PowerPoint Presentation
Chapter 5: The Organization of the Eukaryotic Genome
structure of the chromosome heterochromatin and gene
inactivation the gene concept and eukaryotic gene structure
variations in gene structure repetitive sequencesReference: p.
179-192, 241-242, 279
1
Human chromosomes
If members of these two species interbreed: mitotic divisions of
the zygote may generate a viable adult due to the presence of 2
copies of each relevant gene, but: the hybrid adult will be sterile
due to meiotic failureClosely related species have similar number
of genes but may have very different chromosome numbers
Meiotic failure and sterility in hybrid individuals
Meiosis fails in organisms that do not contain a
chromosomecomplement consisting of an even number of
homologoushaploid sets (i.e. multiples of the diploid number).
Suchorganisms do not produce offspring because some chromosomes
cannot pair during ________ of meiosis.
Mules are the result of a cross between a female horse(2n = 64)
and male donkey (2n = 62) giving rise tooffspring with an odd
number of chromosomes (63).Seedless watermelons occur if a
tetraploid (4n) plant(which produces 2n gametes) is crossed with a
diploid (2n) plant (which produces n gametes) giving rise to
triploid (3n) plants. These hybrids are sterile and hence their
fruits contain no seeds.Meiotic failure and sterility in hybrid
individuals
Important regions of eukaryotic chromosomesCentromere: the point
of junction between sister chromatids.(it is also the attachment
site for mitotic spindle proteins)Telomeres: the stable ends of
linear chromosomes.
Replication origins: sequences where DNA replication begins.
Important regions of eukaryotic chromosomes
During interphase most DNA (~90%) is decondensed into long thin
strands called euchromatin (true chromatin).
decondensation is necessary for transcription decondensed DNA is
still associated with histones
Some DNA (~10%) remains condensed as heterochromatin which is
tightly packed as it is in mitotic chromosomesInterphase
chromatinThe nucleolus is the region of the nucleus where _____
genes are transcribed and ________ are assembled.
Interphase chromatin
Decondensed vs. Condensed ChromatinDecondensed euchromatin and
associated proteinsspilling out of a lysedinterphase nucleus
Condensed mitoticchromosome
The structure of the eukaryotic chromosomeLevels of organization
of DNA in chromosomes:NucleosomesChromatinChromatin fibersCondensed
chromosome
The nucleosome DNA (two turns, 147 bp) wrapped around a cluster
of histone proteins: two of each H2A, H2B, H3, and H4(= core
histone particle) held together by a single histone H1
proteinNucleosomes are thebasic units of DNAorganization.
Isolation of a nucleosomeDigestion of all the linker DNA
sectionsby micrococcal nuclease:only DNA sections that are
associated with the histone nucleosome cores are protected from
nuclease digestion (~147 bp).
Separation of thenucleosome components
Chromatin (11-nm)Chromatin fibers (30-nm)Condensed
chromosome
Human mitoticchromosomesHuman karyotypeFully Condensed
Chromosomes
The condensed chromosome is the highest level of
DNAorganization. DNA is at is maximum condensation stateduring
________.
Interphase Euchromatin
(A) 30-nm chromatin fiber(B) 11-nm chromatin (linked
nucleosomes)
The remodeling of interphase chromatin ATP-dependent enzyme
complexes can displace nucleosomalDNA to expose specific sequences
that can then berecognized by DNA-binding proteins.
chromatin-remodeling complexes use energy from ATP hydrolysis to
push the histone-bound DNA along the histone core, thereby exposing
the underlying DNA
the exposed DNA can then be recognized and bound by
transcription factors that are necessary for gene expression
The remodeling of interphase chromatin
The remodeling of interphase chromatin Multiple rounds of ATP
hydrolysis and DNA sliding candecondense the chromosome to make the
DNA accessible toother nuclear proteins.
