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Molecular Genetics
“Chapter 20”
By the 1950s scientists had determined that
chromosomes contained DNA and that DNA was
the genetic material.
The goal of molecular genetics is to find out how
DNA determines phenotype of an organism.
What happens to DNA during duplication of
chromosomes in mitosis?
Questions
Questions 1-4 page 660
Exploration page 661
The Importance of DNA
Deoxyribonuceic Acid (DNA) – is the carrier of
genetic information in cells
the nucleus of every cell in you body contain
DNA.
This molecule is found in the cells of all
organisms, mushrooms to mammals.
Only molecule known to replicate itself. (permits
cell division)
Guides the repair of worn cells parts and
construction of new ones
Provides for continuity of life
Accounts for diversity of life forms
Regulate the production of cell protein
Genes – are the units of heredity in the chromosome,
hair colour, skin colour, nose length.
Genes are packed within DNA to make you unique.
Unless you are an identical twin you DNA code is one of
a kind
Continuity of Life – is a succession of offspring that
share structural similarities with those of their parents.
Mutations – arise when the DNA within a chromosome
is altered. Most mutations change the appearance of the
organism
Why do we study DNA?
We study DNA for
many reasons,
e.g.,
its central
importance to all
life on Earth,
medical benefits
such as cures for
diseases,
better food crops.
DNA
DNA is often called
the blueprint of life.
In simple terms,
DNA contains the
instructions for
making proteins
within the cell.
Chromosomes and DNA
Our genes are on
our
chromosomes.
Chromosomes
are made up of a
chemical called
DNA.
Historical Profile: James Watson
and Francis Crick
Used X –ray diffraction to help determine the structure of
DNA molecule.
Compared DNA structures in different organisms
DNA consists of two strands of nucleotides.
Each nucleotide contains a deoxyribose sugar, a
phosphate group and a nitrogenous base.
Knew that DNA was made up of sugars, phosphates and
four different nitrogen bases: adenine, guanine, cytosine
and thymine.
Did not know how these bases were arranged.
Did show that the proportion of these nitrogen bases
varied in species, but the proportion stays the same in
the DNA of all species.
Ex: number of adenine is the same as thymine, guanine
is the same as cytosine.
This suggested that the nitrogen bases were arranged in
pairs.
If DNA in your body were stretched out it would reach to
the sun and back appr. 3000 times.
DNA Structure
Watson and Crick’s Model also indicated that the
two strands of DNA form a structure that
resembles a twisted ladder.
The base pairs are the rungs of the ladder and
the sugar-phosphates backbones are the struts.
This structure is called a double helix
Each DNA strand in the double helix twists in a
clockwise direction.
DNA Strand
In the DNA molecule, the bases from one strand are
paired with bases in the other strand.
Purine is always paired with pyrimidine,
Ex: Adenine (purine) + thymine (pyrimidine)
guanine (purine) + cytosine (pyrimidine)
This type of pairing is termed complemetary base
pairing.
Hydrogen bonds between the complementary bases on
opposite strands hold the double helix together.
Although hydrogen bonds are weak, large numbers of
hydrogen bonds are collectively strong, so the DNA
molecule is very stable.
Diagram
Adenine forms two hydrogen bonds with
thymine, while guanine forms three
hydrogen bonds with cytosine.
5’ – ATGCCGTTA – 3’
3’ – TACGGCAAT – 5’
The two strands of nucleotides are antiparallel
(parallel but in opposite directions).
One strand will have a 5’ carbon phosphate
group at one end and a 3’ carbon and hydroxyl
group of a deoxyribose sugar at its other end.
Questions
Questions 1-3 page 663
Activity
Structure of DNA Review
Described as a double helix.
Resembles a twisted ladder.
Sugar and phosphate molecules form the back bone of
the ladder.
Nitrogen bases form the rungs.
Nitrogen bases from one spine of the ladder are
connected with nitrogen bases from the other spine
By means of hydrogen bonds
a hydrogen bond is a weak bond that forms between that
positive charge in the end of one molecule and a
negative charge on the end of another molecule
The backbone of DNA becomes twisted (spiral staircase)
The DNA molecule is made up of individual units
composed of deoxyribose sugars, phosphates, and
nitrogen bases. Each unit is referred to as a nucleotide.
