DNA Structure and Replication

Driving Questions

  • Function of DNA and its cellular organization

  • Structure of DNA and uniqueness of individual DNA

  • Mechanism of DNA copying in living cells; amplification for forensics

  • Usage of DNA profiling and genetic variation in sequences

DNA as Evidence

  • Used by the Innocence Project to absolve wrongly convicted individuals.

  • DNA testing is crucial in court, providing accurate matches between evidence and suspects.

What is DNA?

  • Deoxyribonucleic acid (DNA): hereditary molecule, common to all organisms, instructs individual development.

  • Located in nucleus of eukaryotic cells as chromosomes (DNA + proteins).

Human Chromosomes

  • 23 pairs total; one from each parent.

  • 23rd pair (XX or XY) determines sex.

DNA Structure

  • Composed of nucleotides (sugar, phosphate, base).

  • Forms a double helix: sugars and phosphates create the backbone, bases serve as rungs.

    • Bases: adenine (A), thymine (T), guanine (G), cytosine (C).

  • Complementary base pairing: A pairs with T; C pairs with G.

Genes and DNA Characteristics

  • Genes: units of heredity impacting phenotypes.

  • Each individual's DNA sequence is unique, determined by the arrangement of bases.

DNA Replication

  • Semiconservative mechanism: creates two identical DNA molecules, each with an original and a new strand.

  • Hydrogen bonds are broken by helicase, uncoiling the helix.

  • DNA polymerase adds complementary nucleotides (5' to 3' direction).

Polymerase Chain Reaction (PCR)

  • Technique for amplifying specific DNA segments for study.

  • Requires DNA, nucleotides, polymerase enzyme, and primers.

  • Cycles of heating and cooling replicate DNA, producing billions of copies from few molecules.

DNA Profiling Overview

  • Identifies unique DNA characteristics; focuses on specific genomic portions.

  • Short Tandem Repeats (STRs): effective segments for profiling; vary in length between individuals.

Steps to Create a DNA Profile

  1. Collect and extract DNA from cells (avoid contamination).

  2. Amplify STR regions using PCR.

  3. Separate DNA fragments using gel electrophoresis.

  4. Compare STR patterns visible via fluorescence.

  5. Analyze multiple regions for increased specificity of profiles.

Summary

  • DNA is the hereditary molecule packaged into chromosomes within the cell nucleus.

  • Each human has 23 chromosome pairs inherited from parents.

  • DNA’s double helix structure allows for precise replication and profiling using STRs in forensics.