Chromatin remodeling by chemical modification of histones The
covalent modification of histones can regulate theinitiation of
transcription.
the methylation of certain arginine and lysine residues nearthe
N-tail of histone H3 provides binding sites for proteinsthat
repress the initiation of transcription
the acetylation of specific lysines or the phosphorylation
ofspecific serines destabilize DNA-histone
interactions,decondensing the chromatin to promote the initiationof
transcription
Position effectsThe position of a gene on a chromosome relative
to heterochromatin may affect its expression.The Drosophila white
gene is necessary for normal eye pigmentation. the absence of white
gene product results in white eyes
if the white gene is moved close to heterochromatin DNA by a
chromosomal inversion, the gene may be silenced in some eye cells =
mottled eye
Position effects
One X chromosome is chosen at random to be inactivated at an
early stage of embryonic development.
the inactivated X chromosome may be the paternal (Xp) ormaternal
(Xm) the inactivated chromosome remains as heterochromatin
and(mostly) transcriptionally inert (a Barr body) once inactivation
has occurred, all the descendants of thatcell will have the same X
chromosome inactivatedX chromosome inactivation
X chromosome inactivationFemales are mosaics for all
heterozygous X loci
Calico catTortoiseshell catCoat color is expressed by pigment
genes in melanocytes.An X-linked pigment gene has 2 co-dominant
alleles: XO = orange coatXB = black coat
(Calico cats have colored patches on a white background because
they carry a mutation in an autosomal gene that affects timing of
melanocyte migration to skin surface)Mottled coat color phenotype =
mosaic cats
Interphase chromatinThe heterochromatic regions of individual
interphasechromosomes are well-defined and heritable.What would the
condensation pattern for this chromosome look like in a different
cell type?
Inheritance of chromatin condensation patternsTissue-specific
histone-binding enzymepassed along to daughter cells
Parental histones are distributedrandomly to each daughter
cell,then new histones are produced.
The gene conceptGene: a segment of DNA that is transcribed and
codes for afunctional product: a polypeptide (protein), or an RNA
species as end productSequences within genes: exons: coding
sequences & UTRs introns: non-coding DNA (intervening
sequences) regulatory regions: control gene transcription
levels
The gene concept
promoter
Exons
The dystrophin gene
The dystrophin gene is ~2.6 million nucleotide pairs long. about
0.01% of the human genome: the largest gene known 79 exons code for
a ~3600 amino acid protein only about 0.004% of the gene is
exons
DMD patients have deletions in the dystrophin gene
Eukaryotic gene expression
Alternative splicing can generate different versionsof a gene
product
Genes within genes
One or more genes may be contained within the intron of another
gene.
this is a relatively common feature of nuclear genomes
Example: intron 27 of the human neurofibromatosis type I (NF1)
gene contains three genes: OGMP, EV12B, & EV12A
Overlapping genesThe coding regions of two genes may
overlap.
a single locus can produce more than one product by the use of
different reading frames
found in compact genomes such as viruses & mitochondria
Example: ATPase-6 and ATPase-8 in human mitochondria
Gene densities In mitochondrial andviral genomes: mostof the DNA
encodesa product. In the human nucleargenome: only 1.5%encodes a
product.Human mitochondrial genomeThe human mtDNA is16.6 kb and
codes for tworRNAs, 22 tRNAs, and 13polypeptides necessary forthe
oxidative respiratoryfunctions of the organelle.
Gene densities in four organisms in a typical 50 kb region
Repetitive DNA sequences are noncoding sequences that are
present in multiple copies. simple sequence DNA (satellites) are
clustered repeats interspersed repeats are repetitive sequences
that are notclustered but are scattered throughout the
genome(satellites)(SINEs, LINEs)Repetitive DNA sequences
Examination of the human genome
Long interspersed elements (LINEs): example: Line1 in human 6000
bp ~21% of total human DNA some LINEs are transcribed and
translated, but their products have no known functionExamples of
interspersed repeated sequencesShort interspersed elements (SINEs):
example: Alu sequences in mammalian genomes(characterized by an
Alu1 restriction enzyme site) 300 bp ~13% of total human DNA some
of them are transcribed but not translated function unknownBoth
types originatedas retrotranspositions.
Simple sequence repeatsSatellite DNA: tandemly repeated short
sequences - repeated 1000s of times
5% of human genome:
a 171bp repeat around centromere makes up 1 million bp