Molecular Basis of Inheritance Class 12 Notes: CBSE Biology Chapter 5

The DNA 

DNA (deoxyribonucleic acid) is a double helix structure that was cracked by Watson and Crick based on the results of X-ray crystallography. Each strand of a DNA helix consists of repeating nucleotide units. The nucleotide consists of 3 components: ribose or deoxyribose sugar, nitrogen-based. which is either Purines or pyrimidines and phosphate.

Structure of Nucleotide 

• There are two types of purines which are known as adenine, and guanine. There are three types of pyrimidines: thymine, cytosine, and uracil. 

• All nucleotides are common in DNA and RNA, but uracil is found in RNA, and thymine is only found in DNA. 

• DNA is negatively charged due to the presence of negatively charged phosphate groups. A nitrogenous base binds to the pentose sugar via the glycosidic bond. 

• Two nucleotides join by a 3'5 'phosphodiester bond to form a dinucleotide. A polymer so formed has a free phosphate group at the 5 'end of the ribose sugar known as the 5' end of the polynucleotide chain. The other end of the polymer has a free 3'OH group of ribose sugar. 

• This is known as the 3 'end of the polynucleotide chain. The bond between sugars and phosphates forms the backbone of a polynucleotide chain. The nitrogenous bases are bound to the sugar content and protrude from the backbone.

The salient feature of the double helix structure of DNA is as follows- 

• Two polynucleotides chains that are present, wrap around each other. Here, the backbone is constituted by sugar-phosphate, and bases project inside. 

• The two DNA chains are always antiparallel to each other. Antiparallel means that if one chain has the polarity 5'-3', the other has 3'-5'. 

• The bases which are present in the two strands are paired through hydrogen bonding forming base pairs. Adenine form two hydrogen bonds with thymine whereas cytosine forms three hydrogen bonds with guanine. 

•  The two strands are coiled in the right-handed pattern.  

• The plane of one of the base pair stacks over the other in a double helix. This, in addition to H-bonds, confers the stability of the helical structure. 

Packaging of DNA helix 

Positively charged basic proteins that surround the DNA are known as histones. Histones are found to be rich in basic amino acids such as lysine and arginine. Histones molecules are organized in a specific manner to form a unit of eight molecules called histone octamer. The DNA is negatively charged in nature and is packaged by wrapping around the positively charged histone octamer. This forms a structure called a nucleosome. A nucleosome was found to be containing 200 base pairs of DNA helix. Nucleosomes form a specific repeating unit of a structure which is called chromatin in the nucleus. Chromatin is known to be a thread-like stained body seen in the nucleus. The nucleosomes in chromatin appear as a ‘beads-on-string’ structure when viewed under an electron microscope (EM). The beads on string structure in chromatin are packaged to form chromatin fibers that are further coiled and condensed at the metaphase stage of cell division to form chromosomes. At the high levels, chromatin packaging requires additional proteins. These proteins are the Non-Histone Chromosomal (NHC) proteins. In a typical nucleus, a loosely packed region of chromatin stains light and is referred to as euchromatin. The dense chromatin stains dark is called Heterochromatin. The euchromatin is said to be transcriptionally active chromatin, whereas heterochromatin is inactive. 

Packaging of DNA Helix 

DNA as a Genetic Material 

Griffith performed an experiment which is commonly known as the transforming experiment. He used the two different strains of Pneumococcus. These two different strains were used to infect the mice. The two strains which were used are type III-S (smooth), that contains outer capsule made up of polysaccharide and type II-R (rough) strain do not contain capsule. The capsule protects the bacteria from the host immune system. 

Griffith Experiment

The experiment of Griffith is explained below- 

• The rough strain of Pneumococcus is injected into the mouse. The mouse is alive. 

• The smooth strain of Pneumococcus is injected into the mouse. The mouse dies

• When the heat-killed smooth strain of Pneumococcus is injected into the mouse, the mouse is alive.

• In the last set of experiments, rough strain, and heat-killed smooth strain are injected into the mouse. The mouse dies. 

• This proves that there is some transforming substance present in the heat-killed S strain that is converting or transforming the rough strain into a virulent strain that is responsible for the death of the mouse. The substance that was transformed. later found to be DNA.  

The Genetic Material is DNA 

Alfred Hershey and Martha Chase in 1952 performed an experiment to prove that DNA is the genetic material. They worked on bacteriophages, which are viruses that infect the bacteria. It was found that when the bacteriophage attaches to the bacteria, its genetic material enters into the bacterial cell. The viral genetic material uses the bacterial cell to synthesize more viral particles. Hershey and Chase grew some viruses on a medium that contained radioactive phosphorus and another medium that contained radioactive sulfur. When phosphorus which was radioactive was present in the medium the viruses contained radioactive DNA but not radioactive protein. This is due to the fact that DNA contains phosphorus, but protein does not. Similarly,  when the growth medium contained radioactive sulfur the viruses contained radioactive protein but not radioactive DNA.  This is because DNA does not contain sulfur.  

Hershey and Chase Experiment

These bacteria were only found to be radioactive when infected with viruses that contained radioactive DNA. This indicates that DNA was the material that was transferred from the virus to the bacteria. Bacteria which were infected with viruses containing radioactive proteins were not radioactive. This indicates that the proteins did not get into the bacteria from the virus. So DNA is the genetic material that is transferred from viruses to bacteria. This experiment shows that DNA is the genetic material.

The Central Dogma of Molecular Biology 

It is an explanation of how genetic information transfer in a biological system. It tells us how the DNA replicates and then transcribes into messenger RNA (mRNA). This mRNA will now be translated to make proteins. 

