Control of Gene Expression
Gene Regulation is Necessary
By switching genes off when they are not needed, cells can prevent resources from being wasted. There should be natural selection favoring the ability to switch genes on and off.
A typical human cell normally expresses about 3% to 5% of its genes at any given time.
Cancer results from genes that do not turn off properly. Cancer cells have lost their ability to regulate mitosis, resulting in uncontrolled cell division.
Types of Control in Eucaryotes
Gene expression in eucaryotes is controlled by a variety of mechanisms that range from those that prevent transcription to those that prevent expression after the protein has been produced. The various mechanisms can be placed into one of these four categories: transcriptional, posttranscriptional, translational, and posttranslational.
Transcriptional - These mechanisms prevent transcription.
Posttranscriptional - These mechanisms control or regulate mRNA after it has been produced.
Translational - These mechanisms prevent translation. They often involve protein factors needed for translation.
Posttranslational - These mechanisms act after the protein has been produced.
Heterochromatin and Euchromatin
One X chromosome is inactivated in females by producing a tightly-wound structure called a Barr body.
Regulatory Proteins and Transcription
Proteins called transcription factors function by binding to specific base sequences within regions of DNA called the promoter and the enhancer. The enhancer region may be located at a distance from the gene. These transcription factors are necessary for RNA polymerase to attach. Transcription begins when the factors at the promoter region bind with the factors at the enhancer region creating a loop in the DNA.
Hundreds of different transcription factors have been discovered; each recognizes and binds with a specific nucleotide sequence in the DNA. A specific combination of transcription factors is necessary to activate a gene.
The production of transcription factors is regulated by signals produced from other molecules. For example, hormones activate transcription factors and thus enable transcription. Hormones therefore activate certain genes.
Differential Removal of Introns
Speed of Transport of mRNA Through the Nuclear Pores
Evidence suggests that this time may vary.
Longevity of mRNA
Messenger RNA can last a long time. For example, mammalian red blood cells eject their nucleus but continue to synthesize hemoglobin for several months. This indicates that mRNA is available to produce the protein even though the DNA is gone.
Ribonucleases are enzymes that destroy mRNA.
Messenger RNA has noncoding nucleotides at either end of the molecule. These segments contain information about the number of times mRNA is transcribed before being destroyed by ribonucleases.
Hormones stabilize certain mRNA transcripts.
Prolactin is a hormone that promotes milk production because it affects the length of time the mRNA for casein (a major milk protein) is available.
Ribonulceases destroy the mRNA.
Prolactin is a hormone that promotes milk production because it affects the length of time the mRNA for casein is available.
Preventing Ribosomes From Attaching
Modification of DNA
After fertilization of an ovum, large quantities of rRNA are needed to produce ribosomes. The developing embryo will need many new proteins and the large number of ribosomes can be used to produce these proteins. In frogs, genes that code for the production of rRNA are copied so that there may be more than a million copies of these genes. The genes are located in the nucleolus and enable the fertilized egg to produce rRNA rapidly.
The Immunoglobin Genes