Class 12 Biology Chapter 6 Notes PDF Download
If you are looking for a concise and comprehensive summary of Class 12 Biology Chapter 6, then you have come to the right place. In this article, we will provide you with notes on Molecular Basis of Inheritance, which is one of the most important chapters in CBSE Class 12 Biology syllabus. You can download these notes as a PDF file for free from the link given at the end of this article.
class 12 biology chapter 6 notes pdf download
This chapter deals with the structure, function and regulation of DNA, which is the genetic material in most living organisms. It also explains how RNA acts as a messenger, adapter and catalyst in protein synthesis. Moreover, it discusses how gene expression is controlled in different cells and conditions. Finally, it introduces some applications of molecular biology in human genome project and DNA fingerprinting.
The main topics covered in this chapter are:
Structure of DNA
Replication of DNA
Transcription of DNA
Genetic code and translation
Regulation of gene expression
Human genome project and DNA fingerprinting
Let us now look at each of these topics in detail.
Structure of DNA
DNA stands for deoxyribonucleic acid, which is a long, linear molecule that stores and transmits genetic information in most living organisms. DNA is composed of smaller units called nucleotides, which have three components: a nitrogenous base, a pentose sugar (deoxyribose) and a phosphate group. There are four types of nitrogenous bases in DNA: adenine (A), thymine (T), guanine (G) and cytosine (C).
Nucleotides are linked together by phosphodiester bonds between the phosphate group of one nucleotide and the sugar group of the next nucleotide, forming a polynucleotide chain. The polynucleotide chain has a directionality, with one end having a free phosphate group (the 5' end) and the other end having a free sugar group (the 3' end).
In 1953, James Watson and Francis Crick proposed the double-helical model of DNA, based on the experimental data of Maurice Wilkins, Rosalind Franklin and Erwin Chargaff. According to this model, DNA consists of two polynucleotide chains that run antiparallel to each other, forming a right-handed helix. The two chains are held together by hydrogen bonds between the complementary bases on opposite strands. The base-pairing rules for DNA are: A pairs with T by two hydrogen bonds, and G pairs with C by three hydrogen bonds. The distance between two adjacent base pairs is 0.34 nm, and the helix makes one complete turn every 3.4 nm, containing 10 base pairs.
The double-helical structure of DNA explains its stability, specificity and replication. The stability is due to the stacking of bases and the formation of hydrogen bonds. The specificity is due to the complementary base pairing, which ensures that each strand can serve as a template for the synthesis of a new strand. The replication is due to the semi-conservative mode, which means that each daughter DNA molecule consists of one old strand and one new strand.
Replication of DNA
DNA replication is the process by which DNA makes copies of itself before cell division. It occurs during the S phase of the cell cycle and is semi-conservative, meaning that each daughter DNA molecule consists of one old strand and one new strand.
DNA replication begins at specific sites on the DNA molecule called origins of replication, where the two strands are separated by an enzyme called helicase. This creates a Y-shaped structure called a replication fork, where new nucleotides are added to the growing strands. To prevent the reannealing of the separated strands, proteins called single-strand binding proteins (SSBs) bind to them and keep them apart.
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The synthesis of new strands is catalyzed by an enzyme called DNA polymerase, which adds nucleotides to the 3' end of the existing strand, following the base-pairing rules. However, DNA polymerase can only work in one direction, from 5' to 3'. This creates a problem at the replication fork, where the two strands run in opposite directions. To solve this problem, one strand (the leading strand) is synthesized continuously towards the replication fork, while the other strand (the lagging strand) is synthesized discontinuously away from the replication fork, forming short segments called Okazaki fragments. The Okazaki fragments are later joined together by an enzyme called DNA ligase, forming a continuous strand.
DNA replication is regulated and proofread by various enzymes that ensure the accuracy and fidelity of the process. Some of these enzymes are: DNA primase, which synthesizes short RNA primers that initiate DNA synthesis; DNA topoisomerase, which relieves the supercoiling tension caused by helicase; DNA polymerase I, which removes the RNA primers and replaces them with DNA nucleotides; and DNA polymerase III, which has an exon uclease activity that can correct mismatched bases; and DNA mismatch repair system, which recognizes and repairs errors that escape the proofreading of DNA polymerase III.
