Download The Importance of DNA and RNA - Emmanuel Biology 12

The importance of
DNA and RNA in heredity
Central Dogma of Molecular Biology
DNA is self-replicating.
DNA is transcribed to produce mRNA.
mRNA is translated to produce protein.
Revision – What are DNA and RNA
DNA – deoxyribonucleic acid
RNA – ribonucleic acid
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Both are polynucleotide chains but they
have different functions and structures.
Nucleotides are composed of a base, a
phosphate group and a sugar.
Parts of a Nucleotide
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Base
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Phosphate group
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Nitrogen containing ring compounds
Two types – purine (two rings) or pyrimidine (one ring)
Purines – adenine and guanine
Pyrimidines – thymine, cytosine and uracil
Used to form phosphodiester linkage between 5’ and 3’ carbons of
adjacent nucleotides in order to form polynucleotide chains.
Sugar
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Nucleotides contain a 5 carbon sugar
The sugar used in RNA is b-D-ribose
The sugar used in DNA is b-D-deoxyribose
There is one less oxygen on the sugar used for DNA hence the name
deoxyribonucleic acid.
Subunits of a Nucleotide
Nomenclature (naming) of Nucleotides
BASE
Adenine
Guanine
NUCLEOSIDE
Adenosine
Guanosine
ABBREVIATION
A
G
Cytosine
Uracil
Thymine
Cytidine
Uridine
Thymidine
C
U
T
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Nucleoside refers to base + sugar
Nucleotide refers to base + sugar + phosphate
Key differences between DNA and RNA
DNA
RNA
Sugar
deoxyribose
ribose
Bases
adenine
guanine
cytosine
thymine
adenine
guanine
cytosine
uracil
Structure of
polynucleotide chain
double stranded
single stranded
Structure of DNA
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Watson and Crick modelled the
structure of DNA in 1953 based
on observations of other
scientists including Rosamond
Franklin
Now accepted that DNA is a
double-stranded helix (like a
curved staircase) formed by
cross-linking of two anti-parallel
nucleotide strands with
complementary nucleotide
sequences.
Why complementary base pairs?
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Chargraff’s chromatography data examining the base composition of
DNA from several different organisms indicated that in all cases
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Ratio A:T was 1
Ratio C:G was 1
Further work showed that adenine forms two hydrogen bonds with
thymine while guanine forms three bonds with cytosine.
There are approximately 10 complementary base pairs per helical
turn in the DNA helix.
A purine (double ring) is always paired with a pyrimidine (single
ring) in order for the helix to fit together properly.
Uracil is only found in RNA – it is complementary to the DNA base
adenine and replaces thymidine during the transcription process.
Four Types of RNA
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Messenger RNA (mRNA)
 Copied portion of coding DNA
 Carries genetic information from the DNA out of the
nucleus into the cytoplasm
Transfer RNA (tRNA)
 Transports amino acids to the ribosome during protein
synthesis
Ribosomal RNA (rRNA)
 Structural component of ribosomes
snRNA (snRNA)
 Involved in splicing of pre-mRNA message in the nucleus
to remove introns
Nucleotide bases form the basis of
the genetic code
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The genetic codes consists of four nucleotides (A, C, G, T)
and provides the instructions to make each of the 100,000+
proteins in the human body
The code is read from 5’ to 3’ end of a DNA sequence and is
usually written from left to right
A group of three bases codes for one amino acid
DNA code is copied (transcribed) to produce mRNA, and the
order of amino acids in proteins is determined by the
sequence of the three letter codes in mRNA
The mRNA sequence reads the same as the 5’ to 3’ DNA
sequence except for the substitution of U for T.
Why do we need both DNA and RNA?
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DNA holds all the genetic information/instructions for the
proteins produced in a cell so why do we need RNA.
It is BECAUSE DNA holds all the genetic information, this
means it is EXTREMELY important.
If DNA is damaged in any way, the coding sequence can
change and a MUTATION will arise that will potentially
influence the particular protein and perhaps the whole
cell or organism.
If DNA ventured into the cytoplasm to give instructions
for protein synthesis it would be vulnerable to damage
from chemicals, UV radiation and other mutagens.
RNA acts as a messenger. Damage to mRNA will not
permanently affect function of the cell as the DNA
template is undamaged.
