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MOLECULAR BASIS OF INHERITANCE | Biology | Class 12 | CBSE | Pure English

PW English Medium・113 minutes read

The chapter on Molecular Inheritance delves into the search for genetic material, highlighting key experiments like Griffith's and Hershey-Chase's to establish DNA as the genetic material through its structure, replication, transcription, translation, and role in genetic inheritance. The Human Genome Project aimed to sequence all human genes, addressing ethical concerns and using key techniques like ESTs and sequence annotation, revolutionizing our understanding of human genetics through DNA isolation, sequencing, and fingerprinting.

Insights

The search for genetic material began in 1926, with Griffith's experiment in 1928 being a significant milestone, leading to the identification of DNA as the genetic material through experiments by McLeod, McCarty, and Avery in 1944.
Hershey and Chase's 1952 experiment solidified DNA as the genetic material, distinguishing it from proteins, using bacteriophages and E. coli, proving DNA's role in genetic inheritance.
DNA was initially termed as nuclein by Frederick Miescher, being a negatively charged polymer of deoxyribonucleotides, comprising purines (adenine, guanine) and pyrimidines (cytosine, thymine), forming the nucleotides of DNA.
Watson and Crick's model highlighted DNA's double helical structure with complementary base pairing, showing DNA's stability due to hydrogen bonds and stacking of base pairs, leading to the understanding of DNA's role in genetic inheritance.
The Human Genome Project, initiated in 1990, aimed to identify all human genes, sequence 3 billion base pairs, and address ethical, legal, and social issues, utilizing ESTs and sequence annotation to reveal genetic details and similarities among individuals, culminating in the completion of the project.

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Recent questions

What is the transforming principle in Griffith's experiment?
DNA
Who solidified DNA as the genetic material in 1952?
Hershey and Chase
What is the structure of a nucleotide in DNA?
Deoxyribose sugar, nitrogenous base, phosphate group
What is the Central Dogma theory?
DNA transcribes to RNA, which translates to proteins
What is the lac operon's function in E. coli?
Regulates lactose metabolism

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Summary

00:00

Discovery of DNA as Genetic Material

The chapter being revised is Molecular Inheritance, focusing on genetic material.
The goals of the chapter include exploring genetic material, DNA, RNA, their structures, central dogma, DNA packaging, replication, transcription, translation, genetic code, gene expression regulation, lac operon, human genome project, and DNA fingerprinting.
The search for genetic material began in 1926, with Griffith's experiment in 1928 being a significant milestone.
Griffith's experiment involved two strains of bacteria, R and S, with the S strain being pathogenic due to its capsule.
Griffith's findings suggested a transforming principle transferred from S to R strains, later identified as DNA through experiments by McLeod, McCarty, and Avery in 1944.
McLeod, McCarty, and Avery's experiment involved isolating biochemicals from the S strain and testing their transforming abilities on R strains, leading to the identification of DNA as the genetic material.
The acceptance of DNA as genetic material was solidified by Hershey and Chase's 1952 experiment using E. coli and bacteriophages, where DNA was unequivocally proven as the genetic material.
Radioactive labeling of protein coat and DNA in bacteriophages helped distinguish their roles in infecting E. coli during the experiment.
The experiment involved infecting E. coli with bacteriophages containing radioactively labeled protein coat and DNA, followed by blending, centrifugation, and separation of precipitate and supernatant.
The results confirmed that DNA, not protein, was the genetic material, establishing a clear understanding of DNA's role in genetic inheritance.

22:23

"DNA as Genetic Material: Experimental Evidence"

After centrifugation, the precipitate contains the heavier components, including E. coli cells and the injected virus part, while the supernatant holds the virus part not injected into the E. coli.
The conclusion of the experiment involves two Eppendorf tubes, one with S35 in the protein code of the bacteriophage and the other with P32 in its DNA.
Fluorescence in the supernatant indicates the presence of the radioactively labeled virus part, showing that the protein code is not injected into the E. coli.
In the second experiment, fluorescence in the precipitate reveals the presence of P32, indicating the DNA of the virus, proving DNA as the genetic material.
The search for the genetic material concludes with DNA being established as the genetic material.
DNA was initially termed as nuclein by Frederick Miescher, being an acidic, negatively charged polymer of deoxyribonucleotides.
Deoxyribonucleotides include dATP, dCTP, dGTP, and dTTP, forming the nucleotides of DNA.
A nucleotide comprises a deoxyribose sugar, a nitrogenous base, and a phosphate group, with purines (adenine, guanine) and pyrimidines (cytosine, thymine) as nitrogenous bases.
The structure of a nucleotide involves n-glycosidic and phosphoester bonds within a single nucleotide, while phosphodiester bonds link two nucleotides.
The calculation of phosphodiester bonds in DNA varies based on the type of DNA (double-stranded linear, single-stranded linear, double-stranded circular, single-stranded circular), with specific formulas provided for each case.

