Untitled

Types of Transcription Factors

  • Transcription factors are proteins that bind to DNA and regulate gene expression.

    • Example: Repressor turns genes off by binding to DNA sections.

    • Example: Activator turns genes on.

Alternative Mechanisms for Gene Regulation

  • Some genes are always expressed, leading to potential negative effects if expression is not controlled.

    • Problematic if RNA from such genes persists into later developmental stages in plants.

Regulation via Small RNA Molecules

  • Regulation mechanisms include the action of microRNAs and small interfering RNAs (siRNAs).

Transcription of Regulatory Genes

  1. Hairpin Formation

    • A gene with a sequence identical to another, often on a different chromosome, can lead to a hairpin structure upon transcription.

    • This hairpin can fold into a double-stranded RNA (dsRNA) structure.

  2. Role of Dicer

    • Dicer is an enzyme that processes dsRNA into small pieces, typically around 22 nucleotides long, termed microRNA (miRNA) or siRNA.

  3. RNA Induced Silencing Complex (RISC)

    • RISC binds to one strand of the small RNA.

    • One subprotein in RISC is Argonaute (Arg), which plays a key role in RNA interference (RNAi).

    • miRNA binds to target mRNA strands, leading to translation inhibition or degradation through RNase activity.

Regulation via Small Interfering RNAs (siRNAs)

  • Mechanism differs from miRNAs:

    1. siRNA is involved in silencing through DNA methylation rather than by just targeting existing mRNA.

    2. Another transcription factor activates the relevant gene, producing complementary transcripts that can base pair.

    3. Dicer processes this dsRNA into siRNA, which is then incorporated into RISC.

    4. siRNA leads RISC to bind to active transcription sites on DNA and signals DNA methylation via DNA methyltransferases.

    5. Methylation adds methyl groups to cytosine bases in DNA, resulting in transcriptional silencing (heterochromatin formation).

    • Epigenetics refers to such gene regulation changes that do not alter the DNA sequence.

Methylation Patterns

  • Methylation may be semi-permanent but is sometimes reversible, particularly early in development (demethylation in eggs).

  • Transgenics may face challenges as inserted genes can be silenced due to this methylation process, leading to potential inconsistencies in gene expression.

Gene Expression Control Example: MicroRNA and siRNA

  • Case Study with Corn:

    • A microRNA can regulate a transcription factor leading to significant changes in plant structure.

    • Loss of the microRNA leads to a bushy structure instead of a single stalk.

  • Synthetic MicroRNAs: Researchers create synthetic versions to manipulate plant traits like albinism or flowering patterns, potentially for agricultural applications.

Transposable Elements and Gene Regulation

  • Transposable elements make up a significant portion of genomes, influencing genetic diversity and adaptability.

    • Up to 45% of the human genome and 80-90% in the wheat genome consist of these elements.

  • Barbara McClintock discovered transposons, demonstrating variable phenotypes in corn kernels due to genetic changes via transposons.

Life Cycle of Retrotransposons

  • Long Terminal Repeat (LTR) Retrotransposons: Composed of LTRs, GAG, prot, reverse transcriptase, and integrase.

    • RNA Polymerase synthesizes long mRNA from the LTR, leading to a polyprotein that gets cleaved into functional proteins.

    • Reverse Transcriptase synthesizes DNA from RNA, which then is integrated back into the genome via integrase, thus proliferating transposable elements.

Epigenetics and Transposable Elements

  • Transposons may contribute to significant adaptive changes and genetic diversity but also pose risks, such as disrupting essential gene functions.

  • Transposons respond to environmental changes and stress, playing roles in evolution and expediting genetic variability in populations.