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.