Download DNA, and in some cases RNA, is the primary source of heritable

DNA, AND IN SOME CASES
RNA, IS THE PRIMARY SOURCE
OF HERITABLE INFORMATION
DNA and RNA have structural similarities and
differences that define function
Structure of RNA

Building Blocks

Nucleotide

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

Backbone




Alternating Ribose and Phosphate
Nitrogenous Bases


Ribose sugar
Phosphate group
Nitrogenous bases
A and G – purines (double ring)
C and U – pyrimidines (single ring)
Usually single stranded
3 forms



Messenger
Transfer
Ribsomal
Central Dogma

It was determined that
RNA serves as an
intermediate between
genes and proteins.
This forms the central
dogma of biology cells are governed by
a cellular chain of
command: DNA to
RNA to protein.
Transcription

During transcription one of
the two DNA strands
called the template strand
provides a template for
ordering the sequence of
nucleotides in an RNA
transcript. There are three
basic stages of
transcription: initiation,
elongation and
termination. These three
steps result in the
production of messenger
RNA (mRNA).
http://youtu.be/ztPkv7wc3yU
Transcription
Initiation begins when transcription
factors mediate the binding of the
enzyme RNA polymerase to a promoter
region onto DNA. The combination of
promoter, transcription factors and RNA
polymerase is called the transcription
initiation complex. A promoter called a
TATA box is crucial in forming the
initiation complex in eukaryotes.
Elongation takes place as RNA
polymerase moves along the DNA,
unwinding it as it goes (10 to 20 bases
at a time.) RNA nucleotides are added
(5' to 3' direction) that compliment the
DNA template following Charagaff's
rule - with one exception -adenine's
compliment becomes uracil in the RNA
transcript.
Termination occurs when the RNA
polymerase reaches a special sequence
of nucleotides that serve as a
termination point. In eukaryotes, the
termination region often contains the
sequences AAAAAAAA.
Fig. 17-7
Promoter
Transcription unit
5
3
Start point
RNA polymerase
3
5
DNA
1 Initiation
5
3
RNA
transcript
RNA
polymerase
Template strand
of DNA
3
2 Elongation
Rewound
DNA
5
3
RNA nucleotides
3
5
Unwound
DNA
3
5
5
5
Direction of
transcription
(“downstream”)
3 Termination
3
5
5
3
5
3 end
5
3
RNA
transcript
Nontemplate
strand of DNA
Elongation
Completed RNA transcript
3
Newly made
RNA
Template
strand of DNA
1
Promoter
A eukaryotic promoter
includes a TATA box
Template
5
3
3
5
TATA box
Start point Template
DNA strand
2
Transcription
factors
Several transcription factors must
bind to the DNA before RNA
polymerase II can do so.
5
3
3
5
3
Additional transcription factors bind to
the DNA along with RNA polymerase II,
forming the transcription initiation complex.
RNA polymerase II
Transcription factors
5
3
3
5
5
RNA transcript
Transcription initiation complex
mRNA Editing
The 5' end of mRNA receives a
"cap" which is a modified gaunine
nucleotide that has two additional
phosphate groups - forming GTP.
Capping provides stability to the
mRNA and a point of attachment
for the small subunit of the
ribosome during translation.
The 3' end of mRNA receives a
"poly-A tail" which consists of
about 200 adenine nucleotides. It
provides stability to the mRNA
and also appears to control the
movement of the mRNA across the
nuclear envelope.
RNA splicing removes nucleotide
segments from mRNA. A
transcribed DNA segment contains
two kinds of DNA sequence exons, which are sequences that
express a code for a polypeptide,
and introns, intervening sequences
that are noncoding. Splicing
removes introns and joins exons,
creating an mRNA molecule with a
continuous coding sequence. In
some cases, RNA splicing is
carried out by spliceosomes.
Spliceosomes consist of a variety
of proteins and several small
nuclear ribonucleoproteins
(snRNPs) that recognize the splice
sites.
mRNA Editing
Alternative splicing
allows different mRNA's
to be generated from
the same RNA
transcript. By selective
removing different
parts of an RNA
transcript, different
mRNA's can be
produced, each coding
for a different protein
product. Thus the
number of different
proteins an organism
can produce is much
greater than its number
of genes. Check out the
mRNA processing
activity in your online
textbook. It will help
you understand this
process.
mRNA

