Biology Lecture Notes - RNA & Transcription
RNA & Transcription Review
Topics to be covered include:
Starting & stopping RNA polymerase.
The Genetic Code.
Ribosomes & Protein Synthesis.
Key Enzymes and Proteins in DNA Replication
Helicase: Unwinds the double-stranded DNA.
Single Strand Binding Protein (RPA): Stabilizes unwound single-stranded DNA.
Primase: Synthesizes RNA primers for DNA replication initiation.
Replication Fork: Split the DNA strands for replication (in leading & lagging strands).
Sliding Clamp: Linked to DNA polymerase to hold it onto the template strand.
DNA Polymerase: In prokaryotes, forms a dimer to synthesize both strands.
DNA Ligase: Links Okazaki fragments together on the lagging strand.
Gyrase: Prevents torsional strain ahead of the replication fork.
Problems with Linear Chromosomes
Disadvantage of Linear Chromosomes:
Problem: With each replication cycle, the ends of linear chromosomes shorten due to the inability to fully replicate the lagging strand.
Solution: Telomerase extends chromosome ends to prevent gene loss.
Telomerase:
An enzyme that is an RNA-dependent DNA polymerase.
It adds a repetitive nucleotide sequence (to prevent the loss of essential genetic information) to the ends of chromosomes (telomeres).
It maintains chromosome integrity amidst degradation.
RNA and Transcription Basics
Gene: A segment of DNA that serves as a template for RNA synthesis.
Types of RNA produced:
rRNA: Contributes to ribosome structure.
mRNA: Acts as a template for protein synthesis.
tRNA: Delivers amino acids during translation.
snRNA: Involved in RNA splicing.
snoRNA: Participates in rRNA modification.
miRNA: Regulates gene expression.
lncRNA: Assists in various cellular processes.
siRNA: Involved in RNA interference.
Characteristics of RNA in E. coli (Table 4.2)
Relative Amount & Characteristics:
Ribosomal RNA (rRNA): 80%
Sedimentation Coefficient: 23S
Mass: 1.2 × 10^3 kd
Nucleotides Count: 3700
Transfer RNA (tRNA): 15%
Sedimentation Coefficient: 4S
Mass: 5 × 10^2 kd
Nucleotides Count: 75
Messenger RNA (mRNA): 5%
Sedimentation Coefficient: 2.5S
Mass: 3.6 × 10^1 kd
Nucleotides Count: 120
Transcription Overview
Transcription involves generating RNA from a DNA template.
It occurs in three stages:
Initiation: RNA polymerase binds to the promoter region.
Elongation: RNA strand elongation as nucleotides are added from ribonucleoside triphosphates (ATP, UTP, CTP, GTP).
Termination: RNA polymerase reaches a termination site, leading to the release of the RNA transcript.
RNA synthesis proceeds in the 5' to 3' direction. New bases are always added to the 3' end of the growing RNA strand.
Promotor Regions: In prokaryotes, these are recognized by specific sequences, approximately -10 and -35 bases upstream of the coding region.
Transcription Factors and Mechanisms
General Transcription Factors: Assist in initiating transcription by locating the promoter region. For example, Oct1 and Sox2.
Sigma Factor: A protein that helps in locating the promoter in prokaryotes by binding to the -10 and -35 regions.
Transcription Bubble: Formed during elongation, where unwound DNA remains transiently paired with RNA transcript.
RNA Processing in Eukaryotes
RNA processing begins immediately after transcription in eukaryotes and involves:
Adding a 5' cap for stability in translation.
Adding a 3' poly-A tail to aid in mRNA transport and stability.
Intron removal: Untranslated sequences that must be spliced out to generate functional RNA.
Extensive RNA processing differentiates eukaryotes from prokaryotes.
Alternative Splicing: A mechanism allowing for multiple mRNA variants from a single gene, leading to diverse protein products.
Conclusion and Next Steps
Understand and describe:
Transcription initiation, elongation, and termination.
