Lecture 2: Biology Review II

Date: 8/29/02Overview/Review of:

Mapping Molecular techniques Markers

Genetic Mapping

Definition: A genetic map is an ordering of genes and markers in a linear arrangement corresponding to their physical order along the chromosome. Based on linkage.

Definition: A physical map is an ordering of landmarks on DNA, regardless of inheritance. Measured in base pairs.

Marker

Definition: A marker is a gene or piece of DNA with easily identified phenotype such that cells or individuals with different alleles are distinguishable.

e.g. a gene with known function e.g. a single nucleotide change in DNA

Polymorphism

Definition: A polymorphism is a detectable and heritable variation at a locus.

Definition: A marker is polymorphic if the most abundant allele comprises less than X% of all alleles, usually 95%.

Definition: A mapping population is a population used to map genes.

Natural Populations

Definition: Natural populations are those where mating is not controlled by the experimenter, though the experimenter can choose who to observe.

Only phenotype observable, genotype sometimes unknown, phase is unknown.

Knowns: allele frequencies, genotype frequencies, amount of disequilibrium.

Hardy-Weinberg Equilibrium I

Refers to the equilibrium achieved at a single locus.

Hardy-Weinberg Equilibrium (HWE) is achieved when the allele frequencies and genotype frequencies do not change from generation to generation.

Hardy-Weinberg Equilibrium II

Let pA and pB be the frequencies of allele A and B in the population. Let pAA be the frequency of genotype AA. Similarly, pAB and pBB are genotype frequencies.

Then HWE implies that

pAA = pA2

pAB = 2pApB

pBB = pB2

P(heterozygote) =

Definition: Polymorphism Information Content (PIC)

Measures of Polymorphism

l

ii

pH1

21

l

i

l

ijji

l

ii pppPIC

1 1

22

1

2 21

Uninformative Matings

1 AA : 2 AB : 1 BB

AB ABX

uninformative informative

½ are informative

Classical Linkage Analysis

A few markers. Must have detectable variation. Must be substantially variable in study

population.

Controlled crosses: testcross, backcross, double- haploid

Well-defined parental lines.

Three-Point Testcross Design

X X

F1 F2

testcross

X XASY

btz

ASY

btz

bSz

ASY

btz

dominant recessive

btz

btz

btz

Three-Point Testcross Results

Count the number of recombinant haplotypes produced by F1 parent. Calculate the recombinant fraction for each pair of genes.

1 2 3

1 -- 0.19 0.03

2 -- -- 0.15

3 -- -- --

Map for Three-Point Testcross

1 3 2

0.03

0.19

0.15

Backcross Design

F2

self

no more changes

new recombinant

self

Large-Scale Mapping

Many genetic markers Steps of analysis:

pairwise linkage analysis group into linkage groups order markers in each linkage group

Comparative Mapping

Compare maps of different species. Due to similarities, information can be

transferred between species. Information about how genomes evolve. Uses conserved loci rather than highly

variable loci.

Molecular Techniques: probes

5’ – …AAGCCTAGAGCCCTTAGCCAAAAG… – 3’3’ – …TTCGGATCTCGGGAATCGGTTTTC… – 5’

3’ – *ATCTCGGGAATC – 5’add probe

5’ – …AAGCCTAGAGCCCTTAGCCAAAAG… – 3’*ATCTCGGGAATC

denature

hybridization

Molecular Techniques: restriction enzymes

Definition: An endonuclease is an enzyme (protein that acts as a catalyst to speed up the rate of a biochemical reaction) thatcleaves nucleic acid strands at internal sites (phosphodiester bond).

Definition: A restriction endonuclease is an enzyme that cuts DNA at specific sites that it recognizes.

EcoRI 5’ GAATTC 3’ 3’ CTTAAG 5’

number of cutsites = N/4b

Molecular Techniques: gel electrophoresis

DNA is negatively charged. Proteins can also be charged.

An electric current is passed through a porous medium (agarose, acrylamide) and molecules in the medium respond by moving in electric field, but at different rates based on size and charge.

