Genetics and Evolution- Molecular Basis of Inheritance

What do you mean by transcription?

  • Transcription is the process by which genetic information in DNA is copied into RNA.
  • It is the first step in gene expression and involves the synthesis of a complementary RNA strand from DNA template.
  • RNA polymerase enzyme plays a key role in transcription.
  • The RNA synthesized during transcription is known as messenger RNA (mRNA).
  • Transcription occurs in the nucleus of eukaryotic cells.

Steps involved in transcription

  • Initiation: RNA polymerase binds to the DNA at the promoter region.
  • Elongation: RNA polymerase moves along the DNA template strand and synthesizes the complementary RNA strand.
  • Termination: RNA polymerase reaches the terminator region and detaches from the DNA template.

Types of RNA involved in transcription

  • Messenger RNA (mRNA): Carries the genetic information from the DNA to the ribosomes.
  • Transfer RNA (tRNA): Helps in decoding the information on mRNA and brings specific amino acids to the ribosomes during protein synthesis.
  • Ribosomal RNA (rRNA): Forms the basic structure of ribosomes and catalyzes protein synthesis.

Importance of transcription

  • Transcription is essential for gene expression and protein synthesis.
  • It allows the genetic information stored in the DNA to be utilized in the production of proteins.
  • It regulates the expression of genes in response to environmental cues.
  • Transcription is a crucial step in various cellular processes such as development, differentiation, and response to stress.

Transcription in prokaryotes vs eukaryotes

| Prokaryotes | Eukaryotes | |-|| | Transcription occurs in the cytoplasm. | Transcription occurs in the nucleus. | | No post-transcriptional modifications are required. | Post-transcriptional modifications like splicing, capping, and polyadenylation are necessary. | | mRNA is polycistronic (contains multiple genes). | mRNA is monocistronic (contains one gene). |

Transcription factors

  • Transcription factors are proteins that help in the initiation and regulation of transcription.
  • They bind to specific DNA sequences and interact with RNA polymerase to initiate transcription.
  • Examples of transcription factors include activators, repressors, and enhancers.

Transcriptional control

  • Transcriptional control refers to the regulation of gene expression at the level of transcription.
  • It involves the binding of transcription factors to specific DNA sequences, which either enhance or inhibit transcription.
  • Transcriptional control plays a crucial role in determining cell-specific gene expression patterns.

Transcription regulation in eukaryotes

  • Eukaryotic gene regulation is complex and involves various mechanisms.
  • Promoters, enhancers, and silencers are regions in the DNA that control gene expression.
  • Transcription factors, co-activators, and co-repressors modulate transcriptional activity.
  • Chromatin remodeling and DNA methylation also influence transcription regulation.

Steps involved in transcription

  • Initiation:
    • RNA polymerase binds to the DNA at the promoter region.
    • Promoter region consists of specific DNA sequences that indicate where transcription should begin.
  • Elongation:
    • RNA polymerase moves along the DNA template strand in a 3’ to 5’ direction.
    • It synthesizes the complementary RNA strand in a 5’ to 3’ direction.
    • The DNA double helix is temporarily unwound to expose the template strand.
    • RNA nucleotides are added to the growing RNA strand using complementary base pairing.
  • Termination:
    • RNA polymerase reaches the terminator region.
    • Terminator region signals the end of transcription.
    • RNA polymerase detaches from the DNA template.

Types of RNA involved in transcription

  • Messenger RNA (mRNA):
    • Carries the genetic information from the DNA to the ribosomes.
    • Serve as a template for protein synthesis.
  • Transfer RNA (tRNA):
    • Helps in decoding the information on mRNA during protein synthesis.
    • Carries specific amino acids to the ribosomes.
  • Ribosomal RNA (rRNA):
    • Forms the basic structure of ribosomes.
    • Provides a site for protein synthesis.

Importance of transcription

  • Essential for gene expression:
    • Transcription converts the genetic information stored in DNA into RNA, which is then used to produce proteins.
    • Proteins are responsible for almost all cellular functions and structures.
  • Regulation of gene expression:
    • Transcription allows control over when and where genes are expressed.
    • Gene expression can be regulated by varying the rate of transcription.
  • Cellular processes:
    • Transcription plays a vital role in many cellular processes, including development, growth, and response to environmental stimuli.

Transcription in prokaryotes vs eukaryotes

  • Prokaryotes:
    • Transcription occurs in the cytoplasm.
    • No post-transcriptional modifications are required on mRNA.
    • Prokaryotic mRNA is polycistronic, meaning it contains multiple genes.
  • Eukaryotes:
    • Transcription occurs in the nucleus.
    • Post-transcriptional modifications such as splicing, capping, and polyadenylation are necessary for mRNA maturation.
    • Eukaryotic mRNA is monocistronic, containing one gene per mRNA molecule.

Transcription factors

  • Transcription factors:
    • Proteins that bind to specific DNA sequences called enhancer or promoter elements.
    • They assist in the initiation and regulation of transcription.
    • Examples include activators, repressors, and enhancers.