Transcription of DNA
Transcription is the process by which the information in DNA is copied into RNA, which is a similar but slightly different molecule. RNA stands for ribonucleic acid, which is also composed of nucleotides, but with some differences from DNA. RNA has a ribose sugar instead of deoxyribose, and has a base called uracil (U) instead of thymine. RNA is usually single-stranded, but can form secondary structures by base pairing within itself.
Transcription is necessary because DNA is too large and stable to leave the nucleus and interact with the cytoplasmic machinery for protein synthesis. RNA acts as a messenger, adapter and catalyst in protein synthesis, carrying the genetic code from DNA to the ribosomes, where proteins are made.
Transcription occurs in three stages: initiation, elongation and termination. These stages are catalyzed by an enzyme called RNA polymerase, which synthesizes RNA from one of the strands of DNA (the template strand) in a 5' to 3' direction, following the base-pairing rules, except that U pairs with A instead of T.
In initiation, RNA polymerase binds to a specific sequence on the DNA called the promoter region, which signals the start of transcription. The promoter region usually contains a consensus sequence called the TATA box, which is recognized by a protein called the TATA-binding protein (TBP), which is part of a larger complex called the transcription factor IID (TFIID). The binding of TFIID to the TATA box attracts other transcription factors and RNA polymerase to form the transcription initiation complex, which opens up a small section of DNA and exposes the template strand for RNA synthesis.
In elongation, RNA polymerase moves along the template strand and adds nucleotides to the growing RNA strand, forming a structure called the transcription bubble, which contains about 12-14 base pairs of DNA-RNA hybrid. The transcription bubble moves along with RNA polymerase as it synthesizes RNA, leaving behind a region of unwound DNA called the transcription zone. The transcription zone is restored to its original state by the action of DNA topoisomerase, which relieves the supercoiling tension caused by transcription.
In termination, RNA polymerase stops synthesizing RNA when it reaches a specific sequence on the DNA called the terminator region, which signals the end of transcription. The terminator region can have different mechanisms for terminating transcription, depending on whether it is in prokaryotes or eukaryotes. In prokaryotes, there are two types of terminators: Rho-dependent and Rho-independent. Rho-dependent terminators require a protein factor called Rho, which binds to a sequence on the RNA called the Rho utilization site (rut), and moves along with it until it reaches RNA polymerase and causes it to dissociate from the DNA. Rho-independent terminators do not require Rho, but have a sequence on the RNA that forms a hairpin loop structure followed by a string of U residues, which destabilizes the DNA-RNA hybrid and causes it to separate. In eukaryotes, there are different types of terminators for different types of RNA: polyadenylation signal (AAUAAA) for mRNA, termination codon (UAA, UAG or UGA) for tRNA, and C-rich/G-rich repeats for rRNA.
In eukaryotes, RNA undergoes further processing after transcription, before it can be used for protein synthesis. This processing includes three main steps: splicing, capping and tailing. Splicing is the removal of non-coding sequences called introns from the RNA, and joining of coding sequences called exons. Splicing is carried out by a complex of proteins and small RNAs called the spliceosome, which recognizes the splice sites at the boundaries of introns and exons, and cuts and joins the RNA. Capping is the addition of a modified guanine nucleotide (7-methylguanosine) to the 5' end of the RNA, which protects it from degradation and facilitates its transport and recognition by the ribosome. Tailing is the addition of a poly-A tail (a string of about 200 adenine nucleotides) to the 3' end of the RNA, which also protects it from degradation and regulates its stability and translation.
Genetic code and translation
Genetic code is the set of rules by which the information in DNA or RNA is translated into proteins, which are the functional molecules in living cells. Proteins are composed of smaller units called amino acids, which are linked together by peptide bonds to form a polypeptide chain. There are 20 different types of amino acids in proteins, each with a unique chemical structure and properties.
