Chapter 6: Molecular Basis Of Inheritance CBSE Notes for Class 12 Biology
DNA
DNA, or deoxyribonucleic acid, is a molecule that carries the genetic instructions used in the growth, development, functioning, and reproduction of all known living organisms and many viruses. It's often referred to as the "molecule of life."
Structured as a double helix, DNA consists of two long chains made up of nucleotides. Each nucleotide comprises three components:
1. **A phosphate group:** This provides the backbone of the DNA molecule.
2. **A sugar molecule called deoxyribose:** It links the phosphate groups together.
3. **A nitrogenous base:** There are four types of nitrogenous bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). These bases pair up in a specific manner—A pairs with T, and C pairs with G—forming the rungs of the DNA ladder.
The sequence of these bases along the DNA strands constitutes the genetic information. The unique arrangement of these bases within genes determines the specific traits and characteristics of an organism.
DNA is found in the nucleus of eukaryotic cells (such as those in plants and animals) and in the cytoplasm of prokaryotic cells (like bacteria). It holds the instructions for making proteins, which are the building blocks of cells and play vital roles in almost all biological processes.
Through processes like DNA replication, transcription, and translation, DNA governs the production of proteins and the regulation of cellular activities, ensuring the proper functioning and development of living organisms. Chapter 6: Molecular Basis Of Inheritance CBSE Notes for Class 12 Biology
A gene
A gene is a fundamental unit of heredity and is essentially a segment of DNA that contains the instructions for building one specific protein or a set of related proteins. Genes are responsible for encoding the traits and characteristics of an organism.
The structure of a gene includes specific sequences of nucleotides within the DNA molecule. These sequences determine the order in which amino acids are assembled during protein synthesis. Proteins, in turn, perform various functions in cells and contribute to the structure, function, and regulation of the body.
Genes don't work in isolation; they interact with each other and with environmental factors to determine an organism's traits and behaviors. Mutations, or changes in the DNA sequence, can occur within genes, leading to variations in traits. Some mutations can have noticeable effects on an organism's phenotype, while others may not cause any significant changes.
The study of genes and their functions is a central focus of genetics, helping us understand inheritance patterns, genetic disorders, evolutionary relationships, and the intricate mechanisms governing life processes.Chapter 6: Molecular Basis Of Inheritance CBSE Notes for Class 12 Biology
DNA packaging
DNA packaging is a remarkable process that condenses the long strands of DNA into a compact and organized structure within the cell. This packaging is necessary because the DNA molecules in a human cell, for example, are extremely long—stretching several feet when fully extended—but need to fit within the tiny confines of the cell's nucleus.
The primary levels of DNA packaging involve:
1. **Chromatin:** DNA in its most extended form exists as chromatin, which is a complex of DNA and proteins. The chromatin is further condensed into chromosomes during certain phases of the cell cycle, particularly during cell division (mitosis or meiosis).
2. **Nucleosomes:** The basic unit of chromatin, called a nucleosome, consists of DNA wound around a core of histone proteins. These nucleosomes resemble beads on a string, providing the first level of DNA compaction.
3. **Higher Order Structures:** Beyond nucleosomes, the chromatin undergoes additional levels of coiling and folding, resulting in further compaction. This compaction helps organize the DNA into a more condensed structure, allowing it to fit within the nucleus.
The packaging of DNA is not just about fitting it into a small space; it also plays a crucial role in gene regulation. The level of compaction affects the accessibility of specific regions of DNA to cellular machinery involved in processes like transcription (where the genetic information is copied into RNA) and replication (where DNA is duplicated). Regions of tightly packed DNA might be less accessible for these processes compared to more loosely packed areas.Chapter 6: Molecular Basis Of Inheritance CBSE Notes for Class 12 Biology
RNA world
The RNA world hypothesis is a fascinating theory that suggests RNA (ribonucleic acid) played a crucial role in the early evolution of life on Earth before DNA and proteins became predominant.
This hypothesis proposes that in the primordial soup of early Earth, RNA molecules might have served as both the genetic material and as catalysts for chemical reactions. RNA has the ability to store genetic information, similar to DNA, and also possesses catalytic properties, like proteins.
Several key points support the RNA world hypothesis:
1. **Genetic Role:** RNA can store genetic information and can undergo self-replication. This suggests it could have been the first molecule capable of both carrying genetic instructions and duplicating itself.
2. **Catalytic Properties:** Some RNA molecules, known as ribozymes, have been discovered to exhibit enzymatic activity. This indicates that RNA could have played a role in catalyzing chemical reactions necessary for life.
