NCERT Solutions
Class 12 • Biology • Chapter 5 • Molecular Basis of Inheritance| Nitrogenous Bases | Nucleosides (Base + Sugar) |
|---|---|
| Adenine | Cytidine |
| Thymine | Guanosine |
| Uracil | |
| Cytosine |
According to Chargaff’s rule, A = T and G = C.
- Given: Cytosine (C) = 20%.
- Therefore, Guanine (G) = 20% (Since G = C).
- Total (G + C) = 20 + 20 = 40%.
- Remaining (A + T) = 100 – 40 = 60%.
- Since A = T, Adenine = 60 / 2 = 30%.
5′-ATGCATGCATGCATGCATGCATGCATGC-3′
Write down the sequence of complementary strand in 5’\(\to\)3′ direction.
Rule: A pairs with T, and G pairs with C. The strands are antiparallel.
- Given Strand (5′ \(\to\) 3′): 5′-ATGCATGCATGCATGCATGCATGCATGC-3′
- Complementary (3′ \(\to\) 5′): 3′-TACGTACGTACGTACGTACGTACGTACG-5′
- Sequence in 5′ \(\to\) 3′:
5′-GCATGCATGCATGCATGCATGCATGCAT-3′
5′-ATGCATGCATGCATGCATGCATGCATGC-3′
Write down the sequence of mRNA.
The sequence of mRNA is the same as the Coding Strand (5′ \(\to\) 3′) except that Thymine (T) is replaced by Uracil (U).
mRNA Sequence: 5′-AUGCAUGCAUGCAUGCAUGCAUGCAUGC-3′
The property of Complementary Base Pairing led to this hypothesis.
Explanation: Watson and Crick observed that the two strands of DNA are complementary (A pairs with T, G with C). They suggested that the two strands would separate, and each strand would act as a template for the synthesis of a new complementary strand. As a result, each daughter DNA molecule would have one parental strand and one newly synthesized strand. This is called semi-conservative replication.
- DNA-dependent DNA polymerase: Uses DNA template to synthesize DNA (Replication).
- DNA-dependent RNA polymerase: Uses DNA template to synthesize RNA (Transcription).
- RNA-dependent DNA polymerase (Reverse Transcriptase): Uses RNA template to synthesize DNA (in Retroviruses).
- RNA-dependent RNA polymerase: Uses RNA template to synthesize RNA (in some RNA viruses).
They used Radioactive Isotopes to label specific components of the Bacteriophage (virus):
- Phosphorus-32 (\(^{32}P\)): Used to label DNA because DNA contains phosphorus but protein does not.
- Sulfur-35 (\(^{35}S\)): Used to label Protein Coat because proteins contain sulfur (in amino acids like cysteine/methionine) but DNA does not.
Differentiation: After infection and centrifugation, radioactivity was checked. Bacteria infected with \(^{32}P\)-labeled viruses were radioactive (showing DNA entered the cell), while those infected with \(^{35}S\)-labeled viruses were not (showing protein stayed outside).
| (a) Repetitive DNA | Satellite DNA |
|---|---|
| DNA sequences that are repeated many times in the genome. | A specific type of repetitive DNA that forms a separate peak during density gradient centrifugation. |
| Includes both coding and non-coding sequences. | Usually non-coding and highly repetitive. |
| (b) mRNA (Messenger RNA) | tRNA (Transfer RNA) |
|---|---|
| Linear structure. | Clover-leaf shape (2D) / Inverted L (3D). |
| Carries genetic code from DNA to ribosomes. | Carries specific amino acids to ribosomes for protein synthesis. |
| (c) Template Strand | Coding Strand |
|---|---|
| Strand with polarity 3′ \(\to\) 5′. | Strand with polarity 5′ \(\to\) 3′. |
| Acts as a template for RNA synthesis. | Does not code for RNA; its sequence matches the RNA (except T/U). |
- Site of Protein Synthesis: The ribosome provides the structural platform where mRNA and tRNA come together to synthesize the polypeptide chain.
- Catalytic Role: The large subunit of the ribosome acts as an enzyme (Ribozyme, 23S rRNA in bacteria) to catalyze the formation of Peptide bonds between amino acids.
The lac operon is active as long as Lactose (Inducer) is present. The enzymes produced (\(\beta\)-galactosidase, permease, transacetylase) metabolize the lactose into glucose and galactose.
Shutdown Reason: Once the lactose in the medium is completely consumed/metabolized by the bacteria, there is no inducer left to bind to the Repressor protein. The Repressor becomes active again, binds to the Operator region, and shuts down the operon (Transcription stops).
- (a) Promoter: A DNA sequence where RNA polymerase binds to initiate transcription.
- (b) tRNA: An adapter molecule that reads the genetic code on mRNA and brings the specific amino acid for protein synthesis.
- (c) Exons: The coding sequences or expressed sequences in a eukaryotic gene that appear in the mature/processed mRNA.
It is called a mega project due to its massive scale:
- Huge Cost: Estimated cost was 9 billion US dollars.
- Large Number of Base Pairs: Involved sequencing \(3 \times 10^9\) base pairs.
- Large Data Storage: Required huge data storage devices (3300 books, 1000 pages each), leading to the development of Bioinformatics.
- International Collaboration: Coordinated by US Department of Energy, NIH, Wellcome Trust (UK), and partners from Japan, France, Germany, China, etc.
Definition: DNA fingerprinting is a technique to identify a person based on their unique DNA profile, specifically the Variable Number Tandem Repeats (VNTRs) or satellite DNA.
- Forensic Science: To identify criminals in crime scenes (blood, hair, semen samples).
- Paternity Disputes: To determine the biological parents of a child.
- Evolutionary Biology: To study genetic diversity and evolutionary relationships among populations.
(b) Polymorphism: Variation at the genetic level arising due to mutations. It is the basis of genetic mapping and DNA fingerprinting. (e.g., Allelic sequence variation).
(c) Translation: The process of polymerization of amino acids to form a polypeptide (protein) based on the sequence of codons in mRNA. It occurs in ribosomes.
(d) Bioinformatics: An interdisciplinary field that uses computer science and mathematics to store, retrieve, and analyze biological data (like DNA sequences).