Mitochondrial DNA and Y- chromosome markers - Week 6 Lecture

Topic = Mitochondrial DNA and Y-Chromosome Markers

• Nuclear DNA vs Mitochondrial DNA (mtDNA)

  • Nuclear DNA: 46 chromosomes found in the cell nucleus.

  • mtDNA: exists outside the nucleus in the mitochondria; mitochondria are the powerhouse of the cell.

• Mitochondria basics

  • mtDNA is small and circular, about $16{,}500\text{ bp}$.

  • Contains $37\text{ genes}$, most involved in protein synthesis or respiration (energy production).

  • Multiple copies per mitochondrion; cells contain hundreds to tens of thousands of mitochondria depending on energy requirements.

  • Consequently, there are far more mtDNA copies per cell than nuclear DNA.

• Mitochondrial division and inheritance

  • Mitochondria divide and replicate within the cell to make copies of mtDNA.

  • mtDNA copies are segregated randomly between organelles during division.

  • Inheritance is strictly maternal: sperm mitochondria are located at the base of the tail and provide energy for swimming but are lost upon fertilization; the egg contains all mitochondria for the developing embryo.

  • All children of one mother inherit identical mtDNA; only daughters can pass mtDNA to the next generation.

• Why mtDNA is useful

  • mtDNA can be traced back thousands of years (maternal line) due to its maternal inheritance and high copy number in cells.

  • The double-membrane structure protects mtDNA from degradation, making it particularly useful for historic or badly decomposed samples.

  • Because multiple copies exist per cell and mtDNA is more abundant than nuclear DNA in degraded samples, mtDNA is often recoverable when nuclear DNA is not.

• Evolution and origin of mitochondria

  • Mitochondria began as independent organisms and formed a symbiotic relationship with primitive eukaryotic cells; over time they evolved together.

  • This endosymbiotic origin underpins the unique features of mtDNA (circular genome, maternal inheritance, limited recombination).

• Mitochondrial Eve and population history

  • All modern human mtDNA traces back to a maternal ancestor often referred to as the “Mitochondrial Eve.”

  • Temporal placement: roughly 190{,}000$-$200{,}000\text{ years ago}, in Africa.

  • Her mtDNA lineage spread as descendants migrated out of Africa; mutations began to accumulate over time roughly at a rate of about once every $10{,}000\text{ years}$.

  • The accumulation of mutations creates branching in the mtDNA phylogeny; examples include multiple mutation patterns shown as branching sequences (e.g., GATC/CATC patterns) leading back to Eve.

  • The concept allows researchers to reconstruct ancient human movements and demographic events by examining mtDNA mutations.

• mtDNA movement and ancient DNA analysis

  • Every new mutation forms a new branch in the mtDNA family tree.

  • Ancient DNA is examined to determine where and when particular mutations occurred, illuminating historical population movements.

• Mutations in mtDNA and disease

  • Mutations in mitochondrial genes can cause disease; symptoms often include fatigue and muscle weakness.

  • Mutations are largely confined to the control region of the mtDNA, especially the hypervariable regions HV1, HV2, and HV3.

• mtDNA Control Region and Hypervariable regions

  • mtDNA Control Region (D-loop)

  • Also called the D-loop ( displacement loop).

  • One strand of the loop is displaced by a short stretch of single-stranded DNA, exposing the other strand to replication machinery.

  • Two strands are separated and both can be copied; this region contains no genes and is the most polymorphic portion of mtDNA.

  • Hypervariable Regions

  • HV1: positions $16{,}025$ to $16{,}365\text{ bp}$ (length $342\text{ bp}$).

  • HV2: positions $73\text{ bp}$ to $340\text{ bp}$ (length $268\text{ bp}$).

  • HV3: positions $438\text{ bp}$ to $574\text{ bp}$ (length $136\text{ bp}$).

  • Approximately $3\text{ SNPs per }100\text{ bp}$.

  • Any two people could have about $22\text{ differences}$ across HV1, HV2, and HV3 combined.

  • Sequencing is typically performed on HV1 and HV2, with HV3 tested if needed.

• Cambridge Reference Sequence (rCRS)

  • In 1999, the entire mtDNA of one individual was fully sequenced and became the reference sequence for all mtDNA comparisons.

  • Known as the revised Cambridge Reference Sequence (rCRS).

  • The reference individual was a modern European male; this provides a standardised location and nomenclature for reporting mtDNA variations.

• mtDNA summary for forensics

  • Inherited exclusively from the mother.

  • mtDNA genome length: $16{,}500\text{ bp}$ with $37\text{ genes}$.

  • Multiple mitochondria per cell and multiple mtDNA copies per mitochondrion.

  • mtDNA is protected from degradation and is highly variable in the control region, making it useful for degraded samples.

  • The rCRS is the standard reference sequence for comparison.

• Forensics applications: what mtDNA can help with

  • Ancient relationships (deep ancestry)

  • More recent relationships (family connections over generations)

  • Identification of individuals when nuclear DNA is not available or is too degraded

• mtDNA analysis workflow (forensic workflow)

  • Extract mtDNA from evidence (Q sample).

  • PCR amplify HV1 and HV2 regions (performed separately and preferably after evidence collection).

  • Extract mtDNA from reference sample (K sample).

