Today: Today:

•Mendelian Genetics Wrap-up

•Adding Chromosomes to the Mix

• Lunch

• Inoculate Cultures

• The Eukaryotic Genome

Morgan Discovers Sex-Morgan Discovers Sex-Linked Genes! Linked Genes! (and wins Nobel

Prize, 1933)

?

Sex Determination Happens in a

Variety of Ways

Sex chromosomes (especially the X

chromosome) carry genes for many other

characters.

In humans, the term “sex-linked” generally refers to genes on the

X chromosome.

An Aside: X Inactivation in Female Mammals

In females, one X chromosome is inactivated (at random) and

condenses into a compact Barr body along the inside of the nuclear

envelope. Most genes on this X chromosome are not expressed.

Because it is random which X chromosome forms the Barr body during development,

females are Mosaics of the two cell types.

In females, one X chromosome is inactivated (at random) and

condenses into a compact Barr body along the inside of the nuclear

envelope. Most genes on this X chromosome are not expressed.

Because it is random which X chromosome forms the Barr body during development,

females are Mosaics of the two cell types.

Practice Question: Sex-Linked Chromosomal Inheritance

If you see the If you see the number 74, then number 74, then you do you do notnot have have red-green color red-green color blindness. If you see blindness. If you see the number 21, you the number 21, you are color blind to are color blind to some extent. A some extent. A totally color-blind totally color-blind person will not be person will not be able to see any of able to see any of the numbers. the numbers.

If a color blind man has children with a “wild-type” woman, what are the chances that a daughter of theirs will be colorblind?

What are the chances that their son will be colorblind?

Can females be colorblind? What would the genotype of the parents have to be?

Practice Questions: Sex-Linked Chromosomal Inheritance (Part 2)

More of Morgan’s Complications

Morgan does further crosses between wild type flies (gray bodies and normal wings) and

mutant flies (black bodies and vestigial wings)

b+ = gray

b = black

vg+ = normal wings

vg = vestigial wings

His Results:

High Frequency of Parental Phenotypes

!

????

An Understanding of Linkage Groups allows for LINKAGE MAPPING

One of Morgan’s students, Alfred Sturtevant, develops a method to construct a genetic map.

He hypothesizes that recombination frequencies reflect the distances between genes on chromosomes.

One map unit (or centrimorgan) = 1 % recombination frequency

A sample Genetic Map of a

Drosophila Chromosome

Note that a linkage map is based on recombination

frequencies. As the frequency of cross-over

is NOT uniform over the length of the chromosome, it

portrays the sequence of genes but not their

precise locations.

Thinking Back to Meiosis: Complications of Chromosomal Inheritance- Non-Disjunction

Nondisjunction can result in

ANEUPLOIDY- an abnormal chromosome number

Monosomic- the aneuploid cell has a single copy of a chromosome

Trisomic- the aneuploid cell has three of a given chromosome

If an entire organism has more than two complete chromosome sets it is

POLYPLOID (triploid=3n, tetraploid=4n, etc.)

Red viscacha rat from Argentina = 4n

Aneuploidy results in several human disorders:

3 copies of Chromosome 21 =Down’s Syndrome

(1:700 children born in US)

XYY = no general traits, except tall

XXX = healthy, “normal”

XXY = Klinefelter Syndrome- phenotypically male with normal

intelligence, sterile

XO = Turner syndrome- phenotypically female, sterile,

usually normal intelligence

Chromosomal Alterations can also cause Human Disorders:

One Other Notable Exception: Extranuclear GenesGenes contained in the chloroplasts or mitochondria

are inherited maternally and do NOT display Mendelian inheritance patterns!

In Plants, plastid genes typically responsible

for variegation in leaves.

In animals, defects in proteins involved in the ETC or ATP synthase affect ATP

Synthesis (i.e. mitochondrial myopathy)

Skeletal Muscle: Mitochondrial myopathy, electron microscopy. Number, size and shape of mitochondria are increased and abnormal.

Next: Next: What’s a What’s a

Genome??Genome??

Thinking About Genomes…Thinking About Genomes…

Understanding Genome

Structure and Function! !

Why is genome structure/ function

important?

Remembering Structure…

Nucleosomes are formed of DNA winding around 8 histone proteins, two each of H2A, H2B, H3, and H4. The N-terminus of each

protein extends outward forming a “histone tail”.

Nucleosomes condense into 30nm fibers due to interactions between the histone tails of one nucleosome, the linker DNA,

and the nucleosomes on either side.

Remembering Structure…

During prophase, chromosomes condense further!

