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2005
Q. How many base pairs are there in mitochondrial DNA ?
A. This is a little variable - but the Cambridge Reference Sequence (CRS) has 16,568 base pairs.
Note: it is conventional to describe only one strand of the DNA - the 'L-strand'.
The CRS starts with 'gatca' (bases 1-5) and finishes with 'cgatg' (bases 16565 - 16569)
with base 3106 not being assigned for historical reasons.
Q. What is the Cambridge Reference sequence ?
A. The first complete sequencing was done in Cambridge and a numbering system was developed.
This sequence and the numbering of the bases form the CRS.
All subsequent sequences are now described by comparing them to the CRS.
Q. What sort of mutations are there ?
A. Mutations are of three kinds:-
- Simple mutations when a base changes.
E.g. 'C16223T' when the 'C' in the CRS at 16223 changes to a 'T'.
- Insertions when one or more bases are added.
E.g. '44.C' when an extra 'C' is found between base 44 and base 45. (aka. 44+C)
- Deletions when one or more bases are lost.
E.g. 'C522-' and 'A523-' when a 'CA' at 522 & 523 are lost. (aka. d522-523)
Q. And how is a sequence described ?
A. On this website the description of a sequence is given as a list of mutations such as:-
A263G 309.C 315.C T678C A750G A1438G G3010A A4769G A8860G C9356T A15326G T16519C
Q. How often does a mutation occur ?
A. Mutations are random events, but 1 mutation occurs, on average, perhaps every 1,500 years.
So, the above example of 12 mutations suggests a common ancestor about 18,000 years ago.