Molecular Genetics

Step 1: Understanding the diagram.
The diagram shows the process of transcription inside the nucleus, where a DNA strand is used as a template to form a complementary RNA strand. The labelled part ‘X’ represents the RNA molecule leaving the nucleus.
Step 2: Identification of molecule ‘X’.
The molecule labelled ‘X’ is messenger RNA (mRNA). It is synthesized from DNA during transcription.
Step 3: Role of mRNA in protein synthesis.
mRNA carries the genetic information (in the form of codons) from DNA in the nucleus to the ribosomes in the cytoplasm.
Step 4: Function during translation.
At the ribosome, mRNA serves as a template for assembling amino acids in the correct sequence to form a protein. Each codon on mRNA corresponds to a specific amino acid.
Step 5: Conclusion.
Thus, mRNA acts as a messenger that transfers genetic information from DNA to ribosomes, enabling protein synthesis.

Step 1: Understand CRISPR technology.
CRISPR-Cas9 is a gene-editing tool that allows scientists to cut and modify DNA at specific locations. It works like molecular scissors guided to a target sequence.
Step 2: Role of Guide RNA (gRNA).
Guide RNA is responsible for identifying and binding to the specific DNA sequence that needs to be edited. It has a sequence complementary to the target DNA, ensuring high precision in locating the correct site.
Step 3: Function of Guide RNA.
Once bound, the guide RNA directs the Cas9 enzyme exactly to the target site on the DNA, acting like a navigator.
Step 4: Role of Cas9 enzyme.
Cas9 is an endonuclease enzyme that acts as molecular scissors. It cuts the DNA strands at the specific location identified by the guide RNA.
Step 5: Final outcome of the process.
After the DNA is cut, the cell's natural repair mechanisms allow scientists to insert, delete, or modify genes, enabling precise genetic editing.

Step 1: Understanding CRISPR technology.
CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) is a modern gene-editing tool that allows scientists to precisely modify DNA sequences in an organism.
Step 2: Mechanism of action.
It uses a guide RNA (gRNA) to locate a specific DNA sequence and an enzyme called Cas9 to cut the DNA at that exact location.
Step 3: Modification of genes.
After the DNA is cut, scientists can add, remove, or replace specific genetic material. This leads to changes in the genetic code of the organism.
Step 4: Effect on characteristics.
Since genes control traits (such as color, size, disease resistance), modifying genes results in changes in the organism’s characteristics or phenotype.
Step 5: Conclusion.
Thus, CRISPR enables precise and targeted changes in DNA, allowing scientists to alter the characteristics of organisms efficiently.

Step 1: Understanding recombinant DNA technology.
Recombinant DNA technology involves combining a desired gene (from human DNA) with a vector DNA and then introducing it into a host cell for replication and expression.
Step 2: Identify the DNA used for ligation.
The cut human gene is ligated into a plasmid DNA. Plasmids are circular DNA molecules present in bacteria and are commonly used as vectors in genetic engineering.
Step 3: Identify ‘X’.
‘X’ represents the recombinant DNA (recombinant plasmid) formed after the insertion of the human gene into the plasmid.
Step 4: Explain what happens after insertion into host cell.
When ‘X’ (recombinant plasmid) is inserted into the host bacterial cell, it begins to replicate along with the host DNA. The inserted gene is also expressed, leading to the production of the desired protein.
Step 5: Conclusion.
Thus, plasmid acts as the vector for gene insertion, and introduction of recombinant DNA into the host results in cloning and expression of the desired gene.

Step 1: Understanding gene therapy.
Gene therapy involves introducing a functional gene into cells to replace or repair a defective gene responsible for a disease.
Step 2: Role of vectors.
Vectors are carriers that help transfer the desired gene into target cells. Viruses are commonly used as vectors because of their natural ability to enter cells and deliver genetic material.
Step 3: Function of viruses in gene therapy.
Modified viruses are engineered so that they are harmless but can still carry and insert the required gene into the host cell's DNA. This helps in correcting genetic defects.
Step 4: Analysis of options.
(a) Correct — Viruses insert the desired gene into stem cells.
(b) Incorrect — Viruses do not directly destroy defective genes.
(c) Incorrect — They do not increase stem cell numbers.
(d) Incorrect — They do not convert body cells into stem cells.
Step 5: Conclusion.
Thus, viruses are used because they help in inserting the active gene into stem cells.

Step 1: Understand the structure of a nucleotide.
A nucleotide is the basic structural unit of DNA. Each nucleotide is composed of three essential components which together form the building block of the DNA molecule.
Step 2: Identify the three components.
A DNA nucleotide consists of:

• A deoxyribose sugar
• A nitrogenous base (such as adenine, thymine, cytosine, or guanine)
• A phosphate group

Step 3: Analyze the options.

• (A) Incorrect. A nucleotide contains only one nitrogen base, not two.
• (B) Correct. It correctly lists sugar, base, and phosphate.
• (C) Incorrect. A nucleotide has only one phosphate group in its basic unit.
• (D) Incorrect. Amino acids are components of proteins, not nucleotides.

Step 4: Conclusion.
Thus, the correct components of a DNA nucleotide are deoxyribose sugar, nitrogen base, and phosphate group.
Final Answer:} Deoxyribose sugar, a nitrogen base and a phosphate.