The Central Dogma of Biology
How does the sequence of a strand of DNA correspond to the amino acid
sequence of a protein ? This concept is explained by the central
dogma of molecular biology, which states that:
Why would the cell want to have an intermediate between DNA and the
proteins it encodes?
- The DNA can then stay pristine and protected, away from the caustic chemistry of the cytoplasm.
- Gene information can be amplified by having many copies of an RNA made from one copy of DNA.
- Regulation of gene expression can be effected by having specific controls at each element of the pathway between DNA and proteins. The more elements there are in the pathway, the more opportunities there are to control it in different circumstances.
What is RNA?
RNA has the same primary structure as DNA . It consists of a
sugar-phosphate backbone, with nucleotides attaches to the 1' carbon
of the sugar. The differences between DNA and RNA are that:
There are several different kinds of RNA made by the cell.
- RNA has a hydroxyl group on the 2' carbon of the sugar (thus, the difference between deoxyribonucleic acid and ribonucleic acid.
- Instead of using the nucleotide thymine, RNA uses another nucleotide called uracil:
- Because of the extra hydroxyl group on the sugar, RNA is too bulky to form a a stable double helix. RNA exists as a single-stranded molecule. However, regions of double helix can form where there is some base pair complementation (U and A , G and C), resulting in hairpin loops. The RNA molecule with its hairpin loops is said to have a secondary structure.
- In addition, because the RNA molecule is not restricted to a rigid double helix, it can form many different tertiary structures. Each RNA molecule, depending on the sequence of its bases, can fold into a stable three-dimensional structure.
- mRNA - messenger RNA
- is a copy of a gene. It acts as a photocpoy of a gene by having a sequence complementary to one strand of the DNA and identical to the other strand. The mRNA acts as a busboy to carry the information stored in the DNA in the nucleus to the cytoplasm where the ribosomes can make it into protein.
- tRNA - transfer RNA
- is a small RNA that has a very specific secondary and tertiary structure such that it can bind an amino acid at one end, and mRNA at the other end. It acts as an adaptor to carry the amino acid elements of a protein to the appropriate place as coded for by the mRNA.
- rRNA - ribosomal RNA
- is one of the structural components of the ribosome. It has sequence complementarity to regions of the mRNA so that the ribosome knows where to bind to an mRNA it needs to make protein from.
- snRNA - small nuclear RNA
- is involved in the machinery that processes RNA's as they travel between the nucleus and the cytoplasm. We will discuss these later in the context of eukaryotic gene structure.
The Genetic Code
How does an mRNA specify amino acid sequence? The answer lies in the
genetic code. It would be impossible for each amino aciud to be
specified by one nucleotide, because there are only 4 nucleotides and
20 amino acids. Similarly, two nucleotide combinations could only
specify 16 amino acids. The final conclusion is that each amino acid
is specified by a particular combination of three nucleotides, called
Note the degeneracy of the genetic code. Each amino acid might
have up to six codons that specify it. It is also interesting to note
that different organisms have different frequencies of codon usage. A
giraffe might use CGC for arginine much more often than CGA, and the
reverse might be true for a sperm whale. Another interesting point is
that some species vary from the codon association described above, and
use different codons fo different amino acids. In general, however,
the code depicted can be relied upon.
How do tRNAs recognize to which codon to bring an amino acid? The tRNA
has an anticodon on its mRNA-binding end that is complementary
to the codon on the mRNA. Each tRNA only binds the appropriate amino
acid for its anticodon.