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

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 has important applications in forensic science, law enforcement, archaeology, and paternity testing.

• Applications: Crime scene investigation, identification of human remains, missing persons, archaeology, and paternity testing.

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. Polymerase Chain Reaction (PCR) is used to amplify these regions for analysis.

• STRs: Short DNA sequences (2-6 base pairs) repeated multiple times in a row.

• Example: The sequence "TCTA" repeated 7 times at a locus.

• PCR: Technique to amplify specific DNA regions for profiling.

DNA Markers and Law Enforcement

Law enforcement agencies use DNA markers to compare DNA profiles across individuals. The Combined DNA Index System (CODIS) is a database that stores DNA profiles for criminal investigations.

• CODIS: Database allowing 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
D13S1358	13	Varies
VWA	12	Varies
Additional info:	Other markers	Similar repeat ranges

Genotyping and Allele Labeling

Alleles at STR loci are labeled based on the number of repeats. Individuals inherit one allele from each parent, resulting in a genotype for each marker.

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

• Heterozygous: Two different alleles at a locus (e.g., 7 and 15 repeats).

Probability and Identical Profiles

The probability of two unrelated individuals having the same alleles for all 13 original STR markers is extremely low.

• Probability: $1/10^{10}$ (1 in 10 billion).

• Exception: Identical twins share the same DNA profile.

STR PCR and Fluorescent Dye Labeling

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

• Fluorescent dyes: Used to distinguish different STR markers in a single reaction.

• Size standards: Used to calibrate fragment sizes.

DNA Profiling in Casework

DNA profiles are compared between crime scene evidence and suspect samples. The presence of matching alleles can implicate or exclude individuals.

• Example: Rape case analysis comparing suspect DNA profile to crime evidence.

• Interpretation: Matching profiles suggest involvement; non-matching profiles exclude suspects.

New DNA Profiling Approaches

Recent advances use Single Nucleotide Polymorphisms (SNPs) and DNA phenotyping to predict physical traits and ancestry from DNA evidence.

• DNA phenotyping: Predicts physical appearance from genetic data.

• Forensic genealogy: Uses SNPs and genealogical research to identify suspects.

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. This DNA must fit into a nucleus only about 10 micrometers in diameter.

• Chromosome length: Each chromosome is 19-73 million base pairs.

• DNA packing: DNA is tightly packed to fit into the small nuclear space.

DNA Structure

DNA is composed of nitrogenous base pairs, a sugar-phosphate backbone, and is organized into a double helix.

• Base pair distance: 0.34 nm between adjacent base pairs.

• Double helix: Two strands wound around each other.

Nucleosomes and Chromatin Packing

In eukaryotes, DNA is wrapped around positively charged histone proteins to form nucleosomes, which further coil to form chromatin fibers and chromosomes.

• Nucleosome: Core particle of histone proteins plus 147 base pairs of DNA.

• Chromatin fiber: Multiple levels of DNA packing, including solenoids and metaphase chromosomes.

• Unpacking: Genes must be unpacked from chromatin to be transcribed.

Epigenetics and Gene Regulation

Epigenetic Modifications

Epigenetics refers to heritable changes in gene expression that do not alter the DNA sequence. These changes influence gene expression and phenotypic variation.

• 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 and Transcription

Chromosomal DNA that is tightly packed is inaccessible for transcription and is usually highly methylated.

• Accessible chromatin: Loosely packed, low methylation, active transcription.

• Inaccessible chromatin: Tightly packed, high methylation, inactive transcription.

Heritability of DNA Methylation

DNA methylation patterns are heritable. Maintenance methyltransferases copy methylation patterns to newly replicated DNA strands.

• Maintenance methylation: Ensures epigenetic marks are preserved during cell division.

Summary Table: DNA Profiling vs. Chromosome Structure

Topic	Key Points
DNA Profiling	STRs, PCR, CODIS, probability, forensic applications
Chromosome Structure	DNA length, packing, nucleosomes, chromatin fiber
Epigenetics	DNA methylation, histone acetylation, gene regulation

Additional info: These notes expand on the original content by providing definitions, context, and examples for key concepts in DNA profiling, chromosome structure, and epigenetics, as relevant to a college-level Genetics course.