Control, Genomes and
Environment
Cellular Control – The Genetic Code and
Transcription
What do you remember?
• A DNA Quiz is on your desk!
• You have 5 minutes.
A Problem
• DNA is in the nucleus
• Protein synthesis takes place in the cytoplasm
• How does the code get transferred to the
cytoplasm?
Learning Outcomes
• state that genes code for polypeptides, including
enzymes;
• explain the meaning of the term genetic code;
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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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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Structure of DNA - recap
Adenine = Thymine
(2 Hydrogen bonds)
Cytosine = Guanine
(3 Hydrogen bonds)
Deoxyribose sugar and
phosphate backbone.
Pyrimidines:
Single ring. C and T
Purines:
Double ring. A and G
The Genetic code
• ...is the sequence of nucleotides on mRNA
• It has a number of characteristics. The following
slides will identify them.
• You can use the acronym DUNCS to remember
them.
You will construct this acronym after the next few
slides.
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.
Eg TCT codes for serine in any organism.
For examples of exceptions, see p104 in Purple book
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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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Mind Map p104 PB
Characteristics of the Genetic Code
Include some detail
about each key word
D
S
U
The
Genetic
code
C
N
Overview
• https://www.youtube.com/watch?v=J3HVVi2k2No
Transcription – Learning Outcomes
• describe, with the aid of diagrams, the way in
which a nucleotide sequence codes for the amino
acid sequence in a polypeptide;
Transcription
• The first stage of protein synthesis
• Messenger RNA (mRNA is made)
• Uses one strand (template strand) of DNA as a
template.
• The resulting mRNA strand is a copy of the
coding strand of DNA
Gene expression
When a gene is ‘switched on’ or expressed, it is transcribed into
an mRNA strand which is then translated into a protein.
Proteins, such as enzymes, can
control chemical reactions and
therefore determine how an
organism grows, develops and
functions.
Only some of the many genes in a cell
will be expressed at any one time. This
is key to controlling development and
cell differentiation.
Transcription
(Coding strand)
• Watch the video
• List the steps
What are the steps of transcription?
• 1. DNA unzips / H bonds break;
2. Exposing required, gene / sequence of bases;
3. RNA nucleotides align with DNA;
4. U with A, A with T, C with G, and G with C;
5. This is catalysed by RNA polymerase;
6. mRNA formed (using DNA strand as template);
7. leaves nucleus through pore;
Exam question practice
Plenary
• Translate the DNA sequence below into the
corresponding mRNA sequence:
1. T A C T A G C C G C G A T T T
• AUG AUC GGC GCU AAA
2. T A C A A G C C T A T A A A A
• AUG UUC GGA UAU UUU
Plenary Exam Question
• And revisit the Learning Outcomes
• state that genes code for polypeptides, including
enzymes;
• explain the meaning of the term genetic code;
• describe, with the aid of diagrams, the way in
which a nucleotide sequence codes for the amino
acid sequence in a polypeptide;
Homework
• Genetic Code Homework sheet
• Questions in purple book p105 or green book p97