Nucleic Acids:
FUNCTION as the Genetic Material The Genetic Code
Expression of Genetic Information
Properties of the Genetic Code Composed of triplets
Nonoverlapping
Comma-free (no pauses)
Degenerate (1 aa with more than 1 code)
Colinearity between DNA sequence, mRNA seq and aa seq
Almost universal
Exceptions include (eg):
UGA (stop) codes for tryptophan in mito
CUA (leu) codes for thr in yeast mito
Expression of Genetic Information
Deciphering the Genetic Code Nirenberg and Matthei (1961)
poly (U) = poly (phe)
Random polymers and base ratio
Nirenberg and Leder - trinucleotide binding assay
Khorana - known repeating sequences
A, C, G, T
linear order
A, C, G, U
linear order
20 amino acids
linear order
Relationship?
Change of codes
Coding ratio
The genetic code
• DNA + RNA: 4 bases or nucleotides (A, C, G, T or U)
• Proteins: 20 amino acids
• If 1 base 1 amino acid (e.g., A Met) [coding ratio = 1]
then we have 4
1= 4 codes in mRNA (A, C, G, and U)
•
1 code must code for >1 amino acid so that 4 codes 20 amino acids
e.g., 1 code 5 amino acids (4 x 5 = 20 amino acids) e.g., A Met, Arg, Pro, Tyr, Gly
Ambiguous, confusing, and not satisfying condition How does a cell know that A is for Met, Arg, Pro, Tyr, or Gly?
Courtesy: Prof. Dr. Koh Chong Lek & Prof. Dr. Sam Choon Kook
Cracking or deciphering the genetic code
Marianne Grunberg- Manago Severo Ochoa
Har Gobind Khorana
Evidence from in vitro and in vivo experiments
Heinrich
Matthaei Marshall Nirenberg
Cracking or deciphering the genetic code
Courtesy: Prof. Dr. Koh Chong Lek & Prof. Dr. Sam Choon Kook
5’ AUG 3’
5’ 123 3’
position
Triplet codes or codons The genetic code
• 4 3 = 64 codes or codons in mRNA
• 61 codons have meaning and code for 1 amino acid each
• 3 codons do not code for amino acids UAA ochre
UAG amber UGA opal
nonsense, termination, or stop codons
• AUG, start or initiation codon methionine
Courtesy: Prof. Dr. Koh Chong Lek & Prof. Dr. Sam Choon Kook
Degenerate codes or codons
The genetic code
• 61 codons have meaning and each codes for one amino acid
• 61 codons 20 amino acids
more codon possibilities than amino acids
• Every amino acid coded by at least 1 codon
• Methionine and tryptophan, 1 codon each
• 18 amino acids, coded by 2 to 6 codons arginine, leucine, and serine: 6 codons each
• Degenerate codons
Degenerate codons
2, 3, 4, 6
Courtesy: Prof. Dr. Koh Chong Lek & Prof. Dr. Sam Choon Kook
The genetic code
• Very slightly ambiguous code in vitro system
1 code >1 amino acid
• UUU Phe
UUU Leu (in vitro system)
• Confusing
• Does not happen in in vivo system
i.e., in vivo system, not ambiguous
5’ …AUGAAACCCGGG... 3’
AUG AAA
CCC GGG
5’ …AUGAAACCCGGG... 3’
GGG CGG CCG ACC
CCC AAC AAA GAA UGA AUG
The genetic code
Read in a
non-overlapping fashion
Read in an overlapping fashion
✓
every nucleotide is read once
every nucleotide is read 3 times
Courtesy: Prof. Dr. Koh Chong Lek & Prof. Dr. Sam Choon Kook
5’ …AUGAAACCCGGG... 3’
AUG AAA CCC GGG
The genetic code
Read in a non-overlapping fashion
Commaless or unpunctuated code
i.e., no gaps, every nucleotide is read once
Coding sequence
mRNA
AGGAGG
GUG AUG
UAA UAG
5’ UGA 3’
Specific reading frame or open reading frame (ORF)
Every nucleotide is read once in a non- overlapping fashion within the reading frame
Courtesy: Prof. Dr. Koh Chong Lek & Prof. Dr. Sam Choon Kook
5’ …AUGAAACCCGGGU... 3’
The genetic code
specific open reading frame 5’ …AUGAACCCGGGU.... 3’
A
deletion
5’ …AUGAAAGCCCGGGU.... 3’ insertion or addition Frameshift mutation
the reading frame is altered
the genetic message is jumbled
The genetic code
Nearly or largely universal
• Applicable equally to viruses, bacteria, archaea, and eukaryotes
Codon Normal Altered Source
UGA Stop Trp Human + yeast mitochondria Mycoplasma
CUA Leu Thr Yeast mitochondria AUA Ile Met Human mitochondria AGA
AGG
Arg Stop Human mitochondria
• Exceptions:
Courtesy: Prof. Dr. Koh Chong Lek & Prof. Dr. Sam Choon Kook
3’ 5’
5’ 3’
N C
Colinearity between the linear
nucleotide
sequences of
DNA & RNA
and the linear
amino acid
sequence of
proteins
Mutagenesis
The molecular basis of mutation
Courtesy: Prof. Dr. Koh Chong Lek & Prof. Dr. Sam Choon Kook
GAATTCCGGGCCCTAGCC CTTAAGGCCCGGGATCGG
Genetic information
❖Stored – base sequence
❖Transmitted faithfully
❖Processed to carry out cell functions
❖Regulated
❖Exchanged
❖Altered, repaired
bp:
A-T
G-C
Mutagenesis Mutation
• a heritable change in the genetic material not due to polyploidy or recombination
• mutant
Courtesy: Prof. Dr. Koh Chong Lek & Prof. Dr. Sam Choon Kook
Mutagenesis Mutation
• a heritable change in the genetic material not due to polyploidy or recombination
• mutant
Point mutation
• the smallest change in the genetic material
• a change in a single base pair in the DNA
molecule
Mutagenesis Point mutation
• base substitution
A G
C T
• transition (pu pu, py py), 4
• transversion (pu py), 8
Courtesy: Prof. Dr. Koh Chong Lek & Prof. Dr. Sam Choon Kook
Mutagenesis
Point mutation (sometimes up to 5bp)
• substitution
• deletion (-)
• insertion or addition (+)
Mutagenesis Mutations caused by
• physical
• chemical
• enzymatic methods
Spontaneous or induced mutations Chemical mutagens
induced chemical mutagenesis
Courtesy: Prof. Dr. Koh Chong Lek & Prof. Dr. Sam Choon Kook
Mutagenesis
Effects of base substitutions
• depend on the codons created
Same meaning Silent or synonymous Different or wrong
meaning Missense
No meaning Nonsense
Meaning of codon Mutation
-UGA
ACT- -CGA
GCT- -CGU
GCA-
-CAA
GTT-
Gln
Arg Arg
Stop codon
Nonsense mutation
Missense mutation Silent or synonymous mutation
Ts
Tv Ts
DNA antisense strand
mRNACourtesy: Prof. Dr. Koh Chong Lek & Prof. Dr. Sam Choon Kook
Mutagenesis Point mutation
• substitution
• deletion (-)
• insertion or addition (+) Deletion or insertion destroys the reading frame
frameshift mutations
• the reading frame is altered
• the genetic message is jumbled
The bad red dog and the cat
The sad red dog and the cat The bad red dog and the cat
The sad red doc and the cat The bar edd oga ndt hec at The ban dre ddo gan dth eca t
-d +n
Courtesy: Prof. Dr. Koh Chong Lek & Prof. Dr. Sam Choon Kook
