Genetics Exam Two Study Guide: Linkage, Extranuclear Inheritance, DNA Structure, and Replication

Chapter 5: Linkage and Crossing Over

Independent Assortment and Linkage

During gamete formation, genes may assort independently or be inherited together, depending on their chromosomal location.

• Independent Assortment: Genes located on different chromosomes assort independently, as described by Mendel's Second Law.

• Linkage: Genes located on the same chromosome tend to be inherited together, forming a linkage group.

Crossing Over and Recombination

Crossing over is the exchange of genetic material between nonsister chromatids of homologous chromosomes during prophase I of meiosis. The physical site of exchange is called a chiasma.

• Effect on Allele Segregation: A crossover between a gene and its centromere can alter the segregation pattern of alleles during meiosis.

• Gene Distance and Recombination Frequency: The probability of a crossover event between two genes increases with the physical distance between them. Recombination frequency is used to estimate gene distance on a chromosome.

• Single Crossover Outcomes: In a tetrad, a single crossover between two nonsister chromatids produces two recombinant chromatids and two parental chromatids (unchanged).

Example: If genes A and B are far apart on the same chromosome, the likelihood of a crossover between them is high, resulting in more recombinant gametes.

Chapter 9: Extranuclear Inheritance

Organelle Heredity

Some genetic traits are inherited through DNA located outside the nucleus, primarily in mitochondria and chloroplasts.

• Mitochondrial Inheritance: In most animals, mitochondria are inherited maternally because the egg provides most of the cytoplasm and organelles to the zygote.

• Chloroplast Inheritance: In plants, chloroplasts also exhibit non-Mendelian inheritance patterns.

Reciprocal Crosses and Maternal Effect

• Reciprocal Crosses: Switching which parent provides the egg can change the offspring's phenotype for cytoplasmically inherited traits, revealing extranuclear inheritance.

• Maternal Effect: The mother's nuclear genotype determines the offspring's early phenotype via gene products deposited in the egg. This is distinct from mitochondrial inheritance.

Infectious Heredity

• Definition: Transmission of symbiotic or parasitic microorganisms that affect phenotype, independent of nuclear or organelle DNA.

Endosymbiotic Origin of Organelles

• Evidence: Mitochondria and chloroplasts have circular genomes and bacterial-like features, supporting their origin from ancestral prokaryotes.

Mitochondrial DNA (mtDNA)

• Genetic Content: mtDNA encodes a limited set of proteins, tRNAs, and rRNAs.

• Mutations: Mutations in mtDNA can impair oxidative phosphorylation and ATP production, leading to disease.

• Mutation Accumulation: mtDNA accumulates mutations more rapidly than nuclear DNA, contributing to aging and disease.

Mitochondrial Disorders and Therapy

• Diagnosis: Maternal inheritance, impaired bioenergetic function, reduced ATP, and documented mtDNA mutations are evidence for mitochondrial disorders.

• Mitochondrial Replacement Therapy: Nuclear transfer techniques can reduce transmission of pathogenic mtDNA by replacing defective mitochondria with healthy ones.

Example: Leber's hereditary optic neuropathy (LHON) is a mitochondrial disorder inherited maternally and characterized by vision loss.

Chapter 10: DNA Structure and Genetic Information

Central Dogma of Molecular Biology

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

• Expression: DNA expression produces a phenotype.

• Synthesis: DNA synthesis duplicates genetic material.

• Transmission: Mitosis transmits genetic information to daughter cells.

• Variation: Genetic variation provides the potential for evolutionary change.

Historical Experiments Identifying DNA as Genetic Material

• Griffith: Demonstrated bacterial transformation but did not identify the transforming material.

• Avery, MacLeod, and McCarty: Identified DNA as the transforming principle.

• Hershey and Chase: Used radioactive labeling to show that DNA, not protein, is the genetic material in bacteriophages.

Nucleotides and Nucleic Acid Structure

• Nucleoside: Nitrogenous base + pentose sugar (no phosphate).

• Nucleotide: Nitrogenous base + pentose sugar + one or more phosphate groups.

• DNA Polymerases: Use deoxyribonucleoside triphosphates as substrates. After incorporation, one phosphate remains in the backbone, and pyrophosphate is released.

• Bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T), and Uracil (U). Purines: A, G; Pyrimidines: C, T, U.

