THE GENETIC CODE

If genes are segments of DNA and if DNA is just a string of nucleotide pairs. then how does the sequence of nucleotide pairs dictate the sequence of amino acids in  proteins? The analogy to a code springs to mind at once. By the mid-1960's the genetic code was largely solved. The most important properties of the genetic code are:-

1. The genetic code is a triplet code.
2. The code is non-overlapping
3. The code is degenerate.
4. The code is comma-free.
5. The code is ordered.
6. The code contains start and stop codons 
7. The code is universal.

1. Triplet Code:-

• Twenty different amino acids are incorporated into polypeptides during translation. (20 amino acids commonly found in cellular proteins)
• Thus, atleast 20 different codons must be formed with the four bases available in mRNA.
• If the words are two letters long, then 4X4=16 codons are possible. for example AU, CU OR CC. This is not large enough,
• If  the words are three letters  long then (4cube)4X4X4=64 words are possible. for example AUU, GCG OR UGC. This code provides more than enough words to describe the amino acids. Thus by experimental evidence it was proved that the genetic code is a triplet code. However, if all words are "triplets", then we have a considerable excess to possible words over the 20 needed to name the common amino acids.
• Three bases code for an amino acid. These triplets are termed codons.
• The code is read from a fixed starting point and continues to the end of the coding sequence.
• The reading frame of mRNA is the series of nucleotide triplets that are (used to) read during translation.

             For example, the reading frame of the sequence AAAGGGCCCTTT is (AAA) (GGG) (CCC) (TTT).

2. Non-overlapping:- 

                                 By 1961, it was already clear that the genetic code was non-overlapping. The analysis of mutationally altered proteins. In particularly, the nitrous acid-generated mutants of tobacco mosaic virus showed that only a single amino acid change at one time in one region of the protein. This is predicted by a non-overlapping chains.

3. Degenerate Code:- 

                                  The occurance of more than one codon per amino acid is called degeneracy.

Example:- Leucine, Serine, Arginine are coded each by six different codons. 

Isoleucine has 3 codons.

All the amino-acids except methionine and tryptophan are coded by more than one codon.

The degeneracy is of two types:-

I). Partial degeneracy and

II). Complete degeneracy.

Mitochondrial genome organization e.g. Human

the human genome is the term used to describe the total genetic information (DNA content) in human cells. It comprises of two genomes - a complex nuclear genome and a very simple mitochondrial genome.
        The human mitochondrial genome is defined by a single type of circular double stranded DNA  whose complete nucleotide sequence has been established. It is 16569 bp in length and is 44% (G+C). The 2 DNA strands have significantly different base composition, the heavy(H) strand is rich in guanine, the light(L) strand is rich in cytosine. Human cells typically contain thousands of copies of the mitochondrial molecules, but the number can vary considerably in different cell types.
        During zygote formation, a sperm cell contributes its nuclear genome, but not its mitochondrial genome to the egg cell. The cytoplasm and the mitochondria in the zygote is got from the egg cell. Therefore, the mitochondrial genome is said to be maternally inherited. Males and females both inherit their mitochondrial from mother but males do not transmit their mitochondrial to subsequent generations.
        MITOCHONDRIAL GENES:- The human mitochondrial genome contains 37genes. For 28 of the genes  the heavy strand is the sense strand, for the other nine the light strand is the  sense strand of the 37 genes,
24 specify a mature RNA product, 22-mitochondrial t-RNA molecules and 2-mitochondrial rRNA molecules. The remaining 13 genes encode polypeptides which are synthesized on mitochondrial ribosomes.
        Each of the 13 polypeptides encoded by the mitochondrial genome is a subunit of one of the mitochondrial respiratory complexes, the multichain enzymes of oxidative phosphorylation which are engaged in the production of ATP. However , there are about 100 different polypeptide subunits in the mitochondrial oxidative phosphorylation system and so the vast majority are encoded by nuclear genome and are translated on cytoplasmic ribosomes before being imported into the mitochondria.   Unlike its nuclear counterpart, the human mitochondrial genome is extremely compact:approximately 93%of the DNA sequence represents coding sequence.