RNA processing mechanisms like the addition of a 5' cap, a 3' poly-A tail, and intron splicing.
Differences between prokaryote and eukaryote mRNA processes.
Upcoming Topic: The Genetic Code and Peptide Synthesis.
National Science Foundation (NSF) Graduate Research Fellowship Announcement
In April, the National Science Foundation (NSF) announced its latest class of graduate research fellows.
This fellows program has been a significant initiative since the agency’s inception, contributing to many successful scientific careers.
It has notably fueled the careers of 50 Nobel laureates.
The program receives more than 12,000 applications each year.
This year (2020), NSF selected only 1,000 fellows.
This number is half the usual selection and 1200 fewer than initially promised funding for.
There was public disappointment within the scientific community about this reduced selection.
Two months later, NSF announced an additional round of 500 fellows.
This expansion was attributed to external intervention rather than internal NSF decision-making.
Brian Stone, acting director of NSF, communicated the necessity of the White House's involvement, specifically mentioning Michael Kratsios, director of the White House Office of Science and Technology Policy (OSTP).
Stone expressed gratitude for OSTP's role in facilitating this additional funding.
The fluctuations within the fellowship program illustrate a significant shift in NSF’s historical operations.
The agency, historically designed to avoid political influences and focus on funding nonmedical academic research, has experienced a substantial loss of independence since President Trump took office.
The Trump administration's impact on NSF includes:
Direct orders to abandon long-standing programs.
Termination of over 2,000 grants.
Changes in funding decision criteria, prioritizing the administration’s political agenda.
An increase in the exclusion of diversity, equity, and inclusion (DEI) efforts, as well as climate change research.
NSF is also undergoing operational changes, which include:
Reduced dependence on top academic scientists who are loaned from their institutions.
A realignment of the entire grant-making process to align with Trump’s political priorities.
According to science policy experts, these alterations divert NSF from its foundational principles.
These principles were established in a 1945 report by Vannevar Bush, which emphasized the importance of maintaining U.S. leadership in science through broad funding and training of future scientists.
NSF was intended to embody the idea of funding the best ideas across scientific fields rather than implementing targeted missions typical of other government agencies (e.g., health, energy, space exploration).
RNA & Transcription Review
The Central Dogma of Molecular Biology describes the flow of genetic information: DNA is replicated, DNA is transcribed into RNA, and RNA is translated into protein.
Topics to be covered include:
Starting & stopping RNA polymerase.
The Genetic Code.
Ribosomes & Protein Synthesis.
Key Enzymes and Proteins in DNA Replication
Helicase: Unwinds the double-stranded DNA.
Single Strand Binding Protein (RPA): Stabilizes unwound single-stranded DNA, preventing hydrogen bonds from reforming before replication is complete.
Primase: Synthesizes short RNA primers required for DNA polymerase to initiate DNA synthesis.
Replication Fork: Splits the DNA strands for replication (in leading & lagging strands).
Sliding Clamp: Linked to DNA polymerase to hold it onto the template strand, preventing it from dissociating.
DNA Polymerase: Form a dimer in prokaryotes to synthesize both strands; always adds new nucleotides to the end, synthesizing DNA in the to direction.
DNA Ligase: Links Okazaki fragments together on the lagging strand with a covalent bond.
Gyrase (Topoisomerase): A type of topoisomerase that prevents torsional strain (supercoiling) ahead of the replication fork as DNA is unwound.
Problems with Linear Chromosomes
Disadvantage of Linear Chromosomes:
Problem: With each replication cycle, the ends of linear chromosomes shorten due to the inability to fully replicate the lagging strand. This occurs because DNA polymerase requires an RNA primer, and there is insufficient space at the very end of the lagging strand template to place a final primer, leading to an unreplicated single-stranded overhang. If not fixed, this could lead to the degradation of essential genes and premature aging.
Solution: Telomerase extends chromosome ends to prevent gene loss.
Telomerase:
An enzyme that is an RNA-dependent DNA polymerase (also known as a reverse transcriptase).
