Hello and thank you for your question.
As you probably know, the 'gene' was defined experimentally before the
function of DNA was understood or even known.
Your question is addressed fully in an article that appeared in
Science Magazine in April, 2003.
Genomics. Defining genes in the genomics era.
M Snyder, M Gerstein (2003) Science 300: 258-60.
http://papers.gersteinlab.org/e-print/whatisgene/all.pdf
http://papers.gersteinlab.org/e-print/whatisgene/reprint.html
The article is only a bit over 2 pages long, and is worth careful reading.
Here are the first two paragraphs [citations omitted]:
"A genome is defined as the entire collection of genes encoded by a
particular organism. But what is a gene? Historically, the term gene,
attributed to Johansson, first appeared in the early 1900s as an
abstract concept to explain the hereditary basis of traits. Phenotypic
traits were ascribed to hereditary factors even though the physical
basis of those factors was not known. Subsequently, early genetic
studies by Morgan and others associated heritable traits with specific
chromosomal regions. In the 1930s, Beadle introduced the concept of
'one gene, one enzyme,' which later became 'one gene, one
polypeptide.'
"With the advent of recombinant DNA and gene cloning, it became
possible to combine the assignment of a gene to a specific segment of
DNA and the production of a gene product. Although it was originally
presumed that the final product was a protein, the discovery that RNA
has structural, catalytic, and even regulatory properties made it
evident that the end product could be a nucleic acid. Thus, we now
define a gene in molecular terms as "a complete chromosomal segment
responsible for making a functional product." This definition has
several logical components: the expression of a gene product, the
requirement that it be functional, and the inclusion of both coding
and regulatory regions. According to this definition, it should be
possible to use straightforward criteria to identify genes in the DNA
sequence of a genome. Five such criteria are in common use, but their
application is not straightforward."
According to the article, the five criteria for identifying a gene are:
*Open reading frames
*Sequence features
*Sequence conservation
*Evidence for transcription
*Gene inactivation
I won't quote or paraphrase the article any further, but as the first
paragraph indicates, in the most basic terms: As a matter of
hindsight (now that we know a good deal about the function of DNA and
RNA), what Mendel and his successors observed in their
genetics/heredity experiments was the action of sections of
chromosomes, called genes, in which a stip of DNA provided the coding
whereby RNA could read out the recipe so that units inside the cell
(ribosomes, golgi, etc.) could manufacture the particular proteins and
other molecules that made this pea smooth, that pea wrinkled, and so
forth.
TUTORIAL ON DNA STRUCTURE, REPLICATION, TRANSCRIPTION, AND PROTEIN SYNTHESIS
http://www.ncc.gmu.edu/dna/
To answer your questions more specifically,
(1) (a) Is DNA one long continuous molecule?
For prokaryotes (bacteria) the answer is yes;
"The nucleoid of prokaryotes is one long, single molecule of double
stranded, helical, supercoiled DNA"
http://www.cat.cc.md.us/biotutorials/dna/dna.html
for eukaryotes (plants and animals) the answer is more complicated:
"The DNA in eukaryotic cells is packaged in a highly organized way. It
consists of a basic unit called a nucleosome (def), a beadlike
structure 11 nm in diameter that consists of 146 base pairs of DNA
wrapped around eight histone molecules. The nucleosomes are linked to
one another by a segment of DNA approximately 60 base pairs long
called linker DNA (see Fig. 10). Another histone associated with the
linker DNA then packages adjacent nucleotides together to form a
nucleosome thread 30nm in diameter. Finally, these packaged nucleosome
threads form large coiled loops that are held together by nonhistone
scaffolding proteins. These coiled loops on the scaffolding proteins
interact to form the condensed chromatin seen in chromosomes during
mitosis."
Ibid
(b)is it divided into genes?
Also yes, but as indicated above genes are more logical than physical units.
(2) Chromosomes are visible under a strong microscope as distinct entities.
But how are distinct GENES identified? What marks the beginning and end
of a particular gene?
As the "Defining genes in the genomics era" article indicates, the
definition of a particular gene is more one of function (that's what
the 5 criteria are about) than physical form.
But there is an alternative way of answering this question too - -
research has shown where in the chromosome many genetic traits and
mistakes are found--and these can also be called genes. For example,
http://www.ncbi.nlm.nih.gov/Omim/getmap.cgi
http://www.ncbi.nlm.nih.gov/Omim/getmorbid.cgi
http://www.ncbi.nlm.nih.gov/genome/guide/human/
In this last reference, you can click on one of the chromosomes shown
in the left section of the page and gene maps of that area will
appear. But again, these aren't physical structures - - they're the
sections of DNA that have been associated with particular heritable
functions.
(3) Is there a website that gives a visual representation of the hierarchy
of cells, chromosomes, genes, and DNA?
The best sources are the one cited above:
http://www.ncbi.nlm.nih.gov/genome/guide/human/
and also
http://wsrv.clas.virginia.edu/~rjh9u/dnaprot.html
and its many links.
Search terms used:
"what is a gene
eukaryotic dna "single molecule"
Thanks again for bringing us your question
Google Answers Researcher
Richard-ga |
