MS

Chapter 14 AP Biology

Gene Expression Overview

  • Gene expression refers to the process by which DNA directs the synthesis of proteins that determine the traits of an organism.

  • It consists of two main stages: transcription and translation.

  • This flow of genetic information can be summarized as "DNA -> RNA -> Protein"

Concept 14.1: Genes Specify Proteins

  • The one gene-one enzyme hypothesis was revised to one gene-one protein, as many proteins consist of multiple polypeptides.

  • This is further restated as the one gene-one polypeptide hypothesis.

Basic Principles of Transcription and Translation

  • RNA serves as a critical intermediary between DNA and protein synthesis:

    • RNA has ribose and uracil (U) instead of deoxyribose and thymine (T).

    • Typically, RNA is single-stranded.

  • The two stages to convert DNA information to a protein are:

    • Transcription: synthesis of RNA from DNA

    • Translation: synthesis of polypeptides from RNA information

Central Dogma of Molecular Biology

  • Transcription generates messenger RNA (mRNA).

  • Ribosomes are where translation occurs.

  • In prokaryotes, translation can start before transcription finishes; in eukaryotes, the nuclear envelope separates these processes.

The Genetic Code

  • The genetic code consists of codons (three-nucleotide sequences in mRNA) that code for specific amino acids.

  • There are 64 codons, which include:

    • 61 for amino acids

    • 3 stop codons

  • Codons must be read in the correct reading frame; incorrect reading results in frameshift mutations.

Codons and Amino Acids

  • Codons are read from the 5' to 3' direction; each specifies an amino acid during protein synthesis.

  • Flexible pairing at the third position of codon allows for various tRNAs to recognize multiple codons (wobble pairing).

Concept 14.2: Transcription Details

  • Transcription is catalyzed by RNA polymerase, which unwinds the DNA and assembles RNA nucleotides.

  • The template strand of DNA directs RNA synthesis, producing a complementary RNA strand.

Stages of Transcription:

  • Initiation: RNA polymerase binds to the promoter; transcription starts.

  • Elongation: RNA polymerase adds nucleotides, unwinding the DNA as it goes.

  • Termination: RNA synthesis stops at a terminator sequence.

RNA Processing in Eukaryotes

  • Before mRNA can exit the nucleus and be translated, it undergoes processing:

    • 5' capping: addition of a modified guanine nucleotide.

    • Polyadenylation: addition of a poly-A tail at the 3' end.

    • Splicing: removal of non-coding introns and joining of coding exons.

Concept 14.3: Translation Mechanism

  • Transfer RNA (tRNA) is crucial for translating mRNA into a polypeptide:

    • Each tRNA carries specific amino acids and possesses anticodons for matching mRNA codons.

    • Ribosomes facilitate the coupling of tRNA and mRNA during translation.

Stages of Translation:

  • Initiation: Assembly of mRNA, tRNA (with the first amino acid), and ribosomal subunits.

  • Elongation: Sequential addition of amino acids to the growing polypeptide chain based on mRNA codon sequence.

  • Termination: Occurs when a stop codon in mRNA reaches the ribosome, triggering protein release.

Mutations

  • Mutations are alterations to the genetic material that can affect protein structure and function:

    • Point mutations: changes in a single nucleotide leading to silent, missense, or nonsense changes.

    • Insertions/Deletions: can shift the reading frame, potentially leading to drastic functional changes in proteins.

Conclusion

  • The modern understanding of genes emphasizes their role as sequences coding for specific proteins or functional RNA, reflecting the central role of gene expression in biology.