Molecular Genetics and Cell Division: DNA Structure, Gene Expression, and Meiosis

DNA Structure and Function

Structure of DNA

The structure of DNA is fundamental to its role in heredity and cellular function. DNA is a double helix composed of two antiparallel strands of nucleotides.

• Nucleotides: Each nucleotide consists of a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, cytosine, guanine).

• Bonds: Phosphodiester bonds link nucleotides within a strand; hydrogen bonds connect complementary bases between strands (A-T: 2 bonds, C-G: 3 bonds).

• Antiparallel Orientation: One strand runs 5' to 3', the other 3' to 5'.

• Double Helix: The two strands twist to form a right-handed helix.

Example: The sequence 5'-ATCG-3' pairs with 3'-TAGC-5'.

Bacterial Transformation

Bacterial transformation is the process by which bacteria take up foreign DNA from their environment, leading to genetic change.

• Significance: Demonstrated that DNA is the genetic material (Griffith's and Avery's experiments).

• Application: Used in biotechnology for gene cloning.

DNA vs RNA

• DNA: Double-stranded, deoxyribose sugar, bases A, T, C, G.

• RNA: Single-stranded, ribose sugar, bases A, U, C, G.

• Function: DNA stores genetic information; RNA functions in gene expression (mRNA, tRNA, rRNA).

Prokaryotes vs Eukaryotes

Key Differences

• Prokaryotes: No nucleus, circular DNA, no membrane-bound organelles.

• Eukaryotes: Nucleus present, linear chromosomes, membrane-bound organelles.

• Gene Expression: Transcription and translation are coupled in prokaryotes; separated in eukaryotes.

Central Dogma: Replication, Transcription, Translation

Overview

The central dogma describes the flow of genetic information: DNA → RNA → Protein.

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

• Transcription: DNA is transcribed into RNA.

• Translation: RNA is translated into protein.

Replication

• Purpose: To duplicate the genome for cell division.

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

• Leading vs Lagging Strand: Leading strand synthesized continuously; lagging strand synthesized in Okazaki fragments.

• Requirements: Template DNA, primers, nucleotides, DNA polymerase.

• Location: Nucleus (eukaryotes), cytoplasm (prokaryotes).

• Errors: DNA polymerase proofreads; mistakes can cause mutations.

Equation:

$\text{DNA}_{\text{template}} + \text{dNTPs} \xrightarrow{\text{DNA polymerase}} \text{DNA}_{\text{new}}$

Transcription

• Purpose: To produce RNA from a DNA template.

• Enzyme: RNA polymerase.

• Process: Initiation (promoter recognition), elongation (RNA synthesis), termination (release of RNA).

• Start/Stop Signals: Promoter (start), terminator (stop).

• Location: Nucleus (eukaryotes), cytoplasm (prokaryotes).

• Product: Pre-mRNA (eukaryotes), mRNA (prokaryotes).

Equation:

$\text{DNA} \xrightarrow{\text{RNA polymerase}} \text{RNA}$

mRNA Processing in Eukaryotes

• 5' Capping: Addition of a modified guanine nucleotide to the 5' end.

• Polyadenylation: Addition of a poly-A tail to the 3' end.

• Splicing: Removal of introns, joining of exons.

Translation

• Purpose: To synthesize proteins from mRNA.

• Players: Ribosome, mRNA, tRNA, amino acids.

• Process: Initiation (start codon recognition), elongation (polypeptide synthesis), termination (stop codon).

• Start/Stop Signals: Start codon (AUG), stop codons (UAA, UAG, UGA).

• Location: Cytoplasm (both cell types).

• Product: Polypeptide (protein).

Equation:

$\text{mRNA} \xrightarrow{\text{ribosome}} \text{Protein}$

Applying the Central Dogma

• DNA → DNA: Replication.

• DNA → RNA: Transcription.

• RNA → Protein: Translation.

• Codon Table: Used to translate mRNA codons into amino acids.

Genes and Gene Expression

What is a Gene?

• Definition: A gene is a sequence of DNA that encodes a functional product (RNA or protein).

Gene Expression

• Meaning: The process by which information from a gene is used to synthesize a functional product.

• Importance: Allows cells to produce proteins as needed, enabling cellular function and response to environment.

Mutations and Genetic Variation

Types of Mutations

• Point Mutation: Change in a single nucleotide.

• Insertion/Deletion: Addition or loss of nucleotides.

• Frameshift Mutation: Insertion or deletion not in multiples of three, altering reading frame.

• Most Deleterious: Nonsense and frameshift mutations often have the most severe effects.

Introns vs Exons

• Introns: Non-coding sequences removed during mRNA processing.

• Exons: Coding sequences that remain in mature mRNA.

Gene Expression Regulation

Levels of Control

• Pre-Transcriptional: Chromatin structure (methylation, acetylation), transcription factors, epigenetic modifications.

• Post-Transcriptional (Pre-Translation): mRNA splicing, mRNA lifespan, RNA interference (RNAi).

• Post-Translational: Protein modification (ubiquitin tagging for degradation).

Epigenetic Inheritance

• Definition: Heritable changes in gene expression not due to changes in DNA sequence (e.g., DNA methylation, histone modification).

Chromosomes and Cell Division

Key Terms

• Chromosome: DNA molecule with associated proteins, carrying genetic information.

• Chromatid: One of two identical halves of a duplicated chromosome.

• Homologous Chromosomes: Chromosome pairs with the same genes but possibly different alleles.

• Tetrad: Structure of four chromatids formed during meiosis I by synapsis of homologous chromosomes.

Meiosis vs Mitosis

• Mitosis: Produces two genetically identical diploid somatic cells.

• Meiosis: Produces four genetically diverse haploid gametes.

• Genetic Variation: Meiosis introduces variation via crossing over and independent assortment.

Meiosis: Major Parts

• Meiosis I: Homologous chromosomes separate; crossing over occurs; cells become haploid.

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

• DNA Replication: Occurs before meiosis I, not before meiosis II.

• Tetrads: Formed during prophase I; site of crossing over.

Major Sources of Variation in Meiosis

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

• Independent Assortment: Random orientation of homologous pairs during metaphase I.

• Random Fertilization: Any sperm can fertilize any egg.

Summary Table: Mitosis vs Meiosis

Feature	Mitosis	Meiosis
Number of Divisions	1	2
Number of Cells Produced	2	4
Genetic Identity	Identical	Different
Chromosome Number	Diploid	Haploid
Role	Somatic cell growth/repair	Gamete production

Additional info:

• Telomeres are repetitive DNA sequences at chromosome ends, protecting them from degradation.

• Ubiquitin tags proteins for degradation by the proteasome (post-translational control).

• Acetylation of histones generally increases gene expression; methylation usually represses it.