NT

DNA Structure and Replication

Chapter 7: DNA Structure and Replication

Understanding DNA in Forensics

To identify perpetrators, forensic scientists examine the specific sequence of nucleotide bases along one strand of a person’s DNA. This principle highlights the reliability of DNA testing in judicial contexts. For instance, Roy Brown asserts, "Judges can be fooled and juries make mistakes, [but] when it comes to DNA testing there’s no mistakes.”

Driving Questions

  • What is the function of DNA, and how is DNA organized in cells?
  • What is the structure of DNA, and why is each person’s DNA unique?
  • How is DNA copied in living cells, and how can DNA be amplified for forensics?
  • How does DNA profiling make use of genetic variation in DNA sequences?

DNA as Evidence

The Innocence Project utilizes DNA as evidence to exonerate individuals wrongfully convicted of crimes. Roy Brown was wrongfully convicted of homicide in 1991 and sentenced to 25 years to life in prison until DNA evidence aided in proving his innocence.

  • DNA testing has become a standard component of court cases, providing an extremely accurate method for matching crime scene evidence with suspects.

What is DNA?

Deoxyribonucleic acid (DNA):

  • A hereditary molecule that is inherited from parents to offspring.
  • Common to all living organisms.
  • Functions as the instruction manual detailing how to construct an individual.

DNA Location and Structure

  • Found in the nucleus of eukaryotic cells:
    • Present in the form of chromosomes, which are essentially a single DNA molecule wrapped around proteins.

Human Chromosomes

  • Humans possess 23 pairs of chromosomes, with one chromosome from each pair inherited from the mother and the other from the father.
  • Pair #23 determines the biological sex:
    • XX = Female
    • XY = Male

Genome

  • Refers to the complete set of genetic instructions encoded within all the chromosomes of an organism.

The Structure of DNA

Subunits of DNA

DNA comprises nucleotides, where each nucleotide is comprised of:

  1. A sugar
  2. A phosphate
  3. A base (adenine (A), thymine (T), guanine (G), or cytosine (C))

Shape of DNA

  • Consists of two strands of nucleotides that form a spiral-shaped double helix.
    • Backbone: Formed by alternating sugars and phosphates (outer section of the helix).
    • Rungs: Composed of nucleotide base pairs (inner section of the helix).

Base Pairing

  • Each nucleotide contains one of four bases:
    • Adenine (A)
    • Thymine (T)
    • Guanine (G)
    • Cytosine (C)
  • Base Pairing Rules:
    • Adenine always pairs with Thymine (A-T).
    • Cytosine always pairs with Guanine (C-G).

Unique Base Sequence

  • The sequence of nucleotides along a strand of DNA is unique to each individual.
  • The order of the bases determines an individual’s characteristics, which will later be translated into proteins.

Studying DNA

  • As each person's DNA is unique, it can be extracted from cells to create a DNA profile, which is a visual representation of a person's unique DNA sequence.
  • DNA profiles necessitate multiple copies of an individual's DNA, requiring knowledge about DNA replication.

DNA Replication: Mechanism

  • Semiconservative Mechanism:
    • This mechanism indicates that replication produces two copies of the original DNA molecule, where each resultant molecule comprises one original strand and one new strand

Overview of DNA Replication

  • DNA replication is a natural process that enables cells to create an identical copy of a DNA molecule, resulting in two daughter double helices.
  • This process leverages complementary base-pairing rules (A-T; G-C).

Steps of DNA Replication

  1. Helicase Enzyme: Separates DNA strands by breaking the hydrogen bonds that hold the base pairs together, unwinding the double helix.
  2. DNA Polymerase Enzyme: Reads the template DNA strands and adds complementary nucleotides according to base-pairing rules, matching A to T and C to G.

The Polymerase Chain Reaction (PCR)

  • Purpose: PCR is a laboratory technique employed to amplify (replicate) a specific segment of DNA for detailed examination, functioning like a DNA copy machine.

PCR Lab Procedures

Ingredients Required
  1. DNA sample
  2. Free nucleotides to extend new DNA strands
  3. Polymerase enzyme
  4. Primers: short DNA segments that guide DNA polymerase to the specific DNA region for copying.
Temperature Cycles
  • Heating phase temporarily separates DNA strands.
  • Cooling phase allows DNA polymerase to pair new nucleotides with existing template strands.

PCR Results

  • This process allows for exponential increase in DNA segments, making billions of copies from only a few initial molecules.

DNA Profiling: Overview

  • Definition: DNA profiling involves analyzing a specific section of the genome to identify distinctive traits in an individual's DNA, which is often referred to as a molecular “fingerprint.”
  • This method is less costly and involves less effort compared to sequencing the entire genome.

Structure of Short Tandem Repeats (STRs)

  • Short Tandem Repeats (STRs): Ideal segments used for DNA profiling.
    • Consist of short DNA sequences repeated multiple times along a chromosome. For instance, the sequence AGC may be repeated several times.
  • The number of repeats varies between individuals and can be isolated using PCR.

STR Variation

  • STRs occupy identical loci on chromosomes but can vary in length between individuals, allowing for unique DNA fingerprinting.

Steps in Creating a DNA Profile

Collect Sample

  • Begin by collecting cells and extracting DNA, ensuring that samples maintain integrity without contamination from other individuals.

Amplify Sample

  • Utilize PCR to amplify STR regions, increasing confidence in the results as more regions are amplified.

Separate DNA Fragments

  1. Gel Electrophoresis: A laboratory technique used to separate DNA fragments based on size.
  2. Smaller fragments migrate farther than larger ones within the gel medium.

Compare STR Banding Patterns

  • Utilize fluorescence to visualize bands of DNA. Each individual's unique sequence leads to distinct banding patterns corresponding to varying lengths across multiple STR regions.

Multiple Regions in DNA Profiling

  • Analyzing multiple STR regions significantly reduces the probability that any two individuals (excluding identical twins) will share the same DNA profile, reinforcing the uniqueness of each individual's genetic makeup.

DNA Profiling and the Law

  • DNA evidence is acknowledged as more reliable than other types of evidence.
    • For instance, error rates for bite mark identification may reach as high as 91%, whereas hair analysis can only serve to exclude a suspect without providing a positive identification.
  • With the exception of identical twins, no two individuals possess precisely identical DNA sequences.

Summary

Key Takeaways

  1. DNA serves as the hereditary molecule for all living organisms.
  2. Within a eukaryotic cell, DNA is organized into chromosomes located in the nucleus.
  3. Humans have 23 pairs of chromosomes, with one derived from each parent.
  4. The structure of DNA is a double helix consisting of nucleotides, with the two strands held together by complementary base pairing.
  5. The complementary nature of DNA strands is crucial for accurate DNA replication.
  6. PCR facilitates the amplification of specific DNA sequences for further analysis.
  7. Forensic scientists utilize STRs to generate an individualized DNA profile.