Remembering Structure…

Thinking About Genomes…

In metaphase chromosomes,

the same genes always end up at

the same locations.

What does this tell us about

chromosome packing??

Photo: V. Miszalok, U. Klingbeil, I. Chudoba, V. Smolej

The Importance of Gene Expression

Cell Differentiation! Differences in cell types are due to differential

gene expression.

How might a cell regulate

gene expression??

Regulating Chromatin

Histone Histone acetylation acetylation

(-COCH(-COCH33) ) prevents adjacent adjacent

nucleosomes nucleosomes from binding from binding

to one to one anotheranother

Lysine Residues (amino acids) in the Histone Tails have an Acetyl

group added to them

How does this change the structure of the tail?

Regulating Chromatin

Staining of Acetylated H3 Throughout the Cell Cycle. A field of cells containing interphase, prophase(P), prometaphase (PM) and metaphase (M); Michael J. Hendzel and Michael J. Kruhlak

Other Modifications to Histone Tails

Histones may also be Methylated (CH3)

New Model: Histone Code Hypothesis!

Figure: two different metaphase spreads (human female) with preferential staining.  Barbara A. Boggs, Peter Cheung, Edith Heard, David L. Spector, A. Craig Chinault & C. David Allis

DNA Methylation

Proteins that bind to methylated

DNA may recruit histone

deacetylases!

What would this enzyme do??

Gene Regulation at the Level of Chromatin Structure

Heterochromatin vs Euchromatin??Heterochromatin vs Euchromatin??

New Field: Epigenomics!

Figure: Epigenomics.com

Next Up: Regulating at the

Level of Transcription!

Regulating Transcription with

Transcription Factors

A single protein, TFIID, binds to the TATA box in

the promoter. The correct combination of other small proteins must then bind to TFIID

before the RNA Polymerase can bind and

initiate transcription.

Eukaryotes Also Package Regulator Protein Binding Sites within the

Promoter

Upstream sequences known as

Enhancers, may also

bind proteins and fold over

tohelp initiate

transcription

Proteins that Bind DNA are (relatively) Easy to Find!

Understanding DNA and Protein Structure allows us to recognize motifs, or structures that allow a protein to interact with DNA

Combinatorial Control of

Gene Expression

Post Transcriptional

Regulation: Small RNAs

We carry 250+ genes for micro RNAs

(~20 base pairs long). How might these micro RNA’s affect translation?

Post-Transcription: RNA Interference (RNAi)

Experimental Observation: Injecting Experimental Observation: Injecting dsRNA into a cell can silence the dsRNA into a cell can silence the

corresponding gene!corresponding gene!http://www.nature.com/focus/rnai/animations/animation/animation.htm

Post-Transcription: Alternative Splicing

Post-Transcription

: mRNA Stability                                                                        

Figure: Analysis of H-ferritin mRNA stability in control and PMA-treated THP-1 cells; Biochemical Journal (1996) Volume 319, 185-189

Post-Transcription: mRNA Editing?!!

Post-Translation: Ubiquitin

Chamber of Chamber of Doom?!?Doom?!?

What We’ve Learned from

Our Own Genome, and

Comparing Genomes

http://www.sciencemag.org/sciext/btoy2007/video/bt_video.html

Comparing Prokaryotic and Eukaryotic Genomes

Genome length (base pairs) 4,640,000   12,068,000   Genome length (base pairs) 4,640,000   12,068,000  

Number of protein-coding genes 4,300   5,800   Number of protein-coding genes 4,300   5,800  

Proteins with roles in:          Proteins with roles in:          

Metabolism 650   650    Metabolism 650   650    

Energy production/storage 240   175    Energy production/storage 240   175    

Membrane transport 280   250    Membrane transport 280   250    

DNA  rep./repair/recombination 120   175    DNA  rep./repair/recombination 120   175    

Transcription 230   400    Transcription 230   400    

Translation 180   350   Translation 180   350     

Protein targeting/secretion 35   430    Protein targeting/secretion 35   430    

Cell structure 180   250 Cell structure 180   250   

Essential Components of Multicellular Genomes

Lessons from Genomics: Many Repetitive Sequences

Examples:Examples: Minisatellits (10-40 bp) repeatsMinisatellits (10-40 bp) repeats Microsatellites (1-3 bp) repeatsMicrosatellites (1-3 bp) repeats

CODIS, our national DNA database, uses

Short Tandem Repeats from 13 loci to identify

individuals.

Lessons from Genomics: We have transposons!!

Transposons make Transposons make up >40% of the up >40% of the human genome!human genome!