Genetic information stored in DNA - Information transfer carried out by RNA.
(Cell has 8-10 times as much RNA at any one time than DNA)
The evolutionary choice of DNA as the information storage molecule may be reflected in the differences between DNA and RNA.
The use of Ribose in RNA instead of Deoxyribose
In the chemical structure of ribose, at the 2' C there is a hydroxyl group instead of a hydrogen.
Please see figure of ribose in the text.
The 2' hydroxyl is left free when ribonucleotides are linked to form RNA. This leaves RNA less chemically stable - tendency for hydrolytic cleavage. In contrast, the key to the genetic code is stability over a long period of time.
For same reason, RNA has been more difficult to study than DNA.
Differences between DNA and RNA:
1) The nitrogenous base,Uracil, is used in place of thymine but it acts just like thymine in pairing with adenine.
When RNA is formed using the DNA template (coding strand)
The A is transcribed to U in RNA
The T is transcribed to A in RNA
C to G
G to C
We will cover transcription (synthesis of RNA) later in much detail.
Important point: all three types of RNA are transcribed from DNA (mRNA, tRNA, and rRNA)
2) RNA is much shorter than DNA (recall that 1 DNA molecule spans the length of the entire chromosome.
3) With a few exceptions ( a few viruses) RNA occurs in single stranded form.
The nitrogenous bases can form hydrogen bonds with complementary base pairs.
Intra strand hydrogen bonding is important in secondary structure of RNA
RNAs have stem/loop or hairpin structures because the chains fold back on themselves, creating loops and small base-paired stretches between complementary regions.
One of the most remarkable and unexpected discoveries of recent years is that some RNAs have nucleolytic activity. The RNA enzymes, called ribozymes are able to cleave specific phosphodiester bonds in a manner analogous to protein enzymes.
3 types of RNA - considered one at a time
A primary transcript is transcribed from a sequence of DNA that codes for a specific protein. DNA unwinds, specific enzymes read the bases - ribonucleoside triphosphates are brought in, phosphodiester bonds form - mRNA complete.
The primary transcript retains all of the information of the DNA sequence from which it was transcribed.
mRNA is very short-lived compared to DNA. In prokaryotes, only around for a few minutes. Continuous synthesis of protein requires a continuous synthesis of mRNA. Fluctuating envirnoment, fluctuating needs.
To get from the primary transcript---------->mature mRNA the primary transcript must undergo RNA processing.
The primary transcript of prokaryotic cells is polycistronic.
One primary transcript can code for several different proteins.
RNA processing is much more extensive in eukaryotes.
1) A 5'CAP of 7-methyl guanosine is added.
2) A poly(A) tail is added to the 3' end of the transcript.
3) Introns (intervening sequences) must be cut from the transcript by a process known as RNA splicing.
4) Exons must be joined by DNA ligase.
The universal Genetic Code was deciphered in the early 1960s.
It is a triplet code.
In other words, each sequence of 3 mRNA nucleotides is a codon.
Example:
5'- A U G U U U C G U A C G U A A- 3'
Each codon - codes for a specific amino acid or serves as a stop codon.
Stop codons are: UAA, UAG, UGA
These three codons do not code for an amino acid. They signal as a stop signal for translation.
Start codon is AUG
AUG codes for the amino acid, methionine, so every protein begins with methionine regardless of whether it is a prokaryotic or eukaryotic protein.
There are 3 possible reading frames on a message depending upon where you start. So punctuation is very important (AUG) codon critical.
Because there are 4 different nucleotides, and it is a triplet code--
4^3=64 possible codon triplets for only 20 different amino acids.
Therefore, most amino acids are specified by more than 1 codon-- in other words the genetic code is a degenerate code.
An example of multiple codons coding for one amino acid:
CGU
CGC
CGA
CGG
All four codons code for the amino acid arginine.
The Genetic code also is highly conserved, the same in organisms as diverse as bacteria, plants, and man.
There are as many different mRNAs as there are genes. Most proteins have between 100 and 1,000 amino acids so an mRNA must be at least between 300 and 3,000 nucleotide bases long. mRNA possesses codons and the codons are read by tRNAs.