The Central Dogma of Molecular Biology 

DNA Replication 

DNA replication is the process by which two identical copies of DNA are made from a single DNA molecule. As we know, DNA is a double helix in which two strands are complementary to each other. These two strands of a helix separate at replication time to form two new DNA molecules. Of the two DNA strands formed, one is identical to one of the strands and the other is complementary to the original strand. This form of replication is defined as semi-conservative replication. goes into mitosis, DNA replicates in the S phase of the interface DNA polymerase is the most important enzyme involved in DNA replication is an energy-dependent process During the replication process, the two DNA chains do not separate completely, replication occurs within the small opening in the DNA helix known as the replication fork.DNA polymerase catalyzes the reaction in 5 'to 3' so that in one strand (the template with 3'5 'polarity) the replication is continuous, while in the other (the template with 5'3' polarity) the enzyme DNA ligase then binds to the discontinuously synthesized fragments. The continuously synthesized strand is referred to as the main strand, while the discontinuously synthesized strand is referred to as the lag strand. Replication begins at a specific location in DNA known as the origin of replication.

DNA Replication

Transcription 

It is a process of making RNA, like messenger RNA, from DNA before gene expression or protein synthesis takes place. During transcription, one of the DNA strands acts as a template for mRNA formation. The synthesis of mRNA is carried out by the enzyme RNA polymerase. It generally occurs for a specific stretch of DNA that is most needed for gene expression. In addition to messenger RNA, other forms of RNA such as ribosomal RNA, microRNA, small nuclear RNA can be transcribed in a similar way. It consists of the following three regions: a promoter, a structural gene, and a terminator DNA-dependent RNA polymerase catalyzes polymerase in the 5'3 'direction. The promoter is the region to which the RNA polymerase binds. The terminator defines the end of the transcription.

Process of Transcription

• Transcription consists of a total of three steps- initiation, elongation, and termination.  

• Initiation is the step that involves the binding of RNA polymerase to the promoter. A single type of DNA-dependent RNA polymerase catalyzes the transcription of all types of RNA in bacteria. 

• Elongation is the process of adding nucleotides to form RNA. 

• The termination factor helps in the termination of the transcription. The RNA synthesized after transcription is called the primary transcript. The primary transcript undergoes modifications such as splicing, coating, tailing, etc.

• The primary transcript consists of the introns and exons. Introns are known as splicing. The addition of the polyA tail to the 3 'end of the RNA is referred to as the tail. When capping, an unusual nucleotide (methylguanosine triphosphate) is attached to the 5 'end of the RNA. 

• Some viruses have the property of reverse transcription. You can convert RNA templates into DNA. The enzyme used is known as reverse transcriptase. 

• For example, the human immunodeficiency virus that causes AIDS."

Translation 

This is the process of gene expression or protein synthesis, that occurs in the cytosol. Ribosomes are known to be the cellular organelles that participate in protein synthesis. Messenger RNA, made by the transcription process, is deciphered by ribosomes to form a composite polypeptide. Of amino acids. Messenger RNA is made up of a polymer of nucleotides, or codons. Each codon consists of 3 nucleotides that code for a single amino acid. There are several important components involved in the synthesis of ribosome proteins, messenger RNA, and transfer RNA (tRNA). Transfer RNA is involved in the physical binding of mRNA and the amino acid sequence of proteins.

Translation

It involves 4 main steps- 

• Activation of amino acids- amino acids bind to specific tRNA molecules. 

• Initiation of the polypeptide synthesis- In the process of capping an unusual nucleotide (methyl guanosine triphosphate) is added to  the 5'-end of RNA 

• Elongation of polypeptide synthesis- It involves the addition of amino acids to the growing polypeptide chains 

• Termination of polypeptide synthesis- It involves the end of the translation of protein synthesis. 

Genetic code 

The set of different rules according to which the information encoded in genetic material is translated into proteins in living cells. The outstanding features of the genetic code are: 

• The codon consists of 3 nucleotides. 61 codons code for 20 different types of amino acids. 

• 1 codon codes for a single amino acid. 

• 1 amino acid can be encoded by more than one codon.

Genetic Code

Regulation of Gene Expression 

All the genes in the living cells are not active all the time. They become active when needed. Expression is controlled by genes are known as regulatory genes. Regulation in the eukaryotes can occur at the following different levels- 

• Transcriptional level. 

• Processing level. 

• Transportation of mRNA from the nucleus to the cytoplasm. 

• Translational level. 

Lac Operon or Lactose Operon 

An operon consists of structural genes, operator genes, promoter genes, promoter genes, regulator genes, and repressors.  Lac operon consist of lac Z, lac Y and lac A genes. Lac Z codes for galactosidase, lac Y codes for permease, and lac A codes for transacetylase. When repressor molecules are bound to the operator, genes are not transcribed. When the repressor does not bind the operator and instead inducer binds, transcription is switched on. In the case of the lac operon, lactose is an inducer. So, binding lactose to the operator, switch on the transcription. 

Lactose Operon

Human Genome Project 

The most important and basic features of the Human Genome Project are: 

• The human genome contains 3,164.7 million nucleotide bases. 

• The average gene is 3000 bases, but the size varies. 

• Humans are said to have around 30,000 genes. 

• The functions of more than 50 percent of the discovered genes are unknown. 

• Less than 2 percent of the genome code for proteins. 

• The human genome consists to a large extent of repetitive sequences. 

• Chromosome 1 has the most genes (2968) and Y has the fewest (231).