Genetics Exam 2 Study Guide: Chromosomal Mutations, DNA Structure, and Genome Organization

Chromosomal Mutations and Chromosome Structure

Types of Chromosomes

Chromosomes are classified based on the position of their centromere, which divides them into two arms: the short arm (p) and the long arm (q).

• Metacentric: Centromere is in the middle; arms are of equal length.

• Submetacentric: Centromere is slightly off-center; one arm is longer than the other.

• Acrocentric: Centromere is close to one end; one very short arm and one long arm.

• Telocentric: Centromere is at the very end; essentially only one arm is visible.

p arm: The short arm of the chromosome. q arm: The long arm of the chromosome.

Chromosomal Mutations

Chromosomal mutations involve changes in chromosome structure or number, affecting genetic information.

• Alloploid: An organism with chromosomes derived from two or more different species.

• Allopolyploid: Polyploid organism with chromosomes from different species, often resulting from hybridization.

• Autopolyploid: Polyploid organism with multiple chromosome sets from the same species.

• Euploid: Organism with a chromosome number that is an exact multiple of the haploid number.

• Aneuploid: Organism with a chromosome number that is not an exact multiple of the haploid set (e.g., trisomy, monosomy).

• Monoploid: Organism with only one set of chromosomes (n).

• Unequal Crossing Over: Misalignment during homologous recombination leads to duplications or deletions of genetic material.

• Gene Family: A set of several similar genes, formed by duplication of a single original gene, generally with similar biochemical functions.

Nondisjunction in Humans

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division, leading to aneuploidy.

• Common examples: Down syndrome (trisomy 21), Turner syndrome (monosomy X).

Positional Effect

The expression of a gene can be influenced by its position within the genome, especially if relocated near heterochromatin or regulatory elements.

Gain of Function Mutation

A mutation that results in a gene product with enhanced, new, or abnormal function.

Endosymbiont Theory

This theory proposes that mitochondria and chloroplasts originated as free-living bacteria that were engulfed by ancestral eukaryotic cells.

Cytoplasmic Inheritance

Inheritance of traits controlled by genes located in the cytoplasm, typically in mitochondria or chloroplasts, and usually inherited maternally.

Maternal Effect Genes and Lethal Alleles

• Maternal Effect Gene: The genotype of the mother determines the phenotype of the offspring, regardless of the offspring's genotype.

• Maternal Effect Lethal: Lethal alleles that cause death when inherited from the mother, often due to defective maternal mRNA or proteins.

DNA & RNA Structure and Replication

DNA & RNA Structure

DNA and RNA are nucleic acids composed of nucleotides. DNA is typically double-stranded and forms a double helix, while RNA is usually single-stranded.

• DNA: Deoxyribonucleic acid, contains deoxyribose sugar, bases A, T, C, G.

• RNA: Ribonucleic acid, contains ribose sugar, bases A, U, C, G.

Experimental Protocols to Study Nucleic Acids

• Karyotype/Chromosome Banding: Visualizing chromosomes to detect structural abnormalities.

• Gel Electrophoresis: Separates DNA or RNA fragments by size.

• Dichromatic Shift: Measures changes in absorbance to study nucleic acid structure.

• FISH (Fluorescence In Situ Hybridization): Uses fluorescent probes to detect specific DNA sequences on chromosomes.

A/T Rich Region

Regions of DNA with high adenine (A) and thymine (T) content, often found at origins of replication due to easier strand separation.

oriC

The origin of replication in prokaryotic DNA, where DNA replication begins.

DnaA Protein and DnaA Box

• DnaA Protein: Initiator protein that binds to DnaA boxes to start DNA replication in bacteria.

• DnaA Box: Specific DNA sequences recognized by DnaA protein at the origin of replication.

Primers in Replication

Short RNA sequences synthesized by primase to provide a starting point for DNA polymerase during replication.

RNA and DNA Polymerase Activity

• DNA Polymerase: Enzyme that synthesizes new DNA strands using a DNA template.

• RNA Polymerase: Enzyme that synthesizes RNA from a DNA template during transcription.

Helicase and SSB

• Helicase: Unwinds the DNA double helix at the replication fork.

• SSB (Single-Stranded Binding Proteins): Stabilize single-stranded DNA during replication.

Termination of Replication

Specific sequences and proteins ensure that DNA replication ends properly, preventing over-replication.

Topoisomerase & Supercoiling

• Topoisomerase: Enzyme that relieves supercoiling tension in DNA by cutting and rejoining strands.

• Supercoiling: Over- or under-winding of DNA, affecting its topology and compaction.

Okazaki Fragments

Short DNA fragments synthesized on the lagging strand during DNA replication.

Replication Fork and Leading/Lagging Strand

• Replication Fork: The Y-shaped region where DNA is unwound and new strands are synthesized.

• Leading Strand: Synthesized continuously in the direction of the replication fork.

• Lagging Strand: Synthesized discontinuously as Okazaki fragments, away from the fork.

Semiconservative DNA Replication

Each new DNA molecule consists of one parental and one newly synthesized strand.

Equation:

$\text{Parental DNA} \rightarrow \text{1 old strand} + \text{1 new strand}$

DNA Polymerase I, II, and III

• DNA Polymerase I: Removes RNA primers and fills in gaps with DNA.

• DNA Polymerase II: Involved in DNA repair.

• DNA Polymerase III: Main enzyme for DNA synthesis in prokaryotes.

Ligase

Enzyme that joins Okazaki fragments by forming phosphodiester bonds.

Telomerase

Enzyme that extends telomeres (repetitive DNA at chromosome ends) to prevent loss of genetic material during replication.

Eukaryotic vs Prokaryotic Replication

• Eukaryotic: Multiple origins of replication, linear chromosomes, more complex machinery.

• Prokaryotic: Single origin of replication, circular chromosome, simpler machinery.

Recombination

Exchange of genetic material between homologous chromosomes during meiosis, increasing genetic diversity.

Genome Organization and Chromatin Structure

Viral Genomes

Viruses have diverse genome structures, including DNA or RNA, single- or double-stranded, linear or circular.

Bacterial Genome Structure

Bacteria typically have a single, circular chromosome, sometimes with additional plasmids.

Heterochromatin and Euchromatin

• Heterochromatin: Densely packed, transcriptionally inactive chromatin.

• Euchromatin: Loosely packed, transcriptionally active chromatin.

Lampbrush and Polytene Chromosomes

• Lampbrush Chromosome: Large, extended chromosomes found in oocytes, with loops for active transcription.

• Polytene Chromosome: Giant chromosomes with many DNA strands, found in some insect tissues, useful for gene mapping.

Eukaryotic Genome Compaction

DNA is compacted by wrapping around histone proteins to form nucleosomes, further folded into higher-order structures.

Repetitive Sequences

DNA sequences repeated many times in the genome, including satellite DNA, minisatellites, and microsatellites.

Chromatin Remodeling

Dynamic modification of chromatin architecture to allow access to DNA for transcription, replication, and repair.

Experimental Techniques

PCR (Polymerase Chain Reaction)

PCR is a technique to amplify specific DNA sequences using cycles of denaturation, annealing, and extension.

Equation:

$\text{Number of DNA copies after n cycles} = 2^n$

Summary Table: Chromosome Types

Type	Centromere Position	p/q Arm Ratio	Example
Metacentric	Middle	1:1	Human chromosome 1
Submetacentric	Off-center	1:2	Human chromosome 4
Acrocentric	Near end	1:10	Human chromosome 21
Telocentric	At end	0:1	Mouse chromosomes

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard genetics curricula.