• Sugar Difference: DNA has deoxyribose (H at 2'), RNA has ribose (OH at 2'). Both have a 3' hydroxyl for strand extension.

• Strand Polarity: Nucleic acid strands have 5' and 3' ends. Phosphodiester bonds connect the 3' hydroxyl of one nucleotide to the 5' phosphate of the next.

• Double Helix: DNA is double-stranded, antiparallel, with complementary base pairing (A-T, G-C). Purine-pyrimidine pairing maintains uniform helix diameter.

Comparison of DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strands	Double-stranded	Single-stranded (usually)
Function	Genetic storage	Information transfer, catalysis

RNA Types and Functions

• mRNA (messenger RNA): Encodes the sequence of a polypeptide.

• tRNA (transfer RNA): Brings amino acids to the ribosome during translation.

• rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

• Abundance: rRNA > tRNA > mRNA in most cells.

Fluorescent In Situ Hybridization (FISH)

• Definition: FISH uses labeled nucleic acid probes to detect specific DNA sequences on chromosomes, allowing gene localization.

Chapter 11: DNA Replication

Models of DNA Replication

• Conservative: Parental DNA remains intact; new molecule is entirely new DNA.

• Dispersive: Parental and new DNA are interspersed in both strands.

• Semiconservative: Each daughter DNA molecule contains one parental and one new strand. This is the correct model.

Bacterial DNA Replication

• Origin of Replication: Replication begins at a fixed origin and proceeds bidirectionally.

• Directionality: DNA polymerases synthesize DNA in the 5' to 3' direction, adding nucleotides to a free 3' hydroxyl.

• Requirements: DNA polymerases require a template and a primer.

Bacterial Replication Proteins

Protein	Function
DnaA	Recognizes origin and initiates DNA opening
DNA helicase	Separates parental DNA strands
Single-strand binding proteins	Stabilize separated strands
DNA gyrase	Relieves supercoiling ahead of fork
Primase	Synthesizes RNA primers
DNA polymerase III	Main synthesis of leading and lagging strands
DNA polymerase I	Removes RNA primers, replaces with DNA
DNA ligase	Seals nicks in backbone

Leading and Lagging Strands

• Leading Strand: Synthesized continuously toward the replication fork.

• Lagging Strand: Synthesized discontinuously away from the fork as Okazaki fragments, resulting in semidiscontinuous replication.

• Coordination: Helicase, primase, DNA polymerases, primer removal, and ligase work together at the replication fork.

Eukaryotic DNA Replication

• Complexity: Eukaryotes have more DNA, linear chromosomes, and DNA packaged into chromatin.

• Multiple Origins: Eukaryotic chromosomes have multiple origins of replication for efficient copying.

• Origin Recognition Complex (ORC): Recognizes replication origins and assembles the prereplication complex in early G1 phase.

• Polymerase Roles: Polymerase α (initiates with RNA-DNA primer), polymerase δ (lagging strand), polymerase ε (leading strand), polymerase γ (mitochondrial DNA).

• Nucleosome Dynamics: Nucleosomes are disassembled ahead of the fork and reassembled behind it.

Telomeres and Telomerase

• Telomeres: Repetitive DNA and proteins at chromosome ends protect against degradation and fusion. T-loop structures help shield ends.

• End-Replication Problem: Linear chromosomes lose DNA at ends during replication, causing telomere shortening in most somatic cells.

• Telomerase: A ribonucleoprotein enzyme with an internal RNA template (TERC) and reverse transcriptase protein (TERT) that extends telomeres.

• Activity: Telomerase is inactive in most somatic cells but active in germ cells, stem cells, leukocytes, and many cancer cells.

• Biological Implications: Telomere length and telomerase activity are linked to cellular aging, replicative capacity, and cancer development.

Example: In cancer cells, telomerase is often reactivated, allowing unlimited cell division.

Key Equations

• Recombination Frequency:

$\text{Recombination Frequency (cM)} = \frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}} \times 100$

• Phosphodiester Bond Formation:

$\text{(DNA)n} + \text{dNTP} \rightarrow \text{(DNA)}_{n+1} + \text{PP}_i$

• Semiconservative Replication:

$\text{Parental DNA} \rightarrow \text{1 Parental Strand} + \text{1 New Strand (per daughter molecule)}$

Additional info: Some explanations and examples have been expanded for clarity and completeness.