3. **Transitional Molecule:** RNA might have acted as a transitional molecule between the simpler chemical components present on early Earth and the more complex DNA and proteins found in modern cells.
While this hypothesis has gained significant traction and support within the scientific community, it's important to note that it's still a hypothesis and not yet fully proven. Research continues to explore and validate aspects of the RNA world hypothesis, aiming to understand better how life might have originated and evolved billions of years ago. If confirmed, it would shed light on the early stages of the development of life on our planet. Chapter 6: Molecular Basis Of Inheritance CBSE Notes for Class 12 Biology
DNA replication
DNA replication is the process by which a cell creates an identical copy of its DNA before cell division. This crucial process ensures that each daughter cell receives an accurate and complete set of genetic information.
The replication process involves several key steps:
1. **Initiation:** The process begins at specific sites on the DNA molecule called origins of replication. Enzymes called helicases unwind and separate the two strands of the DNA double helix, creating a replication fork.
2. **Elongation:** Enzymes known as DNA polymerases move along each separated DNA strand, adding complementary nucleotides according to the base-pairing rules (A pairs with T, and C pairs with G). One strand, called the leading strand, is synthesized continuously in the 5' to 3' direction, while the other strand, the lagging strand, is synthesized discontinuously in short fragments known as Okazaki fragments.
3. **Primers and Okazaki Fragments:** RNA primers are synthesized by another enzyme called primase. DNA polymerase then elongates these primers to form Okazaki fragments on the lagging strand.
4. **Joining Fragments:** DNA polymerase replaces the RNA primers with DNA nucleotides and fills in the gaps between Okazaki fragments on the lagging strand. DNA ligase seals the nicks between the fragments, creating a continuous strand of DNA.
5. **Termination:** The replication process continues bidirectionally along the DNA until the entire molecule is copied. The process terminates when the entire DNA molecule has been replicated.
DNA replication is a highly accurate process due to the proofreading capabilities of DNA polymerases, which help detect and correct errors in nucleotide incorporation. This fidelity is crucial for maintaining the integrity of genetic information from one generation of cells to the next.
Errors during DNA replication can lead to mutations, which, depending on their location and impact, can have various effects, ranging from no noticeable change to significant alterations in the functioning of cells and organisms.
The intricate packaging of DNA is dynamic and regulated, allowing cells to control which genes are active or inactive, thereby influencing various cellular processes and the overall function of the organism. Chapter 6: Molecular Basis Of Inheritance CBSE Notes for Class 12 Biology
Transcription
Transcription is the process by which the genetic information encoded in DNA is converted into RNA. This RNA molecule serves as a template for protein synthesis and other cellular functions. The primary steps involved in transcription include:
1. **Initiation:** Transcription begins when RNA polymerase, an enzyme, binds to a specific region of DNA called the promoter. The DNA strands unwind, forming a transcription bubble.
2. **Elongation:** The RNA polymerase moves along the DNA template, synthesizing a complementary RNA strand by adding nucleotides in a sequence determined by the DNA template. RNA nucleotides are matched to the DNA template according to base-pairing rules (A pairs with U in RNA, and C pairs with G).
3. **Termination:** The transcription process continues until it reaches a termination signal in the DNA sequence. At this point, the RNA polymerase releases the newly synthesized RNA molecule, and the DNA strands rejoin.
Transcription produces different types of RNA molecules, each with its specific role:
- **Messenger RNA (mRNA):** Carries the genetic information from DNA to the ribosomes, where it serves as a template for protein synthesis.
- **Transfer RNA (tRNA):** Transfers specific amino acids to the ribosome during protein synthesis, based on the codons on the mRNA.
- **Ribosomal RNA (rRNA):** Forms a part of the ribosome and plays a structural and functional role in protein synthesis.
Transcription is a highly regulated process, and different cells can transcribe different sets of genes based on their specific needs. The regulation of transcription involves complex interactions between regulatory proteins, DNA sequences, and other molecular signals that control when and how genes are transcribed.
Understanding transcription is crucial in comprehending how genetic information is expressed and regulated, influencing various cellular processes and functions in living organisms. Chapter 6: Molecular Basis Of Inheritance CBSE Notes for Class 12 Biology
Genetic Code
The genetic code is the set of rules by which information encoded in genetic material (usually DNA or RNA) is translated into proteins. It specifies how sequences of nucleotides in DNA or RNA are converted into sequences of amino acids in proteins.
The genetic code is based on the sequence of three nucleotides called codons. Each codon represents a specific amino acid or serves as a start or stop signal for protein synthesis. There are 64 possible codons formed by the combinations of four nucleotide bases (A, U, G, C) taken three at a time (4^3 = 64).
