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.
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