NOTE: adenine pairs with thymine and Guanine pairs
with cytosine
The Shape of the Molecule
DNA is a very long
polymer.
The basic shape is
like a twisted ladder
or zipper.
This is called a
double helix.
The Double Helix Molecule
The DNA
double helix
has two
strands
twisted
together.
One Strand of DNA
The backbone of
the molecule is
alternating
phosphates and
deoxyribose
sugar
The teeth are
nitrogenous
bases.
phosphate
deoxyribose
bases
O
O -P O
Nucleotides
O
O
O -P O
O
One deoxyribose together
with its phosphate and
base
make
a
nucleotide
.
O
O -P O
O
Phosphate
Nitrogenous
base
C
O
C
C
C
C
O Deoxyribose
One Strand of DNA
One strand of DNA
is a polymer of
nucleotides.
One strand of DNA
has many millions
of nucleotides.
nucleotide
Searching for the Chemical of
Heredity
Chromosomes – are the long threads of genetic
material found in the nucleus of cells.
Chromosomes are composed of many nucleic acids and
proteins.
Proteins are composed of 20 different amino acids,
which can be organized in an infinite variety.
Nucleotides – are the building blocks of nucleic acids.
Nucleotides are composed of a ribose sugar, phosphate,
and a nitrogen base.
Four nitrogenous bases
DNA has four different bases:
Cytosine C
Thymine T
Adenine A
Guanine G
Two Kinds of Bases in DNA
Pyrimidines are
single ring bases.
N
N C
O C
C
N C
Purines are double
ring bases.
N
N C
C
C
N
N C
N C
Thymine and Cytosine are
pyrimidines
Thymine and cytosine each have one
ring of carbon and nitrogen atoms.
N
O
C
C
O
C C
N
C
thymine
N
O
C
C
N
C
N
C
cytosine
Adenine and Guanine are
purines
Adenine and guanine each have two
rings of carbon and nitrogen atoms.
N
C
Adenine
N
C
C
N
O
N
C
N
N
C
N
C
C
C
N
Guanine
C
N
N
C
Two Stranded DNA
Remember, DNA
has two strands
that fit together
something like a
zipper.
The teeth are the
nitrogenous
bases but why
do they stick
together?
C
N
N
C
N
C
C
C
C
N
N
N
C
C
C
O
The bases attract each
other because of
hydrogen bonds.
Hydrogen bonds are weak
but there are millions and
millions of them in a
single molecule of DNA.
The bonds between
cytosine and guanine are
shown here with dotted
lines
N
Hydrogen Bonds
N
O
Hydrogen Bonds, cont.
When making
hydrogen bonds,
cytosine always
pairs up with
guanine
N
Adenine always
pairs up with
C
thymine
Adenine is bonded
to thymine here
N
N
O
C
N
C
C C
N
C
C
N
N
C
C
O
C
Chargraff’s Rule:
Adenine and Thymine
always join together
A
T
Cytosine and Guanine
always join together
C
G
DNA by the Numbers
Each cell has about 2 m
of DNA.
The average human has
75 trillion cells.
The average human has
enough DNA to go from
the earth to the sun
more than 400 times.
The earth is 150 billion m
or 93 million miles from
DNA has a diameter of
the sun.
only 0.000000002 m.
DNA Replication
For mitosis to occur, DNA must copy itself and be equally divided
between daughter cells.
DNA replication – is the process by which a cell makes an exact
copy of its DNA.
DNA replication is semiconservative.
Semiconservative replication – involves separating the two parent
strands and using them to synthesis two new strands.
The hydrogen bonds between complementary bases break, allowing
the DNA helix to unzip.
Each single DNA strand acts as a template to build the
complementary strand.
DNA replicating
Semiconservatively
Each daughter molecule receives one
strand from the parent molecule plus one
newly synthesized strand.
Separating the DNA Strands
The enzyme DNA helicase unwinds the
helix by breaking the hydrogen bonds
between the complementary bases.
Proteins bind to the separated DNA
strands to prevent them from reforming.
The point at which the two strands are
separating is called the replication fork.
Separating DNA Strands
Building the Complementary
Strands
Need to synthesis two new DNA strands on the template
strands through complementary base pairing.
New strands are sythesized by an enzyme called DNA
polymerase III .
