Unit 2
The Molecule of Life
Genes and Heredity
DNA – A Preview
 The Book of You –Joe Hanson (TED Ed)
What is DNA? The Basics
Objectives
 Describe the DNA molecule (i.e. what it is made of,
importance/purpose, etc)
 Explain how DNA contributes to the diversity of life
forms
 Describe the chemical nature of chromosomes
(proteins and nucleic acids)
 Summarize the work of Watson and Crick in
discovering the DNA molecule
 Explain the structure of DNA (include a diagram)
Criteria for a material to
be hereditary:
 separate particle that can function independently
 found in the nuclear material of all cells
 every cell has the same kind and amount of material
 located in a safe place protected from damage
 able to be passed on to the next generation
 able to copy itself accurately
 carries instructions in chemical code
 able to translate messages from chemical codes into agents
that cause things to happen within the cell/body
The DNA Molecule
 Found in all cells of each living organism
 It is the only molecule that can replicate itself which
permits cell division
 Provides directions that guide the repair of damaged
cells or production of new ones
 Chemical messages carry information from the
nucleus to the cytoplasm where the directions are
carried out
DNA and Diversity
 Offspring are genetically different from their
parents due to exchange of genes during
crossing over and mutations
 In both cases,
the original DNA
code is changed to
produce a
genetically different
offspring
 What is a gene?
Chemical Nature of Chromosomes
Chromosomes are made up of proteins
and nucleic acids
 Protein
 Basic
Unit = amino acid
 Nucleic Acid
 Basic
Unit = nucleotide
Nucleotide
Made of:
Phosphates
 Sugar (deoxyribose)
 One of four different nitrogen bases





Adenine
Thymine
Guanine
Cytosine
Genetic code lies in the sequence of nucleotides, not the
proteins
Watson and Crick
Scientists who discovered the structure of DNA:
• Knowing structure leads to knowledge of how it functions
• Used X-Ray Crystallography
technology (by Rosalind Franklin)
to discover that the
structure was a double helix
• Studied DNA of different
organisms.
• They presented a 3-D model to scientific community in 1953
What they found out…..
 DNA is Double stranded
 Twisted into helix
 Has a beginning and an end
Nucleotides are found in all
organisms but the proportion
of nitrogen bases (A, G, T, C)
varied from species to species
 A=T, C=G suggested they are
arranged in pairs
DNA has 4 levels of
arrangement (structures)
dependent on the stage
of cell cycle
Example:
 as chromatin it is in its
primary structure
 as a chromosome in
prophase it is in its
quaternary structure
DNA Structure
 DNA consists of two molecules that are arranged
into a twisted ladder-like structure called a
Double Helix.
 A molecule of DNA is made up of millions of tiny
subunits called Nucleotides = Basic unit of DNA
 Each nucleotide consists of:
1. Phosphate group
2. Pentose sugar
3. Nitrogenous base
Nucleotides
Phosphate
Nitrogenous
Base
Pentose
Sugar
Genetic code lies in the sequence of nucleotides
Nucleotides
 The phosphate and sugar alternate to form
the backbone of the DNA molecule
 The bases are joined by hydrogen bonds form
the “rungs”.
 There are four types of nitrogenous bases.
Nucleotides
A
Adenine
C
Cytosine
T
Thymine
G
Guanine
Nucleotides
 Each base will only bond with one other
specific base.
 Adenine (A)
 Thymine (T)
 Cytosine (C)
 Guanine (G)
Form a base pair.
Form a base pair.
A
T
C
G
T
A
C
G
A
T
G
C
T
A
DNA Structure
Because of this
complementary base pairing,
the order of the bases in one strand
determines the order of
the bases in the other strand.
Break it Down….
What holds them together?
DNA Structure
 To crack the genetic code found in DNA we
need to look at the sequence of bases.
 The bases are arranged in triplets called
codons.
AGG-CTC-AAG-TCC-TAG
TCC-GAG-TTC-AGG-ATC
DNA Structure
 A gene is a section of DNA that codes for a
protein.
 Each unique gene has a unique sequence of
bases.
 This unique sequence of bases will code for the
production of a unique protein.
 It is these proteins and combination of proteins
that give us a unique phenotype.
Supplementary Resources
 Molecular Genetics
 The Structure of the DNA Molecule
 Build a DNA Molecule
 The Discovery of the Molecular Structure of
DNA
Summary of the Chemical Nature of
Chromosomes
Made up of proteins and
nucleic acids
 Protein

Basic Unit = amino acid
 Nucleic Acid

Basic Unit = nucleotide
*Genetic code lies in the sequence
of nucleotides, not the proteins
Nucleotide made of:
 Phosphates
 Sugar (deoxyribose)
 One of four different
nitrogen bases




adenine
thymine
guanine
cytosine
DNA
Gene
Protein
Trait