Gene Expression | Zoology Optional Notes for UPSC PDF Download

Objectives

Understand the Structure and Functions of Proteins

• Proteins act as a blueprint for synthesizing proteins.
• DNA serves as a template for RNA, which, in turn, serves as a template for protein synthesis.

Know the Process of Gene Expression

• Gene expression involves the conversion of a gene's coded information into the structures present and operating in the cell.
• It encompasses two steps: transcription and translation.

Describe the Process of Transcription

• Transcription is the first step in gene expression.
• During transcription, a complementary mRNA strand is formed based on the template strand of the DNA molecule.
• The sequence of nucleotides in DNA is faithfully copied to a complementary sequence in mRNA.

Describe the Process of Translation

• Translation is the second step in gene expression.
• A sequence of nucleotides in mRNA is translated into the sequence of amino acids in a protein.

Understand How Gene Expression may be Regulated by a Cell

• The regulation of gene expression is crucial for controlling cellular functions.

Structure and Function of Proteins

Proteins and Polypeptides

• Proteins are formed of polypeptides, composed of amino acid subunits.
• There are 20 different amino acids in most proteins.
• Protein diversity arises from the number and order of amino acids, determining the protein's shape.

Functions of Proteins

• Proteins serve various functions:
  • Structural and regulatory components of cells.
  • Enzymes catalyze chemical reactions.
  • Neurotransmitters aid in the nervous system.
  • Antibodies function in the immune system.
  • Hormones change activities of certain cells.

Gene Expression: An Overview

Gene

• The fundamental unit of heredity.
• An ordered sequence of nucleotides on a chromosome that encodes a specific functional product (protein or RNA).
• The process converting a gene's coded information into cellular structures.
• Involves transcription and translation.
• Expressed genes include those transcribed into mRNA and translated into protein, as well as those transcribed into RNA without translation.

The Genetic Code

Genetic Code

• A sequence of nucleotides coded in triplets (codons) along mRNA, determining amino acid sequence.
• DNA sequence predicts mRNA, and genetic code predicts amino acid sequence.
  

Codon Structure

• Codons consist of three bases, providing 64 possible combinations.
• Redundancy in the genetic code protects against harmful mutations.
• Stop codons signal polypeptide termination.

Methionine and Start Codon

• AUG codes for methionine, signaling the beginning of the polypeptide.

Transcription

Transcription Process

• RNA polymerase and transcription factors join the DNA at the promoter.
• DNA double helix opens, allowing complementary base pairing.
• RNA polymerase joins RNA nucleotides, forming a complementary mRNA strand.
• Transcription ends at the terminator, and the mRNA separates from DNA.
• RNA polymerase initiates transcription of a new RNA strand.

mRNA Formation

• Transcription commences in the nucleus as RNA polymerase and associated transcription factors bind to the DNA double helix at a specific DNA sequence known as the promoter (found at the start site, e.g., TATA box & CAAT box).
  
• Subsequently, RNA polymerase opens the DNA double helix, facilitating complementary base pairing.
• RNA polymerase then connects RNA nucleotides to the 3' end of the RNA chain, resulting in an mRNA molecule with a base sequence complementary to the DNA segment. (Note: RNA, containing uracil instead of thymine, forms base pairs with adenine and cytosine in DNA, following A-U and C-G pairing).
• Transcription concludes when RNA polymerase reaches a designated DNA sequence called the terminator (stop site). At this point, the formed RNA molecule separates from DNA. Following this, RNA polymerase initiates the transcription of a new RNA strand, continuing the process.
  

Processing mRNA

Overview

• Transcribed mRNA undergoes processing before leaving the nucleus for translation in the cytoplasm.
• Processing transforms the primary mRNA molecule into a mature mRNA molecule.

Introns and Exons

• Genes in humans often contain introns (non-coding segments) and exons (coding segments).
• Introns do not code for functional proteins, while exons express and code for functional proteins.

Steps of RNA Processing

5' Capping

• Modification of the newly formed mRNA involves adding a methylated guanine nucleotide to the 5' end, forming the 5' cap.
• The 5' cap facilitates mRNA transport to the cytoplasm, attachment to ribosomes, and protects against degradation.