This DNA polymerase builds a new strand by linking
together free nucleotides that have bases
complementary to bases in the template
A short piece of single stranded ribonucleic acid, called a
primer, is attached to the template strand. This gives
DNA polymerase II a starting point to begin synthesizing
a new DNA Strand.
DNA polymerase III adds nucleotides to a
growing strand in only one direction the 5’ – 3’
direction.
This strand is called the leading strand.
(synthesized towards the replication fork)
The other new strand is synthesized in short
fragments away from the replication fork. This is
called the lagging strand.
Another enzyme DNA ligase links the sugarphosphate backbone of the DNA fragments
together.
Building the Lagging Strand
DNA Repair
Complementary strands of DNA are
synthesized, both DNA polymerase I and III act
as quality control checkers by proofreading the
newly synthesized strands.
When a mistake occurs, they backtrack to the
incorrect nucleotide, cut it out and continue
adding.
The repair must be made immediately to avoid
the mistake from being copied in later
replications.
Questions
Questions 1- 7 page 666.
Gene Expression
Specific segments on DNA are called genes
Genes determine inherited traits in an organism
The way the information in a gene is converted
into a specific characteristic or trait through the
production of a polypeptide is called
geneexpression.
Remember that proteins form many structures in
an organism
A second type of nucleic acid is involved in converting
the instructions in a gene into a polypeptide chain.
Ribonucleic acid (RNA) is a polymer of nucleotides
similar tmin eo DNA.
There are 3 main structural difference between RNA and
DNA.
First the sugar in RNA has an extra hydroxyl group and
is called ribose.
Second instead of the base thymine it has uracil.
Third, RNA is a single stranded
Central Dogma
Two stages in gene expression:
1) transcription
2) translation
Transcription the genetic information is
converted from a DNA sequence into messenger
mRNA.
In all cells, the mRNA carries genetic information
from the chromosomes to the site of protein
synthesis.
Translation the genetic information carried by
mRNA is used to synthesize a polypeptide
chain.
This two step process of transferring information
from DNA to RNA and then from RNA to protein
is known as the central dogma of molecular
genetics
EX: DNA
RNS
ATGCAA
UACGUU
The Central Dogma of Molecular
Genetics
Transcription
The DNA sequence of a gene is copied
(transcribed) into the sequence of a single
stranded mRNA molecule.
Transcription is divided into 3 processes:
1) initiation
2) elongation
3) termination
Initiation – an enzyme called RNA polymerase
binds to the DNA at a specific site near the
beginning of the gene.
Elongation – RNA polymerase uses the DNA as
a template to build the mRNA molecule.
Termination – the RNA polymerase passes the
end of the gene and comes to a stop.
Summary of the Process of
Transcription
Question
Questions 1-2 page 669
Translation
Involves protein synthesis
Only 20 amino acids are found in proteins
Codon sequence of three bases in DNA
Start codon (AUG) that signals the start of translation
Stop codons (UAA, UAG, UGA) that signal the end of
translation.
Translation can be divided into the same three stages:
initiation, elongation, termination.
Translation
Initiation – occurs when a ribosome recognizes a specific
sequence on the mRNA and binds to that sight.
The ribosome then moves along the mRNA in the 5’ – 3’
direction adding a new amino acid to the growing
polypeptide chain each time it reads a codon.
Ribosomes synthesize different proteins by associating
with different mRNAs and reading their sequences.
A ribosome must begin reading in the correct place.
(Start codon)
Once the ribosome has bound the mRNA, how
does it get the amino acids that correspond to
the codon?
Second type of RNA molecule known as transfer
RNA (tRNA).
tRNA has a sequence of three bases, the
anticodon that is complementary to the codon of
the mRNA.
Questions
Questions 3-5 page 671
Elongation – The process is similar to a ticker tape
running through a ticker tape machine.
Start codon is AUG. The AUG codon also codes for
methionine, so every protein initially starts with the
amino acid methionine.
The ribosome has two sites for tRNA to attach: A site
and the P site.
The tRNA with the anticodon complementary to the start
codon enters the P sight.
The next tRNA carrying the required amino acids
enters the A site
A peptide bond has formed between the
methionine ans the second amino acid alanine.
The ribosome has shifted over one codon so
that the second tRNA is now in the P site.