It carries its own small RNA template to extend the end of the lagging strand template by repeatedly adding a short, repetitive nucleotide sequence (telomeres).
This extension provides enough space for primase and DNA polymerase to synthesize the remaining section, although the very end may still be lost, it is now a non-coding, repetitive sequence, maintaining chromosome integrity amidst degradation of non-essential information.
RNA and Transcription Basics
Gene: A segment of DNA that serves as a template for RNA synthesis and contains information to make an RNA product (and often a protein product).
Structural Differences between RNA and DNA:
Sugar: RNA contains ribose, while DNA contains deoxyribose.
Bases: Uracil (U) in RNA replaces Thymine (T) in DNA.
Types of RNA produced:
rRNA: 80% of total RNA; contributes to ribosome structure and catalyzes protein synthesis.
mRNA: Acts as a template for protein synthesis (messenger).
tRNA: Delivers specific amino acids during translation (transfer).
snRNA: Involved in RNA splicing.
snoRNA: Participates in rRNA modification.
miRNA: Regulates gene expression.
lncRNA: Assists in various cellular processes.
siRNA: Involved in RNA interference.
Characteristics of RNA in E. coli (Table 4.2)
Relative Amount & Characteristics:
Ribosomal RNA (rRNA): 80%
Sedimentation Coefficient: 23S
Mass: kd
Nucleotides Count: 3700
Transfer RNA (tRNA): 15%
Sedimentation Coefficient: 4S
Mass: kd
Nucleotides Count: 75
Messenger RNA (mRNA): 5%
Sedimentation Coefficient: 2.5S
Mass: kd
Nucleotides Count: 120
Genome Insights
Only about 1-1.5% of the human genome contains information necessary for making proteins.
Approximately 50% of the human genome consists of transposable elements (genetic parasites).
Other significant portions include intergenic regions (about 20%) and large duplications/microsatellites (repetitive sequences).
The term "junk DNA" is often used cautiously, as the function of much non-coding DNA is still being researched, and removal can sometimes have negative consequences.
Transcription Overview
Transcription involves generating RNA from a DNA template. It is fundamentally simpler than DNA replication, as the RNA polymerase itself unwinds the DNA, negating the need for helicase or single-stranded binding proteins.
RNA polymerase reads the DNA template in the to direction and synthesizes the RNA strand in the to direction, always adding new bases to the end of the growing RNA strand.
Genes can be oriented in various directions along the different strands of the DNA double helix.
It occurs in three stages:
Initiation: RNA polymerase binds to the promoter region.
Elongation: RNA strand elongation as nucleotides are added from ribonucleoside triphosphates (ATP, UTP, CTP, GTP). The polymerase moves along the DNA, unwinding it temporarily, and the DNA rewinds behind it.
Termination: RNA polymerase reaches a specific termination sequence within the DNA, leading to the release of the RNA transcript. This is distinct from a stop codon, which is relevant only in translation.
Promotor Regions: These are found in the regulatory region of a gene, adjacent to the coding region. They are recognized by specific sequences, often rich in A's and T's (e.g., the TATAAT box in prokaryotes, approximately -10 bases upstream of the coding region), and they dictate the start point of transcription and the template strand.
Transcription Factors and Mechanisms
Transcription Factors (TFs): Proteins that bind to specific DNA sequences within the regulatory regions of genes to control when and where genes are turned on or off. They can:
Activate (up-regulate) transcription by helping RNA polymerase bind to the promoter and initiate transcription.
Inactivate (down-regulate) transcription by blocking RNA polymerase from binding to the promoter.
General Transcription Factors: Assist in initiating transcription by locating the promoter region (e.g., Oct1 and Sox2 by initially mentioned). They are involved in regulating many genes.
Sigma Factor: A protein that helps in locating the promoter in prokaryotes by binding to the -10 and -35 regions.
Transcription Bubble: Formed during elongation, where unwound DNA remains transiently paired with RNA transcript.
