a) Purine and pyrimidine bases are linked by hydrogen bonds.
b) Pentose sugar is linked to adjacent nucleotide by phosphodiester bonds.

Adenine, Guanine, Cytosine and Uracil.

The DNA as a nucleoid is organized into large loops held by protein. DNA of prokaryotes is almost circular and lacks chromatin organization, hence termed genophore.

In nucleic acids (DNA and RNA), the nitrogenous base is chemically linked to one molecule of sugar forming a nucleotide.
When a phosphate group is attached to the nucleoside, it becomes a nucleotide. The nucleotides are polymerised to form a polynucleotide chain (DNA and RNA).

They inferred each strand of the double strands helps was used as a template for the formation of a new, daughter strand having a complementary sequence of bases.

There are 1.4 million locations where single base DNA differences (Called SNPs - Single Nucleotide Polymorphism - pronounce as snips) occurs in human's genome.
(i) By tracing human history
(ii) By finding chromosomal locations for disease associated sequences

A - \(5^{\prime} \rightarrow 3^{\prime}\) is called a coding strand.
B - \(3^{\prime} \rightarrow 5^{\prime}\) is called a template strand.

1. The genetic code is universal. It means that all known living systems use nucleic acids and the same three base codons (triplet codon) direct the synthesis of protein from amino acids. E.g, mRNA ,(UUU) codon codes for phenylalanine in all cells of all organisms.
2. Some exceptions are reported in prokaryotic, mitochondrial and chloroplast genomes. However similarities are more common than differences.

Structure of the operon: Each operon is a unit of gene expression and regulation and consists of one or more structural genes and an adjacent operator gene that controls transcriptional activity of the structural gene.
i) The structural gene codes for proteins, rRNA and tRNA required by the cell .
ii) Promoters are the signal sequences in DNA that initiate RNA synthesis. RNA polymerase binds to the promoter prior to the initiation of transcription.
iii) The operators are present between the promoters and structural genes. The repressor protein binds to the operator region of the operon.

Both the strands of DNA are not copied during transcription for two reasons.
1. If both the strands act as a template, they would code for RNA with different sequences. This, in turn, would code for proteins with different amino acid sequences. This would result in one segment of DNA coding for two different proteins, hence complicate the genetic information transfer machinery.
2. If two RNA molecules were produced simultaneously, double-stranded RNA complementary to each other would be formed. This would prevent RNA from being translated into proteins.

According to Erwin Chargaff,
a) Adenine pairs with thymine with two hydrogen bonds.
b) Guanine pairs with cytosine with three hydrogen bonds.

The 5' end of the mRNA of prokaryotes has a special sequence which precedes the initial AUG start codon of mRNA. This ribosome binding site is called the Shine - Dalgamo equence or S-D sequence.

There are three nucleotides, called 'anticodon' found on the anticodon arm of tRNA. The anticodon pairs with a codon in mRNA to facilitate picking of correct amino acid from cytoplasm for synthesis of the polypeptide chain.

The assembly of the ribosomal subunits, mRNA and tRNA represent the initiation complex.

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.

When lactose is absent "i" gene regulates and produces repressor mRNA in the absence of lactose, which blocks the transcription. The repressor protein binds to the operator region of the operon and as a result prevents RNA polymerase from binding to the operon. The operon will be switched off (stop) in this situation.

(i) Identify all the genes (approximately 30000) in human DNA.
(ii) To store this information in databases.
(iii) Improve tools for data analysis.  

The simplest type of mutation at the molecular level is a change in nucleotide that substitutes one base for another. Such changes are known as base substitutions which may occur spontaneously or due to the action of mutagens. A well-studied example is sickle cell anaemia in humans which results from a point mutation of an allele of \(\beta \) - haemoglobin gene (\(\beta \) Hb).
A haemoglobin molecule consists of four polypeptide chains of two types, two a chains and two \(\beta \) chains. Each chain has a heme group on its surface. The heme groups are involved in the binding of oxygen. The human blood disease, sickle cell anaemia is due to abnormal haemoglobin. This abnormality in haemoglobin is due to a single base substitution at the sixth codon of the beta-globin gene from GAG to GTG in \(\beta \) -chain of haemoglobin. It results in a change of amino acid glutamic acid to valine at the 6^th position of the \(\beta \) -chain. This is the classical example of 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.