• All 37 mitochondrial genes lack introns and they are tightly packed ( on average one per 0.45kb).
• The coding sequences of some genes ( those encoding 8th and 6th subunits of mitochondrial ATPase) show some overlap 
• In most other cases, the coding sequences of neighbouring genes are contiguous or separated by one or two non-coding bases.
• The mitochondrial genes encodes all the rRNA and t-RNA molecules it needs for synthesizing proteins but relies on nuclear encoded genes to provide all other components.(Such as protein components of mitochondrial ribosomes, amino acyl t-RNA synthetases etc.)
• The only significant region lacking any know coding DNA is the displacement(D) loop region.
• The replication of both H and L strands is unidirectional and starts in the D-loop.
• Unlike the transcription of nuclear genes, in which individual genes are almost always transcription of the mitochondrial DNA starts from the promoters in the D-loop region and continues in the opposing directions for the two different strands round the circle to generate large multigenic transcripts. The mature RNA, are subsequently generated by cleavage of the multigenic transcripts.
• These are 60 mitochondrial sense codons. 

1. Two of the four stop codons, UAA and UAG also act as stop codons. the other two are AGA and AGG which specifies arginine in nuclear genetic code.

• UGA which is a stop codon in the nucleus encodes for tryptophan in the mitocondria.
• AUA specifies methionine in the mitochondria as in nucleus it specifies isoleucine.
• 
  

organs of immune system

there are two types of organ in immune system. primary or central lymphoid organs and secondary or peripheral lymphoid organs. thymus , bursa of fabricius and bone marrow are the primary lymphoid organs while spleen, lymphnodes and mucosa-associated lymphnoids are the secondary lymphoid organs.
organs and tissues are inter-connected wit blood vessels and tissue lymphatics through which lymphocytes circulate.
1). Primary or Central Lymphoid Organs :- In primary lymphoid organs immature lymphocytes which are generated by haematopoiesis get mature and become committed to a particular antigenic specificity. When it matures in primary lymphoid organ lymphocyte becomes immunocompetent, i.e., capable of mounting an immune   response.
I}. Thymus:- Thymus is a greyish, flat, bilobed lymphoid organ situated above heart and extending into the neck on the front and sites of trachea. When it matures it acts as the site of development and maturation of  lymphocytes named tymus-derived lymphocytes or T-lymphocytes or T-cells. 
Each lobe of  Thymus is divided in to a series of lobules,each of which is organized in to two campartments.
1]cortex:outer component
2]medulla:inner component
Cortex is highly dense with thymocytes and medulla is sparsely populated with thymocytes.Thymocytes rapidly divided in the cortex and give rise to T-Lymphocytes.
The T-thymocytes which are produced in thymus only five percent leave the thymus as viable cells and the rest are eliminated due to non productive arrangements and negative selection to eliminate self -reacting T-cells.
Both the cortex and medulla of the thymus contain epithelial cells,dendritic cells and macro phases which contributes the growth and mathuration of thymocytes.

T-limphocytes capable of mounting cell-mediated immune response aganist appropriate antigens.This is effect under the influence of the thymic micro environment and several hormones.The T-lympocytes immediately move from thymus to the secondary or peripheral lympoid organs .

b.sc bio technology 4

       


B.Sc.3-YEAR




SUBJECT:BIO-TECHNOLOGY


PAPER 4:-APPLICATION OF BIO-TECHNOLOGY




TIME-3hrs                          (  MAX MARKS:100)


NOTE:Answer all questions.All answers carry equal marks


1.(a)Write on methods of gene transfer
                      OR
   (b)Write a short notes on principles of exvivo         and invivo gene therapy


2.(a)Write a short notes on plant growth regulators on differentiation?


 (b)Explain briefly production of useful compounds by plant cell culture.
         
3.(a)Explain principle of fermentation technology with examples.
                    (OR)
   (b)Write about screening, isolation and preservation of industrial micro-organisms?


4.(a)Write briefly microbiological quantity of milk.
                       (OR)
   (b)Write about conventional energy sources and non-conventional sources.
5.Write short notes on any three of the following:
(a).methogens and biogas
(b).antibiotics
(c).gene therapy
(d).microbial quality of water

submitted

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