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Molecular Basis of
Heredity
 Discovery
of DNA
 Structure and Function of
DNA
 Replication
 Transcription
 Translation
Discovery of DNA:
Frederick Griffith – 1928
Wanted to know how bacteria caused
pneumonia
 Injected mice with disease-causing
strain  mice died
 Injected mice with harmless strain 
mice lived
 Injected mice with heat killed,
disease-causing strain and harmless
strain mixed together  mice died

Discovery of DNA:
Frederick Griffith – 1928

Transformation:
The heat-killed bacteria passed their
disease-causing ability to the
harmless strain
 One strain of bacteria was changed
into another

Oswald Avery – 1944



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Repeated Griffith’s work
Treated the heat-killed bacteria to enzymes
that broke down everything but DNA 
bacteria still transformed
Treated the heat-killed bacteria to enzyme
that broke down DNA  bacteria did not
transform
Discovered that DNA is the nucleic acid that
stores and transmits genetic information
Alfred Hershey & Martha
Chase – 1952
Bacteriophage – “bacteria eater” (a
virus that infects and kills bacteria)
 Placed a radioactive marker on
phosphorus (DNA) and sulfur (protein)
 Only radioactive phosphorus was
found in the bacteria
 Thus, the virus only injected DNA into
the bacteria not protein

DNA Structure Discovery
Please review DNA discovery notes
handed out in class.
 Portfolio worthy narrative account =


Watson, Crick, Wilkens, and Franklin
controversy newspaper article
Function of DNA
DNA carries information from one
generation to the next
 DNA determines the heritable
characteristics of organisms
 DNA is easily copied

Structure of DNA

Double helix
Anti-parallel
 Complementary
 Sugar phosphate backbone
 Nitrogenous bases in the center held
together with hydrogen bonds
 Chargaff’s Rule = A binds with T, C
binds with G

Structure of DNA

Nucleotide =



A phosphate
group
A deoxyribose
sugar (5 carbon)
A nitrogenous
base
•
•
•
•
Adenine
Thymine
Cytosine
Guanine
Chromosome Structure
“supercoils”
DNA Replication Overview

Each strand of the double helix can be used
as a template for a new strand of DNA



“Semi-conservative” each new DNA
molecules contains one new strand and one
old strand
Prokaryotes = replication is simple; typically
one replication fork (circular DNA)
Eukaryotes = replication is more complex;
hundreds of replication forks
The Cell Cycle

DNA is replicated
during the S phase
of the cell cycle
Replication Enzymes
Gyrase – Unwinds the supercoils
 Helicase – Unwinds the double helix
 Single-strand Binding Proteins –
stabilizes the DNA strands and keeps
them apart
 Primase – Attaches the RNA primer to
the parent DNA strand to begin
replication

Replication Enzymes
(continued)

DNA Polymerase (3 functions) –
1.
2.
3.

Adds new nucleotides to the growing
DNA strand
Proofreads and makes repairs when
needed
Replaces RNA primer with DNA
nucleotides
Ligase – joins and bonds the DNA
fragments together to form a complete
double helix
How Replication Occurs

DNA is synthesized in the 5’  3’
direction only!!!

This means that new nucleotides are
attached to the 3’ carbon of the
deoxyribose molecule.
Replication occurs in the nucleus!
 View the DNA Replication streaming
video now and complete the
replication activities

How Replication Occurs

Depending on how the replication fork
opens :
 Continuous replication  occurs on
the leading strand (new strand is made
continuously in the 5’  3’ direction)
 Discontinuous replication  occurs
on the lagging strand (new strand is
made in fragments called Okazaki
fragments)
DNA Replication VIDEO
Watch DNA Replication streaming
video from PBS.
 http://player.discoveryeducation.com/i
ndex.cfm?guidAssetId=0CB6B02F092A-4035-98B66378AF13F567&blnFromSearch=1&p
roductcode=US

Telomeres

Short repetitive sequence of DNA
• ex. TTTAAGGG (guanine rich)


Protect the ends of the chromosome from
deterioration
Over time there is loss of DNA at the 5’ end of the
lagging strands
• RNA primers cannot be replaced with DNA if there is no
DNA after it for DNA polymerase to bind!

Causes aging in somatic (body) cells!
• Telomerase (enzyme that regenerates telomeres) only
occurs in germ cells (sex cells) and malignant cells!
Turn and Talk

What is the consequence of losing
telomeres on the 5’ end of the lagging
strands of DNA molecules?

What could happen if we could
prevent that loss?
Transcription and
Translation
Structure of RNA

RNA Nucleotide =
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5-carbon sugar (Ribose)
Phosphate group
Nitrogenous base
• Adenine, cytosine, guanine, uracil
• No thymine (only in DNA)

Single stranded molecule


Not a double helix like DNA
Blueprint of DNA (DNA is the Master plan)
Types of RNA

3 main types =

Messenger RNA (mRNA)
• Carries copy of DNA message to the
ribosome to be made into a protein

Transfer RNA (tRNA)
• Transfers amino acids to the ribosome
based on the mRNA coded message

Ribosomal RNA (rRNA)
• Reads the mRNA coded message like a
decoder ring
Transcription Overview
Transcription begins in the nucleus
and ends in the cytoplasm
 To make mRNA  RNA polymerase



Binds to DNA and uses one strand as
a template for a molecule of mRNA
How does it know where to bind?

