DNA and the Genetic Code
June 14, 2011
DNA and the Genetic Code
Protein synthesis
Proteins are biochemical compounds involved in virtually
every process within
cells.
Protein synthesis requires two steps: transcription
and translation.
DNA and the Genetic Code
Protein synthesis (cont.)
DNA and the Genetic Code
DNA (Deoxyribonucleic acid)
DNA is a two-stranded, helical
polymer that contains genetic
instructions used during development
and functioning.
DNA is comprised of 4 nucleotide
bases: guanine (G), adenine (A),
cytosine (C), and thymine (T).
The bases are paired: G on one strand
pairs with C on the other strand;
A pairs with T.
DNA and the Genetic Code
Transcription
Transcription is the synthesis of
messenger RNA (mRNA) from DNA.
The two DNA strands separate from
one another. One strand is used for
replication, the other for transcription.
mRNA is the complement strand of
the original DNA except with T
replaced by U (uracil).
mRNA carries the genetic information out of the nucleus, into
the cytoplasm where it is translated to produce protein.
DNA and the Genetic Code
Translation
Translation is the process in which ribosomes decode mRNA
to produce amino acids.
mRNA is decoded in three-base sections called codons.
A codon codes for a single amino acid, or for “stop!”
There are 64 (43 ) different codons but only 20 amino acids.
⇒ several different codons can specify the same amino acid.
"
&&1
/.
DNA and the Genetic Code
Decoding mRNA
Scientists knew that amino acid sequences were specified by
sequences of bases, but they did not know how the mRNA
sequence was “read” to make proteins.
In the early 1960s, Marshall Nirenberg conducted experiments
using E. coli. He inserted an RNA strand made up only of
uracil (UUUUU...), which produced an amino acid chain made
up only of phenylalanine (Phe-Phe-Phe...).
DNA and the Genetic Code
Decoding mRNA (continued)
This was the first step in breaking the genetic code.
Nirenberg’s mRNA sequence: UUUUUUUUUUUUUUU
Sample mRNA sequence: AUGGCCUAGGUACUAAAU
Questions Nirenberg couldn’t answer with this experiment:
How long are codons (“words”)? Are they the same length?
Do the codons overlap?
Are codons consecutive bases?
Is there “punctuation” between codons?
DNA and the Genetic Code
Decoding mRNA (continued)
H. Gobind Khorana performed a
similar experiment to finish
cracking the genetic code.
of the genetic code. He synthesized artificial message
analyzed the resulting polypeptides. His data are show
the resulting amino-acid couplet also repeats indefinit
use these data (plus Nierenberg’s result) to figure out
code as you can. In particular, show that the code mu
as consecutive, non-overlapping triplets without punc
third position is often degenerate, and that some codo
He used more complex mRNA
sequences to obtain the data
at right:
DNA and the Genetic Code
Your task...should you choose to accept
DNA and the Genetic Code
Your task...should you choose to accept
Just kidding! You don’t have a choice - you have to accept. :)
DNA and the Genetic Code
Your task...which you will accept
Use the previous data from the two experiments to figure out the
genetic code. In particular, show that the code:
is consecutive, non-overlapping triplets.
has no punctuation.
often has a degenerate third position.
contains some codons meaning “stop”.
Also, explain why
coding with doublets is impossible
the only possible reading restart interval less than 9 is 3.
DNA and the Genetic Code
Sample mRNA sequence
AUGGCCUAGGUACUAAAU
DNA and the Genetic Code