DNA Structure and Replication

Introduction

This chapter explores the structure and function of DNA, its role in heredity, the process of DNA replication, and the application of DNA analysis in forensics. Understanding these concepts is fundamental to modern biology and biotechnology.

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 used for forensics?

• How does DNA profiling make use of genetic variation to solve crimes?

DNA as Evidence

Forensic Applications of DNA

• The Innocence Project uses DNA evidence to free individuals wrongly convicted of crimes.

• DNA testing is a standard and highly accurate method in court cases for matching crime scene evidence to perpetrators.

• Forensic scientists identify individuals by analyzing the specific sequence of nucleotides in their DNA.

• DNA evidence is considered more reliable than many other forms of forensic evidence.

Example: DNA profiling can exclude innocent suspects and identify the true perpetrator in criminal investigations.

What is DNA?

Definition and Function

• Deoxyribonucleic acid (DNA) is the hereditary molecule passed from parents to offspring in all living organisms.

• DNA contains the instructions for building and maintaining an organism.

Location and Organization

• In eukaryotic cells, DNA is found in the nucleus.

• DNA is organized into structures called chromosomes.

• Each chromosome consists of a single DNA molecule wrapped around proteins called histones.

Human Chromosomes

Chromosome Number and Inheritance

• Humans have 23 pairs of chromosomes (46 total).

• One chromosome of each pair is inherited from the mother, and the other from the father.

• The 23rd pair determines biological sex: XX for female, XY for male.

Genome

• The genome is the complete set of genetic instructions encoded in the chromosomes of an organism.

Structure of DNA

Nucleotide Structure

• DNA is composed of repeating subunits called nucleotides.

• Each nucleotide consists of:

  • A five-carbon sugar (deoxyribose)

  • A phosphate group

  • A nitrogenous base

Double Helix Shape

• DNA consists of two strands that twist around each other to form a double helix.

• The sugar and phosphate groups form the backbone of each strand.

• The nitrogenous bases form the internal "rungs" of the helix.

Nitrogenous Bases and Base Pairing

• There are four types of nitrogenous bases in DNA:

  • Adenine (A)

  • Thymine (T)

  • Cytosine (C)

  • Guanine (G)

• Bases pair according to specific rules:

  • A pairs with T

  • C pairs with G

• Base pairs are held together by hydrogen bonds.

Base Sequence and Individuality

• The sequence of nucleotides along a DNA strand determines genetic information and individual characteristics.

• Each person's DNA sequence is unique (except for identical twins).

DNA Replication

Overview and Mechanism

• DNA replication is the process by which a cell makes an identical copy of its DNA.

• Replication is semiconservative: each new DNA molecule consists of one original strand and one newly synthesized strand.

Steps of DNA Replication

1. Strand Separation: The enzyme helicase breaks the hydrogen bonds between bases, unwinding the double helix.

2. Nucleotide Addition: The enzyme DNA polymerase reads each template strand and adds complementary nucleotides (A with T, C with G).

Key Enzymes

• Helicase: Unwinds the DNA double helix.

• DNA Polymerase: Synthesizes new DNA strands by adding nucleotides.

Polymerase Chain Reaction (PCR)

Definition and Purpose

• PCR is a laboratory technique used to amplify (make many copies of) a specific DNA segment.

• It enables scientists to study small amounts of DNA by generating billions of copies.

PCR Procedure

• Ingredients: Template DNA, nucleotides, DNA polymerase, primers (short DNA sequences that guide DNA polymerase).

• Steps:

  1. Denaturation: Heating separates the DNA strands.

  2. Annealing: Cooling allows primers to bind to the template DNA.

  3. Extension: DNA polymerase adds nucleotides to synthesize new DNA strands.

• These steps are repeated for many cycles to exponentially amplify the target DNA region.

DNA Profiling

Overview

• DNA profiling identifies unique characteristics in an individual's DNA.

• It is used in forensics to match biological samples to individuals.

Short Tandem Repeats (STRs)

• STRs are short DNA sequences repeated in tandem at specific locations in the genome.

• The number of repeats varies between individuals, making STRs useful for identification.

DNA Profiling Process

1. Sample Collection: Obtain cells (e.g., from cheek swabs) and extract DNA.

2. Amplification: Use PCR to amplify STR regions.

3. Separation: Separate DNA fragments by size using gel electrophoresis. Smaller fragments move faster through the gel.

4. Analysis: Compare the pattern of STR bands to create a DNA profile.

Reliability and Limitations

• Analyzing multiple STR regions increases the certainty of a match.

• DNA evidence can exclude suspects but cannot always positively identify a single individual (except for identical twins).

Summary Table: Key Features of DNA and DNA Profiling

Key Equations and Concepts

• Base Pairing Rule: ,

• Semiconservative Replication: Each new DNA molecule: $1+ new strand

• PCR Amplification: Number of DNA copies after cycles:

Conclusion

DNA is the hereditary molecule of all living organisms, packaged into chromosomes in the nucleus. Its unique sequence allows for individual identification through DNA profiling, which is a powerful tool in both biology and forensic science.