6. MOLECULAR BASIS OF INHERITANCE
GENETIC CODE
§ It is the sequence of nucleotides (nitrogen bases)
in mRNA that contains information for protein synthesis (translation).
§ The sequence of 3 bases determining a single amino
acid is called codon.
§ George
Gamow suggested that
for coding 20 amino acids, the code should be made up of 3 nucleotides. Thus,
there are 64 codons (43= 4 x 4 x 4).
§ Har
Gobind Khorana developed
the chemical method in synthesizing RNA molecules with defined combinations of
bases (homopolymers & copolymers).
§ Marshall
Nirenberg developed cell-free system for protein
synthesis.
§ Severo
Ochoa (polynucleotide phosphorylase) enzyme is
used to polymerize RNA with defined sequences in a
template independent manner.
20 types of amino acids involved in
translation
- Alanine (Ala)
- Arginine (Arg)
- Asparagine (Asn)
- Aspartic acid (Asp)
- Cystein (Cys)
- Glutamine (Gln)
- Glutamic acid (Glu)
- Glycine (Gly)
- Histidine (His)
- Isoleucine (Ile)
- Leucine (Leu)
- Lysine (Lys)
- Methionine (Met)
- Phenyl alanine (Phe)
- Proline (Pro)
- Serine (Ser)
- Threonine (Thr)
- Tryptophan (Trp)
- Tyrosine (Tyr)
- Valine (Val)
The codons for various amino acids
Salient
features of genetic code
·
Codon is triplet (three-letter code).
·
61
codons code for amino acids. 3 codons (UAA, UAG & UGA) do not code for any amino acids. They act as stop codons (Termination codons or non-sense codons).
·
Genetic
code is universal. E.g.
From bacteria to human UUU codes for Phenylalanine. Some exceptions are found
in mitochondrial codons, and in some protozoans.
·
No
punctuations b/w
adjacent codons (comma less code). The codon is read in mRNA in a contiguous fashion.
·
Genetic
code is non-overlapping.
·
An
amino acid is coded by more than one codon (except AUG for methionine & UGG
for tryptophan). Such codons are called degenerate codons.
·
Genetic
code is unambiguous and specific. i.e. one codon specifies only one amino acid.
·
AUG
has dual functions. It codes for Methionine and acts as initiator codon. In eukaryotes, methionine is the first amino acid
and formyl methionine in
prokaryotes.
Mutations
and Genetic Code
-
Relationship between
genes & DNA are best understood by
mutation studies. Deletions & rearrangements in a DNA may
cause loss or gain of a gene and so a function.
- Insertion
or deletion of one or two bases changes
the reading frame from the point of insertion or deletion. It is called frame-shift
insertion or deletion mutations.
-
Insertion/ deletion of three
or its multiple bases insert or delete one or multiple codon. The reading
frame remains unaltered from that point onwards. Hence one or multiple amino acids are inserted /deleted.
-
It proves that codon is a
triplet and is read contiguously.
TYPES OF RNA
· mRNA (messenger RNA): Provide template for
translation (protein synthesis).
·
rRNA
(ribosomal RNA): Structural & catalytic role during translation. E.g. 23S rRNA in bacteria acts as ribozyme.
· tRNA (transfer RNA or sRNA or soluble RNA): Brings amino acids for protein
synthesis and reads the genetic code.
Francis
Crick postulated
presence of an adapter molecule that can read the code and to link with amino
acids.
tRNA is
called adapter
molecule because it has
· An
Anticodon (NODOC) loop that has bases complementary to the codon.
· An amino acid acceptor end to which amino acid binds.
· Ribosome binding loop.
· Enzyme binding loop.
-
For initiation, there is another tRNA called initiator
tRNA.
- There are no tRNAs for stop codons.
- Secondary (2-D) structure of tRNA looks like a clover-leaf. 3-D structure looks like inverted ‘L’.
- Topic 1: The DNA
- Topic 2: The Search for Genetic Material
- Topic 3: Properties of Genetic Material, RNA World
- Topic 4: DNA Replication
- Topic 5: Transcription
- Topic 6: Genetic Code, Types of RNA
- Topic 7: Translation (Protein Synthesis)
- Topic 8: Regulation of Gene Expression, Operon Concept
- Topic 9: Human Genome Project (HGP)
- Topic 10: DNA Fingerprinting
excellent notes
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