Chapter 21
DNA Biology and
Mader, Sylvia S. Human Biology. 13th Edition. McGraw-Hill, 2014.
Points to Ponder
What are three functions of DNA?
Review DNA and RNA structure.
What are the 3 types of RNA and what are their functions?
Compare and contrast the structure and function of DNA and
How does DNA replicate?
Describe transcription and translation in detail.
Describe the genetic code.
Review protein structure and function.
What are the 4 levels of regulating gene expression.
What did we learn from the human genome project and where are
we going from here?
What is ex vivo and in vivo gene therapy?
Define biotechnology, transgenic organisms, genetic engineering
and recombinant DNA.
What are some uses of transgenic bacteria, plants and animals?
21.1 DNA and RNA structure and function
What must DNA do?
1. Replicate to be passed on to the next
2. Store information
3. Undergo mutations to provide genetic diversity
21.1 DNA and RNA structure and function
DNA structure: A review
Double-stranded helix
Composed of repeating nucleotides made of
– a pentose sugar
– phosphate
– a nitrogenous base
Sugar and phosphate make up the backbone while the
bases make up the “rungs” of the ladder
Bases have complementary pairing
– cytosine (C) pairs with guanine (G)
– adenine (A) pairs with thymine (T)
The Bases
Figure 2–22b, c
21.1 DNA and RNA structure and function
DNA structure
21.1 DNA and RNA structure and function
How does DNA replicate?
1. Two strands unwind by
breaking the H bonds
2. Complementary nucleotides
are added to each strand
by DNA polymerase
3. Each new double-stranded
helix is made of one new
strand and one old strand
**The sequence of bases
makes each individual
DNA Replication
• DNA helicases unwind the DNA and separates the
• DNA polymerase bind to the DNA and synthesizes
complementary antiparallel strands
• DNA rewinds into double helix molecules
– New molecules contains one strand of the original DNA
and one newly synthesized strand
21.1 DNA and RNA structure and function
DNA replication
• Cell has repair enzymes that usually fix errors in DNA replication
• A replication error that persists is a mutation
– A permanent change in the sequence of bases that can cause
a change in phenotype and introduce variability
• Most non-infectious disease, conditions, and disorders are due to
mutations in the DNA that change the amino acids in the protein
• Point mutations
– change in 1 base of DNA can be a silent mutation if the amino acids
is not changed
• Insertion mutation
– addition of a base which changes the reading frame
– whole protein after the mutation is wrong
• Deletion Mutation
– removal of a base, alter reading frame, wrong protein is made
21.1 DNA and RNA structure and function
RNA structure and function
Composed of repeating nucleotides
Sugar-phosphate backbone
Bases are A, C, G and uracil (U)
Three types of RNA
– Ribosomal (rRNA):
• joins with proteins to form ribosomes
– Messenger (mRNA):
• carries genetic information from DNA to the
– Transfer (tRNA):
• transfers amino acids to a ribosome where they are
added to a forming protein
21.1 DNA and RNA structure and function
RNA structure
21.1 DNA and RNA structure and function
Comparing DNA and RNA
• Similarities:
– Are nucleic acids
– Are made of
– Have sugar-phosphate
– Are found in the
• Differences:
– DNA is double stranded
while RNA is single
– DNA has T while RNA
has U
– DNA has a deoxyribose
sugar while RNA has a
ribose sugar
– RNA is also found in the
cytoplasm as well as the
nucleus while DNA is not
Gene Expression
DNA provides the cell with blueprints for
synthesizing proteins
• DNA resides in the nucleus and protein
synthesis occurs in the cytoplasm
1. mRNA carries a copy of DNA’s blueprint
into the cytoplasm
2. Other RNA molecules (rRNA and tRNA)
are involved in protein synthesis
21.2 Gene expression
Proteins: A review
• Composed of subunits of amino acids
• Sequence of amino acids determines the shape
of the protein
• Synthesized at the ribosomes
• Important for diverse functions in the body
including hormones, enzymes and transport
• Can denature causing a loss of function
Proteins: A review of structure
21.2 Gene expression
2 steps of gene expression
1. Transcription – DNA
is read to make a
mRNA in the
nucleus of our cells
2. Translation –
Reading the mRNA
to make a protein in
the cytoplasm
Overview of transcription and translation
Genetic code
Made of 4 bases
Bases act as a code
for amino acids in
Every 3 bases on
the mRNA is called
a codon that codes
for a particular
amino acid in
Step 1: Gene Activation
• Uncoils DNA
• Start and stop codes on DNA mark
location of gene
– Locates area for mRNA transcription
Step 2: DNA to mRNA
• Enzyme RNA polymerase transcribes
– binds to promoter (start) sequence
– reads DNA code for gene
– binds nucleotides to form messenger RNA
– mRNA duplicates DNA coding strand, uracil
replaces thymine
Step 3: RNA Processing
• At stop signal, mRNA detaches from DNA
– code is edited (RNA processing)
– unnecessary codes (introns) removed
– good codes (exons) spliced together
– triplet of 3 nucleotides (codon) represents one
amino acid
21.2 Gene expression
1. Transcription
mRNA is made from
a DNA template
mRNA is processed
before leaving the
mRNA moves to the
ribosomes to be read
Every 3 bases on the
mRNA is called a
codon and codes for
a particular amino
acid in translation
Processing of mRNA after transcription
Modifications of mRNA:
• One end of the RNA is capped
• Introns removed
• Poly-A tail is added
important for the nuclear export, translation and
stability of mRNA
Processing of mRNA after
• mRNA Splicing
– Cells use only certain exons rather then all to
form a mature RNA transcript
– Result can be a different protein product in
each cell
– Alternate mRNA splicing may account for the
ability of a single gene to result in two different
proteins in a cell
21.2 Gene expression
2. Translation
1. Initiation
mRNA binds to the small ribosomal subunit and
causes 2 ribosomal units to associate
2. Elongation: polypeptide lengthens
tRNA picks up an amino acid
tRNA has an anticodon that is complementary to the
codon on the mRNA
tRNA anticodon binds to the codon and drops off an
amino acid to the growing polypeptide
3. Termination
a stop codon on the mRNA causes the ribosome to
fall off the mRNA
Overview of transcription and translation
21.2 Gene expression
Regulation of gene expression
1. Transcriptional control (nucleus):
– chromatin density  must decondense to
allow for activation
– transcription factors  DNA-bind proteins
regulate activity of a gene
2. Posttranscriptional control (nucleus)
– mRNA processing
3. Translational control (cytoplasm)
– Differential ability of mRNA to bind ribosomes
4. Posttranslational control (cytoplasm)
– changes to the protein to make it functional
What did we learn from the human genome
project (HGP)?
