Biology 210
GENETICS
6 February, 1998
 

- James Watson in The Double Helix

5.1 The Chemical composition of DNA
5.2 The Physical Structure of the double
       helix
5.3 What a Genetic Material Needs that
       DNA supplies

A few words about the SIZE of genes.

5.1 The Chemical composition of DNA

Remember - there are THREE main components of DNA (in addition to water):
 

1. The purine and pyrimidine nucleotide bases

2. Ribose Sugars

 
Watson-Crick base pairs.  Note that A-T has 2 H-bonds

Also note that purine/pyrimidine bp fit "just right" so that they occupy the same space - compare the other possible schemes:

5.2 The Physical Structure of the double
       helix

There are several different ways to represent the double helix. Shown below is the Watson-Crick "spiral staircase" model:

Figure 5.4 - two different representations of the double helix:

Remember from last time - there are 3 families of DNA helices:

A-DNA family - this is most common for double stranded RNA, RNA/DNA hybrids, as well as for certain DNA sequences, such as long stretches of purines.

B-DNA family - DNA exists in the "B-DNA form", most of the time inside the cells of living organisms.   This is the classical "Watson-Crick" structure.

Z-DNA family - this is much more rare than the other two families, although certains sequences (such as runs of GC repeats (GCGCGC)) can form Z-DNA easily.

5.3 What a Genetic Material Needs that DNA supplies

1. It must be able to replicate accurately.

2. It must have the capacity to carry all the information needed to direct organisation and metabolic activities of the cell.

3. It must be capable of having an OCCASIONAL mutation or change in information.
 

4. A few words about the SIZE of genes & genomes...
 

• Bacterial genes are often around 1000 bp long.        (NOTE: we will talk more about what a gene actually is, on Monday)

• Many human genes are more than 100,000 bp long, and a few (like the gene for muscular dystrophy & cystic fibrosis) are close to 1,000,000 bp long!

 
• The complete GENOME (or all the DNA) from an organism is much generally larger for more complex organisms.  (NOTE, however, that for plants the size can be perhaps 100x to 1000x the size of many animal genomes - this is due to chromosomal duplication, which happens more often in plants than animals.

NOTE: The genome project is actually ahead of schedule, and it is very likely that the first complete sequence of a human genome will be finished within 3 or 4 years from now (probably during the year 2001).  This is based on an article by Richard A. Gibbs ("Hares and tortoises in the race to sequence the human genome: expectations and realities", Trends in Genetics, 13:381-383, (October, 1997)).

 The human genome project has also had a major influence on the rest of biology, as other organisms are being sequenced as goals towards the ambitious end of the 3,000,000,000 bp (or so) nucleotide sequence for the human genome.  In particular, the sequencing of complete bacterial genomes is revolutionising the field of microbiology.   Presently, bacterial genomes are being sequence at a rate of slightly faster than one new genome every month!  As technology improves, this rate will increase.  It is estimated that within the next two years, we will know the complete genomic sequence of most major pathogenic bacteria.
 

Reference: Tang,C.M., Hood,D.W., Moxon,E.R., "Haemophilus influence: the impact of whole genome sequencing on microbiology", Trends in Genetics, 13:399-404, (1997).

Link to a more recent list of sequenced genomes

Link to lecture notes from Autumn 1999

 
 

The following articles are from today's New York Times (6-Feb-98):

• Study Ties Folate, B-6 to Reduced Heart Disease


• Study Places HIV Origins a Decade Earlier


• Fossils From China Are Earliest Known Animals

Back to the GENETICS Syllabus 

Last modified on: 1 February, 2000 by Dave Ussery