When a cell is not actively dividing, its nucleus is occupied by chromatin, which is a compact, orderly tangle of deoxyribonucleic acid (DNA), the carrier of genetic information, twisted around organizing proteins known as histones.

• During cell division, chromatin becomes even more compact and organizes itself into chromosomes. Each chromosome contains a different DNA molecule, and all of the DNA is duplicated so that each new cell receives a complete copy.

Proteins are polypeptides, carbohydrates are polysaccharides, and nucleic acids are polynucleotides. Each nucleotide has three parts: a five-membered cyclic monosaccharides, nitrogen-containing cyclic compound known as a nitrogenous base, and a phosphate group.

There are two classes of nucleic acids, DNA and ribonucleic acid (RNA), where several types of RNA exist. The function of one type of RNA is to put the information stored in DNA to use. Other types of RNA assist in the conversion of the message a specific RNA carries into protein.

The difference between DNA and RNA is found in the sugar portion of the molecules.

• In RNA, the sugar is d-ribose, except hereafter simply referred to as ribose, as indicated by the name ribonucleic acid.

• In DNA, the sugar is 2-deoxyribose, giving deoxyribonucleic acid.

A molecule composed of either ribose or deoxyribose and one of the five nitrogenous bases found in DNA and/or RNA is called a nucleoside. The combination of ribose and adenine, gives the nucleoside known as adenosine, which is the parent molecule of adenosine triphosphate (ATP).

Ribonucleotide is a nucleotide that contains d-ribose—monophosphate examples are AMP, uridine monophosphate (UMP), cytidine monophosphate (CMP), and guanosine monophosphate (GMP).

Deoxyribonucleotide is a nucleotide that contains 2-deoxy-d-ribose (monophosphate examples are dAMP, dTMP, dCMP, and dGMP)

The common abbreviations for some nucleotides are:

• A for adenine,

• G for guanine,

• C for cytosine,

• T for thymine,

• and U for uracil in RNA

In any given species, the amounts of adenine and thymine were always equal, and the amounts of cytosine and guanine were always equal (A = T and G = C). It was also found that the proportions of each (A/T:G/C) vary from one species to another.

According to the Watson–Crick model, a DNA molecule consists of two polynucleotide strands coiled around each other in a helical, screw-like fashion.

• The sugar–phosphate backbone is on the outside of this right-handed double helix, and the heterocyclic bases are on the inside, so that a base on one strand points directly toward a base on the second strand.

• The double helix resembles a twisted ladder, with the sugar–phosphate backbone making up the sides and the paired bases, the rungs.

Wherever a thymine occurs in one strand, an adenine falls opposite it in the other strand; wherever a cytosine occurs in one strand, a guanine falls opposite it on the other strand. This base pairing explains why A and T occur in equal amounts in double-stranded DNA, as do C and G.

The duplication, transfer, and expression of genetic information occur as the result of three fundamental processes: replication, transcription, and translation.

• Replication is the process by which a replica, or identical copy, of DNA is made when a cell divides, so that each of the two daughter cells has the same DNA.

• Transcription is the process by which the genetic messages contained in DNA are read and copied. The products of transcription are specific RNAs, which carry the instructions stored by DNA out of the nucleus and to the sites of protein synthesis.

• Translation is the process by which the genetic messages carried by RNA are decoded and used to build proteins.

DNA replication begins in the nucleus with partial unwinding of the double helix; this process involves enzymes known as helicases.

• The unwinding occurs simultaneously in many specific locations known as origins of replication.

• The DNA strands separate, exposing the bases and effectively forming a “bubble” in which the replication process can begin. At either end of the bubble, where double-stranded DNA and single-stranded DNA meet, are branch points known as replication forks.

• A set of multi-subunit enzymes called DNA polymerases move into position on the separated strands—their function is to facilitate transcription of the exposed single-stranded DNA.

• The NTPs carrying each of the four bases are available in the vicinity. One by one, the triphosphates move into place by forming hydrogen bonds with the bases exposed on the DNA template strand.

• The enzymes move only in the 3′ to 5′ direction along the template strand (and thus new DNA strands only grow in the 5′ to 3′ direction), so that one strand is copied continuously and the other strand is copied in segments as the replication fork moves along.

• In each resulting double helix, one strand is the original template strand and the other is the new copy.

RNA and DNA also differ in size and structure—RNA strands are not as long as DNA molecules. The RNAs are almost always single-stranded molecules (as distinct from DNA, which is almost always double-stranded); RNA molecules also often have complex folds, sometimes folding back on themselves to form double helices in some regions.

• There are also different kinds of RNA, each type with its own unique function in the flow of genetic information, whereas DNA has only one function—storing genetic information. Working together, the three types of RNA make it possible for the encoded information carried by DNA to be put to use in the synthesis of proteins.

  • Ribosomal RNAs :Outside the nucleus but within the cytoplasm of a cell are the ribosomes—small granular organelles where protein synthesis takes place. Each ribosome is a complex consisting of about 60% ribosomal RNA (rRNA) and 40% protein, with a total molecular mass of approximately 5,000,000 amu.

  • Messenger RNAs: The messenger RNAs (mRNA) carry information transcribed from DNA. They are formed in the cell nucleus and transported out to the ribosomes, where proteins will be synthesized. They are polynucleotides of varying length that carry the same code for proteins as does the DNA.

  • Transfer RNAs :The transfer RNAs (tRNA) are smaller RNAs that deliver amino acids one by one to protein chains growing at ribosomes. Each tRNA carries only one amino acid.

In transcription, one DNA strand serves as the template and the other, the informational strand, is not copied. Nucleotides carrying bases complementary to the template bases between a control segment and a termination sequence are connected one by one to form mRNA.

The primary transcript mRNA (or hnRNA) is identical to the matching segment of the informational strand but with uracil replacing thymine.

Introns, which are base sequences that do not code for amino acids in the protein, are cut out before the final transcript mRNA leaves the nucleus.

The genetic information is read as a sequence of codons—triplets of bases in DNA that give the sequence of amino acids in a protein. Of the 64 possible codons, 61 specify amino acids and three are stop codons.

Each tRNA has at one end an anticodon consisting of three bases complementary to those of the mRNA codon that specifies the amino acid it carries.

• Initiation of translation is the coming together of the large and small subunits of the ribosome, an mRNA, and the first amino acid–bearing tRNA connected at the first of the two binding sites in the ribosome.

• Elongation proceeds as the next tRNA arrives at the second binding site, its amino acid is bonded to the first one, the first tRNA leaves, and the ribosome moves along so that once again there is a vacant second site.

• These steps repeat until the stop codon is reached. The termination step consists of separation of the two ribosome subunits, the mRNA, and the protein.