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

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

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: 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.

Genetic Mapping and Recombination Frequency

Genetic distance between genes is estimated by the frequency of recombination events.

Gene Distance and Recombination: The farther apart two genes are on a chromosome, the higher the probability of a crossover occurring between them.
Recombinant vs. Parental Chromatids: In a single crossover event involving two nonsister chromatids, two chromatids become recombinant (containing new allele combinations), while the other two remain parental (unchanged).
Crossover and Centromere: A crossover between a gene and its centromere can alter the segregation pattern of alleles during meiosis.

Chapter 9: Extranuclear Inheritance

Organelle Heredity

Some genetic traits are inherited through organelles such as mitochondria and chloroplasts, which have their own genomes.

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, often maternal.

Types of Extranuclear Inheritance

Reciprocal Crosses: Switching which parent provides the egg can change the offspring's phenotype for cytoplasmically inherited traits, revealing the mode of 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: Transmission of symbiotic or parasitic microorganisms can cause extrachromosomal inheritance.

Endosymbiotic Theory and Organelle Genomes

Endosymbiotic Origin: Mitochondria and chloroplasts are thought to have originated from free-living bacteria, supported by their circular genomes and bacterial-like features.
Mitochondrial DNA (mtDNA): Encodes a limited set of proteins, tRNAs, and rRNAs. Mutations can impair oxidative phosphorylation and ATP production.
Mutation Accumulation: mtDNA accumulates mutations more rapidly than nuclear DNA, contributing to aging and disease.

Mitochondrial Disorders and Therapies

Evidence for Mitochondrial Disorders: Maternal inheritance, impaired bioenergetic function, reduced ATP generation, and documented mtDNA mutations.
Mitochondrial Replacement Therapy: Nuclear transfer techniques can reduce the transmission of pathogenic mtDNA by replacing defective mitochondria with healthy ones.

Chapter 10: DNA Structure and Genetic Information

The Central Dogma and DNA Functions

The central dogma describes the flow of genetic information from DNA to RNA to protein.

Central Dogma:
DNA Functions: Expression (phenotype), synthesis (duplication), transmission (mitosis), and variation (mutation).

Discovery of DNA as Genetic Material

Griffith's Experiment: Demonstrated bacterial transformation but did not identify DNA as 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.

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.
DNA vs. RNA: DNA contains deoxyribose (H at 2'), RNA contains ribose (OH at 2'). Both have a 3' hydroxyl group 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 (usually)	Single (usually)
Function	Genetic storage	Information transfer, catalysis

RNA Types and Detection

mRNA: Encodes proteins; each mRNA specifies a particular polypeptide.
tRNA: Transfers amino acids during translation; participates in synthesizing many proteins.
rRNA: Forms the core of ribosomes; most abundant RNA type.
FISH (Fluorescent In Situ Hybridization): Uses labeled nucleic acid probes to locate specific DNA sequences on chromosomes.

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 only in the 5' to 3' direction, adding nucleotides to a free 3' hydroxyl.
Template and Primer: DNA polymerases require a template strand and a primer with a free 3' end.

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

Multiple Origins: Eukaryotic chromosomes have multiple origins of replication to efficiently copy large genomes.
Origin Recognition Complex (ORC): Recognizes replication origins and helps assemble the prereplication complex in early G1 phase.
Principal Polymerases:
- Polymerase α: Initiates synthesis with RNA-DNA primer
- Polymerase δ: Synthesizes lagging strand
- Polymerase ε: Synthesizes leading strand
- Polymerase γ: Replicates mitochondrial DNA
Chromatin: DNA is packaged into nucleosomes, which must be disassembled and reassembled during replication.

Telomeres and Telomerase

Telomeres: Repetitive DNA and associated proteins at chromosome ends protect against degradation and fusion. T-loop structures help shield ends.
End-Replication Problem: Linear chromosomes cannot be fully replicated at the ends, leading to 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 differentiated somatic cells but active in germ cells, stem cells, leukocytes, and many cancer cells.
Biological Significance: Telomere length and telomerase activity are linked to cellular aging, replicative capacity, and cancer development.