FRSC1030 Foundations in Forensic Science Chapter 10

Key Terms from chapter 10

Allelic ladder

A collection of known DNA fragments representing the most common alleles for specific genetic loci, used ike a molecular ruler to help “measure” the lengths of the fragments in the reference and evidence samples. Used to identify the locus and alleles.

Amelogenin

The primary structural protein in developing tooth enamel, produced by the AMELX (X) and AMELY (Y) genes, that also serves as a genetic marker for sex determination. With the AMELX chromosome allele, a female sample typically has twice the amount of AMEL as a male sample.

Amplicons

Copies of a specific piece of DNA or RNA that are the results of artificial or natural replication, usually created in a lab using techniques like PCR to make millions of copies of a target gene or region.

Amplification

The ability to reliably copy DNA, usually using a technique called PCR.

Annealing

Binding, when two single strands of DNA stick together to form a double bond helix with hydrogen bonds. Can also refer to a process where DNA is heated and cooled in a thermocycler so it can anneal to a short primer and be replicated in a PCR test.

Autorad

Short for autoradiograph, a developed film resembling a bar code, a bar chart, or a ladder, that results from DNA analysis. Each different DNA sample will give a different pattern on an autorad, which can assist in forensic investigations.

Autosomal chromosomes

the 22 pairs of non-sex chromosomes found in the nucleus in humans that carry most of the genetic information and determine body characteristics, other than sex. They are numbered 1 through 22 based on size, and are inherited in equal amounts from both parents.

Autosomal DNA

DNA from the autosomal chromosomes.

Base / base pair

One base and its complement on the other strand of DNA. The four bases are adenine, thymine, guanine, and cytosine.

Buccal swab

A standard method of collecting DNA samples from a person, by swabbing the inside of their mouth.

Capillary electrophoresis (CE)

Analytical technique that separates ions and small molecules in submillimeter diameter capillaries using an applied electric field to move them based on their charge and size.

CODIS

Combined DNA index system, a database that manages and compares DNA profiles from convicted offenders, crime scenes, and missing persons.

Denaturing

Unzipping of the DNA strands, the first stage of the PCR cycle, by heating the sample to around 94 C.

Differential extraction

A technique used to separate fractions, or sources of DNA from different cell types, within a mixture. It’s usually used in sexual assault cases, to separate the vaginal epithelial cells from sperm cells.

DNA dragnet

A police method of collecting voluntary DNA samples from a large number of people to find a suspect in a crime, often when there is no specific individual to target.

DNA profile

The genotypes identified at all the typed STR loci in a sample. They are used in Q vs. K comparisons, and comparisons between crime scene profiles and profiles retrieved from databases.

Dye channel

The different colour channels in an electropherogram, where each dye is attached to different primers to distinguish different alleles for a particular locus. Each STR has a different colour attached to its primer, and each horizontal window of a sorted electropherogram represents one dye channel.

Electropherogram

A visual display of DNA analysis results, represented as peaks, where each peak represents a DNA fragment of a specific size. It’s presented by sorting the output by dye channel, internal sizing standard, and allelic ladder.

European standard set

A standard set of 12 loci adopted by the EU in 2009

Flanking region

A DNA sequence located on either side of a specific gene or locus, crucial for gene regulation and PCR, where primers are designed to bind to the flanking region of a target DNA sequence, which allows for the specific amplification of the DNA segment between the primers.

Fluorescence

The emission of light by a substance that has absorbed and re-emitted it, typically at a longer wavelength, used for visualization enhancement, especially for fingerprints and biological samples.

Gene

The region on a chromosome where the DNA provides instructions for producing a particular protein, where the characteristics inherited from parents are encoded.

Hypervariable regions

A segment of DNA or protein that exhibits a high degree of genetic or sequence variation.

Index

A collection of profiles or samples used to manage and compare different profiles and / or DNA samples, such as the convicted offender index, NDIS, and CODIS.

Internal size standard

A molecular “ruler” used to size each peak by the number of base pairs, which helps correlate peaks to specific STR loci.

Laser microdissection

A technique of physical separation, where portions of the sample are illuminated to induce fluorescence of the cells, which are then separated to isolate different types of cells from each other.

Likelihood ratio

A ratio of probabilities for observing the evidence profile given two specific scenarios.

Locus / loci

The location of a gene on a region of a chromosome.

Migration time

The time that elapses between sample introduction and band detection during gel electrophoresis.

Mitochondrial DNA

DNA found in the mitochondria of a cell, outside the nucleus.

Multiplexing

A method of combining probes in mixtures, allowing simultaneous analysis of multiple loci.

National DNA index system (NDIS)

One part of CODIS, the national level, containing the DNA profiles contributed by federal, state, and local participating forensic laboratories in the United States.

Nuclear DNA

DNA found in the nucleus of a cell

Nucleotide

A unit consisting of a base (A, C, G, or T) connected to a sugar molecule and a phosphate group.

Peak height threshold

A concept in DNA analysis to interpret DNA profiles based on the height of peaks in an electropherogram, used to distinguish valid but small allele peaks from typical background noise, especially in mixed or degraded samples, and determining if there are peaks that should be present but are not.

Polygenetic / polygenic trait

A trait that is determined be many genes, such as eye colour, height, and skin colour.

polymerase chain reaction (PCR)

A technique that involves repeated cycles of unzipping the paired strands of DNA, adding nucleotides to form two complementary strands, and zipping them back together, which produces double the number of DNA molecules every cycle.

Primers

Specialized molecular probes, short synthetic DNA molecules, that bind to target sequences surrounding the region to be copied.

Probes

Compounds that are placed on separated fragments of DNA, and bind specifically to the region of interest, and give off a fluorescent signal as amplification of that area increases. They don’t bind to the repeating sequence, but to the nucleotide sequences on either side.

Restricted fragment length polymorphism (RFLP)

The first method used for DNA profiling, a molecular technique that identifies variations in homologous DNA sequences by using restriction enzymes to cut DNA at specific sites, producing different sized fragments that can be separated by gel electrophoresis.

Restriction enzymes

Enzymes used in RFLP typing, that locate specific nucleotide sequences and snip the DNA at those locations.

Short tandem repeats (STRs)

Segments of DNA where short (2-7 base pairs) sequences of DNA are repeated multiple times in a row. They are highly variable between individuals, making them useful for individual identification.

Thermal cycler

An instrument in which a PCR reaction occurs. Small vials containing the sample and added ingredients (primers, nucleotides, buffers, and enzymes) are placed in the device, and the cycle starts.

Vaginal epithelial cells

Cells that form the lining of the vagina, that shed into the vaginal tract.

Variable number of tandem repeats (VNTR)

Short DNA sequences that repeat in tandem, with the number of repeats varying between individuals, which creates unique DNA profiles, and can play a role in gene regulation and influence protein structure or stability.