|
You
can view a 3-D rotatable image of DNA here...
|
|
The
hereditary material of all multi-cellular organisms is the
famous double helix of deoxyribonucleic acid (DNA). This
contains all of our genes. DNA, itself is made up of four
chemical bases, pairs of which form the "rungs" of the twisted,
ladder-shaped DNA molecules. These four molecules are adenine
(A), thymine (T), cytosine (C), and guanine (G).
All
genes are made up of stretches of these four bases (arranged
in different ways and in different lengths).
|
|
Photo:
National Institute of Health
|
|
|
|
A critical
feature of DNA is the ability of the nucleotides to make
specific pairs: . Adenine pairs with Thymine and Cytosine
pairs with Guanine. We say A is complementary to T (and
U) and C is complementary to G
The
double helical structure of DNA, is due largely to hydrogen
bonding between the base pairs (shown as green dotted lines
in image to left), which link one complementary strand to
the other
Hydrogen
bonds between guanine and cytosine
|
|
|
|
|
|
The
human genome of Homo sapiens is stored on 23 chromosome
pairs. 22 of these are autosomal chromosome pairs, while
the remaining pair is sex-determining. The haploid human
genome occupies a total of just over 3 billion DNA base
pairs that means 6 billion base pairs per diploid cell.
| "The
Human Genome Project (HGP) was one of the great feats
of exploration in history - an inward voyage of discovery
rather than an outward exploration of the planet or
the cosmos; an international research effort to sequence
and map all of the genes - together known as the genome
- of members of our species, --Homo Sapiens"--genome.gov |
|
|
Human
Chromosomes
|
|
|
How many
genes does the Human Genome contain?
The Human
Genome Project has revealed that there are probably about 20,000-25,000
'haploid' protein coding genes. The completed human sequence can
now identify their locations. But only about 1.5% of the genome
codes for proteins, while the rest consists of non-coding RNA
genes, regulatory sequences, introns, and noncoding DNA (once
known as "junk DNA").
Surprisingly,
the number of human genes seems to be less than a factor of two
greater than that of many much simpler organisms, such as the
roundworm and the fruit fly -- see table showing number of genes
for different organisms.
How many
genes do other organisms have?
|
chromosomes
--diploid |
base
pairs |
genome
size (#genes) |
Reference |
| fruit
fly |
8 |
1.65x108 |
13,600 |
|
| Budding
yeast |
16 |
12,462,637 |
6,275 |
|
| human |
46 |
3.3 x 109 |
~21,000 |
|
| human
mitochondria |
|
16,569 |
13 |
|
| rice |
24 |
4.66
x 108 |
46,022
-55,615 |
|
| dog |
78 |
2.4
x 109 |
~25,000 |
|
| mouse |
40 |
3.4
x 109 |
~23,000 |
|
Humans though
have on average three times as many kinds of proteins as the fly
or worm because of mRNA transcript "alternative splicing" and
chemical modifications to the proteins. This process can yield
different protein products from the same gene. --------Fruit
flies share nearly 60% of human genes and are studied by
What do
genes actually do?
Although genes
get a lot of attention, it’s the proteins that perform most life
functions and even make up the majority of cellular structures.
Proteins are large, complex molecules made up of smaller subunits
called amino acids. Chemical properties that distinguish the 20
different amino acids cause the protein chains to fold up into
specific three-dimensional structures that define their particular
functions in the cell.
 |
|
A ribosome
is the component of a biological cell that creates proteins
from all amino acids and RNA representing the protein.
Image:
Wikipedia
|
Mitochondrial
genome
The human
mitochondrial genome, while usually not included when referring
to the "human genome", is of tremendous interest to geneticists,
since it undoubtedly plays a role in mitochondrial disease. It
also sheds light on human evolution; for example, analysis of
variation in the human mitochondrial genome has led to the postulation
of a recent common ancestor for all humans on the maternal line
of descent.
How different
is the human genome from one person to the next?
Scientists
have identified about 1.4 million locations where single-base
DNA differences (SNPs) occur in humans. This information promises
to revolutionize the processes of finding chromosomal locations
for disease-associated sequences and tracing human history.
ref
|
