Comprehensive Genetics Study Notes: DNA, Gene Expression, Mutations, and Mapping

Evidence for Nucleic Acid as Genetic Material

Avery et al. Experiments

The experiments by Avery, MacLeod, and McCarty provided the first conclusive evidence that DNA is the genetic material in cells.

• Key Point: Used Streptococcus pneumoniae (RII and SIII strains) to show that purified DNA could transform non-virulent bacteria into virulent forms.

• Method: Treated cell extracts with enzymes that destroy proteins, RNA, or DNA. Only destruction of DNA prevented transformation.

• Conclusion: DNA is the molecule responsible for heredity.

Hershey & Chase Experiment

This experiment used bacteriophage T2 to confirm that DNA, not protein, is the genetic material.

• Key Point: Labeled phage DNA with 32P and protein with 35S.

• Result: Only 32P (DNA) entered bacterial cells and directed viral replication.

• Conclusion: DNA carries genetic information in viruses.

Tobacco Mosaic Virus (TMV)

• Key Point: Demonstrated that RNA can serve as genetic material in some viruses.

• Example: Hybrid TMV experiments showed that the type of RNA determined the type of virus produced.

Watson, Crick, Wilkins, and Franklin: 3D DNA Model

The structure of DNA was elucidated through X-ray diffraction and model building.

• Key Point: DNA is a double helix with antiparallel strands held together by complementary base pairing (A-T, G-C).

• Contributors: Rosalind Franklin's X-ray images were critical for determining the helical structure.

• Significance: The model explained how DNA could replicate and store genetic information.

DNA Replication

Prokaryotic Circular Chromosomes

• Key Point: Replication begins at a single origin and proceeds bidirectionally.

Rolling Circle Replication (Plasmids)

• Key Point: Used by some plasmids and viruses; a nick in one strand allows continuous synthesis.

Telomeres in Eukaryotes

• Key Point: Telomeres are repetitive DNA sequences at chromosome ends, maintained by telomerase to prevent loss of genetic material during replication.

Recombination

Homologous Recombination (RecA in Bacteria)

Homologous recombination allows exchange of genetic material between similar DNA molecules.

• Key Point: RecA protein mediates strand invasion and exchange in bacteria.

• Importance: Increases genetic diversity and repairs DNA.

Transcription in Prokaryotes vs. Eukaryotes

Promoter Differences

• Prokaryotes: RNA polymerase binds directly to promoter sequences (e.g., -10 and -35 regions).

• Eukaryotes: RNA polymerase requires transcription factors (TFs) to recognize promoters (e.g., TATA box).

Post-Transcriptional Modifications in Eukaryotes

• 5' 7-methylguanosine (7Me-G) cap

• 3' poly-A tail

• Splicing: Removal of introns and joining of exons.

The Genetic Code

The genetic code is a set of rules by which nucleotide sequences are translated into amino acid sequences.

• Key Point: The code is triplet, non-overlapping, and nearly universal.

• Start codon: AUG (methionine)

• Stop codons: UAA, UAG, UGA

Translation

Prokaryotic Translation

• Key Point: Translation can begin before transcription is complete (coupled transcription-translation).

• Shine-Dalgarno sequence: Ribosome binding site in prokaryotic mRNA.

Eukaryotic Translation Differences

• Kozak sequence: Consensus sequence around start codon for ribosome recognition.

• Monocistronic mRNA: Each mRNA usually encodes only one protein.

Regulation of Gene Expression

Prokaryotes: Operons

• lac Operon: Inducible; activated in presence of lactose.

• trp Operon: Repressible; turned off in presence of tryptophan.

Eukaryotes

• Galactose pathway: Genes activated in presence of galactose.

• Steroid hormones: Regulate gene expression by binding to intracellular receptors that act as transcription factors.

• Alternative splicing: Generates multiple proteins from a single gene.

• DNA methylation: Addition of methyl groups to DNA, often silencing gene expression.

Mutations

Point Mutations

• Base substitutions: Replacement of one nucleotide with another.

• Frameshifts: Insertions or deletions that alter the reading frame.

Types of Point Mutations

• Nonsense: Changes codon to a stop codon.

• Missense: Changes codon to code for a different amino acid.

• Silent: No change in amino acid sequence.

Conditional Lethal Mutations

• Key Point: Lethal only under certain environmental conditions (e.g., temperature-sensitive mutations).

Mutations Involving More Than One Base Pair

• Larger deletions: Can cause genetic disorders (e.g., Cri-du-Chat syndrome, monosomy).

• Inversions: Segment of chromosome is reversed.

• Translocations: Segment of one chromosome moves to another.

• Transposition: Movement of DNA segments within the genome.

• Aneuploidy: Abnormal number of chromosomes.

• Polyploidy: More than two complete sets of chromosomes.

Mapping

T4 rII Mapping

• Key Point: Used to determine genetic distances in bacteriophage T4.

• Formula:

$\text{Distance} = \left( \frac{\#\text{WT (Restrictive)}}{\text{Total \# (Permissive)}} \right) \times 100 \times 2$

• Explanation: The factor of 2 accounts for the two possible crossover events.

Bacterial Mapping

• Hfr Mapping: Uses high-frequency recombination strains to map gene order and distance.

• Specialized Transduction (lambda): Only certain genes are transferred by phage.

• Generalized Transduction (P1): Any gene can be transferred by phage.

Eukaryotic Mapping and Inheritance Patterns

• Mendelian Monohybrid Cross: Involves one trait; true-breeding lines.

• Dihybrid Cross: Involves two traits; can show independent assortment.

• Co-dominance: Both alleles are expressed (e.g., AB blood type).

• Epistasis: One gene masks the effect of another.

• Sex Determination: Mechanisms differ among birds (ZW), Drosophila (ratio of X chromosomes to autosomes), and humans (XY system).

• Pedigrees: Used to determine inheritance patterns (X-linked vs. autosomal; dominant vs. recessive).

• Sex-limited vs. Sex-influenced Traits: Traits expressed only in one sex or differently in each sex due to hormonal differences.