Out of the 64 codons, 61 code for amino acids, and the remaining three are stop codons that signal the termination of protein synthesis. The genetic code is degenerate or redundant, meaning that some amino acids are encoded by more than one codon. For instance, the amino acid leucine is specified by six different codons (e.g., UUA, UUG, CUU, CUC, CUA, CUG).
The start codon, AUG (which codes for the amino acid methionine), serves as the initiation signal for protein synthesis in most cases. It also sets the reading frame for the ribosome to start translating the mRNA into a polypeptide chain.
This universal genetic code is shared by nearly all organisms on Earth, from bacteria to plants to humans, suggesting a common ancestry and evolutionary relationship among living organisms. Understanding the genetic code is essential in molecular biology, genetics, and biotechnology, as it underlies the fundamental processes of gene expression and protein synthesis. Chapter 6: Molecular Basis Of Inheritance CBSE Notes for Class 12 Biology
Mutations and Genetic Code
Mutations can impact the genetic code by altering the sequence of nucleotides in DNA, which can lead to changes in the corresponding mRNA and, subsequently, the amino acid sequence in proteins.
There are several types of mutations:
1. **Substitution:** A single nucleotide is replaced by another nucleotide. Depending on where the substitution occurs within a gene, it can result in a different amino acid being incorporated into the protein (missense mutation) or no change in the amino acid sequence (silent mutation). However, if the substitution leads to a stop codon (nonsense mutation), it can prematurely terminate protein synthesis.
2. **Insertion and Deletion:** These mutations involve the insertion or deletion of nucleotides in the DNA sequence. They can cause a shift in the reading frame (frameshift mutation), altering the entire sequence of codons downstream from the mutation site. Frameshift mutations often have a significant impact on the resulting protein sequence, leading to non-functional or truncated proteins.
3. **Duplication and Inversion:** Duplication involves the repetition of a segment of DNA, while inversion reverses the orientation of a segment within the DNA. These structural changes can disrupt the normal coding sequence or regulatory regions, affecting gene expression or protein function.
Mutations can occur spontaneously due to errors in DNA replication or exposure to various external factors like radiation, chemicals, or environmental stressors.
The impact of mutations on the genetic code can range from subtle changes to severe alterations in protein structure and function. Some mutations are neutral or have little effect on an organism's phenotype, while others can lead to genetic disorders, diseases, or changes in an organism's traits.\
Understanding mutations and their effects on the genetic code is crucial in fields like genetics, medicine, and evolutionary biology. It helps scientists comprehend the causes of diseases, genetic variation within populations, and the mechanisms driving evolutionary changes in organisms over time. Chapter 6: Molecular Basis Of Inheritance CBSE Notes for Class 12 Biology
Translation
Regulation of Gene Expression
Regulation of gene expression refers to the complex mechanisms that control when and to what extent a gene is activated or repressed, influencing the production of specific proteins or RNA molecules within a cell.
Several levels of regulation exist, including:
1. **Transcriptional Regulation:** This is the control of gene expression at the level of transcription. Transcription factors, proteins that bind to specific DNA sequences, can either enhance (activators) or inhibit (repressors) the transcription of target genes. Promoters, enhancers, and silencers are regulatory elements that influence the binding of transcription factors to DNA.
2. **Post-transcriptional Regulation:** After transcription, mRNA molecules can be modified, processed, or degraded, affecting their stability and translation efficiency. Processes like alternative splicing, where different exons are joined together, can generate multiple mRNA transcripts from a single gene, leading to different protein products.
3. **Translational Regulation:** This involves controlling the initiation, elongation, or termination of translation. Regulatory factors, such as specific RNA sequences or binding proteins, can affect the accessibility of the mRNA to ribosomes and the efficiency of protein synthesis.
4. **Post-translational Regulation:** After a protein is synthesized, it can undergo modifications such as phosphorylation, glycosylation, or cleavage. These modifications can affect the protein's stability, activity, localization, and interactions with other molecules.
Gene expression regulation is influenced by various internal and external factors, including developmental cues, environmental signals, cell type, and metabolic conditions. It allows cells to respond and adapt to changing conditions by fine-tuning the production of specific proteins needed for different functions.
The dysregulation of gene expression can lead to various diseases, as seen in conditions like cancer, where genes that control cell growth and division are inappropriately activated or suppressed.
Studying gene expression regulation is crucial for understanding development, cellular processes, diseases, and potential therapeutic interventions aimed at modulating gene expression for beneficial outcomes. Chapter 6: Molecular Basis Of Inheritance CBSE Notes for Class 12 Biology
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