[beachcomber (OP)]

Hello

As a layman, I hope you will be able to assist me with a question regarding the human genome / DNA.

It seems that every human being has a specific, set sequence of DNA which comprises the genome. When humans reproduce, these exact sequences are commingled to form another unique set.

In chemistry, combining specific elements results in an unchanging, established and fixed result; 2 atoms of hydrogen + 1 of oxygen produces water.

Presuming that these specific fixed sequences must always produce the same result, how is it that human reproduction produces a new, different sequence from the same bases?

[CliffordK]

The human genome has about 6.4 billion individual base pairs.

The DNA replication mechanism & error correction is extremely accurate, but perhaps one or a couple of uncorrected errors remain with every cell division. 

Some of these errors may completely block a particular protein from being accurately generated.  However, one's cells are diploid, or have two copies of every chromosome/gene, so it is likely that the second copy of the gene is correct. 

This is the reason it is a bad idea to have offspring from close relatives.  It is a greater likelihood that close relatives would share the same mutations.

There are far more mutations in the "untranslated regions" as they would not lead to a lethal mutation.

Meiosis is more than merely choosing 23 haploid chromosomes.  There is also a process called "crossover", where homologous DNA is swapped between DNA strands. giving far more combinations of DNA inside of the chromosomes, giving a greater variety of genes than even the parents had.

[RD]

... There is also a process called "crossover", where homologous DNA is swapped between DNA strands. giving far more combinations of DNA inside of the chromosomes, giving a greater variety of genes than even the parents had.

http://en.wikipedia.org/wiki/Chromosomal_crossover  [ why all siblings are not identical ]


To answer your next question (sort of) ...

J Cell Sci. 2011 Feb 15;124(Pt 4):501-13.
The choice in meiosis - defining the factors that influence crossover or non-crossover formation.

Proteins that influence crossover outcomes

The factors influencing DSB [meiotic Double-Strand Breaks] position are not yet well-understood, but a number of proteins that function downstream of DSB creation, to either promote or prevent crossover formation, have been characterised. Although it is not known exactly how the decision is made to form a crossover or non-crossover, many meiotic proteins influence whether or not a crossover can take place. Here, these proteins will be classified as having either pro-crossover or anti-crossover activities, although at least one of these proteins can be considered to fit into both categories, depending on the context of the protein activity or the model system in which it has been examined. A non-exhaustive list of these genes is presented in Table 1.

http://www.ncbi.nlm.nih.gov/pubmed/21282472

[crossover] control is even more complex than previously suspected.

http://www.ncbi.nlm.nih.gov/pubmed/21782271

[beachcomber (OP)]

Thanks for your prompt replies! My interest was in the way DNA is used to conclusively identify criminals, and then I got to wondering why, if it is so specific, do we not reproduce ourselves identically.
If I understand

"Meiosis is more than merely choosing 23 haploid chromosomes.  There is also a process called "crossover", where homologous DNA is swapped between DNA strands. giving far more combinations of DNA inside of the chromosomes, giving a greater variety of genes than even the parents had."

correctly, our end product selves are a result of these combinations in sub-sets of the DNA.     

[CliffordK]

Correct.

And as mentioned, there are parts of the DNA that do not code for proteins called "untranslated regions".  Since there is no downside from errors in these regions, there is much more variability than in other regions.

These Untranslated regions are used for the DNA fingerprints.

[beachcomber (OP)]

Thanks again CliffordK!

[evan_au]

The same DNA genetic code can also produce different results in different generations.
Some genes are turned on when inherited from the father, and turned off when inherited from the mother (and vice-versa).
These imprinted genes effectively "skip a generation" (or more).