Central Dogma of Molecular Biology
DNA is self-replicating.
DNA is transcribed to produce mRNA.
mRNA is translated to produce protein.
DNA is self-replicating
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In order for genetic information to be passed on
to new cells, chromosomal DNA must be
replicated prior to cell division.
Replication of DNA creates the sister chromatids
found in chromosomes that are preparing to
divide.
This process duplicates the whole chromosome,
and the sister chromatids are then held together
by a common centromere until they are
separated in the process of cell division.
Steps in DNA Replication
1. Unwinding the DNA molecule
2. Making new DNA strands
3. Rewinding the DNA molecule
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Different enzymes are involved in different stages of
DNA replication, and although they are shown as
separate entities in most diagrams, they will tend to
cluster together forming a ‘replication complex’
Unwinding the DNA Molecule
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Replication of DNA begins at a sequence of
nucleotides called the origin of replication.
An enzyme called helicase unwinds the dsDNA
helix and single-stranded binding proteins
(SSBP) react with the ssDNA and stabilize it.
At the same time, DNA gyrase relieves the
strain that unwinding causes on the molecule by
cutting, winding and rejoining DNA strands.
Under an electron microscope the unwound
section looks like a “bubble” and thus is known
as the replication bubble.
Making New DNA Strands
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DNA polymerase III is the major enzyme involved in DNA replication.
It adds nucleotides to the 3’ end of a pre-existing chain of nucleotides thus
generating a new complementary strand of DNA, but it cannot initiate a
nucleotide chain.
An RNA polymerase called primase is needed to start a new nucleotide
chain.
Primase constructs an RNA primer (sequence of about 10 nucleotides
complementary to the parent strand) which DNA polymerase III can then
add nucleotides to.
The unwound DNA exposes two parental strands of DNA which are
antiparallel. This means they are orientated in different directions and must
be replicated by different mechanisms.
The leading strand elongates towards the replication fork (in the direction
of unwinding) by the simple addition of nucleotides to is 3’ end by DNA
polymerase III.
The lagging strand must elongate away from the replication fork. It is
synthesized discontinuously as a series of short segments called Ozaki
fragments. When DNA polymerase III reaches the RNA primer on the
lagging strand, it is replaced by DNA polymerase I, which removes the RNA
primer and replaces it with DNA. DNA ligase then attaches and forms
phosphodiester bonds.
Rewinding the DNA Molecule
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Since each new strand is complementary to its old
template strand, two identical new copies of the DNA
double helix are produced during replication.
In each new helix, one strand is the old template and
the other is new synthesised, therefore replication is said
to be semi-conservative.
The two DNA molecules rewind into the double-helices,
then each double-helix is coiled around histone proteins
and further wrapped up to form separate chromatids
(still joined by a common centromere).
The two chromatids will become separated in the cell
division process to form two separate chromosomes.
Overview of DNA replication
Central Dogma of Molecular Biology
DNA is self-replicating.
DNA is transcribed to produce mRNA.
mRNA is translated to produce protein.
Transcription of DNA
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Process of transcription begins when a section of DNA (a gene) unwinds and the
bases separate exposing two single strands of DNA with unpaired bases.
One of these strands act as a template for the formation of an mRNA molecule (it is
transcribed)
Individual nucleotides of RNA align with the exposed bases on the DNA template
according to base pairing rules. Nucleotides are added to 3’ end of growing RNA
molecule.
The formation of the mRNA molecule is catalysed by the enzyme RNA polymerase.
This molecule is actually referred to as pre-mRNA. It is complementary to the
template strand but requires some post-transcriptional modification.
Post-transcriptional modification of pre-mRNA or nuclear mRNA involves the removal
of introns (non-coding regions within genes) and stitching together of exons (coding
regions of genes). This process is known as RNA splicing.
Following RNA splicing a chemical cap is added to the 5’ end of the molecule and a
poly-A tail (string of A nucleotides) to the 3’ end. The 5’ cap enables efficient protein
synthesis as it is part of the structure recognized by the small ribosomal subunit.
The poly-A tail is also important for initiating translation. It also has a role in
regulating the degradation of mRNA molecules in the cytoplasm.