The genetic code is based on the sequence of nucleotides in mRNA, which is derived from the sequence of nucleotides in DNA. The genetic code is read in groups of three nucleotides called codons, each of which specifies a particular amino acid or a signal for starting or stopping protein synthesis. There are 64 possible codons (4^3), but only 20 amino acids, so some amino acids are coded by more than one codon. This makes the genetic code degenerate, meaning that there is some redundancy or ambiguity in the code. However, the genetic code is also non-overlapping, meaning that each nucleotide belongs to only one codon, and there are no gaps or overlaps between codons. Moreover, the genetic code is universal, meaning that it is shared by almost all living organisms, with some minor exceptions.
The genetic code has some special codons that have specific functions in protein synthesis. These are: start codon (AUG), which initiates protein synthesis and codes for the amino acid methionine; and stop codons (UAA, UAG or UGA), which terminate protein synthesis and do not code for any amino acid.
Translation is the process by which the genetic code in mRNA is converted into a polypeptide chain by using the help of tRNA, rRNA and ribosomes. tRNA stands for transfer RNA, which is a small, cloverleaf-shaped molecule that carries a specific amino acid at one end and has an anticodon at the other end. The anticodon is a sequence of three nucleotides that is complementary to a codon on mRNA. rRNA stands for ribosomal RNA, which is a large, complex molecule that forms the core of the ribosome, where protein synthesis takes place. Ribosomes are composed of two subunits (large and small), each made up of rRNA and proteins. Ribosomes have three sites for binding tRNA: A site (aminoacyl site), where a new tRNA with an amino acid enters; P site (peptidyl site), where a tRNA with a growing polypeptide chain resides; and E site (exit site), where a tRNA without an amino acid leaves.
Translation occurs in three stages: initiation, elongation and termination. These stages involve the interaction of mRNA, tRNA, rRNA and ribosomes, as well as various protein factors and energy molecules.
In initiation, translation begins when a small ribosomal subunit binds to the 5' end of mRNA and scans for the start codon (AUG). When it finds it, it recruits a tRNA with methionine (tRNAMet) to pair with it at the P site. Then, a large ribosomal subunit joins the complex, forming an initiation complex. This requires the help of several initiation factors (IFs) and energy from GTP (guanosine triphosphate).
In elongation, translation continues by adding more amino acids to the growing polypeptide chain. This happens by repeating three steps: codon recognition, peptide bond formation and translocation. In codon recognition, a new tRNA with an appropriate anticodon enters the A site of the ribosome, matching the next codon on mRNA. This requires the help of an elongation factor (EF-Tu) and energy from GTP. In peptide bond formation, the amino acid on the new tRNA is transferred to the amino acid on the tRNA at the P site, forming a peptide bond and extending the polypeptide chain. This is catalyzed by an enzyme called peptidyl transferase, which is part of the large ribosomal subunit. In translocation, the ribosome moves one codon along the mRNA, shifting the tRNAs from the A site to the P site, and from the P site to the E site. The tRNA at the E site is then released from the ribosome. This requires the help of another elongation factor (EF-G) and energy from GTP.
In termination, translation ends when a stop codon (UAA, UAG or UGA) is encountered on mRNA. This codon does not have a corresponding tRNA, but is recognized by a protein factor called a release factor (RF), which binds to the A site of the ribosome and triggers the release of the polypeptide chain from the tRNA at the P site. The ribosome then dissociates into its subunits, releasing the mRNA and the tRNAs. This requires the help of another release factor (RF3) and energy from GTP.
Regulation of gene expression
Gene expression is the process by which genes are turned on or off to produce proteins in response to various signals and conditions. Gene expression is regulated at different levels in prokaryotes and eukaryotes, depending on their complexity and organization.
