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DNA

What is DNA?

Forensic science was revolutionized in the 1980s with the introduction of DNA analysis. This technology has been a great tool in fighting crime, identifying unrecognizable human remains, and paternity testing.

What exactly is DNA?

A scientist looking at a DNA test.

DNA

Cells of your body contain a nucleus, and this is where nuclear DNA can be found. DNA is short for deoxyribonucleic acid. DNA is tightly coiled into chromosomes, which are found inside the nucleus—most nuclei contain 23 pairs of chromosomes.

A diagram of a cell, showing the nucleus, chromosomes, and DNA within it. The DNA is shown close up, revealing the DNA strand and Base Pair Sequences. Cystosine (C) pairs with Guanine (G) and Thymine (T) pairs with Adenine (A).
"DNA, Chromosomes and Genes" by NIH Image Gallery is licensed under CC BY-NC 2.0.

DNA is made up of a building block called a nucleotide. Nucleotides consist of three parts – a deoxyribose sugar, a phosphate, and a nitrogenous base.

"Dioxyribo" refers to the sugar; “nucleic acid” refers to the phosphate and nitrogenous base. These nucleotides bond to create the DNA helical structure. DNA looks like a twisted ladder and when uncoiled would stretch up to two meters long.

DNA's "Ladder"

A diagram of the structure of DNA. The nitrogen bases are held together with hydrogen bonds, which are supported on both sides by the deoxyribose sugars and phosphates.

The sides of the “ladder” consist of alternating 5-carbon deoxyribose sugars the pink pentagons in the image above and phosphates the blue circles . The rungs of the ladder are made of nitrogen bases the yellow hexagons and pentagons bonded together.

Hydrogen bonds hold two matching strands together and the interior of the "twisted ladder" contains four nucleic acid bases. There are 3 billion repeating nucleic acid bases found in each DNA strand.

Knowledge Check #1

True or False: DNA looks like a ladder, or double helix.

  1. True
  2. False

Answer: a. True

Knowledge Check #2

True or False: Genetic technology has been used in forensic science since the 1800s.

  1. True
  2. False

Answer: b. False

Knowledge Check #3

True or False: Matching strands of DNA are held together by hydrogen bonds.

  1. True
  2. False

Answer: a. True

Nitrogen Bases

The nitrogen bases found in DNA include adenine, thymine, cytosine and guanine. These bases will only pair up with their corresponding base. For example, adenine will only pair with thymine (A-T); cytosine will only bond with guanine (C-G). Thanks to this consistent pairing, when DNA is unraveled, one side can be used as a template to create an exact copy of the other side.

Knowledge Check #4

There are at least 7 different types of nucleic acid bases.

  1. True
  2. False

Answer: b. False

Knowledge Check #5

True or False: The nucleic acids adenine and thymine pair, as do cytosine and guanine.

  1. True
  2. False

Answer: a. True

Knowledge Check #6

The nucleic acid bases do not match up in any kind of predictable pattern.

  1. True
  2. False

Answer: b. False

Knowledge Check #7

Determine which base pairs belong on the other side of this DNA strand.

  1. Adenine →
  2. Guanine →
  3. Guanine →
  4. Thymine →
  5. Adenine →
  6. Cytosine →
  7. Adenine →

Answer:

  1. Adenine → Thymine
  2. Guanine → Cytosine
  3. Guanine → Cytosine
  4. Thymine → Adenine
  5. Adenine → Thymine
  6. Cytosine → Guanine
  7. Adenine → Thymine

Genes

The sequence of these nucleic acids creates genes. There are approximately 20,000-25,000 genes in the human body. These genes are simply portions of DNA that code the information to make proteins. Genes can vary in length from 1,000 to 10,000 or more base pairs long. These proteins determine human traits and functions. In humans, these bases are 99.9% the same. Forensic scientists look at the 0.1% that differs, which is about 3 million bases, to create a DNA profile of a person. Except for identical twins, this portion of DNA is unique to each individual.

Knowledge Check #8

The sequence of base pairs for all humans is _________.

  1. 10% the same
  2. 0.1% the same
  3. 99.9% the same
  4. 50% the same

Answer: c. 99.9% the same

Knowledge Check #9

True or False: DNA holds the data used to make the proteins of the body.

  1. True
  2. False

Answer: a. True

Knowledge Check #10

True or False: Each DNA strand is approximately 3 million base pairs long.

  1. True
  2. False

Answer: b. False. Each strand has over 3 billion base pairs, but only 3 million of them are individualized among humans.

Journey into DNA

Go to Journey into DNA to understand more about DNA.

Nuclear DNA

Nuclear DNA is inherited from both parents — half from the mother and the other half from the father. There are two types of DNA found within the nucleus. One type is found on autosomal chromosomes (22 of the 23 chromosome pairs) and is used most often in criminal investigations.

The other type is found on sex-determining chromosomes. The 23rd pair of chromosomes differs between males and females. Females have two X chromosomes for the 23rd pair while males have an X and Y chromosome for the 23rd pair. DNA residing on these chromosomes can be helpful in sexual assault cases where DNA from both a male and female is present and must be differentiated.

A diagram showing the autosomal and sex-determining chromosomes in a male and female.

Mitochondrial DNA

Another source of DNA can be found in the cell's mitochondria, but this DNA is inherited only from the mother. Since this DNA is only inherited through females, mitochondrial DNA can match samples to maternal relatives. Mitochondrial DNA (mtDNA) can be useful in forensic cases where nuclear DNA cannot be used. MtDNA does not degrade as quickly as nuclear DNA and can be obtained from evidence such as a hair without a root, skeletal remains, or old or degraded samples of bodily evidence. However, mtDNA analysis is more costly and time consuming.

Knowledge Check #11

DNA from the nucleus is inherited from ___________.

  1. only the father
  2. only the mother
  3. both the mother and father
  4. neither the mother nor the father

Answer: c. both the mother and father

Knowledge Check #12

Mitochondrial DNA (mtDNA) is inherited from

  1. only the father
  2. only the mother
  3. both the mother and father
  4. neither the mother nor the father

Answer: b. only the mother

Basics of DNA Review

Review the basics of DNA from "DNA - a Molecular Identity" before attempting the lesson exercise.