Class 12th Biology Chapter 5 Molecular Basis Of Inheritance

NCERT Solutions for Class 12 Biology Chapter 5 : This chapter is about Molecular Basis Of Inheritance. We are going to cover the topics like DNA, DNA as genetic material , Replication, transcription, central Dogma, experiment of Griffith,etc. We have prepared this article very carefully and tried to include all those important topics and notes from the chapter which you can use in your preparation for 12th exam or any other upcoming competitive exams.

 

In this chapter we have covered all the topics related to Reproductive Health which are listed below:-

DNA 

Packaging of DNA

DNA as genetic material 

Experiment of Griffith 

Central Dogma of molecular biology 

Genetic material is DNA 

Replication 

Transcription 

 

DNA 
The Full form of DNA is Deoxyribonucleic acid . It is a double helical structure.The structure was discovered by James Watson and Francis Crick based on X ray diffraction data.The strand of a DNA helix is composed of nucleotides of repeating units.Ribose (deoxyribose sugar), nitrogenous base (purines or pyrimidines) and phosphate are the three components of the nucleotides.

The nucleotides components are divided into further types which are:-
1.Purines – Adenine and Guanine
2.Pyrimidine – Thymine, cytosine and uracil 

In DNA and RNA the nucleotides are common whereas Thymine is present in DNA and uracil is present in RNA . Because of the presence of negatively charged phosphate groups the DNA is negatively Through the N-glycosodic linkage the nitrogenous base is linked to the pentoseSugar.To form dinucleotide two nucleotides are linked through 3’-5’ phosphodiester linkage. The 5’-end of polynucleotide chain is referred by a polymer which is formed has a free 5’-end group of ribose sugar at phosphate group. While the 3’-end of polynucleotide chain is referred by polymer which has free 3’-OH  group of ribose sugar. The backbone of a polynucleotide chain is formed by the bonding of sugar and phosphate.

Salient features of DNA helix structure which are as follows:-
1.The DNA helix structure is made up of two polynucleotide and there is a backbone which is constituted by sugar phosphate and bases project inside.
2.The chains of DNA helix structure are two which are anti parallel to each other and if the chain has a polarity of 5’-3’  of one chain then the other has 3’-5’.
3. Three hydrogen bond are formed by cytosine with Guanine , two hydrogen bonds  are formed by Adenine with Thymine and the base pairs is formed through pairing of of hydrogen bonding with the bases  in the two strands.
4. In right hand fashion the two strands are coiled.
5. In addition to H-bonds, in double helix the plane of one base pair stacks over the other which confers stability of helical structure.

The packing of  DNA helix

The DNA which is surrounded by the positively charged basic proteins is called histones. Also the histones are rich in basic amino acids like lysine and arginine. The histone octamer is formed by organising the histones by forming a unit of 8 molecules and this forms a structure which is called as nucleosome. 200 base pairs of DNA helix is contained in nucleosome. The chromatin in nucleus is  formed by the nucleosome which form repeating unit of a structure. In nucleus there is a thread like stained bodies which are chromatin. There is a structure which appears under the EM electron microscope named ‘beads-on-string’ on nucleosome in chromatin. In chromatin the beads on string structure is packed which forms chromatin fibres and coiled further . To form chromosomes they are condensed at metaphase stage of cell division. The non-Histone Chromosomal (NHC) protein are the additional proteins which requires at higher level of chromatin packaging. The euchromatin is defined as the loosely packed region of chromatin which stains light in a typical nucleus, whereas the Heterochromatin is densely chromatin which stains dark. The transcriptionally active is said for Euchromatin and the Heterochromatin is inactive.

DNA as genetic material 
The transforming experiment is a experiment which was performed by Griffith. To infect the mice two different strains of Pneumococcus were used. The outer capsule of type III-S (smooth) strain is made up of polysaccharide and the other type II-R (rough) strain do not contain capsule.

Experiment of Griffith
. When S strain is injected to mouse of Pneumococcus the mouse died.
. When R strain is injected to mouse of Pneumococcus the mouse is alive.
. When the Heat killed S strain is injected to mouse of Pneumococcus the mouse is alive.
. When the Heat killed S strain and R strain is injected to mouse the mouse dies.

