- 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.
- 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.
- 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)
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(Chromosome Structure)
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.
Eukaryotic Chromosomes(Chromosome Structure)
Human Chromosomes(Chromosome Structure)
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(Chromosome Structure)
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.
Homologous Chromosomes(Chromosome Structure)
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.
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.
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. 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. Particularly, interesting prospects are offered by the possibility of using LBCs in studies of transcriptional activity, cytogenetic investigations of karyotype evolution and genome mapping.