The genotype of an organism does not interact directly with the environment. Instead the genome is expressed to create the structural features of the organism and the controlling biochemical control of internal systems such as enzymes.
3.5.1 Comparison of RNA and DNA
3.5.2 DNA transcription
3.5.3 Genetic code
3.5.4 Translation
3.5.5 One gene, one polypeptide hypothesis.
This model illustrate the process of transcription that takes place in the nucleus. The DNA base sequence of the gene is copied into messenger RNA (mRNA)
The DNA helix is opened at the position of the gene.
The helix is unwound by DNA helicase
RNA nucleotides are found in the nucleus space.
One of the polynucleotide chains act as a template for mRNA
Free nucleotides base pair with DNA nucleotides
The phosphodiester bonds on the mRNA chain are formed by RNA polymerase
mRNA is a single polynucleotide chain but the base thymine is replaced by Uracil.
After the mRNA is complete the molecule detach's from the DNA and leaves the nucleus for the cytoplasm ribosomes.
The DNA helix reforms.
'You can treat the genetic code like a dictionary in which sixty-four words in one language (the sixty-four possible triplets of a four-letter alphabet) are mapped onto twenty-one words in another language (twenty amino acids plus a punctuation mark). The odds of arriving at the same 64:21 mapping are less than one in a million million million million million. Yet the genetic code is in fact literally identical in all animals, plants and bacteria that have ever been looked at. All living things are certainly descended from a single ancestor'
R.Dawkins, (1995),River out of Eden.
Well actually the code is nearly Universal. Interestingly the DNA in the mitochondria and chloroplast is slightly different in both prokaryotic and eukaryotic organisms. There are also some Protists in which UAA and UAG code for glutamine rather than acting as stop codons. The significance of these differences is as yet unclear
The genetic code:
Polypeptide is a sequence of bases
Bases are either A T G or C
There are 4 bases if placed in sets of 3 (a triplet).= 43possible triplets of DNA =64 triplets
There are 20 common amino acids
Therefore 64 triplets are mapped to 20 amino acids
However there is a 'punctuation' triplets.
Therefore the mapping of the code is 64: 21
The genetic code is first transcribed into mRNA
The mRNA codons can be mapped to a specific amino acid.
The mapping is 64 triplets: 64 codons: 21
Degenerate code:
DNA is a degenerate code since there are more than one triplet or codon that maps to an amino acid or punctuation.
mRNA codon AUG codes for Methionine and is a START signal for translation.
mRNA codon UAA, UAG, UGA are all stop codons punctuating the code.
GGU, GGC, GGA and GGG all code for amino acid glycine.
Consider what the benefits of a degenerate code might be from an evolutionary perspective?
mRNA is translated into an amino acid sequence (mapping above)
The location of translation is the ribosomes in the cytoplasm.
Ribosomes are also composed of RNA (rRNA) which acts as a catalyst for the translation of the mRNA.
Free ribosomes form polypeptides (proteins ) for internal cell use.
Ribosomes on the endoplasmic reticulum synthesis proteins for secretion.
mRNA from the nucleus locates onto the ribosome.
The start codon (initiation codon) AUG occupies one of two rib some site.
In this image the second site is occupied by CUG codon.
The ribosome moves along the mRNA
One mRNA can have many ribosomes (polysome) which accelerates protein synthesis.
Activation is a process in which Transfer RNA (tRNA) molecules attach to specific amino acids.
The tRNA molecule an anti-codon, three bases that are complementary to the codons on mRNA.
In heterotrophs the amino acids for activation come form consumed protein in the diet.
The first codon (AUG) bonds to the tRNA anti-codon UAC.
This tRNA carried the amino acid Methionine.
The second tRNA (GAC) binds to the second site with mRNA codon CUG
The second tRNA carried the amino acid Leucine.
Note that codon-anticodon binding is antiparallel
The link between the tRNA and the amino acid Methionine is broken.
The bond energy is transferred to form a peptide bond between methionine and Leucine.
The first tRNA is released form the ribosome first site.
This tRNA molecule moves away to pick up more methionine.
The ribosome move one mRNA codon to the right (in this image).
mRNA now occupied site one on the ribosome
The mRNA codon is UGC which has the complementary tRNA of ACG and is charged with Serine.
This occupied site site on the ribosome.
The bond between tRNA and Leucine is broken.
The bond energy is transferred to form a peptide bond between Leucine and Serine.
The tRNA for leucine is released form site one.
Then ribosome shift to the right and the process repeats itself until the stop codon is encountered.
As the amino acid chain is built the polypeptide self assembles into the correct shape. It essentially folds up due to intra-molecular forces such as hydrogen bonds.
Theory: One gene is transcribed and translated to produce one polypeptide.
Some protein are composed of a number of polypeptides and in this theory each polypeptide has its own gene.
e.g. haemoglobin is composed of 4 polypeptides (2 of each type) and there is a gene for each type of polypeptide.
This theory, like so many in biology has exceptions. e.g.
1) Some genes code for types of RNA which do not produce polypeptides.
2) Some genes control the expression of other genes.