Chapter 3.1:
DNA
Unit 3: DNA
If cells are the building
blocks, then DNA is the
blueprint.
1. A Scientist Gone Rogue
Agenda
2. DNA Structure
3. The Four Nitrogenous Bases
4. Functions of DNA
5. Steps in DNA Replication
6. Recombinant DNA
7. DNA vs RNA
A Scientist Gone Rogue
• A simplified description of CRISPR technology
• CRISPR is a technique that allows scientists to make precise edits to any DNA by altering its
sequence.
• Alterations may result in a “knock out” gene which renders the target inactive or introducing or
removing a desired piece of DNA.
• Gene editing with the CRISPR system relies on an association of two molecules and the cells own
internal repair mechanism.
• One of the molecules is a protein, called Cas9, it is responsible for "cutting" the DNA.
• The other molecule is a short RNA molecule which works as a "guide" to bring Cas9 to the position
where it is supposed to cut. Once the cut has been made, the cells repair mechanism is activated
and attempts to repair the damaged DNA.
• This cell repair system introduces deletions, insertions, or modifications thereby editing a genome.
What is a genome?
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A Scientist Gone Rogue
• A Rogue Scientist
• In 2018, the world was shocked by the news
that two genetically modified babies, Lulu and
Nana, had been born.
• A Chinese biophysicist, He Jiankui, attempted to
use CRISPR technology to modify human
embryos making them resistant to HIV resulting
in these two twin girls.
• In 2020, China announced that a third CRISPER
baby had also been born.
A Scientist Gone Rogue
• By engineering mutations into human embryos,
which were then used to produce babies, Jiankui
leapt into an era in which science could rewrite
the gene pool of future generations by altering
the human germ line.
• He also flouted established norms for safety and
human protections along the way.
• There are many concerns about the unintended
consequences of the genetic edits in these
children, what their future holds, and the
consequence of such editing to future
generations.
A Scientist Gone Rogue
• In 2019, He Jiankui was sentenced to three
years in prison and fined three million yuan for
illegally carrying out the human embryo geneediting intended for reproduction.
• Two of his fellow researchers were also
sentenced.
• If we are to understand these emerging
technologies and be able to make informed
decisions about their use, we must at least
understand the structure and function of DNA.
Background Information
• The acronym DNA stands for deoxyribonucleic acid.
• DNA is the molecule that makes up chromosomes and serves as
hereditary information.
• Nucleic Acids
• There are two types of nucleic acids that we will look at here which are
DNA and RNA.
• Remember what you learned in unit 1 - both DNA and RNA are
polymers of nucleotides (i.e., chains of joined nucleotides).
• They form genetic material and are involved in the functioning of
chromosomes and in protein synthesis.
DNA Structure
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Structure
• The shape of DNA is referred to as a double helix made up of
repeating nucleotide units.
Structure
• Nucleotides that make up DNA are composed of
3 key parts:
• i) Phosphoric acid (Phosphate group)
• ii) 5 carbon sugar (Deoxyribose)
• iii) One of the four Nitrogenous bases:
The Four Nitrogenous Bases
• Purines and pyrimidines are nitrogenous
bases that make up the two different kinds of
nucleotide bases in DNA.
• The two-carbon nitrogen ring bases (adenine
and guanine) are purines
• While the one-carbon nitrogen ring bases
(thymine and cytosine) are pyrimidines.
Structure
• When the bases bond together, they form the "rungs"
of the DNA ladder and do so in a set pattern.
• The alternating deoxyribose sugar and phosphates
make up the rails (backbone).
• Adenine always bonds to thymine (two hydrogen
bonds)
• Guanine always bonds to cytosine (three hydrogen
bonds)
Structure
• This bonding of bases is called complementary base
pairing.
• The bases cannot bond any other way because 2
purines would overlap, and 2 pyrimidines would be
too short to form the rungs of the ladder.
• The double strand is held in place by hydrogen bonds
between the bases.
• It is the number and order as well as the type of
bases that determine what kind of organism will
develop.
Structure
• Example: ATCCGATT means something entirely
different than ACCGTTAT, just as the words hate and
heat mean different things even though they contain
the same letters.
• As a DNA strand lengthens, it twists into a double
spiral called a double helix.
Functions of DNA
• DNA
• 1. Replicates itself so each new cell has a
complete, identical copy.
• 2. Controls the activities of a cell by producing
proteins. The combination of proteins
determines the characteristics (phenotype) of
each living organism.
• 3. Undergoes occasional mutations (mistakes in
replication) which accounts for the variety of
living things on Earth.