Promoters  specific sequences in
DNA that signal RNA polymerase to
bind there (also tells when to stop)
mRNA Editing

mRNA must be edited before moving from
the nucleus to the cytoplasm
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Introns – these intervening (non-coding)
sequences must be cut out
Exons – Coding sequences that encode for
a specific protein
No clear understanding why introns must be
removed
Only the mature (“edited”) mRNA moves to
the cytoplasm
The Genetic Code

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Proteins are made using amino acids joined together by
peptide bonds
20 different amino acids
The code consists of 4 letters:
 A, U, C, and G (RNA bases)
The genetic code is read 3 letters at a time
 mRNA “Codon” = 3 bases (AUG)
 tRNA “Anti-codon” = 3 complimentary bases (UAC)
 64 possible 3-base codons (some amino acids have
more than one codon that codes for it)
Each amino acid has an amino group, a carboxyl group,
and an R-group.
 The R-group gives the amino acid it’s unique
personality!!!
 The peptide bond forms between the amino group of
one amino acid and the carboxyl group of another!
The Genetic Code
(continued)

Start codon (for all proteins) =
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AUG  methionine
Several stop codons (do not code for
an amino acid…allows for release of
the protein from the ribosomal
complex)

UGA, UAA, UAG
Translation (or protein
synthesis) Overview
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mRNA serves as instructions for the protein to be
made (made during transcription)
Translation begins when an mRNA molecule attaches
to the ribosomal complex and begins with the 1st
codon (AUG)
tRNA (the “anticodon”) transfers the corresponding
amino acid to the ribosome.
As each codon is read tRNA brings the corresponding
amino acids to the ribosome
The amino acids are bonded to each other via a
peptide bond
Once a stop codon is reached the protein molecule is
released
Ribosomal complex
Mutations
Mutations  Changes in the genetic
material
 2 Types:

Gene Mutation
 Chromosome Mutation

Gene Mutations

Point mutations  Occurs at a single point
in the DNA sequence

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Could change one of the amino acids
Example: AAA TTT (normal)
AAC TTT (mutation)
Frameshift mutations  Addition or deletion
of a nucleotide in the DNA


Changes the “reading frame” of the code
Consequences more serious
Example: AAA TTT (normal)
AAT TT (mutation)
Chromosomal Mutations

Involves changes in the number or
structure of chromosomes.
Deletion
 Duplication
 Inversion
 Translocation

Gene Regulation
Genes are not always “on”
 Genes are regulated to turn “on” and
“off”
 In Prokaryotes:

The Lac Operon (a series of genes
that work together)  breaks down
lactose if present into galactose and
glucose.
 These genes are turned off by
repressors and are only turned on by
the presence of lactose.

Eukaryotic Gene Regulation
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Genes are controlled individually
Have regulatory sequences that are much
more complex than prokaryotic gene
regulation
Why are they more complex?

Cell specialization!!!
• Each cell has DNA for the whole
organism’s functioning, however, only
liver cells need to produce liver
proteins (etc.)
Regulation and
Development
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Differentiation
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Cells become specialized in structure and
function
Hox genes
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Controls the differentiation of cells and
tissues in the embryo (controls the “body
plan”)
• Example: Mouse eye gene inserted into
the “knee” of a fly gene  fly grew an eye
on its leg!!!
Genes have descended from a common
ancestor
Genetic Engineering
Selective Breeding

Humans take advantage of naturally
occurring genetic variations

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Hybridization
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Select desired traits to pass on to the next
generation (domestic animals)
Cross dissimilar individuals to bring out the
best of both organisms (“Hybrid vigor”)
Inbreeding

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Maintains the desired characteristics of a
line of organisms (although not without risk)
Example: Dog breeds
Increasing Variation
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Breeders can increase variation in a population
by inducing mutations
 Radiation and chemicals
 Many mutations are harmful to the organism
New Kinds of Bacteria
 Development of useful strains of bacteria
(digestion of oil)
New Kinds of Plants
 Produces polyploid (multiple sets of
chromosomes) individuals  in plants, larger
and stronger than diploid individuals (fatal in
animals)
Manipulating DNA

Different techniques are used to:
Extract DNA from cells
 Cut DNA into smaller pieces
 Identify the sequence of bases in a
DNA molecule
 Make unlimited copies of DNA

Tools of Molecular Biology
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Makes changes in the DNA code of a living organism
DNA Extraction (SLE A1: banana DNA extraction lab)
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Cutting DNA
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DNA is separated from the rest of the cell
using a simple chemical procedure
Restriction enzymes  cuts specific
sequences of nucleotides
Separating DNA
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Gel electrophoresis  a DNA sample is
placed at one end of a porous gel and an
electric current is applied making the DNA
fragments separate according to size
• Large fragments move more slowly than
short fragments
Using the DNA sequence

Reading the sequence
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Cutting and Pasting
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Creates a series of dye-tagged copies from
which the order tells the exact sequence of
DNA
Recombinant DNA  DNA molecules
produced by combining DNA from different
sources (DNA synthesizers)
Making copies

Polymerase Chain Reaction (PCR)  makes
several copies of the same gene by
repeated heating and cooling
Applications of Genetic
Engineering
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Transgenic Organisms (contains genes from other organisms)
 Transgenic bacteria
• Useful for health (bacteria can be transformed to create
human insulin and other forms of proteins) and
industry (raw materials for plastics and synthetic fibers)
 Transgenic animals
• Used to study genes (example: mice with human
immune systems) and improve the food supply
 Transgenic plants
• Important part of our food supply (25% corn and 52%
soybeans have been modified)
Cloning
 A member of a population of genetically identical cells.
 Easy to do with microorganisms/Hard with multicellular
organisms
• There are ethical concerns too!