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Unit 4: Genetic Information, Variation and
Relationships between Organisms
Lesson 2 The Triplet Code
A S EQU ENC E OF T HR EE D N A B A S ES , C A L L ED A T R I P L ET , C OD ES FOR A
S P EC IFIC AMINO A C I D.
T HE G ENET IC C OD E I S U N I V ER S A L, N ON - OV ER L A PPI N G A N D D EG EN ER A TE .
IN EU K AR Y OT ES , MU C H OF T HE N U C L EA R D N A D OES N OT C OD E FOR
P OL Y P EPT IDES . T HER E A R E, FOR EX A MP L E, N ON - C OD IN G MU L T I PLE
R EPEAT S OF BAS E S EQUENC ES BET W EEN G ENES .
EV EN W IT HIN A G EN E ON L Y S OME S EQU EN C ES , C A L L ED EX ON S , C OD E
FOR A MINO AC ID S EQU EN C ES . W I T HIN T HE G EN E, T HES E EX ON S A R E
S EP A R A TED BY ONE OR MOR E N ON - C O DI N G S EQU EN CES , C A L LED
INT R ONS .
Central Dogma of Genetics
Learning
Outcomes:
1) Describe what
is meant by a
triplet code for
DNA
2) Explain why the
DNA code is
universal, nonoverlapping and
degenerate
3) Discuss how
DNA is also
made of noncoding DNA
even within
genes.
Starter: What does this code tell you?
DNA is made up of a polymer of nucleotides with
4 different types of nucleotide bases. These are
Adenine, Thymine, Cytosine, Guanine. The order
of these bases makes a code, which is read in
groups of three.
Learning Outcomes:
1) Describe what is
meant by a triplet
code for DNA
2) Explain why the DNA
code is universal,
non- overlapping and
degenerate
3) Discuss how DNA is
also made of noncoding DNA even
within genes.
The Triplet code
The sequence of 4 different bases is
highly specific and different for each
gene.
Every 3 bases is called a codon or a
triplet code.
A codon codes for 1 amino acid. (a specific
order of codons will code for a polypeptide)
Q: How many types of Amino acid are there?
Explain how can complex proteins occur then?
Learning Outcomes:
1) Describe what is meant by
a triplet code for DNA
2) Explain why the DNA code
is universal, nonoverlapping and degenerate
3) Discuss how DNA is also
made of non-coding DNA
even within genes.
DNA code is……
1. Universal
All living things use DNA and triplet code as their
genetic code to produce proteins
2. Non-overlapping – each codon is read separately:
TCTGATACTGGAGGT is read as 5 codons.
It is not read as TCT, CTG, TGA, GAT etc.
3. Degenerate
More than one codon can code for the same amino acid.
Learning Outcomes:
1) Describe what is
meant by a triplet
code for DNA
2) Explain why the DNA
code is universal,
non- overlapping and
degenerate
3) Discuss how DNA is
also made of noncoding DNA even
within genes.
Learning Outcomes:
1) Describe what is
meant by a triplet
code for DNA
2) Explain why the DNA
code is universal,
non- overlapping and
degenerate
3) Discuss how DNA is
also made of noncoding DNA even
within genes.
Non-coding DNA
In the DNA sequence, most of the DNA is noncoding. This is both within genes (Introns and
Exons) and between different genes.
It does not code for amino acids, which suggests it
has other roles:
- we don’t know
- codes for ribosomal RNA
- codes for transfer RNA
- codes for siRNA
- codes for regRNA
Learning Outcomes:
1) Describe what is
meant by a triplet
code for DNA
2) Explain why the DNA
code is universal,
non- overlapping and
degenerate
3) Discuss how DNA is
also made of noncoding DNA even
within genes.