This experiment proves that in heat killed S strain there is some transforming substances which converts the rough strain to virulent strain which is responsible for mouse death. Later on, this transforming substance found to be DNA .

Genetic material is DNA 
To prove DNA is a genetic material Alfred Hershey and Martha Chase in 1952 performed an experiment. On bacteriophage they worked which are virus that infects  bacteria. In bacterial cell the genetic material enters when the bacteriophage is attached to the bacteria. There was  some virus which grew by Harshey and chase on a medium which contains radioactive phosphorus and the other medium contains radioactive sulfur.The viruses contained radioactive DNA not the radioactive protein when the radioactive phosphorus was present in the medium it’s because of the DNA contains phosphorus not the protein. Likewise, the viruses contained radioactive protein not the radioactive DNA when growth medium contained radioactive sulfur it’s because sulfur is not contained by DNA.
When they are infected with the viruses having radioactive DNA the bacteria were found to be radioactive only. When passed from the virus to bacteria it indicates that the DNA was the material. The bacteria which was infected by viruses which contains radioactive protein were not radioactive which indicates protein does not enter the bacteria from viruses and DNA is the genetic material which is passed to bacteria from viruses. This experiment shows that DNA is a genetic material.

The Central Dogma of molecular biology 
This is a process which explains about the biological system in which the flow of genetic information takes place. In this process the DNA is replicated and after that it gets converted to messenger RNA (mRNA)  via transcription then this RNA is translated to which forms proteins.

Replication 
It is a process in which two same copies of DNA is produced from a single DNA molecule. Two strands are complementary to each other as DNA is a double helix . At the time of replication the two strands of helix gets separated to form two new DNA molecules. The DNA formed by the two strands one is identical to one of the strands and the other one is complementary to parent strand. This scheme is known as semi conservative replication. DNA is replicated in S phase of interphase before the cell enters the mitosis. In DNA replication the most important enzyme which is involved is the DNA polymerase. The two strands do not gets separated completely during the process of DNA replication, within the small opening of DNA helix the replication occurs which is called as replication fork. The energy dependent process is DNA replication. The DNA polymerase catalyse in 5’-3’ only in  one direction consequently on other strand template with polarity 3’-5’ , the replication is continuous whereas on other with template with polarity 5’-3’ it is discontinuous. The DNA ligase which is a enzyme join the discontinuously synthesized fragments later on. The lagging strand is one  which synthesized discontinuously and the leading strand is one which synthesized continuously. The origin of replication is the specific site on the DNA where the replication begins.

 

Transcription 
Before protein synthesis or gene expression occurs it is a process of formation of RNA like messenger RNA from DNA . The one strand of DNA acts as template from mRNA during transcription. Via RNA polymerase enzyme the synthesis of mRNA occurs. For DNA segment the transcription occurs which is required for gene expression further. Micro RNA , small nuclear RNA ribosomal of other forms of RNA can be transcribed in similar manner other than the messenger RNA. 

There are three regions of transcription unit in DNA which are as follows:-
1. A promoter 
2.  The structural gene
3.  A Terminator

The DNA dependent RNA polymerase catalyse the polimerization in 5’-3’ only in one direction and the strand which has the polarity 3’-5’ is termed as template strand whereas which has the polarity 5’-3’ and sequence is same as RNA at the place of uracil except Thymine is displayed during transcription and is referred as coding strand which does not code for anything. The region where RNA polymerase binds is known as promoter. Towards 3’ -end of the coding strand the Terminator is located which defines the end of transcription.
Initiation, elongation and termination are the three steps of transcription.
Binding of RNA polymerase to promoter is involved in initiation.
The process by which addition of nucleotides to form RNA is called elongation.
After transcription the RNA synthesize and is known primary transcription. Splicing, capping and tailing are the modification where the primary transcription undergoes. There are two types of primary transcription are introns and exons.
Splicing – removal of introns is called splicing.
Capping -  to 5’ -end of RNA and unusual nucleotide (methyl guanosine triphosphate) is added this process is called capping.
Tailing – at the 3’-end of RNA the addition of poly-A tail is called tailing.

Reverse transcription 
Viruses which are able to convert RNA template to DNA have a property of reverse transcription and the enzyme used is called as reverse transcription.
Example of reverse transcription is HIV ( Human Immunodeficiency Virus).

 

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