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Biology Class 12 CBSE

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Chapter 5 – Molecular Basis of Inheritance

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DNA: The Genetic Material

  • DNA (Deoxyribonucleic acid) and RNA (Ribonucleic acid) are the two types of nucleic acids found in living systems.
  • DNA acts as the genetic material in most organisms, while RNA functions as a messenger and genetic material in some viruses (like TMV).
  • A nucleotide consists of three components: a nitrogenous base, a pentose sugar, and a phosphate group.
  • Nitrogenous bases are Purines (Adenine and Guanine) and Pyrimidines (Cytosine, Thymine, and Uracil).
  • In DNA, Thymine is present, whereas in RNA, Uracil is found in place of Thymine.
  • A nitrogenous base is linked to the OH of 1′ C pentose sugar through an N-glycosidic linkage to form a nucleoside.
  • Two nucleotides are linked through 3′-5′ phosphodiester linkage to form a dinucleotide.
  • Erwin Chargaff stated that for double-stranded DNA, the ratios between Adenine and Thymine and Guanine and Cytosine are constant and equal to one.
  • James Watson and Francis Crick proposed the Double Helix model of DNA in 1953.
  • The two chains of DNA have anti-parallel polarity (one runs 5′ to 3′ and the other 3′ to 5′).
  • The bases in two strands are paired through hydrogen bonds (A=T with two bonds, G-C with three bonds).
  • The pitch of the helix is 3.4 nm and there are roughly 10 base pairs (bp) in each turn.

Packaging of DNA

  • In eukaryotes, DNA packaging is carried out by positively charged, basic proteins called histones.
  • Histones are rich in basic amino acid residues Lysine and Arginine.
  • A nucleosome consists of negative DNA wrapped around a positive histone octamer.
  • Euchromatin is loosely packed and transcriptionally active; Heterochromatin is densely packed and inactive.
  • Frederick Griffith (1928) conducted the Transforming Principle experiment using Streptococcus pneumoniae.
  • Avery, MacLeod, and McCarty proved that DNA is the actual transforming substance.
  • Hershey and Chase (1952) gave unequivocal proof that DNA is the genetic material using bacteriophages and radioactive isotopes (32P and 35S).
  • The RNA World hypothesis suggests that RNA was the first genetic material.

Replication and Transcription

  • DNA replication is semi-conservative, as proved by Meselson and Stahl (1958) using heavy nitrogen (15N) in E. coli.
  • DNA polymerase catalyzes polymerization only in the 5′ to 3′ direction.
  • Replication is continuous on the leading strand and discontinuous on the lagging strand.
  • The discontinuously synthesized fragments are called Okazaki fragments, joined by DNA ligase.
  • Transcription is the process of copying genetic information from one strand of DNA into RNA.
  • A Transcription Unit consists of a Promoter, Structural gene, and a Terminator.
  • The DNA strand with 3′ to 5′ polarity acts as the template strand.
  • The strand with 5′ to 3′ polarity is called the coding strand.
  • In eukaryotes, primary transcripts (hnRNA) undergo splicing (removal of introns), capping, and tailing.

Genetic Code and Translation

  • The genetic code is a triplet code; 61 codons code for amino acids and 3 are stop codons (UAA, UAG, UGA).
  • AUG has a dual function: it codes for Methionine and acts as the initiator codon.
  • The code is universal, non-overlapping, and degenerate (one amino acid can be coded by more than one codon).
  • tRNA is called the adapter molecule; it has an anticodon loop and an amino acid acceptor end.
  • Translation is the process of polymerization of amino acids to form a polypeptide.
  • The first phase of translation is aminoacylation (charging of tRNA).
  • Ribosomes consist of two subunits; the larger subunit has two sites for subsequent amino acids to bind.

Regulation and Genomics

  • The Lac Operon was elucidated by Jacob and Monod.
  • It consists of a regulatory gene (i) and three structural genes (z, y, a).
  • The z gene codes for beta-galactosidase; y for permease; a for transacetylase.
  • Lactose acts as the inducer in the Lac Operon.
  • In the absence of an inducer, the repressor protein binds to the operator and prevents transcription.
  • Human Genome Project (HGP) was a 13-year project coordinated by the U.S. Department of Energy and NIH.
  • The human genome contains approximately 3 x 10 to the power of 9 base pairs.
  • The average gene consists of 3000 bases, but sizes vary (Dystrophin is the largest).
  • Less than 2% of the genome codes for proteins.
  • DNA Fingerprinting involves identifying differences in specific regions called Repetitive DNA.
  • VNTRs (Variable Number of Tandem Repeats) are used as probes in DNA fingerprinting.
  • Alec Jeffreys developed the technique of DNA fingerprinting.
  • Polymorphism in DNA sequence is the basis of genetic mapping and fingerprinting.
  • The technique involves isolation, digestion, electrophoresis, blotting, hybridization, and autoradiography.
  • Questions count: 10
  • Passing grade: 4
  • Time limit: 30 minutes
  • Number of allowed attempts: 5
Start Quiz
1.

Describe the structure of a polynucleotide chain with reference to its backbone and linkages.