Genes are a sequence of DNA bases that is
transcribed into RNA molecules and may be
translated into protein
Humans consist of about 3 billion bases (A,T,G,C)
and 25,000 genes
Human genome sequenced in 2003
Many polymorphisms or small regions of DNA that
vary among individuals were identified
Some have not physiological effects
Others contribute to the diversity of human beings and
possibly disease
Genome size is not correlated with the number of
genes or complexity of the organisms
21.3 Genomics
What is the next step in the HGP?
Functional genomics
Understanding how the 25,000 genes function
Understanding the function of gene deserts
(82 regions that make up 3% of the genome
lacking identifiable genes)
Comparative genomics
Help understand how species have evolved
Comparing genomes may help identify base
sequences that cause human illness
Help in our understanding of gene regulation
21.3 Genomics
New endeavors
Proteomics – the study of the structure,
function and interactions of cell proteins
Can be difficult to study because:
protein concentrations differ greatly between cells
protein location, concentration interactions differ from
minute to minute
– understanding proteins may lead to the
discovery of better drugs
Bioinformatics – the application of computer
technologies to study the genome
– May allow scientists to find cause-and-effect
relationships between genetic profiles and
disorders caused by multifactorial genes
How can we modify a person’s genome?
Gene therapy - insertion of genetic material
into human cells to treat a disorder
– Ex vivo therapy – cells are removed for a
person altered and then returned to the patient
– In vivo therapy – a gene is directly inserted into
an individual through a vector (e.g. viruses) or
directly injected to replace mutated genes or to
restore normal controls over gene activity
Gene therapy has been most successful in
treating cancer
Ex vivo gene therapy
-RNA virus that is replicated in a host cell via the enzyme reverse
transcriptase to produce DNA from its RNA genome.
- DNA is incorporated into the host's genome
- Virus replicates as part of the host cell's DNA.
21.4 DNA technology
DNA technology terms
• Genetic engineering
– altering DNA in bacteria, viruses, plants and animal cells
through recombinant DNA technology
• Recombinant DNA
– contains DNA from 2 or more different sources
• Transgenic organisms
– organisms that have a foreign gene inserted into them
• Biotechnology
– using natural biological systems to create a product or to
achieve an end desired by humans
21.4 DNA technology
DNA technology
Gene cloning through recombinant DNA
Polymerase chain reaction (PCR)
DNA fingerprinting
Biotechnology products from bacteria,
plants and animals
21.4 DNA technology
Recombinant DNA – contains DNA from 2 or more
different sources that allows genes to be copies
1. Gene cloning
The gene of interest is inserted into a vector
Vector is typically a plasmid, small accessory rings of
DNA found in bacteria
An example using bacteria to clone the human
insulin gene:
Restriction enzyme
• cut the vector (plasmid) and the human DNA with the
insulin gene
– DNA ligase
• seals together the insulin gene and the plasmid
– Bacterial cells uptake plasmid and the gene is copied
and product can be made
21.4 DNA technology
2. Polymerase chain reaction (PCR)
Used to clone small
pieces of DNA
DNA polymerase to
carry out DNA
deoxyribose, and base)
Important for amplifying
DNA for analysis such
as in DNA fingerprinting
21.4 DNA technology
3. DNA fingerprinting
DNA Fragments are
separated by their
charge/size ratios
Results in a
distinctive pattern for
each individual
Often used for
paternity or to
identify an individual
at a crime scene or
unknown body
21.4 DNA technology
4. Biotechnology products: Transgenic
• Important uses:
– Insulin
– Human growth
hormone (HGH)
– Clotting factor VIII
– Tissue plasminogen activator (t-PA)
– Hepatitis B vaccine
– Bioremediation – cleaning up the
environment such as oil degrading
21.4 DNA technology
4. Biotechnology products: Transgenic plants
• Important uses:
– Produce human proteins in their seeds such as
hormones, clotting factors and antibodies
– Plants resistant to herbicides
– Plants resistant to insects
– Plants resistant to frost
• Corn, soybean and cotton plants are commonly
genetically altered
• In 2001:
– 72 million acres of transgenic crops worldwide
– 26% of US corn crops were transgenic crops
21.4 DNA technology
4. Biotechnology products: Transgenic plants
4. Biotechnology products: Transgenic animals
• Gene is inserted into the egg that when
fertilized will develop into a transgenic
• Current uses:
– Gene pharming: production of pharmaceuticals
in the milk of farm animals
– Larger animals: includes fish, cows, pigs, rabbits
and sheep
– Mouse models: the use of mice for various gene
– Xenotransplantation: pigs can express human
proteins on their organs making it easier to
transplant them into humans