RNA Processing in Eukaryotes
Unlike prokaryotes, where DNA replication, transcription, and translation can occur simultaneously in the cytoplasm, eukaryotes perform DNA replication and transcription in the nucleus, while translation happens in the cytoplasm. This separation allows for extensive post-transcriptional RNA modification.
RNA processing begins immediately after transcription in eukaryotes and involves:
Adding a 5' cap: A modified guanine nucleotide added to the end of the pre-mRNA for stability, protection from degradation, and to aid in ribosome recognition and interaction during translation.
Adding a 3' poly-A tail: A long chain of adenine nucleotides (without a template) added to the end of the pre-mRNA by an enzyme. This tail enhances stability, facilitates mRNA export from the nucleus, and acts as a 'fuse' to signal RNA age, with degradation occurring as the tail shortens over time.
Intron removal (splicing): Untranslated sequences called introns (non-coding regions interrupting the coding sequence) must be precisely excised from the pre-mRNA, and the remaining exons (coding sequences) are ligated together to form functional mRNA. Introns are typically recognized by consensus sequences, such as GU at their end and AG at their end.
Extensive RNA processing differentiates eukaryotes from prokaryotes.
Alternative Splicing: A mechanism allowing for multiple mRNA variants from a single gene by selectively including or excluding certain exons. This leads to diverse protein products from a limited number of genes, significantly increasing proteomic diversity.
Disadvantage: The presence of introns and their subsequent removal represents a significant energy cost during gene expression (transcription and splicing).
RNA Export: After processing, eukaryotic mRNA, bound by various proteins, is actively transported out of the nucleus through nuclear pores to the cytoplasm for translation.
Conclusion and Next Steps
Understand and describe:
The Central Dogma of Molecular Biology.
DNA Replication (including problems with linear chromosomes and telomerase).
Transcription initiation, elongation, and termination (including the role of promoters and termination sequences).
RNA processing mechanisms like the addition of a 5' cap, a 3' poly-A tail, and intron splicing (including alternative splicing).
Differences between prokaryote and eukaryote mRNA processes.
Genome composition and the role of non-coding DNA.
Upcoming Topic: The Genetic Code and Peptide Synthesis.
National Science Foundation (NSF) Graduate Research Fellowship Announcement
In April, the National Science Foundation (NSF) announced its latest class of graduate research fellows.
This fellows program has been a significant initiative since the agency’s inception, contributing to many successful scientific careers.
It has notably fueled the careers of 50 Nobel laureates.
The program receives more than 12,000 applications each year.
This year (2020), NSF selected only 1,000 fellows.
This number is half the usual selection and 1200 fewer than initially promised funding for.
There was public disappointment within the scientific community about this reduced selection.
Two months later, NSF announced an additional round of 500 fellows.
This expansion was attributed to external intervention rather than internal NSF decision-making.
Brian Stone, acting director of NSF, communicated the necessity of the White House's involvement, specifically mentioning Michael Kratsios, director of the White House Office of Science and Technology Policy (OSTP).
Stone expressed gratitude for OSTP's role in facilitating this additional funding.
The fluctuations within the fellowship program illustrate a significant shift in NSF’s historical operations.
The agency, historically designed to avoid political influences and focus on funding nonmedical academic research, has experienced a substantial loss of independence since President Trump took office.
The Trump administration's impact on NSF includes:
Direct orders to abandon long-standing programs.
Termination of over 2,000 grants.
Changes in funding decision criteria, prioritizing the administration’s political agenda.
An increase in the exclusion of diversity, equity, and inclusion (DEI) efforts, as well as climate change research.
NSF is also undergoing operational changes, which include:
Reduced dependence on top academic scientists who are loaned from their institutions.
A realignment of the entire grant-making process to align with Trump’s political priorities.
According to science policy experts, these alterations divert NSF from its foundational principles.
These principles were established in a 1945 report by Vannevar Bush, which emphasized the importance of maintaining U.S. leadership in science through broad funding and training of future scientists.
NSF was intended to embody the idea of funding the best ideas across scientific fields rather than implementing targeted missions typical of other government agencies (e.g., health, energy, space exploration).