Transcriptional control

  • Transcriptional control:
    • Regulation of gene expression at the level of transcription.
    • Involves the binding of transcription factors to enhancer or promoter elements.
    • Transcription factors can either enhance or inhibit transcription.
    • Plays a key role in determining cell-specific gene expression patterns.

Transcription regulation in eukaryotes

  • Eukaryotic gene regulation:
    • Complex and involves various mechanisms.
    • Promoters, enhancers, and silencers are regulatory regions in DNA.
    • Transcription factors, co-activators, and co-repressors modulate transcriptional activity.
    • Chromatin remodeling and DNA methylation also influence transcription regulation.

Mechanism of transcription initiation

  • Initiation of transcription:
    • RNA polymerase binds to the promoter region of the DNA.
    • Transcription factors bind to specific DNA sequences in the promoter region.
    • RNA polymerase and transcription factors form a transcription initiation complex.
    • The DNA double helix unwinds to expose the template strand.
  • Promoter recognition:
    • RNA polymerase recognizes specific DNA sequences called promoter elements.
    • Promoter elements determine the start site and direction of transcription.
    • Different genes have different types of promoter elements.

Regulation of transcription initiation

  • Regulation by transcription factors:
    • Activator proteins bind to enhancer regions and promote transcription.
    • Repressor proteins bind to silencer regions and inhibit transcription.
  • Co-activators and co-repressors:
    • Assist activators or repressors in modulating transcription.
    • They interact with transcription factors and RNA polymerase.
  • Chromatin remodeling:
    • Chromatin structure affects access of RNA polymerase to DNA.
    • Remodeling complexes modify chromatin structure to facilitate or restrict transcription.

Post-transcriptional modifications

  • Modifications of mRNA after transcription:
    • Addition of a 5’ cap at the 5’ end of mRNA.
    • Addition of a poly-A tail at the 3’ end of mRNA.
    • Removal of introns through a process called RNA splicing.
  • Importance of post-transcriptional modifications:
    • Stability and protection of mRNA.
    • Facilitation of mRNA transport from the nucleus to the cytoplasm.
    • Regulation of mRNA translation.

Transcription in Prokaryotes

  • In prokaryotes, transcription occurs in the cytoplasm.
  • It does not involve post-transcriptional modifications like splicing.
  • Prokaryotic mRNA is polycistronic, meaning it contains multiple genes.
  • An example of a prokaryote is bacteria.

Transcription in Eukaryotes

  • In eukaryotes, transcription occurs in the nucleus.
  • It involves post-transcriptional modifications like splicing, capping, and polyadenylation.
  • Eukaryotic mRNA is monocistronic, containing one gene per mRNA molecule.
  • Examples of eukaryotes are plants, animals, and fungi.

Promoter Regions

  • Promoter regions are specific DNA sequences that indicate where transcription should begin.
  • They are located upstream of the gene being transcribed.
  • Promoter elements are recognized by RNA polymerase and transcription factors.
  • Promoter sequences differ among genes.

Transcription Factors

  • Transcription factors are proteins that bind to DNA and regulate the initiation and regulation of transcription.
  • Activator proteins bind to enhancer regions and promote transcription.
  • Repressor proteins bind to silencer regions and inhibit transcription.
  • Transcription factors can either enhance or inhibit transcription.

Transcription Initiation Complex

  • The transcription initiation complex is formed by the binding of RNA polymerase and transcription factors to the promoter region.
  • RNA polymerase unwinds the DNA double helix to expose the template strand.
  • Transcription factors help position RNA polymerase at the correct start site.
  • The initiation complex determines the direction and start site of transcription.

Enhancers and Silencers

  • Enhancers and silencers are regulatory DNA sequences that can enhance or repress transcription.
  • Enhancers are bound by activator proteins, which stimulate transcription.
  • Silencers are bound by repressor proteins, which inhibit transcription.
  • Enhancers and silencers can be located far upstream or downstream from the promoter region.

Co-activators and Co-repressors

  • Co-activators and co-repressors are proteins that assist activators or repressors in modulating transcription.
  • They interact with transcription factors and RNA polymerase.
  • Co-activators enhance transcription by promoting the assembly of the transcription initiation complex.
  • Co-repressors inhibit transcription by interfering with the assembly of the initiation complex.

Chromatin Remodeling

  • Chromatin remodeling refers to changes in the structure of chromatin that make DNA accessible to transcription factors and RNA polymerase.
  • Remodeling complexes modify the position or properties of nucleosomes, which are made of DNA and histone proteins.
  • Histone modification and DNA methylation are involved in chromatin remodeling.
  • Chromatin structure affects the accessibility of promoters to transcription factors.

Post-transcriptional Modifications (5)

  • Post-transcriptional modifications occur after transcription and before mRNA is ready for translation.
  • Addition of a 5’ cap at the 5’ end of mRNA protects it from degradation and aids in ribosome binding.
  • Addition of a poly-A tail at the 3’ end of mRNA also protects it from degradation and aids in mRNA export from the nucleus.
  • RNA splicing removes introns and joins together the exons to form the mature mRNA.
  • Alternative splicing can generate multiple protein isoforms from a single gene.