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DNA Profiling, Chromosome Structure, and Epigenetics: Genetics Study Notes

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DNA Profiling and Genetic Markers

Introduction to DNA Profiling

DNA profiling is a technique used to identify individuals based on their unique genetic makeup. It is widely used in forensic science, paternity testing, archaeology, and the identification of human remains.

  • Applications: Crime scene investigation, missing persons, paternity testing, archaeology.

  • Key Principle: Analysis of specific regions of DNA that vary greatly among individuals.

Short Tandem Repeats (STRs) and PCR

Traditional DNA profiling relies on the analysis of Short Tandem Repeats (STRs), which are short sequences of DNA repeated in tandem.

  • STRs: Short DNA sequences (2-6 base pairs) repeated multiple times at specific loci.

  • PCR (Polymerase Chain Reaction): Used to amplify STR regions for analysis.

  • Example STR: TCTA repeat.

DNA Markers and Law Enforcement

Law enforcement agencies use DNA markers to create profiles for identification and comparison.

  • CODIS: Combined DNA Index System, a database for electronic access to DNA profiles across the US.

  • Markers: 20 microsatellite (STR) loci used as markers (13 original, 7 added in 2017).

STR Marker

Chromosome

Number of Repeats

D8S1179

8

7-20

Other STRs

Various

Varies

Additional info: 19 different alleles can be found at D8S1179 locus; some repeats may be imperfect.

Genotyping and Allele Labeling

Alleles are labeled based on the number of repeats at each STR locus.

  • Genotype Example: Allele 1: 5 repeats, Allele 2: 10 repeats (Genotype: 5;10).

  • Probability: The chance of two unrelated people sharing the same alleles for all 13 original markers is 1 in 10 billion.

  • Identical twins: Cannot be distinguished by DNA profiling as they share identical DNA.

STR PCR Primers and Fluorescent Dye Labeling

STR PCR primers are labeled with fluorescent dyes to allow detection and sizing of amplified DNA fragments.

  • Fluorescent dyes: Enable visualization of STR alleles during electrophoresis.

  • Size ranges: Each STR marker has a characteristic size range and dye color.

Interpreting DNA Profiles

DNA profiles are generated by analyzing the number and size of STR alleles present in an individual's DNA.

  • Heterozygosity: An individual is heterozygous at a locus if two different alleles (peaks) are present.

  • Example: If 8 STR markers show two peaks, the individual is heterozygous for 8 markers.

Case Studies and Applications

  • Paternity Testing: STR profiles are compared between child and potential fathers to determine biological relationships.

  • Crime Scene Analysis: Suspect DNA profiles are matched against evidence to identify perpetrators.

New DNA Profiling Approaches

  • SNPs: Single Nucleotide Polymorphisms are used for DNA phenotyping and genealogical research.

  • DNA Phenotyping: Predicts physical traits from DNA (e.g., eye color, hair color).

  • Forensic Genealogy: Uses SNPs and public databases to identify suspects through relatives.

Chromosome Structure and DNA Organization

Human Chromosomes and DNA Length

Human cells contain 46 chromosomes, with a combined DNA length of approximately 2 meters per cell.

  • Nucleus size: ~10 μm diameter.

  • DNA packaging: DNA must be compacted to fit inside the nucleus, which is about 200,000 times smaller than the DNA length.

DNA Structure

  • Double helix: DNA consists of two strands forming a helical structure.

  • Base pairs: Nitrogenous bases (A, T, C, G) pair via hydrogen bonds.

  • Sugar-phosphate backbone: Provides structural support.

  • Distance between base pairs: 0.34 nm.

Chromatin and Nucleosomes

In eukaryotes, DNA is tightly wrapped around histone proteins to form nucleosomes, which further compact into higher-order structures.

  • Nucleosome: Core particle of 8 histone proteins plus 147 bp of DNA.

  • Solenoid: Nucleosomes coil to form a 30-nm fiber.

  • Chromatin fiber: Multiple levels of packing lead to the formation of metaphase chromosomes.

Gene Expression and Chromatin Remodeling

  • Unpacking DNA: Genes must be accessible (unpacked) for transcription.

  • Epigenetic modifications: DNA methylation and histone acetylation regulate gene transcription.

Epigenetics and Imprinting

Epigenetic Regulation

Epigenetics refers to heritable changes in gene expression that do not involve changes to the DNA sequence.

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

  • Histone acetylation: Addition of acetyl groups to histones, making DNA more loosely packed and accessible for transcription.

  • Imprinting: Epigenetic phenomenon where certain genes are expressed in a parent-of-origin-specific manner.

Chromatin Accessibility

  • Tightly packed chromatin: Inaccessible for transcription, higher methylation.

  • Loosely packed chromatin: Accessible for transcription, lower methylation.

Heritability of DNA Methylation

  • Maintenance methyltransferases: Enzymes that copy methylation patterns during DNA replication.

  • Phenotypic variation: Epigenetic changes can influence gene expression and result in observable differences.

Key Equations and Concepts

  • Probability of identical STR profiles:

  • Distance between base pairs:

  • DNA length per cell:

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