RNA & Transcription Review
The Central Dogma of Molecular Biology describes the flow of genetic information: DNA is replicated, DNA is transcribed into RNA, and RNA is translated into protein.
Topics to be covered include:
Starting & stopping RNA polymerase.
The Genetic Code.
Ribosomes & Protein Synthesis.
Key Enzymes and Proteins in DNA Replication
Helicase: Unwinds the double-stranded DNA.
Single Strand Binding Protein (RPA): Stabilizes unwound single-stranded DNA, preventing hydrogen bonds from reforming before replication is complete.
Primase: Synthesizes short RNA primers required for DNA polymerase to initiate DNA synthesis.
Replication Fork: Splits the DNA strands for replication (in leading & lagging strands).
Sliding Clamp: Linked to DNA polymerase to hold it onto the template strand, preventing it from dissociating.
DNA Polymerase: Form a dimer in prokaryotes to synthesize both strands; always adds new nucleotides to the end, synthesizing DNA in the to direction.
DNA Ligase: Links Okazaki fragments together on the lagging strand with a covalent bond.
Gyrase (Topoisomerase): A type of topoisomerase that prevents torsional strain (supercoiling) ahead of the replication fork as DNA is unwound.
Problems with Linear Chromosomes
Disadvantage of Linear Chromosomes:
Problem: With each replication cycle, the ends of linear chromosomes shorten due to the inability to fully replicate the lagging strand. This occurs because DNA polymerase requires an RNA primer, and there is insufficient space at the very end of the lagging strand template to place a final primer, leading to an unreplicated single-stranded overhang. If not fixed, this could lead to the degradation of essential genes and premature aging.
Solution: Telomerase extends chromosome ends to prevent gene loss.
Telomerase:
An enzyme that is an RNA-dependent DNA polymerase (also known as a reverse transcriptase).
It carries its own small RNA template to extend the end of the lagging strand template by repeatedly adding a short, repetitive nucleotide sequence (telomeres).
This extension provides enough space for primase and DNA polymerase to synthesize the remaining section, although the very end may still be lost, it is now a non-coding, repetitive sequence, maintaining chromosome integrity amidst degradation of non-essential information.
RNA and Transcription Basics
Gene: A segment of DNA that serves as a template for RNA synthesis and contains information to make an RNA product (and often a protein product).
Structural Differences between RNA and DNA:
Sugar: RNA contains ribose, while DNA contains deoxyribose.
Bases: Uracil (U) in RNA replaces Thymine (T) in DNA.
Types of RNA produced:
rRNA: 80% of total RNA; contributes to ribosome structure and catalyzes protein synthesis.
mRNA: Acts as a template for protein synthesis (messenger).
tRNA: Delivers specific amino acids during translation (transfer).
snRNA: Involved in RNA splicing.
snoRNA: Participates in rRNA modification.
miRNA: Regulates gene expression.
lncRNA: Assists in various cellular processes.
siRNA: Involved in RNA interference.
Characteristics of RNA in E. coli (Table 4.2)
Relative Amount & Characteristics:
Ribosomal RNA (rRNA): 80%
Sedimentation Coefficient: 23S
Mass: kd
Nucleotides Count: 3700
Transfer RNA (tRNA): 15%
Sedimentation Coefficient: 4S
Mass: kd
Nucleotides Count: 75
Messenger RNA (mRNA): 5%
Sedimentation Coefficient: 2.5S
Mass: kd
Nucleotides Count: 120
Genome Insights
Only about 1-1.5% of the human genome contains information necessary for making proteins.
Approximately 50% of the human genome consists of transposable elements (genetic parasites).
Other significant portions include intergenic regions (about 20%) and large duplications/microsatellites (repetitive sequences).
The term "junk DNA" is often used cautiously, as the function of much non-coding DNA is still being researched, and removal can sometimes have negative consequences.
