RIBONUCLEIC ACID

RIBONUCLEIC ACID

Ribonucleic acid (RNA) is a nucleic acid that contains a long chain of nucleotide units. It is similar to DNA but contains ribose instead of deoxyribose. Various functions coded in the genes are carried out in the cytoplasm of the cell by RNA. RNA is formed from DNA.

STRUCTURE OF RNA

Each RNA molecule consists of a single strand of polynucleotide unlike the doublestranded

DNA. Each nucleotide in RNA is formed by:

1. Ribose – sugar.

2. Phosphate.

3. One of the following organic bases:

Purines – Adenine (A)

– Guanine (G)

Pyrimidines – Uracil (U)

– Cytosine (C).

Uracil replaces the thymine of DNA and it has similar structure of thymine.

TYPES OF RNA

RNA is of three types. Each type of RNA plays a specific role in protein synthesis. The three types of RNA are:

1. Messenger RNA (mRNA)

Messenger RNA carries the genetic code of the amino acid sequence for synthesis of protein from the DNA to the cytoplasm.

2. Transfer RNA (tRNA)

Transfer RNA is responsible for decoding the genetic message present in mRNA.

3. Ribosomal RNA (rRNA)

Ribosomal RNA is present within the ribosome and forms a part of the structure of ribosome. It is responsible for the assembly of protein from amino acids in the ribosome.

GENE EXPRESSION

Gene expression is the process by which the information (code word) encoded in the gene is converted into functional gene product or document of instruction (RNA) that is used for protein synthesis.

Gene expression involves two steps:

1. Transcription.

2. Translation.

TRANSCRIPTION OF GENETIC CODE

The word transcription means copying. It indicates the copying of genetic code from DNA to RNA. The proteins are synthesized in the ribosomes which are present in the cytoplasm. However, the synthesis of different proteins depends upon the information (sequence of codon) encoded in the genes of the DNA which is present in the nucleus. Since DNA is a macromolecule, it cannot pass through the pores of the nuclear membrane and enter the cytoplasm. But, the information from DNA must be sent to ribosome. So, the gene has to be transcribed (copied) into mRNA which is developed from DNA. Thus, the first stage in the protein synthesis is transcription of genetic code, which occurs within the nucleus. It involves the formation of mRNA and simultaneous copying or transfer of information from DNA to mRNA. The mRNA enters the cytoplasm from the nucleus and activates the ribosome resulting in protein synthesis. The formation of mRNA from DNA is facilitated by the enzyme RNA polymerase.

TRANSLATION OF GENETIC CODE

Translation is the process by which protein synthesis occurs in the ribosome of the cell under the direction of genetic instruction carried by mRNA from DNA. Or, it is the process by which the mRNA is read by ribosome to produce a protein. This involves the role of other two types of RNA, namely tRNA and rRNA. The mRNA moves out of nucleus into the cytoplasm. Now, a group of ribosomes called polysome gets attached to mRNA. The sequence of codons in Mrna are exposed and recognized by the complementary sequence of base in tRNA. The complementary sequence of base is called anticodon. According to the sequence of bases in anticodon, different amino acids are transported from the cytoplasm into the ribosome by tRNA that acts as a carrier. With the help of rRNA, the protein molecules are assembled from amino acids. The protein synthesis occurs in the ribosomes which are

attached to rough endoplasmic reticulum.

GROWTH FACTORS

Growth factors are proteins which act as cell signaling molecules like cytokines and hormones. These factors bind with specific surface receptors of the target cell and activate proliferation, differentiation and/or maturation of these cells. Often, the term growth factor is interchangeably used with the term cytokine. But growth factors are distinct from cytokines. Growth factors act on the cells of the

growing tissues. But cytokines are concerned with the cells of immune system and hemopoietic cells.

Many growth factors are identified. The known growth factors are:

1. Plateletderived growth factor – PDGF

2. Colony stimulating factors – CSF

3. Nerve growth factors – NGF

4. Neurotropins

5. Erythropoietin

6. Thrombopoietin

7. Insulinlike

growth factors – IGF

8. Epidermal growth factor – present in keratinocytes and fibroblasts. It inhibits growth of hair follicles and

cancer cells

9. Basic fibroblast growth factor – present in blood vessels. It is concerned with the formation of new

blood vessels

10. Myostatin – present in skeletal muscle fibers. It controls skeletal muscle growth

11. Transforming growth factors (TGF) – present intransforming cells (cells undergoing differentiation)

and in large quantities in tumors and cancerous tissue. TGF is of two types:

i. TGFα secreted in brain, keratinocytes and macrophages. It is concerned with growth of epithelial cells and wound healing

ii. TGFβ secreted by hepatic cells, T lymphocytes, B lymphocytes, macrophages and mast cells.

When the liver attains the maximum size in adults, it controls liver growth by inhibiting proliferation

of hepatic cells. TGFβ also causes immunosuppression.