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Modern
Molecular
Genetics
By the early 1920’s, scientists
knew that chromosomes were
made up of two substances,
DNA and protein.
1.
2.
In recent years, biochemists have found
that the DNA of chromosomes is the
genetic material that is passed form
generation to generation. (It is known as
the molecule of life.
To demonstrate that DNA was the
substance that determined which traits
were inherited, many experiments
(including the British researcher
Frederick Griffth) were performed
Frederick Griffith
In 1928, Griffith found that a
substance from dead
pneumonia bacteria was
transformed into pneumonia
causing ones.
He called the substance a
transforming factor.
It was later proven that DNA
was the transforming factor.
The transmission of genetic
material from the
pneumonia-causing bacteria
into the harmless
pneumonia bacteria
changed it into pneumoniacausing bacteria.
(I) DNA Structure
A very large molecule consisting of
thousands of smaller, repeating
units known as nucleotides.
DNA is found within the nucleus of
the cell.
(A) DNA Nucleotide
A DNA nucleotide is composed of three parts:
1. A phosphate group
2. A deoxyribose (5-carbon sugar) molecule
3. A nitrogenous base of either adenine, thymine,
guanine, or cytosine
http://bioweb.wku.edu/courses/BIOL115/Wyatt/Biochem/Protein/chime_script1.htm
(B) Watson-Crick Model
In 1935 James Watson and Francis Crick
developed a model of the DNA molecule.
In this model, the DNA molecule consists of two
complimentary chains of nucleotides in a “ladder”
type organization.
The four nitrogenous bases of the DNA molecule
bond together in only one way:
adenine (A) with thymine (T)
cytosine (C) with guanine (G)
James Watson (L) and Francis Crick (R), and
the model they built of the structure of DNA
Double-helix Structure of DNA
Each “step” of the
ladder consists of
nitrogenous bases
bonded together by
weak hydrogen bonds.
The two chains of the
DNA molecule are
twisted to form a
spiral, or double-helix.
(II)
1.
2.
3.
DNA Replication
DNA, unlike any other
chemical compound, can
make exact copies of
itself by a process known
as replication.
In replication, the doublestranded DNA helix
unwinds; the two strands
then separate, or unzip,
by the breaking of the
hydrogen bonds between
pairs of bases.
Free nucleotides in the
nucleus then bond to the
complimentary bases of
the DNA strands.
Replication
produces two
identical DNA
molecules that are
exact copies of the
original molecule.
DNA Replication Animation
Genes and Proteins
Every cell can be thought as a chemical
factory.
Genes, which instruct cells to make
enzymes, are therefore really packages
of information that tell a cell how to make
proteins (long chain of amino acids).
Genes are specific sections of DNA
molecules that are made up of linear
sequences of nucleotides.
(III) RNA (Ribonucleic acid)
RNA is a nucleic acid, like DNA, composed
of nucleotide building blocks.
There are three major differences between
the structure of DNA and RNA:
1. In RNA, ribose is substituted for
deoxyribose.
2. uracil (U) is substituted for thymine (T)
3. RNA consists of only a single strand of
nucleotides.
Genetic Code
A genetic code contains the
information for the sequence of
amino acids in a particular protein.
This code is present in mRNA
molecules and is three bases long.
This is known as a codon.
Ex: UAG - is a codon
Genetic Codes
DNA Sequencing
From DNA to RNA
DNA is copied into RNA by a process
called transcription.
Transcription is similar to DNA
replication:
1. The DNA double-helix opens up.
2. Special enzymes begin to match up
RNA nucleotides with the correct
nucleotides in DNA.
3. A messenger RNA or mRNA molecule
is built.
Messenger RNA (mRNA)
1.
2.
3.
When portions of DNA molecules unwind
and separate, RNA nucleotides pair with
complimentary bases on the DNA strand.
This forms a mRNA that is complimentary
to the DNA strand.
The sequence of nucleotides in the mRNA
contain the genetic code.
The genetic code for each amino acid is a
sequence of three nucleotides forming a
codon.
mRNA
tRNA
Known as transfer RNA
Contains a triplet of
nucleotides called the
anticodon.
At the other end of the
molecule, the amino
acid is attached.
The
anticodon of
tRNA
matches the
codon of the
mRNA.
(IV) Translation
1.
2.
3.
4.
5.
6.
Also referred to as Protein Synthesis.
In the cytoplasm, the mRNA becomes associated with a
ribosome.
Amino acids in the cytoplasm are “picked-up” by
molecules of transfer RNA (tRNA).
Each codon on the mRNA bonds with a corresponding
anticodon on a tRNA, which carries a specific amino
acid.
These amino acids are joined together by peptide
bonds.
The resulting chain of amino acids is a polypeptide.
Protein Synthesis Animation
V. Gene Expression and Cell Differentiation
The human body is made up of many different
types of cells.
All of these cells have the same DNA in them, so
why are they so different from each other?
The answer is that only certain genes are used in
certain cells. The use of the information from a
gene is called gene expression (which genes are
turned on).
Creating the special types of cells through
controlled gene expression is called cell
differentiation.
Without cell differentiation, our bodies
would be made up of only one type of cell.
VI Genetic Engineering
Genetic Engineering- is a new technology
that humans use to alter the genetic
instructions in organisms.
a) Biotechnology- The application of
technology to biological science.
ex: removal of dinosaur DNA from a
mosquito’s last meal.
b) Selective Breeding- A process that
produces domestic animals and new
varieties of plants with traits that are
particularly desirable.
An Example of Selective Breeding
English shorthorn
Brahman
cattle: Good beef but
cattle:
poor heat resistance.
Good
resistance to
heat but poor
beef.
Santa Gertrudis cattle:
Formed by crossing
Brahman and English
shorthorns; has good heat
resistance and beef.
DNA Technology
Makes it possible to put “new” genes
into organisms.
1. Human genes can be inserted into
bacteria.
2. These altered bacteria become
factories that produce human protein.
ex: Gene Splicing
Recombinant DNA
Plasmids
Are small DNA
fragments, are known
from almost all bacterial
cells.
Plasmids carry
between 2 and 30
genes. Some seem to
have the ability to move
in and out of the
bacterial chromosome
Gene Splicing
Allows a scientist to make cuts of DNA from 2
complimentary different organisms, perhaps a frog cell
and a bacterium.
Pieces of DNA from one organism can now be glued, or
spliced, into the DNA of another organism.
Recombinant DNA
Allows scientists to
insert the insulin
gene into bacterial
plasmids.
The bacteria that
contain this gene
produce insulin,
which is used by
people with diabetes.
Cloning
Is a technique that accomplishes the same end result
as asexual reproduction.
It is a way of making identical genetic copies.
Cloning is done by inserting a nucleus from a “parent”
organism’s cell (one that has a complete set of genetic
information from that individual) into an egg cell from
which the nucleus has been removed. The result is an
egg that now contains not 50%, but 100% of the genetic
information from a single parent.
If this new egg cell with all of its genes can be made to
develop normally, the resulting offspring is a clone of
the individual that donated the original cell (In
mammals, the egg would be implanted and develop
inside the body of the female).
In Vitro Fertilization
IVF (illustrated in the diagram at
right) is often used when a
woman's fallopian tubes are
blocked. First, medication is
given to stimulate the ovaries to
produce multiple eggs. Once
mature, the eggs are suctioned
from the ovaries (1) and placed
in a laboratory culture dish with
the man's sperm for fertilization
(2). The dish is then placed in an
incubator (3). About two days
later, three to five embryos are
transferred to the woman's
uterus (4). If the woman does
not become pregnant, she may
try again in the next cycle.