Transcription Overview
Transcription involves generating RNA from a DNA template. It is fundamentally simpler than DNA replication, as the RNA polymerase itself unwinds the DNA, negating the need for helicase or single-stranded binding proteins.
RNA polymerase reads the DNA template in the to direction and synthesizes the RNA strand in the to direction, always adding new bases to the end of the growing RNA strand.
Genes can be oriented in various directions along the different strands of the DNA double helix.
It occurs in three stages:
Initiation: RNA polymerase binds to the promoter region.
Elongation: RNA strand elongation as nucleotides are added from ribonucleoside triphosphates (ATP, UTP, CTP, GTP). The polymerase moves along the DNA, unwinding it temporarily, and the DNA rewinds behind it.
Termination: RNA polymerase reaches a specific termination sequence within the DNA, leading to the release of the RNA transcript. This is distinct from a stop codon, which is relevant only in translation.
Promotor Regions: These are found in the regulatory region of a gene, adjacent to the coding region. They are recognized by specific sequences, often rich in A's and T's (e.g., the TATAAT box in prokaryotes, approximately -10 bases upstream of the coding region), and they dictate the start point of transcription and the template strand.
Transcription Factors and Mechanisms
Transcription Factors (TFs): Proteins that bind to specific DNA sequences within the regulatory regions of genes to control when and where genes are turned on or off. They can:
Activate (up-regulate) transcription by helping RNA polymerase bind to the promoter and initiate transcription.
Inactivate (down-regulate) transcription by blocking RNA polymerase from binding to the promoter.
General Transcription Factors: Assist in initiating transcription by locating the promoter region (e.g., Oct1 and Sox2 by initially mentioned). They are involved in regulating many genes.
Sigma Factor: A protein that helps in locating the promoter in prokaryotes by binding to the -10 and -35 regions.
Transcription Bubble: Formed during elongation, where unwound DNA remains transiently paired with RNA transcript.
RNA Processing in Eukaryotes
Unlike prokaryotes, where DNA replication, transcription, and translation can occur simultaneously in the cytoplasm, eukaryotes perform DNA replication and transcription in the nucleus, while translation happens in the cytoplasm. This separation allows for extensive post-transcriptional RNA modification.
RNA processing begins immediately after transcription in eukaryotes and involves:
Adding a 5' cap: A modified guanine nucleotide added to the end of the pre-mRNA for stability, protection from degradation, and to aid in ribosome recognition and interaction during translation.
Adding a 3' poly-A tail: A long chain of adenine nucleotides (without a template) added to the end of the pre-mRNA by an enzyme. This tail enhances stability, facilitates mRNA export from the nucleus, and acts as a 'fuse' to signal RNA age, with degradation occurring as the tail shortens over time.
Intron removal (splicing): Untranslated sequences called introns (non-coding regions interrupting the coding sequence) must be precisely excised from the pre-mRNA, and the remaining exons (coding sequences) are ligated together to form functional mRNA. Introns are typically recognized by consensus sequences, such as GU at their end and AG at their end.
Extensive RNA processing differentiates eukaryotes from prokaryotes.
Alternative Splicing: A mechanism allowing for multiple mRNA variants from a single gene by selectively including or excluding certain exons. This leads to diverse protein products from a limited number of genes, significantly increasing proteomic diversity.
Disadvantage: The presence of introns and their subsequent removal represents a significant energy cost during gene expression (transcription and splicing).
RNA Export: After processing, eukaryotic mRNA, bound by various proteins, is actively transported out of the nucleus through nuclear pores to the cytoplasm for translation.
Conclusion and Next Steps
Understand and describe:
The Central Dogma of Molecular Biology.
DNA Replication (including problems with linear chromosomes and telomerase).
Transcription initiation, elongation, and termination (including the role of promoters and termination sequences).
RNA processing mechanisms like the addition of a 5' cap, a 3' poly-A tail, and intron splicing (including alternative splicing).
Differences between prokaryote and eukaryote mRNA processes.
Genome composition and the role of non-coding DNA.
Upcoming Topic: The Genetic Code and Peptide Synthesis.