In prokaryotes, gene expression is mainly regulated at the transcriptional level, by controlling when and how much RNA is synthesized from a gene. One of the common mechanisms for transcriptional regulation in prokaryotes is the operon model, which was proposed by Jacob and Monod in 1961. An operon is a group of genes that are transcribed together as a single unit, under the control of a common promoter and operator. The operator is a sequence of DNA that can bind a regulatory protein called a repressor, which can block or inhibit RNA polymerase from transcribing the operon. The repressor can be activated or inactivated by a small molecule called an inducer or a corepressor, depending on whether it is an inducible or a repressible operon. For example, the lac operon in E. coli is an inducible operon that encodes enzymes for lactose metabolism. It is normally turned off by a repressor that binds to the operator, but when lactose is present, it acts as an inducer and binds to the repressor, changing its shape and preventing it from binding to the operator. This allows RNA polymerase to transcribe the lac operon and produce enzymes for lactose utilization.
In eukaryotes, gene expression is regulated at multiple levels: transcriptional, post-transcriptional, translational and post-translational. This is because eukaryotes have more complex genomes and cellular structures than prokaryotes, and need more precise and diverse control over gene expression.
At the transcriptional level, gene expression is regulated by modifying the chromatin structure and by using transcription factors. Chromatin is the complex of DNA and histone proteins that forms the eukaryotic genome. Chromatin can be modified by adding or removing chemical groups (such as acetyl, methyl or phosphate) to the histone tails, which affects the accessibility of DNA to RNA polymerase and other factors. Chromatin can be classified into two types: euchromatin, which is loosely packed and transcriptionally active; and heterochromatin, which is tightly packed and transcriptionally inactive. Transcription factors are proteins that bind to specific sequences on the DNA called enhancers or silencers, which can activate or repress the transcription of a gene, respectively. Transcription factors can also interact with other proteins called coactivators or corepressors, which can enhance or inhibit their effects on transcription.
At the post-transcriptional level, gene expression is regulated by modifying the RNA after it is synthesized from DNA. This includes splicing, capping and tailing, as discussed earlier, as well as editing, degradation and interference. Editing is the alteration of the nucleotide sequence of RNA by inserting, deleting or substituting bases, which can change the meaning of the genetic code. Degradation is the breakdown of RNA by enzymes called ribonucleases (RNases), which can control the amount and stability of RNA in the cell. Interference is the silencing of gene expression by small RNAs called microRNAs (miRNAs) or small interfering RNAs (siRNAs), which can bind to complementary sequences on mRNA and prevent its translation or induce its degradation.
At the translational level, gene expression is regulated by controlling when and how much protein is synthesized from RNA. This can be influenced by various factors, such as the availability and affinity of ribosomes, tRNAs and amino acids; the presence and location of start and stop codons; the secondary structure and stability of mRNA; the interaction of mRNA with proteins called translation factors, which can enhance or inhibit translation; and the feedback inhibition by the products of translation.
At the post-translational level, gene expression is regulated by modifying the protein after it is synthesized from RNA. This includes folding, cleavage, addition or removal of chemical groups (such as phosphate, methyl, acetyl or ubiquitin), interaction with other proteins or molecules, and degradation. These modifications can affect the structure, function, activity, localization and stability of proteins in the cell.
Human genome project and DNA fingerprinting
The human genome project was an international scientific endeavor that aimed to sequence and map the entire human genome, which consists of about 3 billion base pairs of DNA distributed among 23 pairs of chromosomes. The project was launched in 1990 and completed in 2003, with the participation of thousands of scientists from various countries and disciplines. The main objectives and achievements of the project were:
To identify all the genes in the human genome (about 20,000-25,000) and their functions.
To determine the variations in the human genome among different individuals and populations (such as single nucleotide polymorphisms or SNPs).
To compare the human genome with other genomes of animals, plants and microorganisms (such as chimpanzees, mice, rice and yeast).
To develop new technologies and tools for genomic analysis and manipulation (such as sequencing machines, bioinformatics software and gene therapy).
To address the ethical, legal and social implications of genomic research and applications (such as privacy, ownership, discrimination and education).
The human genome project has many applications and challenges in various fields of medicine, biotechnology and ethics. Some of these are:
To diagnose, treat and prevent genetic diseases (such as cystic fibrosis, sickle cell anemia and cancer).