A nucleotide has three components: a nitrogenous base, a pentose sugar, and a phosphate group.
Nitrogenous bases are linked to the pentose sugar through an N-glycosidic linkage to form a nucleoside.
A phosphate group is linked to the 5' OH of a nucleoside through a phosphoester linkage.
Two nucleotides are linked through a 3'-5' phosphodiester linkage to form a dinucleotide.
In RNA, every nucleotide residue has an additional OH group present at the 2' position in the ribose.
The nitrogenous bases are linked to each other by phosphodiester bonds to form the backbone of the chain.
2.

Outline the silent features of the Double Helix model of DNA as proposed by Watson and Crick.  

DNA is made of two polynucleotide chains where the backbone is constituted by sugar-phosphate.
The two chains have anti-parallel polarity, meaning one runs 5' to 3' and the other 3' to 5'.
The two chains are coiled in a left-handed fashion with a pitch of 3.4 nm.
Adenine forms two hydrogen bonds with Thymine from the opposite strand and vice-versa.
Guanine is bonded with Cytosine with three hydrogen bonds.
The plane of one base pair stacks over the other in the double helix, providing stability.
3.

Explain how DNA is packaged in the nucleus of a eukaryotic cell.  

In eukaryotes, the negatively charged DNA is wrapped around a positively charged histone octamer.
Histones are rich in the basic amino acid residues Lysine and Arginine.
A typical nucleosome contains 200 base pairs of DNA helix.
Euchromatin is the densely packed and dark-staining part of the chromatin.
Heterochromatin is transcriptionally inactive, while euchromatin is transcriptionally active.
Chromatin is further packaged to form chromosomes during the metaphase of cell division.
4.

Describe the Hershey-Chase experiment that provided unequivocal proof that DNA is the genetic material.  

They worked with bacteriophages that infect the bacterium E. coli.
Radioactive Phosphorus was used to label the DNA, while radioactive Sulfur labeled the protein coat.
The radioactive phages were allowed to attach to E. coli bacteria to initiate infection.
As the infection proceeded, the viral coats were removed from the bacteria using a blender.
Radioactive Phosphorus was found in the supernatant liquid rather than inside the bacteria.
This experiment proved unequivocally that DNA is the genetic material that enters the bacteria.
5.

Discuss the machinery and enzymes involved in the process of DNA Replication.  

DNA-dependent DNA polymerase catalyzes polymerization only in the 5' to 3' direction.
DNA polymerase can initiate the process of replication on its own without any primer.
On the template strand with 3' to 5' polarity, the replication is continuous.
On the template strand with 5' to 3' polarity, the replication is discontinuous.
The discontinuously synthesized fragments are joined together by the enzyme DNA ligase.
Deoxyribonucleoside triphosphates serve the dual purpose of substrates and energy providers.
6.

Explain the process of transcription in a prokaryotic cell.  

In bacteria, a single RNA polymerase catalyzes the transcription of all types of RNA.
RNA polymerase binds to the promoter and initiates transcription with the help of the sigma factor.
Transcription occurs in the 5' to 3' direction using the 3' to 5' DNA strand as a template.
The newly synthesized mRNA must undergo splicing to remove introns before it can be translated.
Termination occurs when the RNA polymerase reaches the terminator and associates with the rho factor.
Translation can begin much before the mRNA is fully transcribed in bacteria.
7.

Enlist the salient features of the Genetic Code.  

The codon is a triplet, and 61 codons code for amino acids while 3 act as stop codons.
The genetic code is universal, meaning it is the same from bacteria to humans.
One codon can code for more than one amino acid, making the code ambiguous.
The code is degenerate, meaning some amino acids are coded by more than one codon.
The codon is read in mRNA in a contiguous fashion with no punctuations.
AUG has a dual function: it codes for Methionine and acts as an initiator codon.
8.

Describe the structure of tRNA and explain why it is called an adapter molecule.

tRNA has an anticodon loop that has bases complementary to the code on the mRNA.
It has an amino acid acceptor end which binds to specific amino acids.
tRNAs are specific for each amino acid; there are no tRNAs for stop codons.
The secondary structure of tRNA is depicted as a clover-leaf.
The actual 3D structure of tRNA looks like an inverted L-shape.
Every tRNA molecule is identical in sequence regardless of the amino acid it carries.
9.

Explain the regulation of gene expression in E. coli using the Lac Operon model.  

The lac operon consists of one regulatory gene (i gene) and three structural genes (z, y, a).
The i gene codes for the repressor protein which is produced constitutively.
In the presence of the inducer (lactose), the repressor binds to the operator to stop transcription.
The z gene codes for beta-galactosidase which breaks down lactose into glucose and galactose.
The y gene codes for permease, which increases the cell's permeability to lactose.
The lac operon is an example of negative regulation of gene expression.
10.

Outline the various steps involved in the technique of DNA Fingerprinting.  

DNA fingerprinting involves identifying differences in specific regions called repetitive DNA.
These sequences show a high degree of polymorphism, which is the basis of the technique.
The DNA is isolated from cells and digested by restriction endonucleases into fragments.
The fragments are separated by size using the process of gel electrophoresis.
The separated fragments are hybridized with a VNTR probe after being transferred to a membrane.
DNA polymerase is the main enzyme used to identify the final patterns on the membrane.
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