Polyadenylation

• Cleavage of the 3' end involves adding approximately 200 adenosine residues, creating the poly (A) tail.
• The poly (A) tail facilitates mRNA transport to the cytoplasm and translation.

mRNA Splicing

• Introns are removed, and exons join to form a mature mRNA molecule.
• Splicing is carried out by a spliceosome, a complex of RNA and protein.
• Alternate mRNA splicing can result in different mature RNA transcripts from a single gene, contributing to protein diversity.

Small RNA Molecules

• Small RNA molecules regulate mRNA processing, transcription, and translation.
• RNA plays a pivotal role in orchestrating cellular outcomes.

Translation

Process Overview

• Translation is the process by which mRNA codes for protein synthesis at ribosomes.

Ribosome Structure

• The ribosome contains binding sites (A, P, and E) where tRNA molecules bind with the mRNA strand.

tRNA Structure

• tRNA molecules have a cloverleaf shape with areas for binding amino acids and anticodons.
• The anticodon, a three-base sequence, pairs with mRNA codons via complementary base pairing.

Translation Process

1. Initiation

• mRNA binds to the smaller ribosomal subunit, and the larger subunit associates, forming the translation complex.

2. Elongation

• Polypeptide lengthens as tRNA molecules bring amino acids to the ribosome.
• Ribosome moves down mRNA, adding amino acids one at a time.
• Termination occurs when a stop codon enters the A site, requiring a release factor to cleave the polypeptide from the last tRNA.

3. Termination

• Release factor binds to stop codon, cleaving the polypeptide from the last tRNA.
• Ribosome dissociates into two subunits and falls off mRNA.
• Multiple ribosomes, forming a polyribosome, can work on one mRNA simultaneously, increasing protein synthesis efficiency.
  

The Regulation of Gene Expression

Overview

• All cells possess copies of all genes, but actively expressed genes differ among cells.
• Mechanisms regulating gene expression occur at various stages: pretranscriptional, transcriptional, posttranscriptional, translational, and posttranslational.

Pretranscriptional Control (In the Nucleus)

• DNA accessibility to transcription enzymes is crucial.
• Chromosome decondensation or uncoiling is necessary.
• Removal of proteins and chemical modifications protecting DNA precedes transcription.

Transcriptional Control (In the Nucleus)

• Mechanisms determine which genes are transcribed and the transcription rate.
• Transcription factors, influenced by environmental and genetic factors, initiate transcription.
• Control through signaling molecules, intracellular receptors, and cell surface receptors.
• Transcription factors bind to specific DNA promoter elements, activating RNA polymerase.
• Enhancers increase transcriptional activity, interacting with activators, while silencers inhibit transcription, interacting with repressors.
  

Translational Control (In the Cytoplasm)

• Occurs after mRNA leaves the nucleus but before protein synthesis.
• Some mRNAs require additional changes affecting mRNA life expectancy and ribosome binding.
• microRNAs (miRNAs) inhibit translation, while small interfering RNAs (siRNA) mark mRNA for destruction by nucleases.

Posttranslational Control (In the Cytoplasm)

• Occurs after protein synthesis.
• Polypeptide products may undergo additional changes before becoming biologically functional.
• Feedback control: Binding of a product can alter the enzyme's shape, affecting its function.

Transcription Factors

Overview

• Transcription factors in human cells are non-histone DNA-binding proteins.
• They act as fine-tuners, controlling gene expression levels like variable light switches.
• Cells contain various types of transcription factors that, in combination, regulate transcription.
• Differentiation and specialization of cells depend on the activity of specific transcription factors.

Example

• Cells receive signals that turn genes on or off, influencing transcription factors.
• Embryonic hand development illustrates how apoptosis (programmed cell death) gene activation shapes the formation of fingers.

RNA-Directed DNA Synthesis

Central Dogma

• Initially, genetic information was believed to flow from DNA to RNA to protein (central dogma).

RNA-Directed DNA Synthesis

• Retroviruses provide evidence of occasional genetic information flow from RNA to DNA.
• Reverse transcriptase enzyme facilitates this RNA-directed DNA synthesis.
• DNA regions in normal cells may serve as templates for RNA synthesis, which, in turn, acts as a template for DNA synthesis.
• Homology between human and retroviral oncogene sequences suggests therapeutic potential for treating inherited diseases.