The transcriptional control in eukaryotes is very complex. It requires a variety of protein factors that falls into two categories: basal transcription factors and specific transcription fact s. A host of basal transcription factors are required for assembly of transcription apparatus as well as recruitment of RNA POl II to a promoter.
They carry on transcription at a basal rate and do not increase the rate above the basal level.The specific transcription factors increase the rate of transcription in certain cell types and in response to specific signals. Besides, eukaryotic genes are also regulated by specific DNA sequences where these specific transcription factors bind.
Enhancers are such far away DNA sequences where the specific transcription factors bind to increase the rate of transcription. Once the specific factors bind to enhancer sequence, the flexibility of DNA allows bending so that these far away sequences come close to the promoter regions.There are other factors like coactivators and mediators necessary for transcription by the transcription factors.
Eukaryotic gene expression is also controlled by chromatin structure and DNA methylation the chromatin structure at its lowest level of organization is nucleosomes. Histones in the nucleosomes can be modified to result in greater condensation of chromatin making promoters less accessible to transcription factors.
Chemical methylation of DNA through the addition of methyl group to cytosine creates 5-methylcytisine but docs not alter its pairing with guanine. Many inactive mammalian genes are thought to be inactivated through methylation. However, methylation is now thought to have a less active role in gene inactivation.
1. The codon is triplet, ie, the code for each amino acid consists of three nucleotides.
2. The code is degenerate: There are 64 codes for only 20 amino acids which mean more than one codon for each amino acid. This is the degeneracy of the code.
4. The code is practically universal: The genetic code is same in almost all organisms. For example the codon AGG specifies amino acid Arginine in bacteria, animals and plants. But there are some exceptions to it. In mitochondrial genomes “stop codon” UGA is read as amino acid Tryptophan, AUA as Methionine rather than Isoleucine and AGA and AGG as “stop codon” rather than Arginine.
The code is cammaless: The reading of the codons on a mRNA occurs continuously without any punctuation between successive codons.