DNA

              Our  genes are made of deoxyribonucleic acid (DNA). This remarkable molecule contains all the information necessary to make a cell and DNA is able to pass on this information when a cell divides.

In 1953 Watson and Crick proposed a structure for DNA.

Watson and Crick’s conclusions :

1. The DNA structure is a right-handed double helix.

2. The diameter of the helix of DNA is 20A.

3. The length of each nucleotide is 3.4A ; the helix contais ten nucleotides on a loop.

4. DNA consists of two strands, which are antiparallel. Strands have a 5’ head and a 3’ tail.

5. The hereditary information is encoded in DNA in a specific sequence of bases along the polynucleotide chain.

6. The base pairs are held together by hydrogen bonds. These are two H-bonds between A and T and three H-bonds between G and C.

7. The base pairs are located inside the double helix, and sugar-phosphate backbone is located outside.

8. The polynucleotide chains have the same hereditary information.

Each nucleotide consists of three major parts :

- a five-carbon sugar (pentose) ;

- a flat,heterocyclic,nitrogen-containing organic base ;

- a negatively charged phosphate group,which gives the polymer acidic property

                                      

The organic bases are two general types : single-ringed pyrimidines and double-ringed purines. The purines are adenine (A) and guanine (G). The pyrimidines are cytosine (C), thymine (T), and uracil (U).

Since A always pairs with T, G always pairs with C.

Chargaff”s rule

The purine : pyrimidine in double stranded DNA is always 1.

Levels of DNA package are :

- Nucleosome structure (Nucleosomes are the fundamental units of chromatin. The histones form a disk-shaped complex called a nucleosome, which contains two complete turns of double-stranded DNA wrapped around its surface, histones core has 8 proteins – two copies each of the H2A/H2B and H/H4 complexes)

- The 30 nm fibre (It is higher order chromatin organization. Histone H1 binds between nucleosomes to give even more structure to chromatin. At higher ionic strength or in the presence of Mg2+ nucleosome structure becomes more compact and produces a fibre of about 30 nm in diameter. “The string” is called the 30 nm fibre.)

- Chromatin loops : the 30 nm fibre is attached at various points to the nuclear matrix to form a series of loops, containing 30-100 kbp of DNA (the folded fibre model). By adding twists to make these nucleosome and solenoid structure, the DNA is supercoiled. This structure with loops is called “lampbrush chromosome”.

- Interphase chromatin (Chromatin loops are more coiled and form interphase chromatin). During interphase (the period of the cell cycle where the cell is not dividing), two types of chromatin can be distinguished :

          a) Euchomatin, which consists of DNA that is active,e.g., being expressed as protein.

          b) Heterochromatin, which consists of mostly inactive DNA. Heterochromatin can be 

               further distinguished into two types :

                        1) Constitutive heterochromatin, which is never expressed. It is located around 

                             the centromere and usually contains repetitive sequences.

                        2) Facultative heterochromatin, which is sometimes expressed. 

                            Individual chromosomes cannot be distinguished at this stage – they appear 

                             in the nucleus as a homogeneous tangled mix ofg DNA and protein.

- Metaphase chromosomes are higly condensed, duplicated chromosome of dividing cell. In the early stages of mitosis or meiosis (cell division), the chromatin strands become more and more condensed. They cease to function as accessible genetic material (transcription stops) and become a compact transportable form. This compact forms makes the individual chromosomes visible, and they form the classic four arm structure, a pair of sister chromatids attached to each other at the centromere. The shorter arms are called p arms (from the Fench petit,small) and the longer arms are called q arms (q follows p in the Latin alphabet; q-g “grande”). This is the only natural context in which individual chromosomes are visible with an optical microscope.

Rujukan1.      E.S. Klinstova,General Biology Part 1 Cell cycle and Molecular genetics, Publishing House of NNSMA, 2012