Electrophoretic Gel

Molecular Technique: PCR I

Denaturation and hybridization

5’

5’

5’

5’

Elongation & denaturation

5’

5’

Molecular Technique: PCR II

Physical Maps

Banding patterns on chromosomes In-situ hybridization

Denature metaphase chromosomes Add radioactive or fluorescent probe Visualize chromosomes

DNA fragmentation DNA sequence: still not practical for all

organisms

DNA Fragmentation

• Larger fragments better (rare cutters; partial digestion)• Find overlap by sequencing or hybridization.

DNA Vector I

Definition: A cloning vector is a DNA molecule that is capable of self-replicating. Insert the fragment of foreign DNA to make recombinant DNA.

DNA Vector II

phage: virus that infects bacteria (5-25 kb). cosmid: Packaged in lambda phage and infects E.

coli (35-45 kb). yeast artificial chromosome (YAC): has telemere,

centromere, and replication origin (200-2000 kb). bacterial artificial chromosome (BAC) plasmid: extrachromosomal circular DNA

nonessential for cell survival.

How Many Clones?

Let N be the number of clones made.

Let NS be the number of nonoverlapping clones needed to cover the full genome.

N

S

S

N

N

1libraryin not cloneP

SN

P

11log

1logN

More: M. S. WatermanIntroduction to Computational Biology: Maps, Sequences, and Genomes

Genetic Mapping Still Needed

Even if the full sequence is known, mapping is still necessary.

There must be some way to correlate a trait/phenotype with something on the sequence.

Physical Mapping Still Needed

Linkage maps lack resolution Sample more people Better statistics Let recombination accumulate over many

generations.

Even with most precise linkage map can identify a gene to 1 cM (1 Mb in humans).

Morphological Markers

Differences in shape, color, size, etc. Must have one-to-one correspondence with a

controlling gene.

Protein Markers

Definition: An isozyme are proteins with same enzymatic function but different structural, chemical, or immunological characteristics.

Differences: amino acid composition, size, modifications (e.g. phosphorylation).

Differences visualized: gel electrophoresis, mass spectrometry, etc.

DNA Marker: RFLP I

Definition: RFLP is Restriction Fragment Length Polymorphism.

DNA digested with endonuclease. Separate fragments by electrophoresis. Denature strands. Transfer single-stranded DNA to durable membrane

and immobilize (Southern blot). Hybridize labeled probe to the blot. Visualize probe.

DNA Marker: RFLP II

DNA polymorphisms that RFLP identifies: mutation in the restriction site mutation elsewhere to create restriction site insertion/deletion of DNA

RFLP markers are codominant

Mini- and Micro-Satellite Markers

Definition: minisatellites or VNTR (Variable Number of Tandem Repeats) are tandem repeates of sequences 9-100 bp long. Detected by hybridization or PCR.

Definition: microsatellites or SSR (Simple Sequence Repeat) are direct tandem repeated sequences of DNA of 1-6 bp.

STS and EST

Definition: Sequence tagged sites (STS) is a short unique fragment of DNA.

Definition: Expressed sequence tags (EST) are subsets of STSs from cDNA clones. Represent transcribed genes (e.g. usually proteins).

Single-Strand Conformational Polymorphism (SSCP)

Detects changes as small as 1 nucleotide in more than 1000 bp.

Single-stranded DNA is electrophoresed on gel and migrates based on size and shape.

Visualized by Southern blot with specific fragment probe or PCR specific fragment and visualize directly.

Random Amplified Polymorphic DNA (RAPD)

PCR with short probes that bind randomly to sites in the genome.

Good for genomes where little sequence information is available.

Band-present is dominant. Expected number of products = 2fN/16b

Amplified Fragment Length Polymorphism (AFLP)

Cut DNA with frequent- and rare-cutting endonuclease

Anneal adapters to the ends of the frequent-cutter cut sites.

Amplify off adapters with PCR. Use various specific primers to amplify subsets of total.

Visualize on denaturing polyacrylamide gel.

Choosing Markers

High polymorphism. Clear interpretation. Quick typing and easy automation. Personal preference.