did we find out what a gene is?
word "gene" originally meant a piece of DNA in chromosomes that controlled a
given bodily trait.
Basic plan for DNA: information is carried in the ordering
of two pairs of molecules, adenine-thymine (A-T) and cytosine-guanine
(C-G). Billions of such pairs line up in a
specific sequence. When a cell divides, the DNA splits, where
the dashed lines are in the diagram. Because C can only react
with G and A only with T, the new cell creates an identical copy
was discovered in a famous analysis of data that other people were
generating at the time by Francis Crick and James Watson. The most
useful data came from x-ray crystallography, which captures on
film the x-ray scattering produced by crystals of DNA. The patterns,
plus what others had learned about A, T, C, and G, led them to
conclude the these molecules occurred in pairs and that the pair
was held together by a weak bonds involving hydrogen. Each
pair is called a "base pair," and the entire sequence of base
is called the "genome." So now, the question is:
the genetic code is nothing more than the chain of A-Ts and C-Gs,
how do we know what a gene is? That is, where does one segment
(gene) end and another begin?
Discovery Base-Pair Sequences: Gene Mapping
maps were done in fruit flies, because they have a small genome and
because they have been used so often in genetic experiments
mostly complete maps exist for humans (about 3 billion base
how do they know which base pair begins a gene and which ends a
gene? The key is to know the sequence of base pairs that
make a functioning protein. A combination of three approaches is used, and
computers keep track of the information.
short chains of base pairs are known to be markers for the
beginning and ending of base-pair sequences that make a
- A large
series of enzymes exist that can cut DNA into smaller pieces. By
splitting the two strands, one of the strands can be probed with
a single strand from a known gene, for example a bacterial gene
that makes a protein with a known function. The base sequence
that matches the probe sequence is then identified as a gene
an unknown piece of DNA into a bacterium or cell, and test for
any new protein product that appears.
of such sequencing approaches led to the conclusion that the human
may have only about 25,000 genes, which amount to only about 1% of
the total genome. That is only about twice the number that fruit
flies have. Does that make sense? We are more than twice
the animal a fruit fly is. How come we only have twice the genes?
A few species have more genes that people do! Possible
- the proteins
humans make are more complex than those usually
made by fruit flies.
- humans have many more "regulatory
genes," which are genes that turn on or turn off other genes.
genes may work together to make a protein and each combination
can make a different protein.
apparently have a lot of genetic "junk." Huge fractions of our
genome don't seem to do anything. Some seem to be residues
from viral and bacterial infections in our ancestors. So does this
"junk" do anything? One obvious possibility is that it is a source
clear that our understanding is only just beginning. Even if we
were sure what a gene is, we don't know what most of them do, nor
what turns them on or off. We have only begun to consider the
possibility that several genes are involved in making a single
protein, as is the case with the workings of the body's immune