Chromosomes and DNA: A Laymen's Mini-Guide to Understanding Genetics
Dec 1, 2016 - 8:42:37 AM
The code which makes us human, and decides (to an extent) what sort of humans we are, is contained in the DNA (deoxyribonucleic acid) molecules. Each molecule is made up of billions of chemical bases, of which there are four types known by their initial letters A, C, G, and T. These are arranged in complex sequences in two paired strings, making the familiar double-helix. When cells reproduce, the two strings separate and each creates a new counterpart, so that the new DNA is identical to the old.
Along the DNA strings, the combinations of chemical bases operate in groups, some a few hundred bases long, some millions. These groups are the genes, of which humans have about 25,000. Genes control the production of the proteins that make our cells work, and in each cell, although the DNA code is the same, some genes are active and others are dormant. So, in a liver cell, the genes that make the cell do the things a liver cell does are active, and the genes that would make it do the things a skin cell does are not. Genes can also be activated by environmental factors.
DNA molecules come in packages called chromosomes. We have 46 chromosomes in each cell, and they come in pairs, 23 from each parent. 44 (22 pairs) of those chromosomes give us two sets of genetic material, mostly identical (about 99% of DNA is the same throughout the human species). The bits which are different account for the differences between people like the color of eyes, skin, or hair.
These variations can also be responsible for inherited diseases. The way these tendencies are inherited largely depends on whether the trait is dominant or recessive, but many traits are complex and unpredictable. This is one area of much of the important contract research that is currently being undertaken into genetics.
The sperm cells and egg cells are different from other cells, containing 23 chromosomes each—one set of DNA coding, but with the original maternal and paternal chromosomes randomly mixed up. On fertilization these combine in pairs to make the 46 chromosomes of the new individual.
One pair of chromosomes is different, the sex chromosomes which dictate the physical gender of the offspring. These come in two distinct types, X and Y. An X from the mother and a Y from the father will produce a boy (XY), an X from both father and mother will result in a girl (XX).
The human body is made up of trillions of cells. These each perform different functions depending on which genes are activated. Cells have many different structures (called organelles) performing a range of jobs, but at the center of every cell is a nucleus containing the genome, the complete sequence of DNA that is identical in every cell.
One type of organelle is the mitochondria; these do the job of utilizing sugars and oxygen to create energy. They carry their own genetic coding and reproduce within the cell. In some ways, they behave like independent organisms although they could not survive outside the cell. Their origin is unclear, but one theory is that at some very early stage in the evolution of living cells, bacteria were incorporated which took on the job of energy production, freeing the rest of the cell to become more specialized.
At reproduction, sperm cells do not come with accompanying mitochondria, so the developing embryo gets all its mitochondria from the egg cell; hence this genetic package is passed down unchanged through women alone.
Mitochondria, like nuclear DNA, can develop defects which have serious effects on people’s health.
A Fascinating World
Genetics is a complex and intricate world where the building blocks are incredibly small and elusive. Scientists are developing new techniques to study the mechanisms involved in genetic inheritance, and every discovery raises new questions. Huge progress is being made towards discovering and preventing the wide range of problems that can be passed down through the generations. New research is revealing the possibility of a new future all the time.
Freddie Dunn is a student with plans on becoming a Bioinformatics Scientist in the next couple of years. He writes about genome editing and all things DNA and Chromosome related in his articles.
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