BIOLOGY 4 Marks Questions

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Transcription : Genetic information are transferred from DNA to RNA. The process of synthesis of RNA from DNA is called Transcription.
There are three main components of transcriptional unit in DNA.
(i) Promoter,
(ii) Structural gene,
(iii) Terminator.
Schematic Diagram of the Transcription Unit :

Polymorphism: Polymorphism (variation at genetic level) arises due to mutations. New mutations may arise in an individual either in somatic cells or in the germ cells. If a germ cell mutation does not seriously impair individual's ability to have offspring who can transmit the mutation, it can spread to the other members of population (through sexual reproduction). Allelic sequence variation has traditionally been described as a DNA polymorphism if more than one variant (allele) at a locus occurs in human population with a frequency greater than 0.01. In simple terms, if an inheritable mutation is observed in a population at high frequency, it is referred to as DNA polymorphism. The probability of such variation to be observed in non coding DNA sequence would be higher as mutations in these sequences may not have any immediate effect/impact in an individual's reproductive ability. These mutations keep on accumulating generation after generation, and form one of the basis of variability/polymorphism. There is a variety of different types of polymorphisms ranging from single nucleotide change to very large scale changes. For evolution and speciation, such polymorphisms play very important role, and you will study these in details at higher classes.

Among human being, 99.9 percent of base sequence is the same, yet there is a difference in the DNA sequences of any two individuals. The difference in these sequences of DNA make every individuals unique in their phenotypic appearance. The difference in these sequences of DNA can easily be determined by fingerprinting. If one aims to find out genetic differences between two individuals or among individuals of a population, DNA fingerprinting is a very quick way to compare the DNA sequences of these individuals.
In DNA fingerprinting, differences existed in some specific regions/of DNA sequences are identified that are known as Satellite DNA. In DNA fingerprinting. the differences of this satellite DNA is determined. The satellite DNA contains small stretches of base pairs repeated many times, it is therefore, also called repetitive DNA. These repetitive DNA can easily be separated from bulk of genomic DNA by caesium chloride density ultra-centrifugation method in the form of bands. The major peaks are obtained for bulk DNA and lower peaks for satellite/repetitive DNA. This technique was developed by Alec Jeffrey. This technique needs the use of DNA probe. Alec Jeffrey used satellite DNA as probe-as it shows very high degree of polymorphism.
Applications: This technique is now used to:
(1) Identify the criminals in forensic laboratories.
(2) Determine paternity, that is, who the true biological father or mother is of a child.
(3) Verify whether a hopeful immigrant is, as, he or she claims, really a close relative of already an established resident.
(4) identify racial groups to rewrite biological evolution.

It was Jacob and Monad who gave complete explanation of the Lac operon. In Lac operon (here lac means lactose) A poly-cistronic structural gene is regulated by a common promoted and regulator genes. This type of arrangement is very common in bacteria and is known as operon. E Coli bacteria inhabiting in human's intestine derives energy by catabolism of lactose. Jacob and Monad reported that it has a group of three genes in its DNA which are associated with the synthesis of three enzymes needed for lactose catabolism.
When lactose is present in medium these genes become active but become inactive in absence of lactose Jacob and Monad proposed the operon concept for regulation of these genes activity. According to this concept, the regulation of gene activity is made at transcription level by induction or repression. Three enzymes are needed for catabolism and energy production from lactose. These enzymes B-galactosidase, permease and transacetylase.
These enzymes are synthesized from translation of polycistronic m-RNA. The synthesis of this polycistronic m-RNA takes place by transcription of three structural genes located in lac operon. These structural genes or cistrons are Cis-Z, Cis-Y and Cis-A These are placed adjacently and are mutually coordinated. The activity of structural genes are controlled by regulator gene, operator gene and promoter gene. Cis z codes for ẞ galactosidase which hydrolyses the lactose sugar into glucose and galactose. CisY, codes for permease that increases the permeability of cell for ẞ galactosidase. Cis A codes for transacetylase. Therefore the product of all three genes are essential for metabolism of lactose in lac operon.
When lactose is present in the medium as inducer, it enters the cell through the action of permease and joins the repressor produced by regulator gene. As a result repressor can not bind to operator. This allows the RNA polymerase assess to the promoter and transcription of a poly cistronic mRNA from structural genes is resulted which translate the required enzymes for metabolism of lactose. Therefore the lac operon is example of the best understood regulatory mechanism of enzyme induction.
Sometimes lactose on binding with repressor alters its structure and switches off the transcription joining the operators. In this situation, lactose is called co-repressor as it activates repressor to make operator inactive.

