Sezione distesa di un cromosoma

One level of regulation of transcription in eukaryotes is to affect the state of packing of nucleosomes. The 30 nm fiber is not accessible for transcription, while the 11 nm fiber is accessible. The transition from one state to the other is affected by posttranslational modification of histone "tails" that hang off the main part of each histone. This is shown in the cartoon below.

The modifications include acetylation of many different lysine residues, methylation of arginine and lysine residues, and phosphorylation. Specific modifications are made or removed by enzymes specialized for a particular modification. Histones stick to DNA because they are positively charged, allowing them to be attracted to the negatively-charged sugar-phosphate backbone of DNA. Acetylation of histone tails makes them less positively charged, and tends to promote a looser configuration of chromatin compared to the state in which the acetylation is not present.

Histone acetyltransferases (HATs) add acetyl groups to histone tails, promoting an open chromatin configuration (the 11 nm fiber). Histone deacetylases (HDAC) remove acetyl groups from histone tails, promoting an inaccessible chromatin configuration (the 30 nm fiber)

EUCROMATINA ED ETEROCROMATINA La variabilità della densità della cromatina nei nuclei delle cellule è causa di una colorazione differenziale di alcune regioni cromosomiche. Il materiale colorato più intensamente viene chiamato eterocromatina mentre la sua controparte meno colorata è chiamata eucromatina La maggior parte dei geni eucariotici è localizzata nell’eucromatina. Euchromatin and HeterochromatinExamination of nuclei from many different eukaryotes reveals that the nucleus contains two types of chromatin: euchromatin, which is relatively loosely condensed and transcriptionally active, and heterochromatin, which is more tightly condensed and transcriptionally inactive. Heterochromatin also tends to replicate later in S phase than does euchromatin.In condensed chromosomes, we typically see heterochromatin around the centromere. Often about a third of a condensed chromosome is pericentric heterochromatin. There is also usually telomeric heterochromatin, as shown in the figure below.

In condensed chromosomes, we typically see heterochromatin around the centromere. Often about a third of a condensed chromosome is pericentric heterochromatin. There is also usually telomeric heterochromatin, as shown in the figure below.

The figure below points out some additional differences between euchromatin and heterochromatin: heterochromatin contains satellite sequences, short sequences repeated millions of times, as well as middle-repetitive transposible genetic elements. It has a very low density of transcribed genes. The figure also points out specific histone modifications that differ between euchromatin and heterochromatin: methylation of lysine 9 of histone H3 in heterochromatin, and methylation of lysine 4 of histone H3 in euchromatin.

A large number of chromosome rearrangements have been recovered in Drosophila. Some chromosome rearrangements that have a one breakpoint in euchromatin and one in heterochromatin exhibit position-effect variegation, as shown in the images below. There is an X chromosome inversion that has one breakpoint near the site of the white gene and the other breakpoint in the pericentric heterochromatin of the X chromosome. Flies bearing this inversion show a mottled or variegated expression of the white gene, as shown in the photographs and drawings.

Position-effect variegation in this case is interpreted to be a spreading of heterochromatin formation into the region including the white gene. In the uninverted chromosome, there is a barrier of some kind that marks the euchromatin-heterochromatin boundary. When a new euchromatin-heterochromatin junction is created as a consequence of the inversion, it lacks a boundary, and some of the euchromatin, including that containing the white gene, becomes heterochromatic and transcriptionally inactivated. This phenomenon provides an opportunity to investigate the formation of heterochromatin.Position-effect variegation is sensitive to genetic and environmental modifiers. For example, as shown below, X0 males bearing the variegating X chromosome show more inactivation of white, while XYY males bearing the variegating X chromosome show less.

Another suppressor of position-effect variegation is Su(var)205, also known as HP1 or heterochromatin protein 1.

La proteina HP1 È una proteina cromosomica non istonica estremamente conservata nella scala evolutiva È composta da 206 aa HP1 è codificata dal gene Su(var)2-5 , un soppressore della PEV. Possiede 3differenti domini strutturali : all’ N-term è presente un CD attraverso il quale HP1 lega l’ Histone H3 metilato in lisina 9, al C-term c è un CSD coinvolto nelle interazioni proteina-proteina inclusa la dimerizzazione di HP1 e ad unire questi domini terminali c è una Hinge region che è responsabile del legame di HP1 con il DNA telomerico e conferisce flessibilità strutturale.

Notice that heterochromatin is a distinct state that can be described by a particular combination of histone modifications (generally reduced methylation and acetylation, but enrichment for methylation of histone H3).

The amplification of the genome in the salivary glands (called polytenization) is uneven. Euchromatin amplifies, but most of the heterochromatin hardly amplifies at all. As shown in the figure below, this results in a chromosome set in which the five major chromosome arms (X, 2L, 2R, 3L, and 3R) are seen emerging from the pooled heterochromatin, called the chromocenter. The euchromatin of each chromosome arm is amplified to about 1,000 copies. The mitotic chromosomes (not to scale) are shown as an inserted figure in the upper left.

CROMOSOMI POLITENICI DI DROSOPHILA In alcuni tessuti di alcuni organismi, i cromosomi si duplicano senza che avvenga la separazione dei cromatidi fratelli che si accumulano l’uno vicino all’altro, formando un fascio di elementi allineati. I cromosomi che ne derivano sono detti POLITENICI. L’esempio più straordinario di cromosomi politenici si trova nelle ghiandole salivari della larva di Drosophila. Quando questi cromosomi sono colorati, la cromatina più densa si colora più intensamente, creando un alternarsi di bande chiare e scure. I cromosomi politenici omologhi si appaiano. Tutti i centromeri dei cromosomi politenici di drosophila sono concentrati nel cromocentro.

The primary constriction occurs at the centromere of a mitotic chromosome

The kinetochore forms in mitosis at the centromere

Variazioni della struttura dei cromosomi Delezioni Duplicazioni Inversioni Traslocazioni

Inversione Un’inversione è una mutazione cromosomica che si verifica quando un segmento cromosomico viene escisso e poi reintegrato nel cromosoma dopo rotazione di 180 gradi rispetto all’orientamento originale Conseguenze di un’inversione alla meiosi: si verificano gravi conseguenze genetiche se avviene un crossing-over entro l’inversione.

Conseguenze di un’inversione paracentrica: l’appaiamento dei cromosomi omologhi richiede la formazione di anse che comprendono i tratti invertiti chiamate anse di inversionei.

Due gameti possiedono una serie completa di geni e sono vitali. Gli altri due gameti non sono vitali.

Conseguenze di un’inversione pericentrica: l’appaiamento dei cromosomi omologhi richiede la formazione di anse che comprendono i tratti invertiti chiamate anse di inversionei.