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Sunday, December 2, 2012

Chromosomes & Genes

I’ve noticed that more and more it is assumed that people know what the terms chromosomes, genes, genetic codes and genomes mean but I suspect that a simple and clear explanation of what they actually are would not be out of order for many.

Chromosomes are found in the nucleus of every cell of our body and they contain the blueprints that carry all the information needed to build our bodies from conception and then direct our internal chemistry all through our life. 

Our body is made up of discrete cells and the picture on the left shows a typical cell. In its nucleus there are 23 chromosomes.  Each chromosome is made up of a matched pair of chromatids, one from each of our parents and they are held together by a short section near the middle called the centromere. 

The drawing shows one chromatid spewing out the very long single molecule of DNA that is normally curled up very tightly inside of it. The two intertwining strands of DNA each have short molecutes called nucleotides or bases attached at regular intervals. Each base tends to stick like a magnet to the base opposite it on the other strand and that's how they are held together. There are just 4 kinds of bases usually abbreviated as G, A, T, and C and they act like a kind of alphabet and spell out different instructions depending on their position and order of attachment. Sections of the DNA, typically about 10,000 bases long, are marked off as genes and each DNA molecule has about 1000 genes more or less.

When a cell needs an enzyme [a type of protein molecule] to activate a process, it sends a chemical message to ‘turn on’ the gene that has the information to produce it.  When a gene is ‘turned on’, the section of DNA where it’s located opens up [unzips], the information is copied and a copy of the information is sent back to the cell so it can assemble the enzyme needed. When enough of the enzyme is made, a message is sent for the DNA to zip up again.

Of course things are not quite so simple as that – each chromosome happens to have two DNA molecules and they match up gene for gene, one from each parent.  The dominant gene usually is the one that wins out, sometimes modified by its partner’s.

Scientists have learned how to make records of the sequencing of the bases in all of our chromosomes – it’s called coding the genome –  and most of the genes have been identified and their functions sorted out.  However, there are long sections of the DNA molecules in chromosomes that are not genes and yet serve some purpose biochemically not yet fully understood.

When we have children, we can pass on only 23 single chromatids to each offspring, with our partner passing on the other 23 to pair up with them to make our child's chromosomes. In preparation, our chromatid  pairs [one from each parent] get entangled by crossing over one another, physically breaking apart and exchanging parts with each other to form two different ‘scrambled’ chromatids as shown in a simple case on the left. The whole process is called ‘recombination’ and Mother Nature thus ensures that each child will receive a unique chromatid containing something from each of the formerly paired ones.  So we are all totally ‘one-of–a–kind’ unique unless of course we have an identical twin and then epigenetics steps in and we become unique.  Rie

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