Functions of DNA
• Steps in DNA Replication
• 1. The DNA molecule becomes untwisted by
enzymes breaking the bonds (topoisomerase).
• The two strands that make up DNA become
unzipped and each side acts as a template.
• (**Important** The weak hydrogen bonds
between the nitrogenous base pairs are broken
by the enzyme helicase.)
Functions of DNA
• Steps in DNA Replication
• 2. New complementary nucleotides, always
present in the nucleus, move into place and pair
with complementary bases on the exposed
strands.
• (**Important** The enzyme DNA polymerase
assists with the complementary base pairing.)
• A joins to T
• C joins to G
Steps in DNA Replication
• Steps in DNA Replication
• 3. The adjacent nucleotides, through their sugarphosphate components become joined together
along the newly forming chain.
(**Important**The enzyme ligase glues the
alternating sugar phosphate backbone
together).
• 4. When the process is finished, 2 complete
DNA molecules are present, identical to each
other and to the original molecule.
• 5. Both new DNA strands will now wind back up
into their helical shape.
Steps in DNA Replication
• DNA replication is called semiconservative because each new double helix is composed of an old
(parental) strand and a new (daughter) strand.
• Enzymes assist the unwinding process, join the nucleotides, and assist the rewinding process.
• When errors are made in replication a mutation can arise.
Steps in DNA Replication
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Recombinant DNA
• Definition: DNA having genes from 2 different
organisms, often produced in the laboratory by
introducing foreign genes into a bacterial plasmid
which makes a new combination of DNA.
• A vector is used to introduce recombinant DNA.
• A plasmid is the most common vector (they are small
rings of DNA found in bacteria).
Recombinant DNA
• The plasmid must be removed from the bacteria and
must have a foreign gene inserted into it.
• An enzyme (restriction enzyme) breaks the plasmid
DNA.
• The new foreign DNA can now be attached to the
plasmid.
• The enzyme, ligase, acts like the glue and sticks the
foreign DNA to the plasmid and makes it whole again.
• The plasmid DNA is then put back into the bacteria.
• Every time the bacteria replicates it produces a plasmid
with the foreign gene.
• Eventually there are many copies of the foreign gene.
Recombinant DNA
• Viral DNA can also be used as a vector to carry
recombinant DNA into a cell.
• When a virus containing recombinant DNA infects a cell,
the viral DNA enters.
• Here it can direct the reproduction of many more viruses.
• Each virus derived from a viral vector contains a copy of
the foreign gene, therefore viral vectors allows cloning of
a particular gene.
• Viral vectors are also used to create genomic libraries.
• A genomic library is a collection of engineered viruses
that carry all the genes of a species.
• It takes about 10 million viruses to carry all the genes of
a mouse.
Summary
• Segments of DNA (particular genes) can be inserted into bacteria and the bacteria will go on its
merry way and produce these genes.
• If desired genes are used - like those that produce certain chemicals (vaccines, antibodies, etc.)
then these proteins become much more available.
• Protein hormones like insulin can be made using yeast cells.
• Interferons, proteins used in cancer treatments to help the immune system are now mass-produced
this way.
Summary
• Uses of Recombinant DNA
• 1. Generate a DNA library, which will catalogue all the base sequences of known genes.
• 2. Identify specific genes (in 1998, the genes that mutate and cause prostate cancer were
identified).
• 3. Produce synthetic copies of genes to mass produce chemicals such as insulin.
• 4. Insert genetic material into chromosomes that will help regulate cell function to make the
organism genetically "better" (gene therapy).
Summary
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DNA vs RNA
• Both are nucleic acids made up of
nucleotides.
• Types of RNA:
• mRNA - messenger RNA
• Carry coding sequences for protein
synthesis
• tRNA - transfer RNA
• Carry amino acids to the ribosomes
during protein synthesis
• rRNA - ribosomal RNA
• Form the core of ribosomes
Practice
• To learn about the similarities and differences between DNA and RNA, label each statement below
as “DNA”, “RNA”, or “Both”
• 1. Nucleotides contain phosphate group Both
10. Single stranded
RNA
• 2. Deoxyribose sugar DNA
• 3. Adenine pairs with uracil RNA
• 4. Located in the nucleus only DNA
• 5. Double stranded DNA
• 6. Monomers contains nitrogen
Both
• 7. Cytosine pairs with guanine Both
• 8. Located in nucleus and cytoplasm RNA
• 9. Adenine pairs with thymine DNA
1. A Scientist Gone Rogue
Summary
2. DNA Structure
3. The Four Nitrogenous Bases
4. Functions of DNA
5. Steps in DNA Replication
6. Recombinant DNA
7. DNA vs RNA