Refer to the diagram on the next page.
Protein Synthesis
Termination – eventually the ribosome
reaches one of the 3 stop codons: UGA
UAG, UAA.
Since they do not code for an amino acid,
there are no corresponding tRNAs
Questions
Questions 1-11 page 676
DNA and Biotechnology
Recombinant DNA – fragment of DNA
composed of sequences originating from at least
two different sources.
Ex: Gene that encodes insulin has been
introduced into bacterial cells so that they
become living factories producing this vital
hormone.
Genetic transformation- the introduction and
expression of foreign DNA in an organism.
DNA Sequencing
DNA can be sequenced in a test tube
using isolated segments of DNA.
This technology requires a primer, DNA
polymerase and 4 DNA nucleotides.
The Human Genome Project
Plans to produce maps of the entire genetic
makeup of a human being.
Human genome consists of approx. 30 000
genes
Goal to help combat genetic disorders.
The project also opened a box of ethical
questions, legal dilemmas and societal
problems.
Enzymes and Recombinant DNA
Anther way to identify specific segments of DNA
is by creating genetic linkage maps.
Once a segment of DNA has been identified,
molecular biologists may use enzymes to isolate
that segment or modify it.
The DNA fragment may then be used to create
recombinant DNA or be transferred to another
organism.
Restriction Endonucleases
Known as restriction enzymes, are like molecule
scissors that can cut double stranded DNA at
specific base –pair sequences.
Each type recognizes a particular sequence of
nucleotides that is known as its recognition site.
Molecular biologists use these enzymes to cut
DNA in a predictable and precise way.
Methylases
Are enzymes that modify a restriction
enzyme recognition site by adding a
methyl group to one of the bases in the
site.
Important tools in recombinant DNA.
They protect the gene fragment from being
cut in the wrong spot.
DNA Ligase
To create recombinant DNA, pieces of
DNA from two sources must be joined.
Using restrictive enzymes and methylases,
molecular biologists can engineer
fragments of DNA that they want.
These segments of DNA are joined
together by DNA ligase.
Transformation
Is any process by which foreign DNA is
incorporated into the genome of the cell.
Vector – is the delivery system used to
move the foreign DNA into a cell.
Transgenic – a cell or an organism that is
transformed by DNA from another species.
Transformation of Bacteria
Most common to be transformed.
Used to study gene transformations, and gene functions
First step is to isolate the DNA fragment to be tranferred.
DNA is then introduced to a vector.
Plamids are small circular double stranded DNA molecules that
occur naturally in the cytoplasm of bacteria. Can be used as a
vector.
Refer to page 684 Figure 9
Questions
Questions 1-9 page 686
Mutations and Genetic Variation
Mutations are changes in the sequence of DNA.
Are sources of new genetic variation
Point mutations – are changes in a single base
pair of a DNA sequence. They may or may not
change the sequence of amino acids.
Gene Mutations – change the amino acids
specified by the DNA sequence, and they often
involve more than a single base pair.
Different Types of Mutations
One type of point mutation, called a silent
mutation, has no effect on the operation of the
cell. ACA changed to ACU both code for
threonine.
Missense mutation – arises when a change in
the base sequence if DNA alters a codon
leadign to a different amino acid
Ex: sickle cell anemia
Nonsense mutation – occurs when a change in
the DNA causes a stop codon.
Deletion – is a mutation which occurs when nucleotides
are removed from the DNA sequence.
Insertion – is the placement if an extra nucleotide in a
DNA sequence.
Frameshift mutation – a mutation that causes the
reading frame of codons to change.
Translocation – the transfer of a fragment of DNA from
one site in the genome to another location.
Inversion – the reversal of a segment of DNA within a
chromosome.
Causes of Genetic Mutations
Some are simply caused by error of genetic
machinery and are known as spontaneous
mutations.
Ex: DNA polymerase misses a base or two
causing point mutations.
Mutations might arise from mutagenic agents.
These are induced mutations
Ex: UV radiation, cosmic rays, X-rays,
chemicals.
Case Study
Case study page 689-690
Questions
Questions 1-10 page 694
Review questions page 702-709
Homework
Case Study – page 518-519 (old text)
Homework
Review Questions
Page 522 Questions 1-8