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DNA
CHAPTER 12
THE SECRET OF LIFE
Discovery

Gregor Mendel explained inheritance…
 Where
did this information come from? Where was it
stored? Scientists knew there were both proteins and
nucleic acids (DNA) in the nucleus of cells, but which
was involved in inheritance?
 Griffith:
found that one strain of bacteria
could be transformed into another
 Avery: identified the transforming molecule
 Hershey & Chase: experiments provided
definitive evidence that DNA was the
transforming factor
Discovery

Rosalind Franklin used XRay Diffraction to discover
the structure of DNA

Other scientists (Wilkins,
Watson & Crick) stole this
work, and used it to take
credit for themselves, this
later won them a Nobel
Prize
DNA

DNA: Deoxyribonucleic Acid
 Classified
as a nucleic acid –
genetic material! Blueprint for
living organisms! Passed from
generation to generation!

Nucleotide: basic structural
unit of DNA, made of a
1.
sugar (deoxyribose)
2.
a phosphate
3.
a nitrogenous base
DNA

The structure of DNA is called a double helix –
twisted ladder

The phosphate of one group bonds to the
sugar of another – creating a backbone on
the outside of the molecule [sides of the
ladder]

The nitrogen bases pair with each other on
the inside [rungs of the ladder]

The whole molecule is held together by
hydrogen bonds
DNA

The two sides of DNA
are antiparallel –
meaning they are
parallel to each other
but are oriented in
the opposite direction
of each other

The direction of one
strand is 5’ to 3’ and
the other is 3’ to 5’
DNA

Nitrogen bases are
either -

Purines: double ringed
bases
 Guanine
(G) and
Adenine (A)

Pyrimadines: single
ring bases
 Cytosine
(C) and
Thymine (T)
DNA

A purine base always binds to a pyrimidine base
to ensure equal distance between the two rails of
the ladder

Complementary base pairing  each base will
only bond with 1 other specific base
A
bonds to T [double hydrogen bond]
C
bonds to G [triple hydrogen bond]
DNA

Order of bases in one strand
determines the order of the
bases in the other strand

Chargaff’s Rule: within a
species the amount of
guanine nearly equals the
amount of cytosine, and the
amount of adenine nearly
equals the amount of
thymine
K’NEX
Following the
directions on your
handout, build your
model
PHOSPHATE
SUGAR
ASSEMBLED
NUCLEOTIDE
ADENINE
DOUBLE
HYDROGEN
CYTOSINE
TRIPLE
HYDROGEN
GUANINE
THYMINE
DNA Replication

DNA is copied during
mitosis and meiosis, which is
how each cell gets a copy
of your DNA

Semiconservative
Replication: the parent
strands separate, serve as
templates, and create 2
new molecules of DNA (1
strand of parental DNA
and 1 strand of new DNA)
DNA Replication

DNA helicase: enzyme responsible
for unwinding and unzipping the
double helix.

Hydrogen bonds are broken creating
two single strands

Free Floating nucleotides (in the
nucleus) match up to the parent
strands

DNA polymerase: adds new
nucleotides to the single strands and
proofreads
DNA Replication

During replication, one strand is the leading
strand, one is the lagging strand

The leading strand replicates continuously

The lagging strand replicates away from the
fork
 It
is synthesized discontinuously in small segments
called Okazaki Fragments  each about 100-200
nucleotides long
DNA Replication

In prokaryotic cells, DNA
is a loop (circular) in the
cytoplasm. The DNA
strand is opened at one
origin of replication

Eukaryotic DNA unwinds
in multiple areas during
replication
Ribonucleic Acid

Nucleic acid, single strand

Contains the sugar ribose

Has Uracil instead of Thymine
A
–U
C
–G
 Three
major types of RNA
 mRNA,
rRNA, tRNA
Ribonucleic Acid

Messenger RNA (mRNA): long strands of RNA
nucleotides that are formed complementary to
one strand of DNA.
 They
travel from the nucleus to the ribosome to
direct the synthesis of a specific protein

Ribosomal RNA (rRNA): associates with proteins
to form ribosomes in the cytoplasm

Transfer RNA (tRNA): smaller segments of RNA
nucleotides that transport amino acids to the
ribosome
DNA

DNA is read 3 bases at a time – nitrogen bases
spell out a message that creates proteins
 Amino
acids make up proteins and proteins
act as building blocks for cells and enzymes

Codon: sequence of three nucleotides that form
a unit of genetic code in a DNA or RNA molecule

DNA is TRANSCRIBED into RNA, RNA is
TRANSLATED into proteins  CENTRAL DOGMA
Transcription

Transcription: synthesis of DNA code into mRNA
 The
DNA is unzipped in the nucleus
 An
enzyme called RNA polymerase moves
along the template DNA strand
 It
reads the template strand and a
complementary mRNA strand is synthesized
[uracil is substituted for thymine]
 The
mRNA is released and moves into the
cytoplasm through nuclear pores in the nucleus
Transcription

mRNA code is significantly shorter than DNA

Introns: interrupting sequences that DO NOT
code for anything.

