DNA Structure and Replication

Chapter 7: DNA Structure and Replication

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 wrongly convicted of crimes.

  • DNA Testing: Standard practice in court cases for accurate matching of crime scene evidence to potential perpetrators. This method is recognized as extremely reliable and precise.

What is DNA?

  • Deoxyribonucleic Acid (DNA):

    • A hereditary molecule passed from parents to offspring.

    • Common to all living organisms.

    • Serves as the instruction manual for constructing an individual.

What is the Genetic Material?

  • Properties of DNA:

    • Experiments in 1928 & 1952 indicated DNA as the genetic material.

  • Historical Context:

    • The race to discover DNA's structure began after Watson & Crick published their model in 1953.

  • Structure:

    • DNA is characterized as a double-stranded, complementary helix.

The Structure of DNA: Subunits

  • Nucleotides:

    • DNA comprises nucleotides which are made up of three components:

    • A sugar (deoxyribose)

    • A phosphate group

    • A nitrogenous base

Nitrogenous Bases of DNA

  • Types:

    • Adenine (A)

    • Cytosine (C)

    • Guanine (G)

    • Thymine (T)

  • Chargaff's Rules:

    • A pairs with T (A=T)

    • G pairs with C (G≡C)

Sugar-Phosphate Backbone

  • Backbone Structure:

    • The sugar-phosphate backbone of DNA is maintained via covalent bonds, which are responsible for the stability of the nucleic acid structure.

  • Hydrogen Bonds:

    • Base pairing occurs through hydrogen bonds between paired nucleotides.

  • Key Measurements:

    • Each base pair is approximately 0.34 nm apart.

The Discovery of DNA Structure

  • Nobel Prize Recognition (1962):

    • Awarded to Watson, Crick, Wilkins, and Franklin for their groundbreaking work on DNA’s structure.

  • Significance:

    • Their discovery has compelling implications for understanding genetic information and replication mechanisms.

DNA Replication

  • Mechanism:

    • Replication is semiconservative: each parental DNA strand directs the formation of a new complementary strand.

  • Enzymatic Role:

    • DNA Polymerase: The enzyme responsible for DNA replication, catalyzing the addition of nucleotides.

DNA Replication Process

  • 1. Separating Strands:

    • Functions through the breaking of hydrogen bonds by helicase, unwinding the double helix.

  • 2. Adding Nucleotides:

    • DNA polymerase uses base-pairing rules (A to T, C to G) to add complementary nucleotides.

The Polymerase Chain Reaction (PCR)

  • Definition:

    • A laboratory technique designed to amplify a specific segment of DNA for analysis.

  • Lab Procedure:

    • Key ingredients:

    • DNA sample

    • Free nucleotides

    • DNA polymerase

    • Primers (short DNA sequences that guide amplification)

  • Temperature Cycles:

    • Heating: Separates the DNA strands.

    • Cooling: Allows nucleotide pairing to occur.

  • Results:

    • Enables billions of copies to be generated from a minimal starting quantity of DNA.

DNA Profiling: Overview

  • Purpose:

    • Identifies unique characteristics in individual DNA.

  • Genome Analysis:

    • Instead of sequencing the entire genome (costly), specific portions are analyzed for profiling.

DNA Profiling: Short Tandem Repeats (STRs)

  • Characteristics:

    • STRs are repeating sequences of DNA that vary in length among individuals.

  • Application in PCR:

    • STRs can be specifically targeted using PCR, which aids in profiling.

Making a DNA Profile

  • 1. Collection:

    • Obtain cell samples and ensure no contamination occurs.

  • 2. Amplification:

    • Amplify STR regions using PCR; more amplified regions yield higher certainty in profiling.

  • 3. Separation of DNA Fragments:

    • Utilize Gel Electrophoresis: A method to separate DNA fragments by size, with smaller fragments traveling further in the gel.

  • 4. Comparing Banding Patterns:

    • Visualization via fluorescence reveals distinct patterns for each individual's DNA.

Implications of DNA Profiling

  • Multiple STR Analysis:

    • Examines various STR regions, significantly reducing the probability of coincidental matches.

  • Legal Reliability:

    • DNA evidence is deemed more reliable than traditional evidence forms, minimizing false positives.

    • Comparative statistics: Error rates for bite mark identification can reach up to 91%, whereas hair analysis can only exclude suspects, not positively identify them.

Summary

  • DNA Overview:

    • DNA is the hereditary molecule essential for all living organisms and organizes into chromosomes within the nucleus of eukaryotic cells.

  • Human Genetics:

    • Humans possess 23 pairs of chromosomes, inheriting one chromosome from each parent.

  • Double Helix Structure:

    • Comprised of two strands of nucleotides held by base-pairing rules.; this complementary pairing is crucial for replication.

  • PCR Significance:

    • Facilitates the replication of specific DNA sequences and is instrumental in forensic analysis utilizing STRs for DNA profiling.