Overview Gene Expression

Gene Expression P1

Overview of Gene Expression

• Gene Expression: The process by which a gene's information is converted into the structures and functions of a cell, specifically through the synthesis of proteins.

  • mRNA Destruction: If mRNA is destroyed, the gene cannot be expressed, and therefore the specific protein, which can be enzymes or structural proteins, is not synthesized.

Components of Expression

• Gene: A portion of DNA responsible for making a specific protein, which may include:

  • Enzymes

  • Transmembrane proteins

  • Peripheral proteins

• Protein Functionality: The effectiveness of gene expression depends on whether the corresponding protein is produced in its functional form or not.

Examples Illustrating Gene Expression

• Kangaroo Example:

  • Functional vs. Nonfunctional Proteins:

    • Functional protein: Produces a specific trait (e.g., pigment).

    • Nonfunctional protein: May result from structural changes in the protein that affect its active site.

  • This shows how gene expression determines observable traits, such as color in animals.

Cases of Gene Expression

• Case 1: Protein is synthesized but is nonfunctional due to changes in its primary structure (amino acid sequence) affecting its shape and function.

  • Consequences: The gene is not expressed as the protein cannot perform its designated function.

• Case 2: Protein is not synthesized at all due to destruction of mRNA.

  • Consequences: No gene expression occurs; the enzyme is absent, and thus its function is not fulfilled.

Regulation of Expression

• Transcription Factors:

  • These proteins can enhance or inhibit the process of transcription, influencing whether genes are expressed or not.

  • Absence of Transcription Factors: If certain transcription factors are absent, genes may be silenced because mRNA will not be produced.

Specific Examples of Gene Regulation

• E. coli and Tyrosine:

  • E. coli can obtain tyrosine from its host (e.g., human intestines).

    • If enough tyrosine is present, the gene for synthesizing tyrosine in E. coli is silenced to prevent wasteful production.

    • If tyrosine is absent, the gene becomes active, and E. coli synthesizes its tyrosine.

Additional Influencing Factors

• Hormones: Some hormones can regulate gene expression positively or negatively through their presence or absence affecting the transcription machinery.

• Chromatin Structure:

  • The compactness of DNA within chromatin (DNAs and proteins) influences gene accessibility for transcription mechanisms.

  • Modifications of chromatin can inhibit or promote gene expression.

Summary of Mechanisms Affecting Gene Expression

• mRNA and Protein Stability:

  • mRNA can be degraded or proteins can be destroyed, which directly impacts gene expression.

  • Regularly assessing mRNA stability is crucial in understanding how gene expression is controlled at the molecular level.

Conclusion

• Important concepts in gene expression include the distinction between functional and nonfunctional proteins, the role of mRNA, and how external factors such as transcription factors, hormones, and even the inherent properties of DNA such as chromatin structure can influence the overall expression outcomes.

Gene Expression P2

DNA Methylation

• Definition: Addition of a methyl group (CH3) to DNA, affecting gene expression.

• Function:

  • Methyl groups added to cytosine nucleotide result in gene silencing, inhibiting transcription.

  • Conversely, removing methyl groups "activates" the genes, allowing transcription to occur.

• Significance:

  • Regulates gene expression, preventing unnecessary protein synthesis and conserving energy.

  • Example: E. coli activates or deactivates genes related to trypsin based on intestinal needs.

Chromatin Modification

• Definition: Structural changes to chromatin (DNA + proteins) that impact gene accessibility.

• Role of Acetyl Groups:

  • Presence of acetyl groups added to histones exposes DNA, permitting transcription.

  • Absence of acetyl groups tightens histone-DNA interaction, restricting gene expression.

• Mechanism: Acetylation vs. Methylation

  • Acetylation: Increases accessibility, allows transcription.

  • Methylation: Decreases accessibility, prevents transcription.

RNA Interference (RNAi)

• Definition: Regulatory mechanism controlling gene expression post-transcriptionally.

• Key Components:

  • MicroRNAs (miRNAs) and small interfering RNAs (siRNAs) are involved with enzymes forming silencing complexes.

• Mechanism:

  • RISC (RNA-induced silencing complex) degrades target mRNA or inhibits translation, silencing the gene.

  • Prevents the production of proteins from specific genes when not needed.

Importance in Gene Regulation

• Both DNA methylation and chromatin modification are vital for adaptive responses to cellular needs.

• They ensure that proteins are synthesized only when required, maintaining metabolic efficiency.

• Understanding these processes is fundamental for advancements in genetic research, biotechnology, and medicine.

Gene Expression P3

RNA Induced Silencing Complex (RISC)

• RISC: RNA Induced Silencing Complex

• Function: Can lead to the destruction of mRNA or inhibit protein synthesis, resulting in a halt in function.

Chromatin Modification

• Chromatin can be modified to influence gene expression.

• Acetylation of Histones:

  • Addition of acetyl groups to histones

  • Converts chromatin into euchromatin, making genes accessible for transcription and translation.

  • Promotes gene expression.

• DNA Methylation:

  • Addition of methyl groups, primarily to cytosine in DNA.

  • Highly methylated genes are turned off.

  • Removal of methyl groups can reactivate genes.

Gene Expression and Methylation Signals

• Signaling for methylation can be affected by environmental conditions and protein levels.

• Example: High protein levels can switch off genes to prevent wasteful protein synthesis.

Understanding Mutation

• Definition of Mutation:

  • Change in the base sequence of DNA or alteration in chromosome number.

  • e.g., Down syndrome: Presence of 47 chromosomes due to nondisjunction during meiosis.

Types of Mutations

• Base Substitution Mutation:

  • One base is swapped for another, potentially altering amino acid coding.

  • Can be silent (no change in amino acid) or affect protein function.

• Deletion Mutation:

  • Base is removed, causing a shift in the reading frame (frameshift mutation) which can change all codons downstream.

• Addition Mutation:

  • An extra base is added, also leading to reading frame shifts.

Codon and Amino Acids

• mRNA codons consist of three bases coding for specific amino acids.

• Changes in the DNA sequence lead to changes in codons, thus altering the resulting protein form.

Silent Mutations

• Occurs when a substitution does not change the amino acid sequence, maintaining protein function (e.g., GCA codes for alanine and GCU also codes for alanine).

Nonsense Mutations

• Results in the creation of a stop codon, terminating protein synthesis prematurely, which can lead to nonfunctional proteins.