Exons: The coding regions where the genes
are located.

The introns are removed prior to translation.
Translation

Translation: code is read and translated to
make a protein
 Once
in the cytoplasm, mRNA moves to a
ribosome
 Here,
tRNA molecules interpret the mRNA codon
sequence
 tRNA
(clover shaped) is activated by an enzyme
by attaching a specific amino acid to each tRNA
 Anticodon:
unit of genetic code in tRNA that
corresponds to a complementary match the
codon in mRNA
Translation

The activated tRNA anticodon binds to the mRNA
codon in the ribosome

Additional activated tRNA’s continue to move into
the ribosome, binding to additional mRNA codons as
the mRNA moves through the ribosome like a ribbon.

As each codon and anticodon come together, the
mRNA bonds the amino acids on each tRNA, and
creates a growing polypeptide (protein) chain

This continues until a stop codon is reached and
then protein synthesis stops

The amino acid chain (polypeptide) folds into a
protein
Mutations

Sometimes errors
occur when
chromosomes are
copied

There may be little to
no effect because
the cell has some
repair mechanisms

Mutation: a
permanent change
in a cell’s DNA
Mutations
If a trait that hasn’t been
present in either family
suddenly appears, we
can guess a mutation
took place
 A mutation causes a
change in a child’s trait
only when it takes place
in the parents’ sex cells

Mutations


Mutations in the autosomal cells do not get
passed on
Mutations that occur in sex cells are passed on
to the organism’s offspring and will be present in
every cell of the offspring.
Mutations

Point Mutation: chemical change in just 1 base pair

Substitution: one base is exchanged for another
 Missense:
DNA will code for wrong amino acid
 Nonsense:
change codon to stop codon, causes
translation to stop early


Frameshift Mutation: the gain or loss of a nucleotide

Insertion: addition of a nucleotide to the DNA sequence

Deletion: removal of a nucleotide to the DNA sequence
These all disrupt normal protein production and can
sometimes cause genetic disorders
Causes of Mutations

Some are spontaneous

Some are caused from things
outside of the cell
 Radiation,
Mutagens,
Various chemicals nicotine, pesticides
Genetic Engineering

Genetic Engineering: manipulating the DNA of
one organism in order to insert the DNA into
another organism
 Jellyfish
fluorescence added to mosquito DNA

Has applications in human health and agriculture

Can be used to increase or decrease the
expression of specific traits

Biotechnology: The use of genetic engineering to
solve problems
Genetic Engineering

Glow in the dark cats - skin cells from Turkish Angora
female cats and used a virus to insert genetic instructions
for making red fluorescent protein

Venomous cabbage - gene that programs poison in
scorpion tails and looked for ways to combine it with
cabbage

Medicinal eggs - genetically modified hens that produce
cancer-fighting medicines in their eggs. The animals have
had human genes added to their DNA so that human
proteins are secreted into the whites of their eggs, along
with complex medicinal proteins similar to drugs used to
treat skin cancer and other diseases.

Web spinning goats - Researchers inserted a spiders’
dragline silk gene into the goats’ DNA in such a way that
the goats would make the silk protein only in their milk.
This “silk milk” could then be used to manufacture a weblike material called Biosteel.
Genetic Engineering

Clones: Using the DNA of one organism to
create another exact copy of the
organism
 Cloning
is a way of producing living
things with identical desirable traits.

Breeding: the bringing together of
two living things to produce offspring

Selective breeding: can bring out the
desired traits of living things
 examples:
dog breeding, agriculture,
hybridization
Genetic Engineering

Knowing what to expect tells us which living
things to breed for certain traits.

Inbreeding: process in which closely related
organisms are bred to ensure inheritance of
desired traits and elimination of undesired traits
 Can

be harmful, inheritance of recessive traits
Test Cross: organism with unknown genotype
bread with organism that is homozygous
recessive for desired trait
Genetic
Engineering

Designer Babies – using
genetic engineering to
choose the traits of your
offspring

Savior Siblings - children
conceived with the initial
purpose of acting as
donors for a sick brother
or sister.
Genetic Engineering

Gene Therapy – technique aimed at correcting
mutated genes that cause diseases
 Scientists
insert a normal gene into a chromosome
to replace a dysfunctional gene

Restriction Enzymes – cut a DNA molecule at a
particular place
 essential

tools for recombinant DNA technology
Recombinant DNA – combines segments of DNA
from different sources
Genetic Engineering

Gel Electrophoresis – electric
current is used to separate
DNA fragments according to
size
The Human Genome Project

The genetic
blueprint for a
human being

The mapping of the
genome was
finished in 2003, and
scientists are
continuing to
discover what each
gene does and
how it functions.