Download Nucleic Acids and the Genetic Code

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Introduction to nucleotides
Nucleotides are nitrogen-containing organic substances
that form the basis of the nucleic acids DNA and RNA.
All nucleotides contain the following three groups:
a phosphate
group
a nitrogencontaining base
a pentose
sugar
In DNA the sugar is deoxyribose, whereas in RNA the
sugar is ribose.
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Bases
There are five bases, split into two types:

adenine (A) and
guanine (G) are
purine bases.
A

thymine (T),
cytosine (C) and
uracil (U) are
pyrimidine bases.
T
G
C
U
DNA contains A, G, T and C, whereas RNA contains
A, G, U and C.
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Identifying bases
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Base pairing rules
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Formation of nucleotides
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Formation of polynucleotides
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Determining the structure of DNA
The double-stranded
structure of DNA was
determined in 1953 by the
American biologist James
Watson and the British
physicist Francis Crick.
X-ray diffraction studies by
British biophysicist
Rosalind Franklin strongly
suggested that DNA was a
helical structure.
The Austrian chemist Erwin Chargraff had earlier showed
that DNA contained a 1:1 ratio of pyrimidine:purine bases.
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Structure of DNA
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How is DNA packaged?
In eukaryotic cells, DNA is packaged as chromosomes in
the nucleus.
There is around 2 m of DNA in a
cell, so to fit it needs to be
tightly coiled and folded.
Eukaryotic DNA is associated
with proteins called histones.
Together, these form chromatin
– the substance from which
chromosomes are made.
In prokaryotic cells, DNA is loose in the cytoplasm –
there are no histones or chromosomes.
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Structure of eukaryotic chromosomes
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DNA replication
The discovery of DNA’s structure by Watson and Crick
provided evidence that complementary base pairing was
key to DNA’s ability to replicate.
Scientists proposed that DNA ‘unzipped’
as hydrogen bonds between base pairs
were broken.
New polynucleotide strands could then
be synthesized using the originals as a
template.
Several hypotheses were proposed as
to the specific mechanism by which new
strands are created.
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Mechanisms of DNA replication
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Meselson–Stahl DNA experiment
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DNA replication
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DNA replication enzymes
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DNA replication
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Errors in replication: gene mutations
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The genetic code: timeline
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What is the genetic code?
The genetic code of an organism is the sequence of bases
along its DNA. It contains thousands of sections called genes
or cistrons. Each gene codes for a specific polypeptide.
one gene/cistron
thousands more bases
in gene (not shown)
All polypeptides are made from amino acids, so the
sequence of bases in a gene must code for amino acids.
The genetic code is almost universal – the same sequence
of bases codes for the same amino acids in all organisms.
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The triplet code
Given that there are four bases in DNA, and these code for
20 amino acids, what is the basis for the genetic code?

If one base = one amino acid,
possible amino acids = 4

If two bases = one amino acid,
possible amino acids = 16 (4×4)

If three bases = one amino acid,
possible amino acids = 64 (4×4×4)
The existence of a three-base (triplet) code was confirmed
by experiments by Francis Crick and his colleagues in 1961.
The triplet code is degenerate, which means that each
amino acid is coded for by more than one triplet.
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What is mRNA?
When a polypeptide is required, the triplet code of its gene is
converted into a molecule of messenger RNA (mRNA).
This process is called transcription and is the first stage of
protein synthesis.
Like DNA, mRNA is a nucleic
acid, but it differs in that:

it is single stranded,
not double stranded

it contains ribose instead of
deoxyribose

it contains uracil instead of thymine.
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mRNA strand
during
transcription
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Transcription and codons
During transcription, the mRNA is built up by complementary
base pairing, using the DNA as a template. The DNA’s base
triplets are converted into mRNA codons.
What are the codons in the mRNA transcribed from this
sequence of DNA base triplets?
DNA
TAC GCA GAT
TAC
mRNA
AUG CGU CUA AUG
The genetic code is non-overlapping: each base is only
part of one triplet/codon, and each triplet/codon codes just
one amino acid.
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The genetic code
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What is translation?
Once a molecule of mRNA has been transcribed, it moves
out of the nucleus via a nuclear pore.
In the cytoplasm, the
mRNA combines with a
ribosome – the cellular
structure on which the
polypeptide chain will
be built in a process
called translation.
ribosome
mRNA
strand
How are the correct amino acids transported to the ribosome,
and how are they linked together in the correct order?
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What is tRNA?
In the cytoplasm, amino acids become attached to transfer
RNA (tRNA) molecules. Each tRNA is specific for one amino
acid.
amino acid
3’ end
attachment site
Each tRNA molecule
5’ end
has a sequence of
three bases called an
hydrogen bond
anticodon. These
are complementary
to codons on the
mRNA molecule.
What is the anticodon
for the codon A U G?
UAC
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nucleotides
anticodon
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What happens during translation?
tRNA molecules attach to the ribosome,
and their anticodons pair up with the
appropriate codons on the mRNA.
The amino acids transported by the
tRNA link together, and the tRNA
molecules then return to the cytoplasm.
The ribosome moves along
the mRNA, and amino acids
continue to join together until
all the codons have been
translated and the
polypeptide is complete.
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Stages of protein synthesis
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From DNA to amino acids
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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