Why cells regulate their genes

Chapter 12 followed a gene’s information from DNA to RNA to protein. Cells don’t run that process flat out for every gene. They choose which genes to express, when, and how much, and those choices shape everything from a bacterium’s diet to the cell types in your body.

Regulated genes and constitutive genes

Most genes are regulated: the cell switches them on, turns them up or down, or shuts them off as conditions change. Gene regulation makes sure each protein is made at the right time, in the right amount, and in the right cell type.

A smaller group of genes is expressed at a fairly steady level all the time. These are constitutive genes, also called housekeeping genes. They usually code proteins a cell needs constantly to stay alive, such as certain enzymes of metabolism.

When biologists say a gene is “turned on,” they mean it’s being transcribed into mRNA. A gene that’s “turned off” has little or no mRNA made from it.

What regulation buys an organism

It saves energy

Building proteins takes a lot of energy and raw material, so making one that isn’t needed is waste. The gut bacterium E. coli shows the payoff. It can feed on lactose, the sugar in milk, but only with two proteins: a transporter that brings lactose into the cell and an enzyme that breaks it down.

With no lactose around, the bacterium makes almost none of either protein. When lactose appears, it makes lots of both. Once the lactose is used up, it shuts those genes off, breaks down most of the proteins, and reuses their amino acids to build proteins it does need.

It makes different cell types

Recall from Chapter 10 that cell differentiation turns unspecialized cells into specialized ones such as muscle cells, neurons, and skin cells. These cells carry the same genome but have very different proteomes. Some proteins are made in only one cell type, and others are made in all of them but in different amounts. Gene regulation sets each proteome, and the proteome gives each cell its shape and its job.

It moves an organism through development

Multicellular organisms also turn genes on and off as they develop. Hemoglobin, the oxygen-carrying protein in red blood cells, is built from globin polypeptides, and mammals have several globin genes that code slightly different versions. An embryo, a fetus, and a newborn each switch on a different set of them.

The embryonic and fetal hemoglobins bind oxygen more tightly than the hemoglobin made after birth. That lets a developing baby pull oxygen from its parent’s blood across the placenta, even though it isn’t breathing yet.

Check yourselfA liver cell and a skin cell from the same person contain the same genes. Why do they make different proteins?

Gene regulation. Each cell type expresses its own set of genes, in its own amounts, so each ends up with a different proteome, and that proteome gives the cell its structure and function.

Where along the way genes are regulated

A protein-coding gene’s product passes through several steps on its way to becoming a working protein, and a cell can step in at any of them. Bacteria and eukaryotes lean on different steps.

Transcription is the most efficient place to regulate because it’s the first step. A cell that doesn’t transcribe a gene spends nothing on that gene’s mRNA or protein.

After translation, a cell can still adjust a protein. Feedback inhibition, from Chapter 6, switches an enzyme off without changing how much of it exists. Eukaryotes can also tag a protein with ubiquitin so it’s destroyed quickly once it’s in excess or no longer needed. Cells use this to keep tight control of proteins such as transcription factors and the proteins that move a cell through division.

Check yourselfAn enzyme’s level stays steady, but it stops working once its product builds up. At which level is this regulated?

Post-translation. The enzyme’s amount hasn’t changed, so neither have transcription and translation. Its activity has been shut off, as in feedback inhibition.