A human being is built of bricks called cells eg skin cell ,muscle cell , fat cell , nerve cell etc. Each cell has a brain called the nucleus . Outside the nucleus is a jelly like stuff called the cytoplasm where proteins are made and a cell membrane to keep the whole cell separate from its neighbours .
The brain or nucleus of the cell contains 46 rope like structures called chromosomes ..
If we arrange them according to size we find that they come in pairs, . There are 22 pairs in men and 23 in women . The first 22 are named according to numbers 1 to 22 and the last pair in women are called XX . In men there are 22 number pairs and the unpaired chromsomes are an X and a very short one called Y . One of each kind of chromosome comes for the mother and the other from the father . So Chromosme no.1s could be labelled 1m and 1f ( 1m from mother , 1f from father , 2m from the mother 2f from the father and so on for the other 22) -see picture below .
In the above figure the inside of a nucleus is shown with only 6 chromosmes for reasons of space but there are a total of 46 in each cell nucleus.
Lets look at say Chromosome 1m which comes from the mother .
It consists of two strings which are attached near the middle . Each string is called a chromatid and the attachment point is called a centromere
Take a peek at each chromatid string in the lower left figure above . The string is like a spiral staircase called a double helix . It is made of two thread strands coiled round each other . Imagine two people going up side by side on this staircase holding on to the arm rail . This armrail is the sugar molecule called deoxyribose .The sugar is called deoxy because it has oxygen taken away .
Each step below their feet is made of two nucleotides linked to each other . There are only four nucleotides altogether here colored differently called A -green , G-blue , C-red , T-yellow . Adenosine , Guanine , Cytidine and Thymidine.
Adenosine on one of the threads is always attached to Thymidine on the other and vice versa and Guanosine always links up with Cytidine . This total staircase structure of the four nucleotides AGCT and the sugar deoxyribose is called DNA -deoxyribosenucelic acid . It is the basic structure of life .
Outside the nucleus in mammal cells and in bacteria there is another form of genetic material called RNA where the sugar still has oxygen and its called ribosenucleic acid . Bacteria differ from us -they dont have the cytidine but have another compund called Uracil .
Now look again at the bottom right figure above . Reading the left side of the figure from top to bottom we see it reads AGTC . There are only four letters in the genetic code . Further down it could read GGTACTTAA etc.
Since A is matched with T always in each step of the ladder we call it a base pair and the same for C-G pair . There are in the 46 chromosomes over 3 billion ie 3,000,000,000 base pairs ! if it takes 20 minutes to read one pair after chemical analysis or say 3 an hour then it would 1b hours to read all of them and as they are 10,000 hrs in a year it ould take 100,000 years or 1000 men 100 years to decode the whole lot . It is however estimated that the whole genetic code will be decoded within five years .
How is this genetic code translated into building proteins ? Something called messenger RNA comes from the cytoplasm into the nucleus see figure below .
It breaks open a chromatid and starts reading the code on one of the strands called the sense strand . It does this by buidling an opposite nucleotide for each one on the ladder . For example if it sees an A it builds a T for itself and if the next letter is another A it adds another T to the structure its building so now it has TT .
The gene staircase must have some kind of sequence to tell it where to start lets say AAA and where to stop CCC , so if the messenger RNA sees a sequence of 9 letters AAACAACGGATACCC it knows that this protein is coded by 9 letters . So it ignores the AAA as it is a start sequence , copies the other 9 to make GTTGCCTAT and then encounters CCC which tells it to stop . It takes the GTTGCCTAT to the cytoplam where there are protein making factories called Ribosomes .
A gene is that part of the DNA which codes for building a protein . Some genes can be short and others have very long codes . If radiaton damages a part of the code then the mRNA will translate it wrongly and make a defective protein . Surprisingly most of the DNA in between the genes on the chromosmes appears to consist of nonsense DNA and is not translated by the mRNA .
A protein is made up of blocks called amino acids and there are 20 or so of them in humans . Each amino acid is coded by three nucleotides so we can group our structure brought out by the mRNA above into three parts ie GTTGCCTAT into GTT GCC TAT . GTT might be for the amino acid valine , GCC for glycine and TAT for say tyrosine . So the protein that is formed is these three linked in order valine-glycine-tyrosine ie a three peptide protein .
If the mRNA had brought back GCC TAT GTT GTT them it would have been a four peptide protein glycine-tyrosine-valine-valine etc. So each protein has its own codon on the Chromosme which is then translated by the mRNA and the protein made from it .
The aids virus is like mRNA . It invades the cell and gets into the nuclues and then breaks open the chromatid and stitches itself into the genetic code . Many years later it activates itself , tells the mRNA to make millions of its Aids copies and then go to the ribosomes and coat each of them with a bit of protein which is then released to the outside and invades more cells .
Most of the proteins built are structural proteins eg for muscle , skin etc. but some large proteins are catalysts called enzymes . They provide a large surface area for molecules to attach and come together and react at body temperatures for eg. glucose to be broken down to water and carbon dioxide . If the body temparature falls too low even the enzymes are not effective and the reactions stop and the person can die . Read the page again to make sure you understand the topic .