Slide 1: Molecular Basis of Inheritance - Structure of mature mRNA

  • mRNA (messenger RNA) is a crucial molecule involved in the process of gene expression.
  • It carries the genetic information from DNA in the nucleus to the ribosomes in the cytoplasm.
  • The structure of mature mRNA is adapted for efficient protein synthesis.

Slide 2: Components of mRNA

  • mRNA is composed of three main components:
    • 5’ Untranslated Region (UTR): Contains regulatory sequences and the translation start codon.
    • Coding Region: Contains nucleotide triplets, called codons, which specify the amino acid sequence of the protein.
    • 3’ UTR: Contains sequences involved in the termination of translation and mRNA degradation.

Slide 3: Structure of mRNA

  • The structure of mature mRNA can be divided into three parts:
    1. Cap Structure: A modified guanine nucleotide is added at the 5’ end, forming a 5’ cap. This cap protects the mRNA from degradation and assists in ribosome binding.
    2. Coding Region: Contains a series of codons that specify the sequence of amino acids in the protein.
    3. Poly-A Tail: A string of adenine nucleotides is added at the 3’ end, forming a poly-A tail. This tail provides stability to the mRNA and facilitates its export from the nucleus.

Slide 4: 5’ Untranslated Region (UTR)

  • The 5’ UTR is found at the 5’ end of mature mRNA.
  • It contains regulatory sequences involved in various processes, such as:
    • Initiation of translation
    • mRNA stability
    • Localization and transport of mRNA

Slide 5: Coding Region

  • The coding region of mRNA is the portion that is translated into a protein.
  • It is composed of a series of nucleotide triplets called codons.
  • Each codon corresponds to a specific amino acid or a stop signal.
  • For example, the codon AUG codes for the amino acid methionine, which is often the start codon for protein synthesis.

Slide 6: Reading Frame

  • The reading frame of an mRNA determines how the codons are interpreted during translation.
  • There are three possible reading frames in an mRNA molecule.
  • The correct reading frame is established by the start codon and is maintained until a stop codon is reached.

Slide 7: Open Reading Frame (ORF)

  • An open reading frame (ORF) is a coding region that contains a start codon followed by one or more codons and ends with a stop codon.
  • The ORF represents a potential gene or protein-coding sequence.

Slide 8: 3’ Untranslated Region (UTR)

  • The 3’ UTR is found at the 3’ end of mature mRNA.
  • It contains sequences involved in mRNA stability, termination of translation, and the degradation of mRNA.
  • It also plays a role in the post-transcriptional regulation of gene expression.

Slide 9: Cap-Dependent Translation Initiation

  • Cap-dependent translation initiation is the most common mechanism of translation initiation in eukaryotes.
  • The 5’ cap structure interacts with translation initiation factors and ribosomes to facilitate the binding of ribosomes to the mRNA.
  • This process ensures efficient translation of the mRNA into a protein.

Slide 10: Poly-A Tail

  • The poly-A tail is a string of adenine nucleotides added at the 3’ end of mature mRNA.
  • It enhances the stability of the mRNA and protects it from degradation.
  • The poly-A tail also plays a role in the export of mRNA from the nucleus and in the regulation of mRNA translation and decay.
  1. Role of 5’ Untranslated Region (UTR)
  • Contains regulatory sequences that help in the initiation of translation.
  • Several upstream open reading frames (uORFs) present in the 5’ UTR can control the protein expression level.
  • Example: The 5’ UTR of the ATF4 mRNA contains uORFs that regulate its translation in response to stress signals.
  1. Role of Coding Region
  • Contains codons that specify the sequence of amino acids in a protein.
  • The genetic code is universal, with the same codons representing the same amino acids across species.
  • Example: The codon sequence AUG specifies the amino acid methionine in all organisms.
  1. Role of 3’ Untranslated Region (UTR)
  • Contains sequences that influence mRNA stability and degradation.
  • Binding sites for regulatory proteins and microRNAs are often found in the 3’ UTR.
  • Example: The binding of microRNAs to the 3’ UTR of target mRNA leads to translational repression or mRNA degradation.
  1. Translation Initiation Factors
  • Proteins that assist in the binding of ribosomes to the mRNA and the initiation of translation.
  • Examples: eIF4E, eIF4G, and eIF4A form a complex that interacts with the 5’ cap structure.
  • eIF2 plays a key role in the recognition of the start codon and the recruitment of the small ribosomal subunit.
  1. Ribosomes and Translation
  • Ribosomes are large complexes of proteins and rRNA that carry out translation.
  • Consist of a small and large subunit, which come together on the mRNA during translation initiation.
  • The large subunit has the peptidyl transferase activity required for peptide bond formation between amino acids.
  1. Start Codon
  • The start codon serves as the initiation signal for translation.
  • The most common start codon is AUG, which codes for methionine.
  • Alternative start codons, such as GUG and UUG, can also be used in certain contexts.
  1. Stop Codons
  • Stop codons (UAA, UAG, and UGA) signal the termination of translation.
  • They do not code for any amino acid and do not have tRNAs associated with them.
  • Release factors recognize the stop codons and promote the release of the newly synthesized protein.
  1. Reading Frame Shifts
  • Reading frame shifts occur when there is a deletion or insertion of nucleotides in the coding region.
  • This alters the sequence of codons and can lead to a completely different protein being synthesized.
  • Example: Frameshift mutations can result in severe genetic diseases, such as Duchenne muscular dystrophy.
  1. Alternative Splicing
  • Alternative splicing is a mechanism that generates multiple mRNAs and proteins from a single gene.
  • Different combinations of exons can be included or excluded during mRNA processing.
  • Example: The Dscam gene in Drosophila undergoes extensive alternative splicing, resulting in over 38,000 different protein isoforms.
  1. Regulation of mRNA Stability
  • The stability of mRNA can be regulated by various factors, including RNA-binding proteins and miRNAs.
  • Stabilizing elements in the 3’ UTR can prevent mRNA degradation.
  • Example: AU-rich elements (AREs) in the 3’ UTR of many mRNAs determine their stability and turnover rate.

