomeres are repetitive sequences at the ends of eukaryotic chromosomes, protecting them from degradation.

• PCR is a laboratory technique to amplify DNA, involving three steps:

  • Denature

  • Anneal

  • Extension

Central Dogma of Biology

Overview

The central dogma describes the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein.

• Transcription: DNA to RNA

• Translation: RNA to protein

Transcription

• RNA is synthesized by RNA polymerase using the DNA template strand.

• Transcription factors and promoters regulate gene expression.

RNA Processing (Eukaryotes)

• Splicing: Removal of introns and joining of exons by the spliceosome.

• 5' cap: Added to the 5' end for stability and ribosome binding.

• Poly-A tail: Series of adenine nucleotides added to the 3' end for stability and export from the nucleus.

Translation

• Occurs on ribosomes, which are made of protein and rRNA.

• tRNAs bring amino acids to the ribosome, matching codons in mRNA with anticodons in tRNA.

• Three sites in the ribosome: A (aminoacyl), P (peptidyl), E (exit).

• Mutations can affect the protein sequence:

  • Missense: changes one amino acid

  • Silent: no change in amino acid

  • Nonsense: introduces a stop codon

  • Frameshift: insertion or deletion alters reading frame

Gene Expression and Regulation

Prokaryotic Gene Regulation

• Genes are organized into operons (e.g., lac operon).

• RNA polymerase binds to the promoter to initiate transcription.

Eukaryotic Gene Regulation

• Promoters and transcription factors regulate gene expression.

• Chromatin structure (euchromatin vs. heterochromatin) affects accessibility.

• Epigenetic modifications (DNA methylation, histone acetylation) can alter gene expression without changing DNA sequence.

• Alternative splicing allows for multiple proteins from one gene.

• MicroRNAs (miRNAs) and RNA-binding proteins can regulate mRNA stability and translation.

Cell Cycle, Mitosis, and Meiosis

Cell Cycle

• Consists of G1, S, G2, and M phases.

• Checkpoints ensure proper DNA replication and division.

Mitosis

• Division of a nucleus into two genetically identical daughter nuclei.

• Phases: prophase, metaphase, anaphase, telophase, cytokinesis.

• Key structures: centromere, spindle fibers, chromatids, chromosomes, kinetochores.

Meiosis

• Produces haploid gametes (sperm and egg) from diploid cells.

• Involves two divisions: meiosis I and meiosis II.

• Homologous chromosomes pair and undergo crossing over in prophase I.

• Independent assortment and recombination increase genetic diversity.

Table: Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of daughter cells	2	4
Genetic composition	Identical	Unique
Chromosome number	Diploid	Haploid
Function	Growth, repair	Gamete production

Mendelian Genetics

Basic Principles

• Mendel's First Crosses: Monohybrid, dihybrid, and test crosses.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes for different traits assort independently during gamete formation.

• Dominant vs. recessive traits: Dominant alleles mask recessive alleles in heterozygotes.

Punnett Squares and Pedigree Analysis

• Punnett squares predict offspring genotypes and phenotypes.

• Pedigree analysis tracks inheritance patterns in families.

Non-Mendelian Genetics

• Codominance: Both alleles are expressed (e.g., AB blood type).

• Incomplete dominance: Heterozygote phenotype is intermediate (e.g., snapdragon flower color).

• Polygenic inheritance: Multiple genes influence a trait (e.g., skin color, height).

• Sex-linked traits: Traits associated with genes on sex chromosomes (e.g., X-linked traits).

Example: In a dihybrid cross (AaBb x AaBb), the phenotypic ratio is typically 9:3:3:1 if genes assort independently.

Additional info: This study guide covers foundational concepts in molecular genetics, gene expression, and cell division, which are essential for understanding heredity and variation in living organisms.