Biomedical Engineering Reference
In-Depth Information
Table 3.1
The number of chromosomes found in selected species/
cells, along with their predicted/estimated (approximate) number of
genes
Cell/species
No. chromosomes
No. genes
E. coli
1
4 400
S. cerevisiae
16
6 200
Fern
1 200
13 600
Fruit fl y
18
13 000
Mouse
40
30 000-35 000
Rat
42
23 000
Dog
78
19 300
Human
46
30 000-35 000
shorter than chromosomal DNA, are often present in multiple copy number and tend to house genes
coding for proteins required within these organelles. Human mitochondrial DNA, for example, is
6600 base pairs (6.6 kbp) in length and houses 37 genes. Such DNA molecules are believed to be
vestiges of chromosomes from ancient bacteria that gained entry into early eukaryotic cells.
The genomes of different species are organized into different numbers of chromosomes, as is
evident from Table 3.1. Chromosomes present in all cells contain both coding regions (i.e. genes,
which are stretches of DNA that encode the specifi c amino acid sequence of a particular polypep-
tide or the exact nucleotide sequence of a tR NA or rR NA) and non-coding regions. Coding regions,
as we will subsequently see, often represent only a small fraction of total genome sequences.
In close association with gene sequences are regulatory elements, i.e. stretches or regions of
DNA that mark the beginning or end of a gene or a series of related genes or which regulate
the level of gene expression (Figure 3.7). A characteristic regulatory sequence upstream (i.e. on
the 5
'
side) of a gene is termed the promoter region (P), which RNA polymerases (the enzymes
responsible for transcribing the gene into RNA) identify and bind. Immediately adjacent to this
is a characteristic sequence that represents the starting point for transcription (T
C
). Immediately
downstream of the gene is a transcriptional termination site (t
C
). The intervening sequence, of
course, represents the precise stretch of DNA that is copied into RNA and is often called the
transcriptional unit. The gene sequence will often contain start and stop signals or sequences (T
L
and t
L
) that ultimately dictate the precise stretch of transcriptional unit actually translated into
polypeptide (Figure 3.7). Other regulatory regions controlling gene expression can also be present,
either upstream and/or downstream of the gene itself. In addition to genes and their associated
t
L
t
C
T
C
T
L
5'
3'
P
final coding sequence
transcriptional unit sequence
Figure 3.7
Generalized gene organization within the genome. Refer to text for details
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