From these, the centromeric index, the arm ratio and the relative length of the chromosome can be calculated. Relative length is defined as the length of the entire chromosome multiplied by 100 and divided by the total length of all the chromosomes in the complement. In holocentric chromosomes, since the centromere is diffuse, only the actual lengths of the chromosomes are taken into account.
Euchromatic bands (including Q, G, and R bands) form a pattern of positive (darkly stained or brightly fluorescent) and negative (weakly stained or weakly fluorescent) bands along the entire length of the higher vertebrate chromosome. Thus, chromosome bands are actually a visible expression of the functional and structural division of chromosomes.
DNA - THE GENETIC MATERIAL
DNA FINGERPRINT
Thus, an individual's DNA sequence fingerprint is a molecular manifestation that distinguishes one individual from another. Jeffrey and his colleagues coined the term in 1988 while working on the simultaneous detection of variable DNA loci by hybridization of specific multilocus probes to restriction fragments. In fact, the hybridization of DNA fragments with specific oligonucleotide probes has made DNA fingerprinting an important technique.
In DNA typing, genomic DNA is cut with restriction enzymes, followed by electrophoresis and hybridization of the DNA fragments with multilocus probes. However, in DNA profiling, specific DNA sequences are first amplified with oligonucleotide primers and then separated. In humans, there are minisatellites, the satellite DNA existing in relatively short regions of 1 to 5 kb, consisting of 20 to 50 repeating units, each containing about 100 base pairs.
These are distinct from the more common tandemly repeated satellite DNA regions, which are 20 to 100 kb in length.
DNA CONTENT
The amount of DNA per gene is greater in the human genome due to introns, which can range from one to many, even more than 20. A large part of most eukaryotic nuclear genomes consists of repetitive DNA, consisting of single sequence elements that are repeated many times, either in tandem arrays or dispersed throughout the genome. A single copy of DNA makes up most genes and consists of sequences that are not repeated elsewhere.
Several amphibians and flowering plants have a genome ten times larger than that of humans, even though the number of genes is almost the same.
DNA STRUCTURE
The individual nucleotides are joined together to form a polymer called a polynucleotide and is formed by attaching one nucleotide to another through the phosphate groups. In the DNA nucleotide, the carbon atoms in the sugar molecule are numbered in a clockwise direction. They are always numbered in the same way with the carbon of carboxyl group (-C=O) appearing at one end of the chain form numbered as 1'.
The numbering of the carbon atoms is important because it indicates where on the sugar the other nucleotide components are attached. In fact, the blank is used to distinguish the sugar carbon from the carbon and nitrogen atoms in the nitrogenous base, which are simple forms of 1, 2, 3, and so on. Nucleotides in a DNA strand are covalently attached to each other in a linear fashion.
In a single nucleotide, the base (purine or pyrimidine) is always bonded to the 1' carbon atom and the phosphate groups are bonded to the 5' carbon atom of the pentose sugar. The nucleotides are linked together by connecting the α-phosphate group, attached to the 5' carbon of one nucleotide, to the 3' carbon of the next nucleotide in the chain. In DNA, the deoxyribose has hydrogen (-H), while in RNA, the ribose has a hydroxyl group (-OH) at carbon atom number 2' in the molecule.
RNA in the cell usually exists as a single polynucleotide chain, while DNA is always in the form of two polynucleotides twisted around each other to form a double helix.
THE DOUBLE HELIX
The two strands of the DNA molecule together are relatively stable because of the many such bonds. Heat causes the two strands of the DNA double helix to separate, a process called DNA melting or DNA denaturation. The discovery of the double helix by Watson and Crick in Cambridge in 1953 was one of the greatest events in the history of molecular biology.
The double spiral fulfills several conditions, such as if the polynucleotide is coiled or folded in some other way, then the different atoms must not be placed too close to each other and the new chemical must occur between the atoms at the proper distance. The double helix model also took into account the results obtained by scientists Erwin Chargaff in the US and Rosalind Franklin in the UK. Taking all these aspects into consideration, Watson and Crick concluded that the only structure that fit the facts was the "double helix".
For the discovery of the "double helix", Watson and Crick shared the Nobel Prize in 1962 together with Maurice Wilkins. Nucleotides are linked together to form DNA strands, and the linear sequence of nucleotides within a strand is known as the primary structure of DNA. The double helix polynucleotides are complementary, the sequence of one determines the sequence of the other.
These forms of DNA received increasing attention as it is now clear that the nucleotide sequence is one of the factors influencing the shape taken by a segment of the double helix.
