RNA is said to be as the first genetic material because
1. RNA can directly code for protein synthesis of protein and hence can easily expresses the character. It is the genetic material in many viruses.
2. RNA can also act as a catalyst; there are some important biothemical reactions in living systems that are catalyzed, by RNAs and not as proteins.
3. Many essential life processes like splicing translation, etc have evolved around RNA.

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.

Steps in DNA printing:
i) Extraction of DNA: The process of DNA fingerprinting starts with obtaining a sample of DNA from blood, semen, vaginal fluids, hair roots, teeth, bones, etc.,
ii) Polymerase chain reaction (PCR) In many situations, there is only a small amount of DNA available for fingerprinting. If needed many copies / f the DNA can be produced by PCR (DNA amplification).
iii) Fragmenting DNA: DNA is treated with restriction enzymes which cut the DNA into smaller fragments at specific sites.
iv) Separation of DNA electrophoresis: During electrophoresis in an agarose gel, the DNA fragments are separated into bands of different sizes. The bands of separated DNA are sieved out of the gel using a nylon membrane (treated with chemicals that allow for it to break the hydrogen bonds of DNA so there are single strands).
v) Denaturing DNA: The DNA on gels is denatured by using alkaline chemicals or by heating.
vi) Blotting: The DNA band pattern in the gel is transferred to a thin nylon membrane placed over the 'size fractionated DNA strand' by Southern blotting.
vii) Using probes to identify specific DNA: A radioactive probe (DNA labeled with a radioactive substance) is added to the DNA bands. The probe attaches by base pairing to those restriction fragments that are complementary to its sequence. The probes can also be prepared by using either. 'fluorescent substance' or 'radioactive isotopes'.
viii) Hybridization with probe: After the probe hybridizes and the excess probe washed off, a photographic film is placed on the membrane containing 'DNA hybrids'.
ix) Exposure film to make a genetic/ DNA Fingerprint: The radioactive label exposes the film to form an image (image of bands) corresponding to specific DNA bands. The thick and thin dark bands form a pattern of bars which constitutes a genetic fingerprint.

i) Comparative studies of mutations (sudden change in a gene) and corresponding alteration in amino acid sequence of specific protein have confirmed the validity of the genetic code.
ii) The simplest type of mutation at the molecular level is a change in nucleotide that substitutes one base for another and are known as base substitutions which may occur spontaneously or due to the action of mutagens.
iii) An example is sickle cell anaemia in humans which results from a point mutation of an allele of B-haemoglobin gene (βHb)
iv) A haemoglobin molecule consists of four polypeptide chans of two types, two a chains and two B chains. Each chain has a heme group on its surface. The heme groups are involved in the binding of oxygen.
v) The human blood disease, sickle cell anaemia is due to abnormal haemoglobin due to a single base substitution at the sixth codon of the beta globin gene from GAG to GTG in' β-chain of haemoglobin. It results in a change of amino acid glutamic acid to valine at the 6th position of β-chain.
vi) This is point mutation that results in the change of amino acids residue glutamic acid to valine. The mutant haemoglobin undergoes polymerisation under oxygen tension causing the change in the shape of the RBC from biconcave to a sickle shaped structure.