Course 23.3Lesson 3 of 3
A bacterium switches the lac operon on only when lactose is present. The repressor sits on the operator and blocks the polymerase, and lactose, the inducer, takes the repressor off. The Human Genome Project read the whole sequence of human DNA. DNA fingerprinting compares the VNTR bands of people, and uses them in paternity tests and forensic work.
Try the code and the simulations beside the lesson.
A cell has all its genes in all its cells, but it does not use them all at once. A liver cell and a nerve cell have the same DNA, and they make different proteins. A bacterium makes the enzymes that digest a sugar only when the sugar is there. The control of when and how much of a gene is used is regulation of gene expression.
In a eukaryote, regulation can act at several levels: when the first transcript is made (transcription), how it is processed (splicing), how the mRNA leaves the nucleus (transport), and how it is translated (translation). In a prokaryote, the control of the start of transcription is the main control.
In 1961, François Jacob and Jacques Monod described how a bacterium controls the use of the sugar lactose. They gave us the idea of the operon, a group of genes that work together under one control. The lac operon of E. coli has these parts.
| Part | What it is |
|---|---|
| i gene (regulatory gene) | Makes the repressor protein, all the time |
| Promoter (p) | Where RNA polymerase binds |
| Operator (o) | Where the repressor binds. It lies next to the promoter |
| z gene | Codes for beta-galactosidase, which splits lactose into glucose and galactose |
| y gene | Codes for permease, which lets lactose into the cell |
| a gene | Codes for transacetylase |
The three genes z, y and a are transcribed together, in one mRNA, so the transcript is polycistronic. All three enzymes are needed to use lactose.
So the operon is switched by its own substrate. Because the repressor switches the operon off, this is negative regulation. A very low level of the enzymes is always present. This is how the first lactose enters the cell.
Glucose and galactose are not inducers of the lac operon.
You should find that a little lactose already takes most of the repressors off, and that the activity of the operon is small, but not zero, without lactose.
This part is not in the NEET syllabus, but A-Level and IB ask it. In the cell, a small part of the lactose is changed to allolactose, and allolactose is the real inducer. Also, even if lactose is present, the operon is only fully on when glucose is absent. This second control is through the molecules cAMP and CAP.
The genome is the whole of the DNA of an organism. The Human Genome Project (HGP) was a very large project. It began in 1990 and was completed in 2003. Its goals included these.
There were two ways to work. In the expressed sequence tags (ESTs) method, only the genes that are expressed as RNA are found. In sequence annotation, the whole DNA is sequenced, and the functions are assigned to its parts afterwards. The DNA was cut into small pieces, which were cloned in bacteria and yeast with special vectors, the BAC (bacterial artificial chromosome) and the YAC (yeast artificial chromosome). The pieces were read by automated machines that work on a method of Frederick Sanger, and a computer put them in order by their overlaps. Such work is part of bioinformatics.
Some of its findings are these.
Older prints of NCERT say that there are about 30,000 genes. More recent counts give about 20,000 genes that code for proteins.
Some parts of the DNA that do not code for any protein, the satellite DNA, are made of short pieces repeated again and again. The number of repeats differs from person to person. A repeat that varies like this is called a variable number of tandem repeats (VNTR). This variation is the basis of DNA fingerprinting, which was developed by Alec Jeffreys.
The steps are these.
Every cell of a person has the same pattern, whether it is from blood, hair or skin. A child gets half of its bands from each parent. This is why DNA fingerprinting is used in paternity tests and in forensic work. In the picture, the child has three bands (8.0, 3.6 and 2.2 kb) that the mother does not have. Man A has all three, and so he can be the father. Man B has none of them.
The program finds the bands of a child that did not come from the mother, and checks whether a man has them all.
The next program shows the lac operon as a switch.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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