Difference between Prokaryotic and Eukaryotic DNA :

S.No.	Prokaryotic DNA	Eukaryotic DNA
1	It is found in cytoplasm, mitochondria and chloroplast.	It is found in nucleus.
2	Much less in amount, less than 0.1 Pg.	Much more (> 1 Pg) in amount than prokaryotic DNA.
3	It is usually circular, double stranded.	It is usually linear, double stranded.
4	It is naked, without histone.	It is coated with histone.
5	It can code for fewer proteins.	It can code for far more proteins.
6	Non-coding introns are absent.	DNA is mosaic of exons (coding regions) and introns (noncoding region).
7	G : C contents are more than A : T.	A : T contents are more than G : C.
8	On beating it is denatured as a tangled mass.	It is denatured into 2 separate strands.
9	It exists singly.	It exists in pairs.
10	Repeated sequences absent.	Present.
11	Very little part of DNA is functionless.	Greater part of DNA is noncistronic and functionless.

The information of sequence of amino acids exist in the coded language of DNA which is transcribed in mRNA. During transcription, this genetic information is coded in the sequence of nucleotides in m-RNA and determines the sequence of amino acids in protein molecules. The transmission of genetic information between sequence of nitrogen bases of m-RNA and 20 amino acids is known as genetic code. The credit of discovery of genetic code goes to Nirenberg et. al. (1961).
Of all nucleotides, there are only four types of nitrogen bases (A, G, C and U). These four letters are the complete alphabet of genetic dictionary and the sequence of amino acid in protein always follow the sequence of nitrogen bases. The living cells usually have 20 types of essential amino acid in different types of protein and this specific sequence of amino acid in a specific m-RNA depends on the presence of specific type of nitrogen bases that is A, G, C, U sequence.
The characteristics of genetic code are as follow :
1. The code is always triplet : Each codon of the genetic code dictionary is made by specific sequence of three nitrogen bases in m- RNA.
2. The code is comma less : There is no punctuation or comma between two successive code. The codon is read in m-RNA in a contiguous fashion and when the chain is terminated, the terminating codons occupy that place. They are UAA, UAG and UGA.
3. The code is non-ambiguous : Genetic codon does not show ambiguity i.e. a specific codon codes only for one amino acid although some genetic code have been found to code different amino acids in abnormal conditions.
4. The code is non-overlapping : Since the DNA molecule is a long chain of nucleotides it could be read either in an overlapping or non-overlapping manner. In the non-overlapping code, six nucleotides would code for two amino acids, while in the overlapping code up to four could be coded. In the non-overlapping code, each letter is read only once while in the over-lapping code it would be read three times, each time as a part of a different word.
5. The code is universal : It has become proved that genetic code is universal i.e. the genetic code is valid for all organisms ranging from bacteria to man (Except mitochondria and ciliate protozoa i.e.
(i) In yeast mitochondria UGA codes for tryptophan, although in the nuclear genes UGA is a termination codon.
(ii) In ciliate protozoa (Mycoplasma capricolum) in this genetic code, codon UAA and UAG specify glutamine instead of stop signals
The code is degenerated : There are 64 possible codons in a triplet code, of which 61 have been shown to code amino acids. Since only 20 amino acids take part in protein synthesis it is obvious that there are many more codons than amino acid types. Except for tryptophan and methionine, which have a single codon each, all other amino acids involved in protein synthesis have more than one codon. This is called degeneracy of codons. 

A molecule of Nitrogen base joins the molecule of pentose sugar. It results into the formation of nucleosides. Similarly when phosphoric acid joins the nucleoside it forms nucleotide. i.e. a nucleotide has three components :
1. Pentose sugar (ribose in RNA and deoxyribose in DNA)
2. A phosphate group
3. Nitrogen base
1. Pentose sugar : Depending on the type of pentose sugar, nucleic acid are of two types :
Nucleic acid with ribose sugar is ribonucleic acid (RNA)
Nucleic acid with deoxyribose is deoxyribonucleic acid (DNA) 


The main difference between the two sugars lies at second carbon atom which has -OH group in ribose sugar and hydrogen atom in deoxyribose sugar. Sugar acts as a link to join the nitrogen base and phosphate group.
2. Phosphate or phosphoric acid : This is the ortho-phosphoric acid that connects the two successive nucleotides. This bond is called as phospho-ester bond which joins the nucleoside at 3' carbon atom of sugar to that of 5' carbon atom of successive nucleosides
3. Nitrogen base :
Pyrimidine : These include thymine, cytosine and uracil of these DNA has thymine and cytosine while RNA has cytosine and uracil.
Purine : They are Adenine and Guanine in DNA and RNA both.
One important feature of the structure of double helical DNA is that DNA is made of two polynucleotide chains which has a backbone of sugar phosphate with nitrogen bases projected inside. The two chains exhibit anti-parallel polarity which means if the one chain has the polarity in 5' to 3' end direction then other has 3' to 5' end.