How do scientists disable a gene in mice?

 

If they are disabling a gene in one cell, how does that affect the entire organism?

 

Also, is the genome for a mammal present in each cell?

 

Or is the genome all of an organism's DNA in each cell.. does this mean there's 3 billion base pairs in each cell?

There are a number of techniques that can be used to knock-out or knock-down genes in recombinant animals. One of these utilises the Cre-loxP system which relies on homologous recombination occuring during sexual reproduction of selected transgenic animals.

http://en.wikipedia.org/wiki/Gene_knockout

 

Genes can also be conditionally knocked out based on their cell-type specific expression by removing specific promoter and regulatory regions of the gene in question. I think there are other systems which can induce gene silencing, but I'm not too familiar with those, perhaps one of the resident experts can fill in the gaps here.

 

As to your final question, every somatic cell contains a full complement of the organisms genome, so yes in humans that is 23 pairs of chromosomes constituting (2x) 3.2 billion base pairs. Germ cells contain only a haploid complement of and thus only contain 3.2 billion base pairs. In addition the mitochondrial genome contains a further 16.6 kilobases of DNA, which technically also contributes to our complete genome.