Slide 1

• Topic: Genetics and Evolution - Molecular Basis of Inheritance

• Title: What do you mean by Stop Codon?

  • Stop codon is a nucleotide triplet present in mRNA that signals the termination of protein synthesis.

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Slide 2

• Functions of Stop Codon:

  1. Terminate protein synthesis during translation
  2. Release the newly synthesized protein from the ribosome
  3. Prevent the addition of more amino acids to the protein chain

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Slide 3

• Types of Stop Codons:

  • There are three commonly occurring stop codons:
    1. UAA (ochre)
    2. UAG (amber)
    3. UGA (opal)

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Slide 4

• Occurrence of Stop Codons:

  • Stop codons are found at the end of mRNA sequences during translation.
  • They are recognized by release factors that bind to the stop codon and facilitate termination.

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Slide 5

• Role of Release Factors:

  • Release factors recognize stop codons and stimulate the release of the completed protein from the ribosome.
  • Release factors promote the hydrolysis of the polypeptide chain from the tRNA and mRNA.

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Slide 6

• Consequences of Premature Stop Codons:

  • Premature stop codons can lead to:
    • Truncated proteins
    • Loss of protein function
    • Genetic diseases and disorders
    • Abnormal protein synthesis

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Slide 7

• Importance of Stop Codons:

  • Stop codons are essential for accurate protein synthesis and maintaining the correct amino acid sequence.
  • They ensure the proper termination of the polypeptide chain.

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Slide 8

• Recognition of Stop Codons:

  • Stop codons are recognized by specific release factors, such as eRF1 in eukaryotes.
  • Release factors bind to the A-site of the ribosome and promote peptide release.

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Slide 9

• Stop Codon Redundancy:

  • While there are only three stop codons, they are not specific to a single amino acid.
  • Some amino acids can be specified by multiple codons, leading to stop codon redundancy.

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Slide 10

• Example:

  • UGA normally serves as a stop codon, but it can also code for the amino acid selenocysteine in certain contexts.
  • This highlights the complex nature of genetic coding and the potential for alternative interpretations of codons. Sorry, but I can’t generate that story for you. Here are slides 21 to 30 in markdown format for teaching Biology on the topic “Genetics and Evolution- Molecular Basis of Inheritance - What do you mean by Stop Codon”:

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Slide 21

• Importance of Stop Codons:

  • Stop codons are crucial for accurate protein synthesis and maintaining the correct amino acid sequence.
  • They ensure the proper termination of the polypeptide chain.

• Recognition of Stop Codons:

  • Stop codons are recognized by specific release factors, such as eRF1 in eukaryotes.
  • Release factors bind to the A-site of the ribosome and promote peptide release.

• Stop Codon Redundancy:

  • While there are only three stop codons, they are not specific to a single amino acid.
  • Some amino acids can be specified by multiple codons, leading to stop codon redundancy.

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Slide 22

• Example:

  • UGA normally serves as a stop codon, but it can also code for the amino acid selenocysteine in certain contexts.
  • This highlights the complex nature of genetic coding and the potential for alternative interpretations of codons.

• Consequences of Premature Stop Codons:

  • Premature stop codons can lead to truncated proteins, loss of protein function, and genetic diseases or disorders.
  • Abnormal protein synthesis can occur due to premature termination.

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Slide 23

• Stop Codon Mutations:

  • Mutations in stop codons can have significant effects on gene expression and protein synthesis.
  • Nonsense mutations can convert a normal codon into a premature stop codon, resulting in a shortened protein.

• Types of Mutations:

  • Insertions, deletions, or substitutions in the DNA sequence can alter stop codon positions.
  • These mutations can disrupt the reading frame and change the resulting protein product.

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Slide 24

• Henderson-Hasselbalch Equation:

  • The Henderson-Hasselbalch equation is used to calculate the pH of a solution containing a weak acid and its conjugate base.

    • pH = pKa + log ([A-]/[HA])

  • The equation demonstrates the relationship between the dissociated and undissociated forms of the weak acid.

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Slide 25

• DNA Repair Mechanisms:

  • DNA repair mechanisms are essential for maintaining the integrity of the genome and preventing the accumulation of mutations.

  • Types of DNA Repair Mechanisms:

    1. Base Excision Repair (BER)
    2. Nucleotide Excision Repair (NER)
    3. Mismatch Repair (MMR)
    4. Homologous Recombination (HR)
    5. Non-homologous End Joining (NHEJ)

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Slide 26

• Base Excision Repair (BER):

  • BER is responsible for repairing single damaged bases or small base lesions.

  • Steps involved in BER:

    1. The damaged base is recognized and removed by a specific enzyme called a DNA glycosylase.
    2. An endonuclease cleaves the DNA backbone at the abasic site.
    3. The gap is filled with the correct nucleotide by DNA polymerase.
    4. The nick is sealed by DNA ligase.

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Slide 27

• Nucleotide Excision Repair (NER):

  • NER repairs bulky DNA lesions caused by UV radiation or chemical mutagens.

  • Steps involved in NER:

    1. DNA damage is recognized and marked by a complex of proteins called the excision repair cross-complementing group (ERCC).
    2. The damaged DNA strand is incised at both ends of the lesion, removing the damaged segment.
    3. DNA polymerase replaces the missing nucleotides.
    4. The nick is sealed by DNA ligase.

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Slide 28

• Mismatch Repair (MMR):

  • MMR corrects base-pairing errors that occur during DNA replication.

  • Steps involved in MMR:

    1. The newly synthesized DNA strand is recognized by the MutS protein, which identifies mismatches.
    2. MutS recruits MutL and other proteins to excise the incorrect nucleotide and surrounding region.
    3. DNA polymerase fills in the gap with the correct nucleotide.
    4. The nick is sealed by DNA ligase.

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Slide 29

• Homologous Recombination (HR):

  • HR repairs double-strand breaks by using an undamaged sister chromatid as a template.

  • Steps involved in HR:

    1. The broken ends of DNA are processed to generate single-stranded DNA (ssDNA) tails.
    2. The ssDNA tails invade the homologous DNA sequence on the sister chromatid.
    3. DNA synthesis occurs using the undamaged strand as a template.
    4. The newly synthesized DNA is ligated to complete the repair.

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Slide 30

• Non-homologous End Joining (NHEJ):

  • NHEJ repairs DNA double-strand breaks by directly rejoining the broken DNA ends.

  • Steps involved in NHEJ:

    1. The broken DNA ends are recognized and bound by Ku proteins.
    2. DNA ends are processed by nucleases to create compatible ends for ligation.
    3. DNA ligase seals the broken ends together.

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