Comparison of transcription in
eukaryotes and prokaryotes
Central Dogma of Molecular Biology
DNA is self-replicating.
DNA is transcribed to produce mRNA.
mRNA is translated to produce protein.
Translation of mRNA
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mRNA exits the nucleus through nuclear pores and binds to
ribosomes within the cytoplasm.
Translation of mRNA begins with the sequence AUG (start codon).
Transfer RNAs (tRNA) bring specific amino acids to the ribosome
and these are added to the growing polypeptide chain by
condensation polymerisation. New amino acids are added to the
carboxyl (COOH) end of the polypeptide.
The tRNA drops away from the mRNA and acquires another specific
amino acid from the pool in the cytoplasm. Each tRNA can only
carry one type of amino acid.
Translation ends when the ribosome reaches a stop codon – the
tRNA molecules corresponding to the stop codons UAG, UGA and
UAA don’t carry a amino acid.
The mRNA is then released from the ribosome.
Structure of tRNA molecule
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Once a tRNA gene is
transcribed, the RNA that is
produced folds to form the
shape of a three-leafed clover.
This is a functional tRNA
molecule.
This tRNA is charged with the
amino acid lysine at the amino
acid attachment site. The
anticodon UUU will bind to the
complementary AAA sequence
in the mRNA.
Gene Regulation
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All the somatic cells in your body contain the same
chromosomes and therefore the same DNA and same
genes.
However, these cells are able to have different shapes
and sizes and perform different functions and change
throughout your lifespan.
These differences are possible because of different
mechanisms that control the expression of individual
genes. These mechanisms are collectively referred to as
mechanisms for gene regulation.
How are genes regulated?
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There are many steps involved in the expression
of gene, therefore there are many different
mechanisms for regulating expression:
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The structure of genes varies
The rate of transcription can be regulated
Post-transcriptional modifications can influence which
protein is produced
The rate of translation can be regulated
The activity of the protein product (enzyme) can be
regulated
Role of different mechanisms in
gene regulation
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Gene structure
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All genes contain an upstream promoter region. This consists of a binding site
for RNA polymerase and other base sequences known as upstream promoter
elements (UPEs). UPEs initiate transcription. Genes vary in the number and
type of UPEs. A gene with only one UPE will be weakly expressed. A gene with
many UPEs is actively transcribed.
Other DNA sequences known as enhancers increase the rate of transcription.
Genes which code for the production of essential proteins are often present as
multiple copies.
Genes can be permanently inactivated in some cells by changes in the
chromosome’s structure.
Transcription rate
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DNA binding proteins called transcription factors, regulate the rate at which a
gene is transcribed. These proteins bind with the upstream region of the gene
and stimulate transcription.
Transcription factors may be activated by hormones.
Role of different mechanisms in
gene regulation
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Post-transcriptional modifications
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Translation
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Some pre-mRNAs can be modified in more than one way. Pre-mRNA may be
spliced differently in different tissues, leading to different protein products.
Cells can regulate the amount of translation which occurs by controlling the lifespan of mRNA; mRNA may be inactivated after only a short time being
translated, or may survive longer in the cell and be translated many times.
Protein activity
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Gene expression may be regulated by controlling the activity of the proteins
produced in translation. For example, enzyme inhibitors may inactivate an
enzyme until it is needed.
Some proteins may control the production of other proteins – e.g. repressor
proteins can bind to promoter region of DNA and prevent transcription.
Other factors in gene regulation
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It is important to realize that the
environment of a cell can also influence
the expression of genes. This includes
factors such as:
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Light
Temperature
Ions
Hormones
Lac Operon – an example of gene
regulation in E. Coli
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The bacterium Escherichia coli is capable of producing the enzyme
b-galactosidase which splits lactose to produce glucose and
galactose.
This enzyme is only produced when the bacteria encounters lactose.
When lactose is not present, a protein binds to the promoter region
of the b-galactosidase gene and prevents transcription (RNA
polymerase cannot access the promoter). This protein is referred to
as a repressor protein.
When lactose is present in the growth medium of the bacteria, it
enters the cell and binds to the repressor protein causing it to be
removed from the DNA and allowing transcription to occur.
The gene is ‘on’ or ‘off’ depending on the nutrients available to the
cell.