genetic code

1. GENETIC CODE AND MUTATIONS
DR.HAMISI MKINDI,MD.
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2. SESSION OUTLINE
• Introduction
• What is genetic code?
• The standard genetic code.
• Characteristics of a genetic code
• Mutations

3. INTRODUCTION
• Genetic information is stored in the nucleic acids
(DNA and RNA ), and genetic expression Is
achieved through protein synthesis.
• Though the linear sequence of nucleotides in the
DNA contain the information for protein
sequences, proteins are not made directly from
the DNA.
• Instead, mRNA molecule is synthesized from the
DNA and directs the formation of the protein.

5. • RNA is composed of four nucleotides ie
adenine, guanine, cytosine and uracil.
• Three adjacent nucleotides constitute a unit
called CODON.
• Codon codes for an amino acid which are
building blocks of protein.
• Polymer of amino acids make up a protein.

6. PRIMARY PROTEIN STRUCTURE

7. DEFINITION OF GENETIC CODE
• The dictionary that identifies the
correspondence between a sequence of
nucleotide bases and a sequence of amino
acids.
• Is the particular sequence of nucleotides that
tells which amino acids are to be linked
together to form a protein

8. • It consist of 64 triplets of nucleotides that
codes for 20 amino acids.
• Each of these triplets are known as codons
• There are 64 codons, three of which do not
code for amino acid but indicate the end of a
protein.
• 61 codons codes for 20 amino acids

9. • The AUG(meth) codon is found at the
beginning of every mRNA and indicates the
start of a protein.
• Methionine and tryptophan are the only two
amino acids that are coded for by just a single
codon (AUG and UGG, respectively)
• The other 18 amino acids are coded for by two
to six codons.

10. CENTRAL DOGMA OF MOLECULAR
BIOLOGY

11. The standard genetic code

12. The standard genetic code in DNA
language

13. CHARACTRISTICS OF GENETIC CODE
1. ALMOST UNIVERSAL:
• The genetic code is virtually universal insofar as
its specificity has been conserved from very
early stages of evolution, with only slight
differences in the manner in which the code is
translated.
• An exception occurs in mitochondria, in which a
few codons have meanings different than those
shown . For example, UGA codes for tryptophan
(Trp)

14. 2.NON OVERLAPPING ,NON
PUNCTUATED
• meaning that the code is read from a fixed
starting point as a continuous sequence of
bases, taken three at a time without any
punctuation between codons.
• For example, AGCUGGAUACAU is read as AGC
UGG AUA CAU.

15. NON-OVERLAPPING, NON PUNCTUATED
CHARACTERISTIC OF CODONS

16. 3. DEGENERATES:
• A single amino acid can be coded by more than
one amino acid.
• It can be
 two fold degeneracy eg for phenylalanine
 Three fold degeneracy eg for isoleucine
 Four fold degeneracy eg for alanine
 Six fold degeneracy eg for leucine

17. • There are only two codons that code for only
one amino acid. These are:
AUG Met
UGG Tryp

18. • In general, the third nucleotide in a codon is
less important than the first two in
determining the specific amino acid to be
incorporated
• and this accounts for most of the degeneracy
of the code.

19. 4.SPECIFICITY
• The genetic code is specific (unambiguous),
because a particular codon always codes for
the same amino acid.
• For any specific codon, a single amino acid is
indicated

20. MUTATIONS
• DEFINITION: A mutation is a permanent
change in the nucleotide sequence that codes
for a certain amino acid.
• In germ cells, are transmitted to the next
progeny and may give rise to inherited
diseases
• If it occur in somatic cells, are not transmitted
to the next progeny but are important in
causation of cancers.

21. Types of mutations
1. Point mutations
2. Trinucleotide repeat expansion
3. Splice site mutation
4. Frame shift mutation

22. 1. POINT MUTATION
 A mutation is called a point mutation when
only one base in the DNA is substituted for
another one.
 Can be transition type or transversion type

24. Effects of point mutation
Can be,
1. silent mutations
2. missense mutation
3. nonsense mutation

25. SILENT MUTATIONS
 A point mutation is classified as silent when
the base substituted codes for the same
amino acid.
 There is no detectable effect
 Eg a codon change from CGA to CGG does not
affect the proteins because both of these
codones specify arginine

26. MISSENSE MUTATION
 A point mutation is classified as missense
when the base substituted codes for a
different amino acid.
e.g. In sickle cell disease, a sixth amino acid is
valine(GUG) rather than glutamic acid(GAG)

27. NONSENSE MUTATION
 A point mutation is classified as nonsense,
when the substituted base codes for a STOP
signal.
 Causes the premature termination of a
polypeptide chain which is usually non
functional

29. 2.TRINUCLEOTIDE REPEAT EXPANSION
• Occasionally, a sequence of three bases that is
repeated in tandem will become amplified in
number so that too many copies of the triplet
occur
• If this happens within the coding region of a
gene, the protein will contain many extra
copies of one amino acid.
• This results in change in size and structure of
the protein.

30. • For example, amplification of the CAG codon
leads to the insertion of many extra glutamine
residues in the huntingtin protein, causing the
neurodegenerative disorder Huntington
disease

32. 3.SPLICE SITE MUTATION
 Is a genetic mutation that inserts, deletes or
changes a number of nucleotides in a specific
site at which splicing takes place during the
processing of precursor messenger RNA into
mature mRNA
 The deletion of splicing site results in one or
more introns remaining in mature mRNA and
may lead to production of abnormal proteins

33.  Splice site mutations can result in exons being
skipped (removed) or introns retained
 This leads to an altered reading frame.
 Results in diseases such as Thalassemia.

34. 4.FRAME SHIFT MUTATION
 This type of mutation happens when one,
two or more nucleotide bases are added or
deleted from the coding message.
 Causes insertion or deletion mutations
 This alters the reading frame.

35. CONSEQUENCES:
1. An altered peptide chain due to added
amino acids, or a truncated product
due to creation of a stop signal.
2. If three bases are deleted, an amino
acid is lost. If three are added, an amino
acid is gained.
3.Loss of three nucleotides can result in
pathologies such as cystic fibrosis.

37. REFERENCES
• Lippincotts illustruted reviews biochemistry
• Lehninger principles of biochemistry
• Harper’s biochemistry 26th edition
• https://www.youtube.com/watch?v=2BwWav
ExcFI