The mode of DNA replication was determined in 1958 by Meselson and Stahl. They designed an experiment to distinguish between semi-conservative, conservative and dispersive replications. In their experiment, they grew two cultures of E.coli for many generations in separate media. The 'heavy' culture was grown in a medium in which the nitrogen source (NH₄CI) contained the heavy isotope ¹⁵N and the 'light' culture was grown in a medium in which the nitrogen source contained light isotope 14N for many generations. At the end of growth, they observed that the bacterial DNA in the heavy culture contained only ¹⁴N and in the light culture only ¹⁴N. The heavy DNA could be distinguished from light DNA (¹⁵N from ¹⁴N)with a technique called Cesium Chloride (CsCI) density gradient centrifugation. In this process, heavy and light DNA extracted from cells in the two cultures settled into two distinct and separate bands (hybrid DNA).
The heavy culture (¹⁵N) was then transferred into a medium that had only NH⁴CI and took samples at various definite time intervals (20 minutes duration). After the first replication, they extracted DNA and subjected it to density gradient centrifugation. The DNA settled into a band that was intermediate in position between the previously determined heavy and light bands. After the second replication (40 minutes duration), they again extracted DNA samples, and this time found the DNA settling into two bands, one at the light band position and one at intermediate position. These results confirm Watson and Crick's semi-conservative replication hypothesis.

According to the classical concept of gene introduced by Sutton in 1902, genes have been defined as discrete particles that follow Mendelian rules of inheritance, occupy a definite locus in the chromosome and are responsible for the expression of specific phenotypic character. They show the following properties:
(i) Number of genes in each organism is more than the number of chromosomes; hence several genes are located on the same chromosome.
(ii) The genes are arranged in a single linear order like beads on a string.
(iii) Each gene occupies a specific position called locus.
(iv) Genes may exist in several alternate forms called alleles.
(v) Genes may undergo sudden change in positions and composition called mutations.
(vi) Genes are capable of self-duplication producing their own copies.

1. The process protein synthesis actually consists of two stages-transcription and translation.
2. In eukaryotic cells, transcription takes place in the nucleus. During transcription, DNA is used as a template to make a molecule of messenger RNA (mRNA). The molecule of mRNA then leaves the nucleus and reaches ribosome in the cytoplasm, where translation occurs.
3. During translation, the genetic code in mRNA is read and used to make a protein.
4. These two processes are summedup by the central dogma of molecular biology

(a) Replicating fork of DNA
(b)

(c) Source of Energy:
NTP's are used in the synthesis of RNA primers and ATP is used as an energy source for some of the enzymes, needed to initiate and sustain DNA synthesis occurs at the replication fork.
Enzymes involved in replication :
(i) DNA Helicase
(ii) DNA polymerase
(iii) Topoisomerase/DNA Gyrase
(iv) DNA ligase
(v) Primase
(d) Both the strands of (parental) DNA act as template for the synthesis of new strands
1. On the templates strand with \(3^{\prime} \rightarrow 5^{\prime}\) polarity the new strand (leading strand) is synthesized as a continuous stretch. It is called continuous synthesis (DNA polymerase carryout polymerisation).
2. On the other strand \(5^{\prime} \rightarrow 3^{\prime}\) polarity DNA synthesized as short stretches; it is called discontinuous synthesis.
3. Later the short stretches of DNA are joined by DNA-ligases into a continuous strand called lagging strand.

Human genome project was considered to be a mega project because
1. It had a specific goal to sequence for everybase pair present in the human genome. (Human Genome has 3.1647 billion nucleotide base pairs).
2. It took around 13 years of completion and got accomplished in year 2006.
3. It was a large-scale project, which aimed at developing new technology and generating new information in the field of genomic studies.
4. As a result of it, several news areas and avenues have opened up in the field of genetics, biotechnology and medical sciences.
5. It provided clues regarding the understanding of human biology.