45:22

"DNA Structure and Central Dogma Theory"

Wilkins and Franklin used x-ray crystallographic studies to suggest DNA's double helical structure.
Chargaff's base composition studies revealed adenine always equals thymine and cytosine always equals guanine.
The ratio of A:T and C:G in DNA is always 1, while the total purines equal the total pyrimidines.
Prokaryotes have GC-rich DNA, while eukaryotes have AT-rich DNA due to different A+T/C+G ratios.
Watson and Crick's model highlights complementary base pairing, with A pairing with T and C pairing with G.
The stability of DNA's helical structure is due to hydrogen bonds and stacking of base pairs.
DNA consists of two anti-parallel polynucleotide chains with sugar-phosphate backbones and nitrogenous bases.
The helical structure of DNA has a pitch length of 3.4 nanometers and a helical diameter of 2 nanometers.
The Central Dogma theory states DNA transcribes to RNA, which translates to proteins, influencing phenotype.
Reverse Central Dogma is observed in retroviruses like HIV, where RNA can be transcribed into DNA.

01:09:40

From Nucleosomes to Chromosomes: DNA Packaging Journey

A nucleosome contains 200 base pairs of DNA and covers 200 base pairs of DNA.
A diagram of a nucleosome shows an octamer of histones with DNA wrapped around it, including core DNA, linker DNA, and histone H1 acting as a clamp.
The DNA in a nucleosome takes 1.75 turns on the histone octamer, giving it a beads-on-string appearance.
DNA condenses to form nucleosomes, which further condense into chromatin fibers and ultimately chromosomes, seen in dividing cells at metaphase.
Non-histone chromosomal proteins, acidic in nature, are used in higher levels of packaging.
During packaging, some DNA parts are densely condensed as heterochromatin, while others are loosely packed as euchromatin, affecting gene expression.
DNA and RNA differ in structure, with DNA being double-stranded, containing deoxyribose sugar and thymine, while RNA is single-stranded, with ribose sugar and uracil.
RNA, initially the first genetic material, evolved into DNA for genetic storage, while RNA now acts as a catalyst and has a structural role.
DNA replication is semi-conservative, with one parental and one newly formed strand, proven by the Meselson-Stahl experiment using E. coli and cesium chloride centrifugation.
Replication in prokaryotes occurs in the cytoplasm, while in eukaryotes, it happens in the nucleus, involving enzymes like topoisomerase, helicase, and DNA polymerase, starting from the origin of replication (ori).

01:35:23

DNA Replication and Transcription Process Overview

Replication fork forms when only half of the DNA is opened up due to insufficient energy for helicase to open it all at once.
Proteins like helix destabilizing protein and single-stranded binding proteins prevent complementary base pairing and maintain the DNA strands single-stranded.
Primase enzyme, with RNA polymerase activity, forms RNA primers acting as foundations for DNA replication.
RNA primers are essential as DNA polymerase lacks initiating capacity, allowing only RNA polymerase to start the process.
DNA polymerase attaches nucleotides and forms phosphodiester bonds, utilizing charged nucleotides that serve as substrates and energy sources.
DNA polymerase synthesizes new DNA strands only in the 5' to 3' direction, creating a new strand continuously on the leading strand and discontinuously in fragments on the lagging strand.
Lagging strand's Okazaki fragments are later joined by DNA ligase, ensuring the completion of the new DNA strands.
After replication, DNA repairing involves removing RNA primers, proofreading, and attaching broken segments with DNA ligase, ultimately yielding two DNA molecules.
Transcription involves forming RNA from DNA, with different types of RNA (mRNA, rRNA, tRNA) being transcribed from specific gene segments like cistrons.
A transcription unit comprises a promoter region, structural gene, and terminator on the DNA segment responsible for mRNA formation and subsequent protein synthesis.