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



Messenger RNA (mRNA) - is a single
strand of RNA that provide the template
used for sequencing amino acids into a
polypeptide.
A triplet group of three adjacent
nucleotides on the mRNA, called a codon,
codes for one specific amino acid.
There are 64 possible ways that four
nucleotides can be arranged in triplet
combinations, there are 64 possible
codons.
However, there are only 20 amino acids,
and thus, some codons code for the same
amino acid. The genetic code, to the right,
provides the decoding for each codon.
Of the 64 codons, 61 code for amino
acids; 3 triplets are "stop" signals to end
translation.
Using the genetic code chart you can
determine the amino acid for each codon.
For example, if the codon is AUG, the
amino acid will be Methionine.
tRNA

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
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

Transfer RNA (tRNA) - is a short RNA
molecule (about 80 nucleotides) that is
used for transporting amino acids to their
proper place on the mRNA template.
Molecules of tRNA are not identical, each
carries a specific amino acid on one end
and has an anticodon on the other end.
The anticodon base-pairs with the
complementary codon on mRNA.
Flexible pairing at the third base of a
codon is called wobble and allows some
tRNAs to bind to more than one codon.
The tRNA molecule looks like a cloverleaf.
Because of hydrogen bonds, tRNA
actually twists and folds into a threedimensional molecule.
The correct match between a tRNA and
its amino acid is done by the enzyme
aminoacyle-tRNA synthetase. The amino
acid attaches to the 3' end of the tRNA
molecule.
rRNA




Ribosomal RNA (rRNA) - molecules are the
building blocks of ribosomes. The two ribosomal
subunits (large and small) are made of proteins
and rRNA.
Remember the nucleolus is the area of the
nucleus that actively produces/transcribes rRNA
and assembles the rRNA together with proteins
to form large and small subunits of ribosomes.
Together the two subunits form a complete
ribosome. Ribosomes coordinate the activities
of mRNA and tRNA during translation.
Ribosomes have four binding sites –




one for mRNA,
one for a tRNA that carries a growing
polypeptide chain (P site, for "polypeptide”)
one for a second tRNA that delivers the next
amino acid that will be added to the polypeptide
chain ( A site, for "amino acid”)
one for a third tRNA that has recently lost its
amino acid to growing polypeptide chain and is
about to be discharged from the ribosome (E site,
for "exit").
Translation


http://youtu.be/D5vH4Q_tAkY
During translation mRNA
synthesizes polypeptides. This
process takes place in three
stages: initiation, elongation, and
termination.
Initiation - The small ribosomal
subunit binds to the 5' end of
mRNA and a special initiator
tRNA which carries the amino
acid methionine. The small subunit
moves along the mRNA until it
reaches the start codon (AUG).
Proteins called initiation factors
bring in the large ribosomal
subunit that completes the
translation initiation complex. The
initiator tRNA is sitting in the P
site.
Translation

Elongation - During this stage amino
acids are added one by one to the
preceding amino acid. Each addition
involves proteins called elongation factors
and occurs in three steps: codon
recognition, peptide bond formation and
translocation. A new tRNA that
complements the mRNA enters into the A
site (codon recognition). A peptide bond
attaches the amino acid(s) from the tRNA
in the P site to the amino acid in the A site
(peptide bond formation). The tRNA in
the P site no longer has an amino acid
attached to it. The ribosome moves down
the mRNA strand (translocation) placing
the 1st tRNA that has no amino acid in the
E site, the 2nd tRNA with the growing
polypeptide in the P site, and leaving the
A site empty. A new tRNA can now enter
the A site. The tRNA in the E site leaves
the ribosome. This continues until
termination.
Translation

