A chromomere, also known as an idiomere, is one of the serially aligned beads or granules of a

eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

chromosome A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...

, resulting from local coiling of a continuous

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

thread. Chromomeres are regions of chromatin that have been compacted through localized contraction. In areas of

chromatin Chromatin is a complex of DNA and protein found in eukaryote, eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important r ...

with the absence of transcription, condensing of DNA and protein complexes will result in the formation of chromomeres. It is visible on a chromosome during the

prophase Prophase () is the first stage of cell division in both mitosis and meiosis. Beginning after interphase, DNA has already been replicated when the cell enters prophase. The main occurrences in prophase are the condensation of the chromatin retic ...

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

and

mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

. Giant banded ( Polytene) chromosomes resulting from the replication of the chromosomes and the synapsis of homologs without cell division is a process called endomitosis. These chromosomes consist of more than 1000 copies of the same chromatid that are aligned and produce alternating dark and light bands when stained. The dark bands are the chromomere. It is unknown when chromomeres first appear on the

. Chromomeres can be observed best when chromosomes are highly condensed. The chromomeres are present during leptotene phase of prophase I during meiosis. During zygotene phase of prophase I, the chromomeres of homologs align with each other to form homologous rough pairing (homology searching). These chromomeres helps provide a unique identity for each homologous pairs. They appear as dense granules during leptotene stage There are more than 2000 chromomeres on 20 chromosomes of maize. Drosophila polytene chromosomes 2

Physical properties

Chromomeres are organized in a discontinuous linear pattern along the condensed chromosomes (pachytene chromosomes) during the prophase stage of

. The linear pattern of chromomeres is linked to the arrangement of genes along the chromosome. Chromomeres contain genes and sometimes clusters of genes within their structure. Aggregates of chromomeres are known as chromonemata. Cohesive proteins SMC3 and hRAD21(plays a role in sister chromatid cohesion) are found within chromomeres at high concentrations, and maintain the proper structure of chromomeres. The protein XCAP-D2 is also present at high concentrations within the chromomere, and acts as a condensin component. High concentrations of

tandem repeat In genetics, tandem repeats occur in DNA when a pattern of one or more nucleotides is repeated and the repetitions are directly adjacent to each other, e.g. ATTCG ATTCG ATTCG, in which the sequence ATTCG is repeated three times. Several protein ...

s of the

heterochromatin Heterochromatin is a tightly packed form of DNA or '' condensed DNA'', which comes in multiple varieties. These varieties lie on a continuum between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a rol ...

protein HP1β builds up within the chromomere. In regions where loops attach to chromomeres, there is hyper

acetylation : In chemistry, acetylation is an organic esterification reaction with acetic acid. It introduces an acetyl group into a chemical compound. Such compounds are termed ''acetate esters'' or simply ''acetates''. Deacetylation is the opposite react ...

histone In biology, histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei and in most Archaeal phyla. They act as spools around which DNA winds to create structural units called nucleosomes ...

4. The extra acetylation loosens chromatin from a condensed form, making it more accessible to proteins involved in transcription.

Functions

Chromomeres are known as the structural subunit of a chromosome. The arrangement of chromomere structure can aid in control of gene expression. Maps of chromomeres can be made for use in genetic and evolutionary studies. Chromomeric maps can be used to locate the exact position of genes on a chromosome. Chromomeric maps can be used to analyze chromosome aberrations, and find correlations between the aberrations and their effects on genes near breakpoints.

Lampbrush chromosomes and chromomeres

Chromomeres display different properties and behaviours when associated with lampbrush chromosomes. Although found all across the lampbrush chromosome, they are not organized in a clear pattern along as they are in normal pachytene chromosomes of meiosis. The two sister chromatids of a lampbrush chromosome separate fully, forming lateral loops that extend from chromomeres, and act as transcription complexes. The lateral loops are areas where the chromosomes are transcriptionally active. The loops define where the chromomere will be positioned. The chromomere is an organization of regions of non-transcribed chromatin. These regions of chromatin that have not been transcribed are located at the ends of the loops that were formed by the sister chromatids of a lampbrush chromosome. Each chromomere can have up to several pairs of loops from lampbrush chromosomes originating from it, as well as micro-loops that cannot be detected with a

light microscope The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible spectrum, visible light and a system of lens (optics), lenses to generate magnified images of small objects. Optical microscopes ...

. Chromomeres of lampbrush chromosomes act as structural units, and are not considered to be genetic units participating in transcription. The arrangement of a chromosome into chromomeric units happens almost simultaneously with the start of transcription. The loop structures extending from chromomeres are maintained by a high level of transcriptional activity and structural proteins of the chromosome. Chromatin within the chromomere are held in position by a variety of histone modifications and epigenetic markers. Methyl-CpG-binding protein 2 (MeCP2) is a protein that binds to methylated DNA. MeCP2 has been found to associate most strongly with transcriptionally inactive chromomere domains. MeCP2 binding patterns to chromomere domains are proportional to the density of chromomeric DNA found on a chromosome. The binding of MeCP2 to chromomeric regions represents regions of chromatin that are highly dynamic on the lampbrush chromosome.

References

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External links

"Preparation and analysis of spermatocyte meiotic pachytene bivalents of pigs for gene mapping" - Nature

* ttps://doi.org/10.1007%2FBF00327327 "Chromonema and chromomere" - Springerlink Chromosomes