Question 1

What is the importance of gene expression? What are the steps involved in it?

Accepted Answer

Gene expression is important because it allows the genetic information stored in DNA to be converted into functional products, mainly proteins, which determine the phenotype of an organism. It controls cellular functions and responses to the environment. The steps involved in gene expression are:
1. Transcription: The process where the DNA sequence of a gene is copied into messenger RNA (mRNA).
2. RNA processing (in eukaryotes): The primary RNA transcript undergoes modifications such as capping, splicing, and polyadenylation to become mature mRNA.
3. Translation: The mRNA is decoded by ribosomes to synthesize a polypeptide chain (protein).
4. Post-translational modifications: The polypeptide may undergo folding and chemical modifications to become a functional protein. — Gene expression is the process by which information from a gene is used to synthesize functional gene products. Transcription initiates the process by producing RNA from DNA. In eukaryotes, RNA processing ensures the mRNA is mature and ready for translation. Translation then converts the mRNA code into a protein sequence. These steps ensure that the genetic code is accurately expressed as proteins that perform cellular functions.

Question 2

Describe the process of regulation of gene expression in prokaryotes by giving example of lac operon.

Accepted Answer

In prokaryotes, gene expression is regulated to conserve energy and resources by producing enzymes only when needed. The lac operon in E. coli is a classic example of gene regulation. It controls the metabolism of lactose.

The lac operon consists of structural genes (lacZ, lacY, lacA) that code for enzymes β-galactosidase, permease, and transacetylase, respectively, an operator, a promoter, and a regulatory gene that produces a repressor protein.

When lactose is absent, the repressor binds to the operator, preventing RNA polymerase from transcribing the structural genes, so enzymes are not produced.

When lactose is present, it acts as an inducer by binding to the repressor, causing it to change shape and release from the operator. This allows RNA polymerase to transcribe the genes, producing enzymes to metabolize lactose.

Additionally, positive control involves an activator protein (CAP) that binds to DNA in the presence of cAMP, enhancing transcription when glucose is low.

Thus, the lac operon is an inducible system regulated by both negative and positive controls to ensure enzymes are synthesized only when lactose is available and glucose is scarce. — The lac operon regulation involves the repressor protein blocking transcription in absence of lactose and releasing it in presence of lactose. The activator protein enhances transcription when glucose is low, ensuring efficient use of energy sources. This dual regulation exemplifies how prokaryotes control gene expression in response to environmental changes.

Question 3

What would be the effect of loss of all proteins from a cell on DNA replication?

Accepted Answer

Loss of all proteins from a cell would severely impair DNA replication because proteins such as DNA polymerase, helicase, primase, single-strand binding proteins, topoisomerase, and DNA ligase are essential for the replication process. Without these proteins, the DNA strands cannot be unwound, stabilized, primed, synthesized, or joined properly, leading to failure of DNA replication. — DNA replication requires multiple enzymes and proteins to perform specific functions. Helicase unwinds the DNA helix, single-strand binding proteins stabilize the unwound strands, primase synthesizes RNA primers, DNA polymerase extends the new DNA strand, and ligase joins Okazaki fragments. Without these proteins, replication cannot proceed.

Question 4

How is the structure of DNA affected by UV rays? Discuss the molecular basis of the type of mutation caused by this type of radiation and the mechanism used by cells to correct them.

Accepted Answer

UV rays cause damage to DNA primarily by inducing the formation of thymine dimers, where two adjacent thymine bases bond covalently, distorting the DNA helix structure. This distortion blocks DNA replication and transcription.

The mutation caused is a type of substitution mutation because the thymine dimer can lead to incorrect base pairing during replication.

Cells repair this damage using nucleotide excision repair mechanisms. In this process, enzymes recognize the distortion, excise the damaged DNA segment containing the thymine dimer, and DNA polymerase fills the gap using the complementary strand as a template. DNA ligase then seals the nick, restoring the DNA to its original state. — UV radiation causes thymine dimers that distort DNA structure, leading to mutations if unrepaired. The nucleotide excision repair pathway is a critical mechanism that maintains DNA integrity by removing damaged nucleotides and replacing them with correct ones, preventing mutations from becoming permanent.

Question 5

Differentiate between the following
(a) Leading strand and lagging strand
(b) Transcription and translation
(c) Transition and transversion mutation
(d) Codon and anticodon

Accepted Answer

(a) Leading strand vs Lagging strand:
- Leading strand is synthesized continuously in the 5'→3' direction.
- Lagging strand is synthesized discontinuously in Okazaki fragments.

(b) Transcription vs Translation:
- Transcription is the process of synthesizing RNA from DNA.
- Translation is the process of synthesizing polypeptide chains from mRNA.

(c) Transition vs Transversion mutation:
- Transition mutation is the substitution of a purine for another purine (A↔G) or a pyrimidine for another pyrimidine (C↔T).
- Transversion mutation is the substitution of a purine for a pyrimidine or vice versa.

(d) Codon vs Anticodon:
- Codon is a triplet of nucleotides on mRNA that codes for an amino acid.
- Anticodon is a triplet of nucleotides on tRNA complementary to the codon. — These distinctions are fundamental to understanding DNA replication, gene expression, and mutation types. Leading and lagging strands differ in synthesis mode due to DNA polymerase directionality. Transcription and translation are sequential steps in gene expression. Transition and transversion mutations differ in the type of base substitution. Codon and anticodon pairing ensures correct amino acid incorporation during translation.

Question 6

Which of the following types of radiations is least likely to be harmful to cells?
(a) Gamma rays
(b) Ultraviolet rays
(c) X rays
(d) Alpha rays

Accepted Answer

Alpha rays are least likely to be harmful to cells because they have low penetration power and can be stopped by a few centimeters of air or a sheet of paper, thus causing less damage to cells compared to gamma rays, X rays, and ultraviolet rays which have higher penetration and ionizing power. — Gamma rays and X rays are highly penetrating ionizing radiations causing severe cellular damage. Ultraviolet rays cause DNA damage such as thymine dimers but have less penetration than gamma and X rays. Alpha particles have high ionizing power but very low penetration, making them less harmful unless ingested or inhaled.

Question 7

In which of the following DNA repair mechanism is apyrimidinic or apurinic (AP) site formed?
(a) Excision repair
(b) Mismatch repair
(c) Both of the above
(d) None of the above

Accepted Answer

Excision repair. In excision repair, damaged bases are removed creating an apurinic or apyrimidinic (AP) site, which is then processed to restore the correct DNA sequence. Mismatch repair corrects base pairing errors without forming AP sites. — Excision repair involves removal of damaged bases, leaving AP sites that are recognized and processed by specific enzymes. Mismatch repair targets incorrectly paired bases but does not create AP sites.

Basic Processes

Basic Processes — Study Notes

Introduction

DNA as Genetic Material

Structure of DNA

Practice Questions — Basic Processes

All 12 Chapters in Biotechnology