Mitochondrial Dna And Ancestry - Tufts University printable pdf download

Day 1 : M itochondrial D NA a nd A ncestry

Genomic DNA is found in the cell nucleus and is passed down equally from both parents. Genomic DNA codes

for all of your traits such as eye color, hair color, and just about anything that is inherited. Your cells also have

another type of DNA called mitochondrial DNA. Mitochondrial DNA (mtDNA) is independent of nuclear DNA.

While nuclear DNA is located in the nucleus of the cell, mitochondrial DNA is located in the mitochondria.

Mitochondria is the organelle present in all cells which converts sugars into energy. Mitochondria is unusual in

that i t i s t he o nly o rganelles w hich h as i ts o wn c ircular l oop o f D NA. H uman m itochondrial D NA i s m uch s maller

than human nuclear DNA. Human nuclear DNA has around 3 billion base pairs or nucleotides, while human

mitochondrial D NA o nly h as 1 6,600 b ase p airs.

Mitochondrial DNA is inherited from the mother alone, rather than being inherited from the father and the

mother. Additionally, recombination (or crossing over) does not occur in mitochondrial DNA (mtDNA). For both

of t hese r easons, t he s equence o f m itochondrial D NA s tays t he s ame o ver g enerations, a nd t hus i s a u seful t ool

for l ooking a t m aternal a ncestry. S cientists h ave f ound t hat t he m itochondrial g enome m utates 5 -‐10 t imes f aster

than n uclear D NA. O ne t ype o f m utation, c alled a s ingle n ucleotide p olymorphism ( SNP), i s a s ingle b ase p air i n a

DNA s equence t hat h as b een s wapped o ut f or a d ifferent n ucleotide. B elow i s a n e xample o f a S NP.

Source: K . M innehan t hesis p resentation.

In the above example, the fourth nucleotide from the left side is different. Person 1 has a guanine/

cytosime ( G/C), a nd p erson # 2 h as a a denine/thymine ( A/T).

Scientists c ompare a ll D NA s equences t o t he D NA s equence o f o ne i ndividual f rom C ambridge E ngland k nown a s

the Cambridge Reference Sequence or “rCRS”. This individual was chosen to serve as a comparison point for all

other DNA sequences. This is similar to using a standard unit like a meter to measure the length of an object.

The Cambridge Reference Sequence is the genome all scientists use as their standard for comparison; it is

scientists’ “ ruler” f or c omparing a n i ndividual’s D NA s equence t o a s tandard.

Looking B ack i n T ime

By l ooking a t t he p atterns o f S NPs i n m itochondrial D NA, s cientists c an t race m aternal a ncestry b ack h undreds o f

thousands o f y ears. S cientists d o t his b y c omparing t he p atterns i n m itochondrial D NA b etween d ifferent g roups

of p eople. I f t wo p eople h ave v ery s imilar p atterns o f S NPs i n t heir D NA, t hey a re m ost l ikely r elated i n m aternal

lineage.

As our ancestors traveled throughout the world they settled in different communities. After a few generations

of reproduction, people in similar settings evolved with similar patterns of mtDNA. Since the patterns are not

Developed a t T ufts U niversity w ith f unding f rom D avid R . W alt’s H HMI P rofessor’s A ward a nd t he C amille a nd

Henry D reyfus F oundation.

Mitochondrial Dna And Ancestry - Tufts University