Repetitive DNA is defined by its rapid rate of renaturation. It consists of short sequences that are repeated in identical or related copies in the genome. Highly repetitive DNA consists of very short sequences repeated many times in tandem in large clusters. It is called simple sequence DNA. It is usually less than 10% in mammalian genomes but may be up to 50% in Drosophila virilis. It is often possible to fractionate this due to its distinct buoyant density. This kind of fraction is called satellite DNA.

Satellite DNAs are found in regions of heterochromatin. Heterochromatin is tightly coiled up and inert in contrast with the euchromatin that makes up most of the genome.

Heterochromatin is frequently found at the centromeres.

Arthropods have satellites with a short repeating unit - only 7 bp.
Each satellite has arisen by a lateral amplification of a very short sequence.

These satellites represent very long stretches of DNA of very low sequence complexity, within which constancy of sequence can be maintained.

Mammalian satellites consist of a hierarchy of repeats.

6 gigabases fit in the human cell, and stretches to about 6 feet long.!
4 megabases fit in the E. Coli, 1.3 microns

DNA must be compressed exceedingly tightly to fit into the space available. In contrast with the customary picture of DNA as an extended double helix, structural deformation of DNA to bend or fold it into a more compact form is the rule rather than the exception.

In a bacterium the DNA packing density is 10 mg/ml.
In the phage T4 head the density is > 500 mg/ml.
So between 500 times and 10 times the density of water.
Heavy as lead baby!

Condensing Viral Genomes Into Their Coats

• Virus particles hold their DNA/RNA in capsids
• The position of the RNA within the capsid is determined by its binding to the proteins of the shell.

During mitosis the sister chromatids move to opposite sides of the cell. Their movement depends on the attachment of the chromosome to microtubles, which are connected at their other end to the poles.

The microtubules comprise a cellular filamentous system, reorganized at mitosis so that they connect the chromosomes to the poles of the cell. The sites in the two regions where the microtubule ends are organized are called MTOC's - microtubule organizing centers.

The region of the chromosome that is responsible for its segregation at mitosis and meiosis is called the centromere. It is associated with two important features.

It contains the site at which the sister chromatids are held together prior to the separation of the individual chromosomes. This appears as a constricted waist where the four arms of the chromosomes are connected.

The term centromere has been used in both the functional and structural sense to describe the feature of the chromosome responsible for its movement. The centromere is pulled towards the pole during mitosis and the attached chromosome is dragged along. Thus the chromosome is MERELY a structure for attaching a large number of genes to the apparatus for division. Quantum thinking baby, yeah!

The centromere is essential for segregation as shown by the behavior of chromosomes that have been broken. A single break generates one piece that keeps the centromere - the top of the wishbone as it were - and another, an acentric fragment that lacks it.

When translocations generate chromosomes with more than one centromere, aberrant structures form at mitosis, since the two centromeres on the same sister chromatid can be pulled towards different poles, breaking the chromosome.

In C-banding centromeres show as darkly stained regions.

This kinetochore appears to be directly attached to the microtubles.

Fragments of DNA have been isolated by virtue of their ability to confer mitotic stability on these plasmids.

A CEN fragment is defined by its ability to confer stability upon such a plasmid.

Centromeres are interchangeable. They are used simply to attach the chromosome to the spindle. They play no role in distinguishing one chromosome from another.

Telomeres caps the end of DNA. Chromosome ends generated by breakage are "sticky" and tend to react with other chromosomes.

It must lie at the end of a chromosome.
It must confer stability on a linear molecule.
The human telomere repeat is ccctaa, in general form it is

Cn(A/T)m where n > 1 and m is 1-4.

Telomerase is a specialized example of a reverse transcriptase, an enzyme that synthesizes a DNA sequence using an RNA template.
Telomeres are essential for survival of the chromosome.

Any segment of DNA that has an origin should be able to replicate. Event though plasmids are rare in eukaryotes, it is possible to construct them by suitable manipulation in vitro. This has been accomplished in yeast.

S. Cerevisiae mutants can be transformed to the wild phenotype by addition of DNA that carries a wild-type copy of the gene. Some yeast DNA fragments, when circularized, are able to transform defective cells very efficiently. These fragments can survive in the cell as self replicating plasmids.

A high-frequency transforming fragment possesses a sequence that confers the ability to replicate efficiently in yeast.
This segment is called an ARS (for autonomously replicating sequence). ARS elements are derived from authentic origins of replication and initiation occurs at the locations of ARS elements in a chromosome.

Synthesis of Okazaki fragments requires priming, extension, removal of RNA, gap filling, and nick ligation.

A eukaryotic genome is divided into multiple replicons, and the origin in each replicon is activated once and only once in a single division cycle.

One round of replication either inactivates or destroys the factor and another round cannot occur until further factor is provided.