PACKAGING OF DNA INTO CHROMOSOMES - THE NUCLEOSOME MODEL
Within living cells, DNA is associated with a variety of proteins that influence its final tertiary structure. The different forms of DNA are distinguished from each other by different dimensions to their double helices. In a nucleosome, eight histone molecules (a pair each of H2A, H2B, H3 and H4) form a ball-like structure around which DNA is wrapped almost twice.
A single histone molecule of histone H1 binds to 'linker' DNA outside the nucleosome to hold the DNA in place. When treated with DNase, the linker DNA is digested and a nucleosome core particle containing approximately 150 bp of DNA is released. The DNA molecule is complexed with proteins to form a structure called chromatin and is coiled in a highly organized manner.
The next thickness, about 25 nm, is due to further coiling of the nucleosomes, which form a hollow coil called a solenoid. The interaction of H1 of one nucleosome with H1 of another causes chromatin coiling followed by supercoiling. The diameter of the supercoils is the same as the diameter of the chromosomes during cell division.
The central scaffold consists largely of the enzyme topoisomerase II, which has the ability to pass one strand of DNA through another strand.
SATELLITE AND REPETITIVE DNA
After such treatment, the chromosomes have a central core with dense staining of the non-histone protein called the scaffold. The solenoids are arranged in loops that emerge from the central matrix of the scaffold, which itself is in the form of a spiral. The loops attach to the scaffold from specific areas along the DNA called scaffold binding regions.
During the early stages of meiotic division, bead-like localized thickenings were found along the chromosomes. When the chromosome molecule is split into fragments, the single-copy DNA, rich in GC content, is near the main DNA band while the repetitive DNA migrates to satellite band position. Minisatellite DNA refers to shorter clusters of 100bp to 20 kb such as telomeric repeats of 10 to 15 kb.
There are some repetitive DNA sequences that have no function in the life cycle but play an important role in evolution. These are not found in constant positions in the DNA and are called mobile elements. Such mobile DNA elements, which are present in both prokaryotes and eukaryotes, can cause mutations when they move to new locations in the genome.
The process by which these sequences are copied and inserted into a new location in the genome is called transposition.
CHROMOSOMAL ALTERATIONS
CHANGES IN CHROMOSOMAL STRUCTURE
Thus, the formation of a loop-like structure of the duplicated region occurs when such homologs pair. If the duplicated segments are small, these may have no effect on the viability of the individual, but they do exhibit some phenotypic effects. In fact, it is believed that this happened with the various globin genes, the genes that code for the components of the protein hemoglobin.
Ancestral gene duplications may have led to the divergence of duplicated genes in their function during the course of evolution. If the order of the genes on the chromosomes is reversed, it is called an inversion. An inversion can be caused by a simultaneous break at two points in the chromosome, followed by the joining of the fragment in the reversed orientation.
When two breaks occur in one chromosome and one in the non-homologous chromosome, the fragment of the first chromosome can join at an intercalary position in the second chromosome. The size of the chromosome and the position of the centromere may change as a result of such a change. Even the gene's function is suppressed when it is translocated into a heterochromatic region.
As in reciprocal homozygous, both pairs of each of the non-homologous chromosomes carry the same reciprocal translocation, so there is no problem in meiosis.
VARIATION IN CHROMOSOME NUMBER
There are many cases of aneuploids such as Down syndrome (trisomy on chromosome 21), Edward syndrome (trisomy on chromosome 18), Patau syndrome (trisomy on chromosome 13), in addition to Klinefelter (47, XXY) and Turner ( 45, XO) syndrome. In many mammalian species, including humans, the X chromosomes differ from other chromosomes in that only one chromosome is active in a given cell. In normal females, only one X is active in a given cell and the other X is heterochromatic and remains in a condensed state throughout interphase.
The inactivated X forms a structure called a Barr body (named after its discoverer Murray Barr) that can be identified in a cell. By counting the number of Barr bodies in a cell, chromosomal abnormalities involving X chromosomes can be determined. If x is the haploid number of chromosomes, the organisms with three sets of chromosomes (3x) are called triploids.
Plants can often self-fertilize, so that a single new polyploid plant with an even number of chromosomal sets (tetraploid, hexaploid, etc.) can still reproduce. This is an important feature because different sets of chromosomes in a polyploid often have different origins. The polyploids that receive all their chromosomal sets from the same species are called autopolyploids.
If diploid or unreduced pollen from a diploid organism fertilizes a diploid egg of the same species, the offspring are autotetraploids AAAA where A is a complete chromosomal set of type A.
SEX CHROMOSOMES