Termination - This process
occurs when a stop codon
in the mRNA reaches the A
site of the ribosome. The A
site accepts a protein
called a release factor.
The release factor causes
the addition of a water
molecule instead of an
amino acid. This reaction
releases the polypeptide,
and the translation
assembly then comes
apart.
Phenotypes determined by protein
activities…



In 1909, British physician
Archibald Garrod first
suggested that genes
dictate phenotypes through
enzymes that catalyze
specific chemical reactions
He thought symptoms of
an inherited disease reflect
an inability to synthesize a
certain enzyme
Linking genes to enzymes
required understanding
that cells synthesize and
degrade molecules in a
series of steps, a
metabolic pathway
One-Gene-One-Enzyme



George Beadle and Edward Tatum exposed
bread mold to X-rays, creating mutants that
were unable to survive on minimal medium as a
result of inability to synthesize certain molecules
Using crosses, they identified three classes of
arginine-deficient mutants, each lacking a
different enzyme necessary for synthesizing
arginine
They developed a one gene–one enzyme
hypothesis, which states that each gene dictates
production of a specific enzyme
Fig. 17-2
EXPERIMENT
No growth:
Mutant cells
cannot grow
and divide
Growth:
Wild-type
cells growing
and dividing
Minimal medium
RESULTS
Classes of Neurospora crassa
Wild type
Class I mutants Class II mutants Class III mutants
Condition
Minimal
medium
(MM)
(control)
MM +
ornithine
MM +
citrulline
MM +
arginine
(control)
CONCLUSION
Wild type
Precursor
Gene A
Gene B
Gene C
Class I mutants Class II mutants Class III mutants
(mutation in
(mutation in
(mutation in
gene B)
gene A)
gene C)
Precursor
Precursor
Precursor
Enzyme A
Enzyme A
Enzyme A
Enzyme A
Ornithine
Ornithine
Ornithine
Ornithine
Enzyme B
Enzyme B
Enzyme B
Enzyme B
Citrulline
Citrulline
Citrulline
Citrulline
Enzyme C
Enzyme C
Enzyme C
Enzyme C
Arginine
Arginine
Arginine
Arginine
Mutations



Mutations are changes in
genetic material of a cell.
Mutations can occur
spontaneously or they can be
caused by mutagens.
Mutagens are physical or
chemical agents that cause
mutations. Point mutations are
chemical changes in just one
base pair of a gene.
Point mutations can be
divided into two general
categories:


Substitution
Frameshift
Substitutions

Substitution- occurs when the DNA
sequence contains an incorrect
nucleotide in place of the correct
nucleotide.



Silent mutations have no effect on the
amino acid produced by a codon
because of redundancy in the genetic
code. The new codon that results from
the mutation codes for the same amino
acid. This occurs most often when the
nucleotide substitution results in a change
of the last of the three nucleotides in a
codon.
Missense mutations still code for an
amino acid, but not the right amino acid.
The effect can be minor, or it may result
in a defective protein. The hemoglobin
protein that causes sickle-cell disease is
caused by a missense mutation.
Nonsense mutations change an amino
acid codon into a stop codon, nearly
always leading to a nonfunctional
protein.
Substitution Mutations
1.
Silent
2.
Missense
3.
Nonsense
Frameshift

Frameshift- occurs as the
result of a nucleotide deletion
or insertions. Such mutations
cause all subsequent
nucleotides to be displaced
one position. If a frameshift
mutations occurs in a DNA
segment whose transcription
produces mRNA, all codons
following the transcribed
mutation will change.


deletion - occurs when a
nucleotide is omitted to the
nucleotide sequence.
insertion - occurs when a
nucleotide is added to the
nucleotide sequence.
Mutations



Alterations in DNA sequence
can lead to changes in the
type or amount of protein
produced and the consequent
phenotype
DNA mutations can be
positive, negative or neutral
based on the effect or the
lack of the effect they have
on the resulting nucleic acid or
protein and the phenotypes
that are conferred by the
protein
Mutations are the primary
source of genetic variation –
especially in prokaryotes