Structure and functions of nucleic acids.

Date: 2015-10-07; view: 465.

THE BASIC THEORETICAL ITEMS OF INFORMATION

There are two kinds of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

The acids are polymers. Their monomer (unit) named nucleotide. Each nucleotide contain of nitrogenous base, and sugar-phosphate backbone. The nitrogenous bases are of two types: purines (adenine, guanine) and pyrimidines (thymine, cytosine and uracil). Sugar-phosphate backbone contain of sugar (ribose or deoxyribose) and phosphate group. Nucleotides attach each other within the chain of nucleic acid by phosphodiethereal links.

DNA is different from RNA by some traits:

James Watson and Francis Crick accomplished the structure of DNA molecule in 1953. They discovered that:

1. Each DNA molecule is made of two antiparallel strands, which are twisted around one another to form a double helix.

2. The uprights of this ladder are composed of sugar and phosphate part of the nucleotides.

3. The rungs are made up of the nitrogen bases of nucleotide. Each nucleotide is situated parallel to axis of spiral.

4. Each rung consists of a purine matched with pyrimidine. The matching of the bases is highly specific. Adenine matches Thymine only and Guanine matches Cytosine only. Thus A – T and C – G pairs are the only ones that can be formed.

5. The two strands are held together by weak hydrogen bonds. Two hydrogen bonds are formed between A and Tand threehydrogen bonds are formed between C and G.

There are Chargaff's rules (Erwin Chargaff) that reflect principle of complementary correspondence between nucleotides A and T, C and G.

1) The ratio of A to T is 1 to 1 and of G to C is 1 to 1.

2) Quantities of purines nitrogenous bases and pirimidines nitrogenous bases are equal.

3) Wide variations exist in the (A+T)/(G+C) ratio.

There is polymorphism among molecules of DNA and there are some conformations of DNA.

B-form is ordinary, regular helix. It has 10 nucleotides per complete one turn (whorl) of helix.

A-form has an irregular helix (it has changeable quantity of nucleotides per complete one turn).

C-form has less of nucleotides per complete one turn (whorl), than B-form.

Z-form has the helix twisted to the left (all other forms of DNA have helix twisted to the right).

The ribbon model of DNA.

The main characteristic of DNA is the its possibility to undergo replication. Replication is process of DNA synthesis. As a result of the replication from one original double helix (molecule of DNA) two daughter double helixes DNA are formed. During replication hydrogen bonds between two strands are broken and new strand is built according the role of complementation. Some ferments take part in process of replication. The main ferments are named DNA-polimerase I, DNA-polimerase III, primase, helicase.

The DNA strands are antiparallel (two strand of DNA run in opposite directions: from 3' to 5' for one and from 5' to 3' for the other). But replication proceeds in the 5' to 3' direction on EACH strand. Thus one strand will form a continuous copy (leading strand), while the other will form a series of short Okazaki fragments. Then the fragments connect each other. The synthesis of DNA occurs in the region of “replication fork”.

The method of DNA replication is known as method of semiconservativereplication since each new double helix retains one strand of the original DNA double helix.

Biological significance of the replication is keeping of constant kit of heredity material in the rows of changing generations of cells and organisms.

Functions of DNA are:

1. Kipping of heredity information

2. Replication of heredity information

3. Realization of heredity information

4. Transmit of heredity information

RNA is localized within cytoplasm (ribosome, mitochondria) and within nuclei (chromatin, nucleolus). RNA is synthesized on the base of DNA according role of complementation. The main ferment for synthesis is RNA-polimerase. The method of RNA synthesis is known as method of conservativesynthesis since new strand doesn't have any components of the original RNA strand.

There are three types of RNA: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).

Messenger RNA produced 0n DNA and most of it passes out into the cytoplasm. This RNA carries information that is necessary for protein synthesis in ribosome. From 5% to 10% of all cell RNA there is mRNA.

Ribosomal RNA is synthesizes on the DNA on regions of nucleolus. rRNA is a component of ribosomes. About 80% of all cell RNA there is rRNA.

Transfer RNA produced on DNA and has the form of a clover leaf. About 80% of all cell RNA there is tRNA. tRNA consist of 75 – 90 nucleotides. It has 4 key regions:

- Carrier end. A specific amino acid joins it.

- Recognition end. It has 3 unpaired ribonucleotides that are named anticodon. These nucleotides are complementary to nucleotides of mRNA called codon. Anticodon reads its appropriate codon and temporarily joins it during protein synthesis.

- Enzyme site. It is meant for a specific charging enzyme, which catalyzes the union of a specific amino acid to tRNA molecule (aminoacilsynthetase).

- Ribosome site. It is meant for attachment to a ribosome.