A) Genes: Structure, Replication & Mutation

 The size of DNA molecules presents an interesting biological
puzzle. Given that these molecules are generally
much longer than the cells or viral particles that contain them,how do they fit into their cellular or viral packages?  

          Cellular DNA contains genes and intergenic regions,
both of which may serve functions vital to the cell. The
more complex genomes, such as those of eukaryotic
cells, demand increased levels of chromosomal organization,
and this is reflected in the chromosomes’ structural
features. We begin by considering the different
types of DNA sequences and structural elements within

Genes Are Segments of DNA That Code For Polypeptide Chains & RNAs:

Our understanding of genes has evolved tremendously
over the last century. Classically, a gene was
defined as a portion of a chromosome that determines
or affects a single character or phenotype (visible
property), such as eye color. George Beadle and Edward
Tatum proposed a molecular definition of a gene
in 1940. After exposing spores of the fungus Neurospora
crassa to x rays and other agents known to
damage DNA and cause alterations in DNA sequence
(mutations), they detected mutant fungal strains
that lacked one or another specific enzyme, sometimes
resulting in the failure of an entire metabolic
pathway. Beadle and Tatum concluded that a gene is a

segment of genetic material that determines, or codes  for, one enzyme: the one gene–one enzyme hypothesis.
Later this concept was broadened to one gene–
one polypeptide, because many genes code for a
protein that is not an enzyme or for one polypeptide
of a multi subunit protein.
           The modern biochemical definition of a gene is
even more precise. A gene is all the DNA that encodes
the primary sequence of some final gene product, which
can be either a polypeptide or an RNA with a structural
or catalytic function. DNA also contains other segments
or sequences that have a purely regulatory function.
Regulatory sequences provide signals that may denote
the beginning or the end of genes, or influence the
transcription of genes, or function as initiation points
for replication or recombination. Many genes in eukaryotes and a few in bacteria
and archaea are interrupted by noncoding DNA segments
and are therefore considerably longer than this
simple calculation would suggest. 
    How many genes are in a single chromosome? The
Escherichia coli chromosome, one of the bacterial
genomes that have been completely sequenced, is a circular
DNA molecule (in the sense of an endless loop
rather than a perfect circle) with 4,639,675 bp. These
base pairs encode about 4,300 genes for proteins and
another 157 genes for structural or catalytic RNA molecules.
Among eukaryotes, the approximately 3.1 billion
base pairs of the human genome include almost 29,000
genes on 24 different chromosomes.


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