02:01:08

DNA Transcription: Process and Key Components

Transcription is distinct from replication as only one DNA strand acts as a template for RNA formation.
A cistron or transcription unit comprises a promoter, structural gene, and terminator.
The coding strand serves as a frame of reference for the promoter and terminator positions.
Promoter functions as the RNA polymerase binding site, while the terminator halts the process.
RNA polymerase, specifically DNA-dependent RNA polymerase, is crucial for transcription.
Prokaryotes have one RNA polymerase, while eukaryotes possess three types for various RNA formations.
Transcription initiates with RNA polymerase binding to the promoter, followed by RNA formation on the template strand.
Sigma factor aids in recognizing the promoter, while the rho factor terminates the process.
Post-transcriptional modifications in eukaryotes involve capping, tailing, and splicing of hnRNA.
Genetic code, akin to a language, involves codons (triplets of nucleotides) coding for amino acids, deciphered by Nirenberg and extended by Khurana.

02:25:21

Genetic Code: Universal, Degenerate, Unambiguous, Translation Process

The genetic code consists of 64 codons, with 61 coding for amino acids and 3 serving as stop codons.
The genetic code is universal, meaning one codon codes for one amino acid across all organisms.
The code is degenerate, with some amino acids being coded by multiple codons due to synonyms.
It is unambiguous, with each codon specifically coding for one amino acid.
The start codon, AUG, serves a dual purpose of initiation and coding for methionine.
Stop codons are UAA, UAG, and UGA, signaling the end of translation.
Translation, the formation of proteins on mRNA, occurs in the cytoplasm in both prokaryotic and eukaryotic cells.
Three types of RNA are involved in translation: mRNA, rRNA, and tRNA.
Ribosomes, acting as protein factories, are responsible for protein synthesis.
Translation involves initiation, elongation, and termination stages, with ribosomes translocating on mRNA and forming peptide bonds between amino acids.

02:49:02

Gene Expression Regulation in Prokaryotes and Eukaryotes

In prokaryotes, replication, transcription, and translation occur in the cytoplasm, simplifying gene expression regulation.
Transcription initiation is crucial in prokaryotes; once initiated, the process continues without hindrance.
Transcription and translation are coupled in bacteria, with mRNA formation leading directly to translation.
Eukaryotic gene expression regulation involves four hurdles: transcription, processing of hnRNA, mRNA transport to the cytoplasm, and successful translation.
The operon concept, introduced by Jacob and Monod, is found in prokaryotes and consists of a cluster of genes responsible for specific metabolic functions.
The lac operon in E. coli is switched off in the absence of lactose due to repressor protein binding to the operator site, inhibiting transcription.
In the presence of lactose, the lac operon is switched on as lactose binds to the repressor protein, preventing its binding to the operator site and allowing transcription of structural genes.
The Human Genome Project, a 13-year project starting in 1990, aimed to identify all human genes, sequence the 3 billion base pairs, and address ethical, legal, and social issues.
Two major approaches in the Human Genome Project were ESTs, focusing on coding parts, and sequence annotation, which involved sequencing the entire DNA.
DNA isolation, fragmentation, amplification, and sequencing using automated DNA sequencers were key steps in the Human Genome Project, revealing details like the number of base pairs, genes, and the similarity among individuals.

03:10:01

"DNA Fingerprinting: Uncovering Unique Genetic Patterns"

3 x 10^9 is a large number, leading to the use of density-based centrifugation with cesium chloride on DNA.
Centrifugation resulted in bulk genomic DNA and satellite DNA, a repetitive portion within the 3 x 10^6 base pairs.
Satellite DNA, a repetitive sequence in the human genome, can be mini or microsatellite based on the length of repeating units.
Mini satellites like VNTRs and microsatellites like SSRs and STRs are examples of repetitive DNA.
VNTRs form the basis of DNA fingerprinting, with varying repeat numbers in individuals except for monozygotic twins.
DNA fingerprinting involves DNA isolation, treatment with restriction endonucleases, and separation of fragments via gel electrophoresis.
Denaturation, Southern blotting, hybridization with a radioactively labeled DNA probe, and autoradiography complete the DNA fingerprinting process, revealing unique patterns for each individual.

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