  • PCR amplify HV1 and HV2 regions.

  • Sequence HV1 and HV2 amplicons on both strands (forward and reverse).

  • Confirm sequence with both strands.

  • Note differences from Cambridge reference sequence (rCRS).

  • Compare Q and K sequences.

  • Compare with a database to determine haplotype frequency.

• Comparison to the Cambridge sequence

  • Both test (Q) and reference (K) samples are compared to the revised Cambridge Reference Sequence (rCRS).

  • Differences are noted, and results are compared between sequences.

• Mitochondrial haplogroups and haplotypes

  • A haplotype is a set of alleles inherited from a single parent (here, the mother).

  • All your mtDNA comes from Mum → maternal haplotype.

  • Haplogroups are broader historical classifications defined by points in history where mutations arose and sequences diverged.

  • Haplogroups can be traced from the rCRS back to Mitochondrial Eve.

• World distribution of mtDNA haplogroups

  • Haplogroups are distributed across the world and can be used to define ethnic groups.

  • The slide shows a world-wide map of haplogroups (examples include A, B, C, D, E, F, G, H, HV, L-lineages, M, N, P, Q, R, T, U, V, W, X, Y, Z and others with regional labels).

  • Some regions show strong regional clustering (e.g., African, American, Polynesian areas are very large in scope).

• Y chromosome: basics

  • The Y chromosome is the second smallest human chromosome and contains about $60{,}000{,}000$ bases.

  • It is passed from fathers to sons, enabling paternal lineage tracing.

• Y-chromosome as a tool for paternal ancestry

  • Helps determine paternal ancestry and population movements through time.

• Y chromosome structure and key regions

  • Pseudoautosomal regions (PAR) at the tips allow pairing with the X chromosome during cell division.

  • The non-recombining region of the Y (MSY) is male-specific DNA.

  • The SRY gene in MSY determines male development.

  • PAR (pseudoautosomal regions) are shared between X and Y chromosomes.

• Y chromosome testing: markers and applications

  • Forensic testing relies on STRs (short tandem repeats) and SNPs in the euchromatin region.

  • STRs are more variable and therefore more informative for distinguishing individuals.

  • SNPs are more stable and thus more informative for ancestry studies.

• Y-chromosome STRs (Y-STRs)

  • There are over 400 Y-STRs known.

  • Only 9 form the minimal haplotype, which is a core set of markers used in some analyses.

  • Commercial Y-STR kits typically include about 11 markers.

  • Y-STRs are valuable for identifying male lineages and differentiating male contributors in mixed samples.

  • Forensic context: useful in rape cases where samples are often mixed male/female DNA.

• Y-STRs: advantages and limitations

  • Advantages: enables amplification of male-only DNA, highly useful in mixed samples typical of rape cases.

  • Limitations: useful in paternity cases when the father is unavailable; cannot exclude other male family members because close paternal relatives share many Y-STR haplotypes.

• Y-chromosome SNPs and haplogroups

  • SNPs accumulate over history, forming new haplogroups with each new mutation.

  • All haplogroups eventually trace back to the Y-chromosome “Adam.”

  • Tracing Y-chromosome haplogroups reveals how human populations moved over time.

• Y-chromosome haplogroups (examples and phylogeny)

  • Major haplogroups and branches include major lines such as R1, R1a, R1b, J2, J1, E, O, Q, C, etc., with subclades illustrated (e.g., 11a, 11c, R1, R1a, R1b, J2, J1, E3b, E3a, K, SO on).

  • The diagram shows how branches diverge over time according to new mutations.

• mtDNA vs Y-chromosome: summary for forensic applications

  • mtDNA is inherited from the mother and can be used to trace maternal ancestry; useful when samples are degraded or when maternal lineage is needed across many generations.

  • Y-chromosome DNA is inherited from the father and is used to trace paternal ancestry; useful in identifying male lineages and in mixed-sex crime samples where male contributions need to be isolated.

  • Both mtDNA and Y-chromosome data can reveal ancestry and population history spanning thousands of years.

• Key exam-style questions to prepare for (as listed in the source)

  • What are the characteristics of mitochondria and mtDNA?

  • Why is mtDNA so useful in archaeological studies?

  • Which regions of mtDNA are useful in forensics?

  • What is the Cambridge sequence and how is it used?

  • What is the structure of the Y chromosome and which parts are relevant in forensics?

  • What are the advantages and limitations of Y-chromosome studies?

  • How can we trace back to mitochondrial Eve and Y-chromosome Adam?

• Quick references and formulas to remember

  • mtDNA genome length: $16{,}500\text{ bp}$; $37\text{ genes}$.

  • Copy number per cell: $\text{mtDNA copies per cell} \in [10^2, 10^4]$ (approximately).

  • Hypervariable regions lengths and SNP density: HV1 length $\approx 342\text{ bp}$; HV2 $\approx 268\text{ bp}$; HV3 $\approx 136\text{ bp}$; about $3\text{ SNPs per }100\text{ bp}$; typical total differences across HV1, HV2, HV3 between two individuals around $\approx 22\text{ differences}$.

  • Mutation rate in Eve context: roughly $1\text{ mutation per }10{,}000\text{ years}$ along the mtDNA phylogeny.

  • mtDNA inheritance: maternal only; Y-chromosome inheritance: paternal only.