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

Structure of DNA

DNA Composition and Bonds

The structure of DNA is fundamental to its function as the genetic material in all living organisms.

• DNA (Deoxyribonucleic Acid) is a double helix composed of two antiparallel strands of nucleotides.

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

• Phosphodiester bonds link nucleotides within a strand (between the 3' OH of one sugar and the 5' phosphate of the next).

• Hydrogen bonds hold the two strands together: A pairs with T (2 H-bonds), C pairs with G (3 H-bonds).

Example: The complementary base pairing ensures accurate DNA replication.

Bacterial Transformation

Discovery of DNA as Genetic Material

• Transformation is the process by which bacteria take up foreign DNA from their environment.

• Griffith's experiment (1928) and Avery, MacLeod, and McCarty (1944) demonstrated that DNA is the hereditary material.

DNA vs RNA

Key Differences

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

• RNA: Single-stranded, ribose sugar, bases A, U, C, G (uracil replaces thymine).

• RNA is more versatile (mRNA, tRNA, rRNA) and less stable than DNA.

Prokaryotes vs Eukaryotes

Cellular Differences

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

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

Replication, Transcription, and Translation

Overview of Central Dogma

• Replication: DNA → DNA (copying genetic material).

• Transcription: DNA → RNA (synthesis of RNA from DNA template).

• Translation: RNA → Protein (synthesis of polypeptides from mRNA).

Purpose: To accurately transmit genetic information and express genes as functional proteins.

Replication: Leading vs Lagging Strand

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

• Lagging strand: Synthesized discontinuously as Okazaki fragments, later joined by DNA ligase.

Key Enzymes in Replication

• Helicase: Unwinds DNA helix.

• Primase: Synthesizes RNA primers.

• DNA Polymerase: Adds nucleotides to the growing DNA strand; requires a primer and template.

• Ligase: Joins Okazaki fragments.

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

Transcription

• RNA Polymerase: Synthesizes RNA from DNA template.

• Initiation: RNA polymerase binds to promoter region.

• Elongation: RNA strand grows in 5' to 3' direction.

• Termination: RNA polymerase detaches at terminator sequence.

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

mRNA Processing in Eukaryotes

• 5' Cap added for stability and ribosome binding.

• Poly-A tail added to 3' end for stability.

• Splicing: Removal of introns, joining of exons.

Translation

• Initiation: Ribosome assembles at start codon (AUG).

• Elongation: tRNAs bring amino acids; peptide bonds form.

• Termination: Stop codon signals release of polypeptide.

Location: Cytoplasm (on ribosomes).

Signals for Start and Stop

• Transcription Start: Promoter sequence (e.g., TATA box).

• Transcription Stop: Terminator sequence.

• Translation Start: Start codon (AUG).

• Translation Stop: Stop codons (UAA, UAG, UGA).

DNA to Protein: The Genetic Code

• DNA triplets are transcribed to mRNA codons, which are translated to amino acids using the codon table.

Mutations

Types and Effects

• Point mutations: Single nucleotide changes (silent, missense, nonsense).

• Insertions/Deletions: Can cause frameshifts, often more deleterious.

• Most deleterious: Frameshift and nonsense mutations (can truncate or inactivate proteins).

Introns vs Exons

Gene Structure in Eukaryotes

• Exons: Coding sequences retained in mature mRNA.

• Introns: Non-coding sequences removed during RNA splicing.

Gene Expression and Regulation

Definition and Importance

• Gene expression: The process by which information from a gene is used to synthesize a functional product (protein or RNA).

• Regulation ensures genes are expressed at the right time, place, and amount.

Levels of Gene Regulation

• Pre-transcriptional: Chromatin structure (methylation, acetylation), transcription factors.

• Transcriptional: Promoter/enhancer activity, transcription factors.

• Post-transcriptional (pre-translation): mRNA splicing, mRNA lifespan, RNA interference (RNAi).

• Translational: Regulation of translation initiation.

• Post-translational: Protein modification (ubiquitin tagging, phosphorylation).

Epigenetic Inheritance

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

Chromosomes and Cell Division

Key Terms

• Homologous chromosomes: Chromosome pairs, one from each parent, similar in size and gene content.

• Chromosome: DNA molecule with part or all of the genetic material.

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

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

Meiosis vs Mitosis

Comparison Table

Feature	Mitosis	Meiosis
Number of divisions	1	2
Resulting cells	2	4
Ploidy	Diploid (2n)	Haploid (n)
Genetic identity	Identical	Different
Cell type	Somatic	Gametes

Major Parts of Meiosis I and II

• Meiosis I: Homologous chromosomes separate (reductional division), crossing over occurs, tetrads form, cells become haploid.

• Meiosis II: Sister chromatids separate (similar to mitosis), resulting in four haploid cells.

• DNA replication occurs before meiosis I, not before meiosis II.

Sources of Genetic Variation in Meiosis

• Crossing over (prophase I): Exchange of genetic material between homologous chromosomes.

• Independent assortment (metaphase I): Random orientation of homologous pairs.

• Random fertilization: Any sperm can fertilize any egg.

Summary Table: DNA, RNA, and Protein Synthesis

Process	Starting Material	Product	Main Enzyme(s)	Location
Replication	DNA	DNA	DNA Polymerase	Nucleus (eukaryotes)
Transcription	DNA	RNA	RNA Polymerase	Nucleus (eukaryotes)
Translation	mRNA	Protein	Ribosome	Cytoplasm

Key Equations

• Chargaff's Rule:

$\text{[A]} = \text{[T]},\ \text{[C]} = \text{[G]}$

• Central Dogma:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Additional info: This guide expands on the review outline by providing definitions, process details, and tables for comparison and synthesis. For codon table usage, refer to a standard genetic code chart to translate mRNA codons to amino acids.