To develop personalized medicine based on individual genetic profiles (such as pharmacogenomics).
To understand the molecular basis of complex traits and behaviors (such as intelligence, personality and mental disorders).
To trace human ancestry and evolution (such as mitochondrial DNA and Y-chromosome analysis).
To create genetically modified organisms (such as transgenic animals and plants).
To protect human rights and dignity (such as informed consent, confidentiality and respect).
DNA fingerprinting is a technique that uses DNA to identify individuals based on their unique genetic makeup. DNA fingerprinting is done by using restriction enzymes to cut DNA into fragments of different sizes; gel electrophoresis to separate the fragments by size; and hybridization probes to label and detect specific fragments that are variable among individuals. These fragments are called restriction fragment length polymorphisms (RFLPs), and they form a pattern of bands on the gel that is unique for each individual. This pattern is called a DNA fingerprint, and it can be used to compare and match samples from different sources.
DNA fingerprinting has many applications and limitations in various fields of forensics, paternity testing and genetic diseases. Some of these are:
To identify suspects or victims of crimes (such as murder, rape or kidnapping).
To establish biological relationships or inheritance (such as paternity, maternity or kinship).
To diagnose or screen for genetic disorders or mutations (such as Huntington's disease or cystic fibrosis).
To face the challenges of accuracy, reliability and validity (such as human error, contamination or fraud).
To respect the ethical, legal and social issues of privacy, consent and discrimination (such as confidentiality, ownership and rights).
Conclusion
In this article, we have provided you with notes on Class 12 Biology Chapter 6, which covers the molecular basis of inheritance. We have discussed the structure, function and regulation of DNA and RNA, as well as the processes of replication, transcription and translation. We have also explained the genetic code and its features, as well as the mechanisms of gene expression in prokaryotes and eukaryotes. Finally, we have introduced some applications of molecular biology in human genome project and DNA fingerprinting.
We hope that these notes will help you to revise and prepare for your CBSE Class 12 Biology exam. You can download these notes as a PDF file from the link given below. If you have any questions or feedback, please feel free to contact us.
Happy learning!
FAQs
What is the difference between DNA and RNA?
What are the three stages of transcription?
What are the special codons in the genetic code?
What is an operon and how does it regulate gene expression in prokaryotes?
What are the steps involved in DNA fingerprinting?
Answers
DNA and RNA are both nucleic acids that store and transmit genetic information, but they have some differences in their structure and function. DNA has a deoxyribose sugar, a thymine base, a double-stranded helix and a stable molecule that stays in the nucleus. RNA has a ribose sugar, a uracil base, a single-stranded molecule that can form secondary structures and a less stable molecule that can leave the nucleus.
The three stages of transcription are initiation, elongation and termination. In initiation, RNA polymerase binds to the promoter region on DNA and forms the transcription initiation complex. In elongation, RNA polymerase synthesizes RNA from one of the strands of DNA (the template strand) in a 5' to 3' direction. In termination, RNA polymerase stops synthesizing RNA when it reaches the terminator region on DNA and releases the RNA.
The special codons in the genetic code are the start codon (AUG) and the stop codons (UAA, UAG or UGA). The start codon initiates protein synthesis and codes for the amino acid methionine. The stop codons terminate protein synthesis and do not code for any amino acid.
An operon is a group of genes that are transcribed together as a single unit, under the control of a common promoter and operator. The operator is a sequence of DNA that can bind a regulatory protein called a repressor, which can block or inhibit RNA polymerase from transcribing the operon. The repressor can be activated or inactivated by a small molecule called an inducer or a corepressor, depending on whether it is an inducible or a repressible operon.
The steps involved in DNA fingerprinting are: extraction of DNA from a sample; digestion of DNA by restriction enzymes; separation of DNA fragments by gel electrophoresis; transfer of DNA fragments to a membrane by blotting; hybridization of DNA fragments with labeled probes; detection of hybridized fragments by autoradiography; and comparison and matching of DNA fingerprints.
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