- Horace

Part 1: Chromosomal Localisation of DNA Structures

Outline:

Introduction: DNA Compaction in chromosomes (again!)

A Brief overview of eukaryotic chromosome architecture

Fragile X-syndrome and nucleosome positioning

Centromeric DNA

A few more words about curved DNA

yeast Autonomously Replicating Sequences

Human centromeric DNA

 

Telomeric DNA

Telomeric DNA Sequences

4-stranded DNA structures

Telomerase enzyme

Heterochromatin vs. Euchromatin

Heterochromatin and repetitive DNA sequences

Euchromatin and the organisation of genes

Introduction to Eukaryotic Chromosomes

A brief REVIEW (stuff you should've learned in Biology 101 or High School biology)

• One DNA molecule per chromosome

 

 
 
 

The DNA for human chromosome # 1 would be more than 2 cm long, in its B-DNA form!
 
 

• A comparison of genome sizes in eukaryotes

• Anatomy of chromosomes - there are four important parts in metaphase chromosomes (telomeres, centromeres, and heterochromatin & euchromatin):

A Fragile-X syndrome and nucleosome positioning

Other Formats: Links: Order this document J Biol Chem 1996 Oct 4;271(40):24325-8  Nucleosome assembly on methylated CGG triplet repeats in the fragile X mental retardation gene 1 promoter. Godde JS, Kass SU, Hirst MC, Wolffe AP Laboratory of Molecular Embryology, NICHHD, National Institutes of Health, Bethesda, Maryland 20892-5430, USA. Expansion and methylation of CGG repeat sequences is associated with Fragile X syndrome in humans. We have examined the consequences of CGG repeat expansion and methylation for nucleosome assembly and positioning on the Fragile X Mental Retardation gene 1 (FMR1) gene. Short unmethylated CGG repeats are not particularly favored in terms of affinity for the histone octamer or for positioning of the reconstituted nucleosome. However, upon methylation their affinity for the histone octamer increases and a highly positioned nucleosome assembles with the repeat sequences found adjacent to the nucleosomal dyad. Expansion of these CGG repeats abolishes the preferential nucleosome assembly due to methylation. Thus, the expansion and methylation of these triplet repeats can alter the functional organization of chromatin, which may contribute to alterations in the expression of the FMR1 gene and the disease phenotype. PMID: 8798682, UI: 96394576

Centromeric DNA

There are certain DNA sequences that are associated with the centromeres of chromosomes.  Knowledge of this was essential in the construction of Yeast Artificial Chromosomes (or YACs, as they're usually called).

here's an oversimplified view of the attachment of the kinetochore

 
 

Telomeric DNA

telomeric DNA can fold back on itself.  This is necessary to allow for DNA synthesis (this isn't a problem for circular chromosomes!).

The ends of the chromosomes get shorter every time the cells divide, because part of the bases are used to template off of themselves.  Thus, after every round of replication, the chromosome gets a bit shorter.  This is kind of like "planned obscelence", where the cells basically have so many divisions and then they fall apart.  However (fortunately!) the cells have a mechanism for extending the length of the telomeres - the cells have an enzyme which can make the telomeres longer.  The TELOMERASE enzyme is a ribozyme - that is, it contains a necessary piece of RNA which serves as the template for synthesizing the new strand.  It has been found that in many cases cancer cells have a mutation such that the telomerase gene is overexpressed, thus allowing the cells to "live forever".   Early results from clinical trials show that by specifically inhibiting the activity of the telomerase protein, they can slow or completely stop the growth of many types of cancer.  More recently, the idea of using telomerase gene therapy to prevent people from getting old has received much attention in the media.

Figure 6.26 from Hartl & Jones (page 250).

 

Two types of chromatin:

• Heterochromatin - where the DNA is more condensed, and usually there is not much transcriptional activity.  Some heterochromatin will remain condensed throughout the cell cycle.

 

 

Euchromatin - this is where the "active" genes are - usually this region is much less condensed.

• Highly repetitive DNA -
Dispersed - e.g., Alu family
about 300 bp long
500,000 copies in humans
(about 5% of the human genome)
dispersed throughout the chromosomes
Localised highly repetitive sequences
about 2-10 bp long
present in millions of copies, often in large blocks
(about 6% of the human genome)
associated with heterochromatin
usually very high A+T content

• Middle repetitive DNA

 
makes up more than 40% of the human genome
position varies due to transposable elements
Includes the following types of sequences:

• Dinucleotide repeats

• microsatellite DNA

• TRInucleotide repeats

associated with many diseases
(e.g., Fragile X, muscular distrophy)

Back to the Lekture notes