Role of Genes
Genes provide the instructive and regulative blueprint that is expressed in form of proteins that have several important structural and functional roles indispensable for survival. These proteins are encoded by genetic material in form of nucleobase sequences. Any alteration or mutation in these genetic sequence affects the functioning of these proteins.
Utilization of Genetic Information
Life begins as a single-celled zygote for many multicellular organisms, rest of the cells are formed by cleavage from this zygote so they possess the same genetic material. This genetic information in zygote is sufficient to make a whole organism that contains billions of cells that are cloned from it.
In the process of development the cell number increases and they are specified according to their cell fate and they start to differentiate. Growth in terms of increase in cell number and their differentiation is a highly regulated process.
Though all the cells comprise the same genetic information, they become differentiated to perform a particular function better and in this process utilize only a subset of the genes. In each specialized cells, genes are differently regulated with some being “switched off” and some being activated or “switched on”.
Genes: Switched On and Switched Off
Some genes are activated in all cells as they perform some basic functions important for survival like housekeeping genes or genes for enzymes of cellular respiration.
Other genes are switched on only in the specialized cells of a tissue. For instance the genes for insulin in pancreas, genes for beta and alpha globins in RBC and genes for digestive enzymes.
In prokaryotes also they may not require a gene to active all the time. For instance, glucose is the preffered energy source but in its unavailability if lactose is available then the genes required for catabolism of lactose will be switched on.
A genetic switch can be regulated or tripped by different molecular mechanisms like activation of transcription of the gene, increased translation or post-translational modifications of proteins that may activate it.
Regulation of Growth and Replication
Cell growth is an actively regulation process, there are many checkpoints along the cell cycle that monitor major activities involved in growth and replication. This complex process occurs with the coordinated interaction of several factors like growth factors, cell death, environmental signals, cell removal and contact inhibition. Failure of any of these signals may lead to abnormal patterns of growth or replication as seen in case of cancer. Normal growth can also be stopped prematurely.
Skin Color
Skin color shows an example of how genes are regulated. It can vary according to the presence of pigment melanin present in cells. The expression of skin color results from the interplay of several genes like tyrosinase which starts of the reaction by converting tyrosine to L-DOPA. The synthesis of melanin is also affected by exposure to UV radiation as it also protects the skin against such damagaing radiations. An increased pigmentation can result from sunlight exposure by directly increasing melanocytes or due to gene activity or can result from the combined effect of both. The difference in skin pigmentation intensity varies more due to the activity levels of pigment cells.
Blood Sugar Levels
Glucagon and insulin are proteins that help regulate blood sugar levels and are synthesised and secreted by pancreas. Insulin acts on elevated blood sugar levels, while glucagon functions when the blood sugar levels decrease. Insulin comprise of A and B chain, that are cleaved from a single protein precursor after translation. Increase in blood sugar level increases insulin synthesis mainly by 2 processes: by activating and increasing transcription of insulin gene and increasing protein translation.
Beta Globins
Hemoglobin is the iron pigment responsible for oxygen transport in the blood. It is comprised of 2 alpha and 2 beta polypeptide chains that are encoded by genes on two different chromosomes. In case of humans, the cluster of beta-globin is found on chromosome 11 and comprise of 1 pseudogene and 5 active genes. Each of these active genes at a specfic developmental stage produces a beta-globin.
The developmental order of these genes include epsilon, gamma-G, gamma-A, delta and beta. The beta chain of embryonic hemoglobin is formed by epislon globin, that is soon replaced by gamma globins in 3 months. At birth, The 2 adult hemoglobins present at birth are beta globin and delta globin. The LCR (Locus Control Region) is a regulatory site that activates these genes and this is also crucial to activate transcription of any of the beta-globin genes.
References
- Functions of DNA methylation: islands, start sites, gene bodies and beyond. Nat Rev Genet . 2012 May 29;13(7):484-92.
- Genetic control of pubertal timing. Curr Opin Pediatr . 2008 Aug;20(4):458-64.
- Genetic control of flowering time in rice: integration of Mendelian genetics and genomics. Theor Appl Genet . 2016 Dec;129(12):2241-2252.