i) Although human genome contains 3 billion nucleotide bases, the DNA sequences that encode proteins make up only about 5% of the genome.
ii) An average gene consists of 3000 bases, the largest known human gene being dystrophin with 2.4 million bases.
iii) The function of 50% of the genome is derived from transposable elements such as LINE and ALU sequence.
iv) Genes are distributed over 24 chromosomes. Chromosome 19 has the highest gene density. Chromosome 13 and Y chromosome have lowest gene densities.
v) The chromosomal organization of human genes shows diversity.
vi) There may be 35000-40000 genes in the genome and almost 99.9 nucleotide bases are exactly the same in all people.
vii) Functions for over 50 percent of the discovered genes are unknown.
viii) Less than 2 percent of the genome codes for proteins.
ix) Repeated sequences make up very large portion of the human genome. Repetitive sequences have no direct coding functions but they shed light on chromosome structure, dynamics and evolution (genetic diversity).
x) Chromosome 1 has 2968 genes, whereas chromosome 'Y' has 231 genes.
xi) Scientists have identified about 1.4 million locations, where single base DNA differences (SNPs - Single nucleotidepolymorphism - pronounce as 'snips') occur in humans. Identification of 'SNIPS' is helpful in finding chromosomal locations for disease associated sequences and tracing human history.

A transcriptional unit in DNA is defined by three regions, a promoter, the structural gene, and a terminator. The promoter is located towards the 5' end. It is a DNA sequence that provides binding site for RNA polymerase. The presence of promoter in a transcription unit defines the template and coding strands. The terminator region located towards the 3' end of the coding strand contains a DNA sequence that causes the RNA polymerase to stop transcribing. In eukaryotes the promoter has AT-rich regions called TATA box (Goldberg- Rogness box) and in prokaryotes, this region is called Pribnow box. Besides promoter, eukaryotes also require an enhancer. The two strands of the DNA in the structural gene of a transcription unit have opposite polarity. DNA dependent RNA polymerase catalyses the polymerization in only one direction, the strand that has the polarity 3'\(\rightarrow\)5' acts as a template, and is called the template strand. The other strand which has the polarity 5'\(\rightarrow\)3' has a sequence same as RNA (except thymine instead of uracil) and is displaced during transcription. This strand is called coding strand.
The structural gene may be monocistronic (eukaryotes) or polycistronic (prokaryotes). In eukaryotes, each mRNA carries only a single gene and encodes information for only a single protein and is called monocistronic mRNA. In prokaryotes, clusters of related genes, known as operon, often found next to each other on the chromosome are transcribed together to give a single mRNA and hence are polycistronic.

In eukaryotes, organization is more complex. Chromatin is formed by a series of repeating units called nucleosomes. Kornberg proposed a model for the nucleosome, in which 2 molecules of the four histone proteins H2A, H2B, H3 and H4 are organized to form a unit of eight molecules called histone octamere. The negatively charged DNA is wrapped around the positively charged histone octamere to form a structure called nucleosome. A typical nucleosome contains 200 bp of DNA helix. The histone octameres are in close contact and DNA is coiled on the outside of nucleosome. Neighbouring nucleosomes are connected by linker DNA (HI) that is exposed to enzymes. The DNA makes two complete turns around the histone octameres and the two turns are sealed off by an HI molecule. Chromatin lacking HI has a beads-on-a-string appearance in which DNA enters and leaves the nucleosomes at random places. HI of one nucleosome can interact with HI of the neighbouring nucleosomes resulting in the further folding of the fibre. The chromatin fiber in interphase nuclei and mitotic chromosomes have a diameter that vary between 200-300 nm and represents inactive chromatin. 30 nm fibre arises from the folding of nucleosome, chains into a solenoid structure having six nucleosomes per turn. This structure is stabilized by interaction between different HI molecules. DNA is a solenoid and packed about 40 folds. The hierarchical nature of chromosome structure is illustrated. Additional set of proteins are required for packing of chromatin at higher level and are referred to as non-histone chromosomal proteins (NHC). In a typical nucleus, some regions of chromatin are loosely packed (lightly stained) and are referred to as euchromatin. The chromatin that is tightly packed (stained darkly) is called heterochromatin. Euchromatin is transcriptionally active and heterochromatin is transcriptionally inactive.

(i) Replication begins at the initiation site called the site of 'origin of replication(ori). In prokaryotes, there is only one origin of replication, whereas in eukaryotes with giant DNA molecules, there can be several origins of replication (replicons).
(ii) Replication occurs within a small opening of the DNA helix called as replication fork. Unwinding of the DNA strand is carried out by DNA helicase.
(iii) Thus, in one strand (template strand with polarity 3' ➝ 5') the replication is. continuous and is known as the leading strand while in the other strand (coding strand with polarity 5'➝3') replication is discontinuous, known as the lagging strand The discontinuously synthesized fragments of the lagging strand (called the Okazaki fragments) are joined by the enzyme DNA ligase.

(iv) As they move away in both directions, newly synthesized complementary nucleotides are paired with the existing nucleotides on the parent strand and covalently bonded together by DNA polymerase.
(v) Formation of new strand requires a primer (a short stretch of RNA) for initiation. The primer produces a 3' OH end on the sequence of ribonucleotides, to which deoxyribonucleotides are added. The RNA primer is ultimately removed by the exonuclease activity of DNA polymerase.
(vi) Finally, when all the nucleotides are in position, gaps are sealed by the enzyme DNA ligase.
(vii) At the point of origin of replication, the helicases and topisomerases (DNA gyrase) unwind and pull apart the strands, forming a Y-Shaped structure called the replication fork.
(viii) The two strands of a DNA helix have an antiparallel orientation. The enzyme DNA polymerase can only catalyse the addition of a nucleotide to the new strands in the 5' ➝ 3' direction, as it can only add nucleotides to the 3' carbon position.