Chromosome Structure

chromosome structure

Chromosome Structure

Chromosome Structure

  • In eukarytoic cells, chromosomes are composed of single molecule of DNA with many copies of five types of histones.
  • Histones are proteins molecules and are rich in lysine and arginine residues, they are positively charged. Hence they bind tightly to the negatively-charged phosphates in the DNA sequence. { Chromosome Structure }
  • A small number of non-histone proteins are also present, these are mostly transcription factors. Transcription factors regulate which parts of DNA to be transcribed into RNA. 
  • During most of the cell’s life cycle, chromosomes are elongated and cannot be observed under the microscope. 
  • During the S phase of the mitotic cell cycle the chromosomes are duplicated. 
  • At the beginning of mitosis the chromosomes are duplicated and they begin to condense into short structures which can be stained and observed easily under the light microscope.
  • These duplicated condensed chromosomes are known as dyads.
  • The duplicated chromosomes are held together at the region of centromeres. 
  • The centromeres in humans are made of about 1-10 million base pairs of DNA.  
  • The DNA of the centromere are mostly repetitive short sequences of DNA, the sequences are repeated over and over in tandem arrays. 
  • The attached, duplicated chromosomes are commonly called sister chromatids. 
  • Kinetochores are the attachment point for spindle fibers which helps to pull apart the sister chromatids as the mitosis process proceeds to anaphase stage. The kinetochores are a complex of about 80 different proteins. { Chromosome Structure }
  • The shorter arm of the two arms of the chromosome extending from the centromere is called the p arm and the longer arm is known the q arm. 

Bacterial Chromosome(Chromosome Structure)

Chromosome Structure

Bacterial chromosomes contain circular DNA molecule unlike the linear DNA of vertebrates.Most of chromosomes are circular DNA molecules and there are no free ends to the DNA.  The bacterial DNA is packaged into a single chromosome into a continuous loop. The DNA is folded or coiled to fit into the cell. The compaction of the DNA involves the binding of proteins to the DNA that help form initial loops which is then coiled. 

Prokaryotic Chromosome

Prokaryotes like the bacteria and archaea typically have a single circular chromosome. The chromosome size of most bacteria is from only 160,000 base pairs to 12,200,00 base pairs.Some bacteria in exceptions contain a single linear chromosome. The base sequences in prokaryotic chromosomes are less than in eukaryotic cells. Bacterial chromosomes have a single origin of replication from which the replication starts. In some archaea there are multiple replication origins. The prokaryotic genes are organized into operons and it usually it does not contain introns. Nucleus is absent in prokaryotes, the DNA is organized into a structure called the nucleoid. The DNA of the archaea are more organized, they are packaged within structures similar to eukaryotic nucleosomes. The chromosomes in the prokaryotes and plasmids are generally supercoiled like that of the eukaryotes. The DNA are released into the relaxed state for the process of transcription, replication and regulation. { Chromosome Structure }

chromosome structure

Eukaryotic Chromosomes
In eukaryotes the chromosomes are multiple large, linear and are present in the nucleus of the cell. Each chromosome typically has one centromere, one or two arms that project from the centromere, the arms are usually not visible during most of the time. Most of the eukaryotes have a small circular genome in the mitochondria. Some of the eukaryotes have small linear or circular chromosomes in the cytoplasm. To fit into the compartment in which it is contained the DNA has to be condensed and the degree to which it is condensed is expressed as its packaging ratio. Packaging ratio is the length of the DNA divided by the length into which it is packaged. The chromosomes of eukaryotic organisms consists of complexes made of DNA and protein, and it is organized in a condensed manner. This condensation permits the large amount of DNA to be stored in the nucleus of the cell.{ Chromosome Structure }

Human Chromosomes

Humans chromosomes are of two types autosomes and sex chromosomes. Genetic traits that are linked to the sex of the  person are passed on through the sex chromosomes. The rest of the genetic information is present in the autosomes. Humans have 23 pairs of chromosomes in their cells, of which 22 pairs are autosomes and one pair of sex chromosomes, making a total of 46 chromosomes in each cell. Many copies  of mitochondrial genome are present in human cells.    

Sex Chromosomes

Sex chromosomes differ in form of size, behavior from the ordinary chromosome. The sex chromosomes determine the sex of an individual during reproduction. These sex chromosomes differ between the male and the females. Females have two copies of X chromosome, males have one X chromosome and one Y chromosome.  In the process of sexual reproduction in humans, two different gametes fuse to form a zygote. { Chromosome Structure }

Homologous Chromosomes

Homologous chromosomes are also known as homologs or homologues. The homologous chromosomes are pairs of chromosomes that are approximately of the same length, position of centromere, and pattern of staining, genes for the same characteristic are at a corresponding loci. In an organism one of the homologous chromosome is inherited from the mother and the other from the father. These chromosomes are usually not identical, but they carry the same type of genes.During the process of mitosis the daughter chromosomes carry the same sequence of nucleotide, assuming there are no errors during the replication process.  The genome in diploid organisms is composed of homologous chromosomes One of homologous pair is the maternal chromosome and the other is the paternal chromosome. During the process of meiosis the homologous chromosomes cross over. Homologous chromosomes are not identical but they are similar. The genes are carried in the same order, but the alleles for the trait may not be similar. 

Polytene chromosome

chromosome structure

Polytene chromosomes are giant chromosomes common to many dipteran (two-winged) flies. They begin as normal chromosomes, but through repeated rounds of DNA replication without any cell division (called endoreplication), they become large, banded chromosomes. For unknown reasons, the centromeric regions of the chromosomes do not endoreplicate very well. As a result, the centromeres of all the chromosomes bundle together in a mass called the chromocenter.

Polytene chromosomes are usually found in the larvae, where it is believed these many-replicated chromosomes allow for much faster larval growth than if the cells remained diploid. Simply because each cell now has many copies of each gene, it can transcribe at a much higher rate than with only two copies in diploid cells.{ Chromosome Structure }

The polytene chromosomes at the right are from the salivary glands of the fruit fly Drosophila melanogaster. the bands on each chromosome are like a road map, unique to each chromosome and well defined enough to allow high resolution mapping of each chromosome.{ Chromosome Structure }

Lampbrush Chromosome

chromosome structure

Lampbrush Chromosomes (LBCs) are present in the oocytes of birds, lower vertebrata and invertebrates during the prolonged prophase of the first meiotic division. Their name stems from their similarity to bottle brushes. { Chromosome Structure }Lampbrush chromosome of the early prophase is a bivalent, made up of two conjugating homologues. The axis of each homologous chromosome is formed by sister chromatids that are differentiated into regions of transcriptionally active and inactive chromatin. Transcription activity of LBCs is observed as a mantle of symmetrically distributed side loops along the chromosome axis. Changes in transcriptional activity are reflected in changes in their morphology. Transcriptional activity of LBCs is directly connected with physiological processes of the body and shows in the morphological structure of the chromosomes. The use of cytogenetic techniques and in situ hybridization have made it possible to identify unique and repeating sequences as well as DNA replication proteins in LBCs. { Chromosome Structure }Particularly, interesting prospects are offered by the possibility of using LBCs in studies of transcriptional activity, cytogenetic investigations of karyotype evolution and genome mapping.

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