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Unlike prokaryotes, multiple gene-regulating mechanisms operate in the nucleus before and after RNA transcription, and in the cytoplasm both before and after translation.
Histones are small proteins packed inside the molecular structure of the DNA double helix. Tight histone packing prevents RNA polymerase from contacting and transcribing the DNA. This type of overall control of protein synthesis is regulated by genes that control the packing density of histones. X-chromosome inactivation occurs when dense packing of the X chromosome in females totally prevents its function even in interphase. This type of inactivation is inherited and begins during embryonic development, where one of the X chromosomes is randomly packed, making it inactive for life.
Activator-enhancer complex is unique in eukaryotes because they normally have to be activated to begin protein synthesis, which requires the use of transcription factors and RNA polymerase. In general, the process of eukaryotic protein synthesis involves four steps:
1. Activators, a special type of transcription factor, bind to enhancers, which are discrete DNA units located at varying points along the chromosome.
2. The activator-enhancer complex bends the DNA molecule so that additional transcription factors have better access to bonding sites on the operator.
3. The bonding of additional transcription factors to the operator allows greater access by the RNA polymerase, which then begins the process of transcription.
4. Silencers are a type of repressor protein that blocks transcription at this point by bonding with particular DNA nucleotide sequences.
5. The processing and packaging of RNA both in the nucleus and cytoplasm provides two more opportunities for gene regulation to occur after transcription but before translation.
Adding extra nucleotides as a protective cap and tail to the RNA identifies the RNA as an mRNA by the ribosomes, and prevents degradation by cell enzymes as it moves from the nucleus into the cytoplasm.
RNA splicing occurs when “gaps” of nonprotein-code-carrying nucleotides called interons are removed from the code-carrying nucleotides, called exons, which are then connected to shorten the RNA molecule for conversion into tRNA and rRNA. The number of interons regulates the speed at which the RNA can be processed.
After the extra nucleotides have been added as a cap and tail and the RNA has been spliced, it moves to the cytoplasm where additional mechanisms of gene regulation exist.
The longevity of the individual mRNA molecule determines how many times it can be used and reused to create proteins. In eukaryotes, the mRNA tends to be stable, which means it can be used multiple times; which is efficient, but it prevents eukaryotes from making rapid response changes to environmental disruptions. The mRNA of prokaryotes is unstable, allowing for the creation of new mRNA, which has more opportunities to adjust for changing environmental conditions.
Inhibitory proteins prevent the translation of mRNA. They are made inactive when bonded with the substance for which they are trying to block production.
Post-translation control involves the selective cutting and breakdown of proteins that prevent the formation of the final product. In both cases, the hormone or enzyme required to finish or activate the final product may be rendered inactive.
Although much has been learned about inheritance since Mendel's time, the fundamentals remain the same. In sexual reproduction the offspring inherit one half of their genes from the father and one half from the mother. The chance of inheriting a particular gene can be estimated by pedigrees and Punnet squares. Every trait, feature, or characteristic is controlled by genes or a combination of genes. Numerous gene-regulating mechanisms activate and inactivate organism functions.
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