Slide 21: Regulation of mRNA Translation

  • Translation of mRNAs can be regulated at various levels to control gene expression.
  • Regulatory factors can influence the initiation, elongation, and termination of translation.
  • Examples of regulation include:
    • Binding of regulatory proteins to specific RNA sequences
    • Interference of translation initiation by microRNAs
    • Alteration of ribosomal subunit availability or activity

Slide 22: Post-transcriptional Modifications

  • mRNA molecules undergo post-transcriptional modifications before they are fully functional.
  • Modifications include:
    • Addition of a 5’ cap and a 3’ poly-A tail
    • Splicing of introns
    • Editing of mRNA sequences by enzymes

Slide 23: Splicing of Introns

  • Introns are non-coding regions within a gene that are transcribed into mRNA but are later removed.
  • Splicing is the process by which introns are removed and exons are joined together.
  • Splicing is catalyzed by the spliceosome, a complex of proteins and small nuclear RNAs (snRNAs).
  • Alternative splicing can lead to the production of multiple protein isoforms from a single gene.

Slide 24: RNA Editing

  • RNA editing is a post-transcriptional modification that changes the nucleotide sequence of an mRNA molecule.
  • It is carried out by enzymes called RNA-editing enzymes.
  • The most common form of RNA editing is the conversion of adenosine (A) to inosine (I) within mRNA molecules.
  • RNA editing can affect the function and properties of encoded proteins.

Slide 25: Non-coding RNAs

  • Non-coding RNAs (ncRNAs) are RNA molecules that do not code for proteins.
  • They have various functions in the cell, including:
    • Regulating gene expression
    • Controlling mRNA stability and translation
    • Mediating RNA splicing and editing
  • Examples of ncRNAs include microRNAs, long non-coding RNAs (lncRNAs), and small interfering RNAs (siRNAs).

Slide 26: MicroRNAs (miRNAs)

  • MicroRNAs (miRNAs) are small ncRNAs that negatively regulate gene expression.
  • They base-pair with target mRNAs, leading to mRNA degradation or translational repression.
  • miRNAs play important roles in developmental processes, disease, and cellular homeostasis.
  • Dysregulation of miRNA expression has been implicated in various diseases, including cancer.

Slide 27: Long Non-coding RNAs (lncRNAs)

  • Long non-coding RNAs (lncRNAs) are RNA molecules that are longer than 200 nucleotides and do not code for proteins.
  • They have diverse functions and can interact with DNA, RNA, and proteins.
  • lncRNAs play roles in gene expression regulation, chromatin organization, and cellular processes.
  • Dysregulation of lncRNAs has been associated with a wide range of diseases.

Slide 28: Small Interfering RNAs (siRNAs)

  • Small interfering RNAs (siRNAs) are small double-stranded RNA molecules that regulate gene expression.
  • They are involved in a process called RNA interference (RNAi).
  • siRNAs guide the degradation of target mRNAs or inhibit their translation by binding to complementary sequences.
  • RNAi is a powerful tool for gene silencing and has applications in research and therapy.

Slide 29: Riboswitches

  • Riboswitches are RNA structures located in the untranslated regions of certain mRNAs.
  • They can bind small molecules and regulate gene expression.
  • Binding of the small molecule to the riboswitch induces a conformational change that affects mRNA stability or translation.
  • Riboswitches are involved in the regulation of metabolic pathways in bacteria and some eukaryotes.

Slide 30: Inference of Protein Function from mRNA Structure

  • The structure and sequence of mRNA can provide insights into the function of the encoded protein.
  • Conserved sequence motifs in the coding region can indicate functional domains or regions.
  • The presence of regulatory elements in the UTRs can suggest post-transcriptional regulation.
  • Comparative genomics and computational analysis are used to infer protein function from mRNA structure and sequence data.