Transcription is the first step in gene expression, where DNA is copied into RNA. It's a crucial process that determines which genes are active in a cell. RNA polymerase and promoter regions play key roles in starting transcription.
RNA comes in different types, each with a specific job. mRNA carries genetic info to ribosomes, tRNA brings amino acids, and rRNA helps build proteins. In eukaryotes, mRNA undergoes processing to become functional, including splicing and adding protective structures.
Transcription Initiation
RNA Polymerase and Promoter Regions
- RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template
- Promoter regions are specific DNA sequences located upstream of a gene that signal the starting point for transcription
- RNA polymerase binds to the promoter region to initiate transcription
- Promoter regions contain specific sequences recognized by RNA polymerase and transcription factors (TATA box)
Transcription Factors and Gene Regulation
- Transcription factors are proteins that help regulate gene expression by binding to specific DNA sequences
- Transcription factors can act as activators enhancing transcription or repressors reducing transcription
- The binding of transcription factors to regulatory sequences (enhancers, silencers) influences the recruitment and activity of RNA polymerase
- The combination of transcription factors present in a cell determines which genes are expressed and at what level
RNA Types
Messenger RNA (mRNA)
- mRNA is the type of RNA that carries the genetic information from DNA to the ribosomes for protein synthesis
- mRNA is synthesized during transcription and serves as a template for translation
- The sequence of nucleotides in mRNA determines the amino acid sequence of the protein product
- mRNA molecules are typically short-lived and are degraded after translation
Transfer RNA (tRNA) and Ribosomal RNA (rRNA)
- tRNA molecules are responsible for carrying specific amino acids to the ribosomes during protein synthesis
- Each tRNA has an anticodon sequence that pairs with the corresponding codon on the mRNA
- tRNAs are charged with their specific amino acid by aminoacyl-tRNA synthetases
- rRNA is a component of ribosomes, the cellular machinery responsible for protein synthesis
- rRNA molecules provide structural support to the ribosome and catalyze peptide bond formation between amino acids
mRNA Processing
Introns, Exons, and Splicing
- Eukaryotic genes often contain non-coding sequences called introns interspersed between coding sequences called exons
- The primary transcript (pre-mRNA) undergoes splicing to remove introns and join exons together
- Splicing is carried out by the spliceosome, a complex of proteins and small nuclear RNAs (snRNAs)
- Alternative splicing allows for the production of different mRNA variants from the same gene leading to protein diversity
5' Cap and Poly-A Tail
- A 5' cap (7-methylguanosine) is added to the 5' end of the mature mRNA molecule
- The 5' cap protects the mRNA from degradation and facilitates its transport to the cytoplasm and recognition by ribosomes
- A poly-A tail, consisting of a string of adenine nucleotides, is added to the 3' end of the mRNA
- The poly-A tail enhances mRNA stability, aids in export to the cytoplasm, and influences translation efficiency
- The addition of the 5' cap and poly-A tail occurs co-transcriptionally in the nucleus before the mature mRNA is exported to the cytoplasm
Key Terms to Review (22)
RRNA: Ribosomal RNA (rRNA) is a type of RNA that is a fundamental component of ribosomes, which are the cellular structures responsible for protein synthesis. rRNA plays a crucial role in translating messenger RNA (mRNA) into proteins by facilitating the binding of transfer RNA (tRNA) and catalyzing the formation of peptide bonds between amino acids, thus linking it to the larger framework of nucleic acids and proteins in living organisms.
MRNA: mRNA, or messenger RNA, is a single-stranded molecule that carries genetic information from DNA to the ribosome, where proteins are synthesized. This process is crucial as it translates the genetic code into functional proteins, which are essential for numerous biological functions and processes.
TRNA: tRNA, or transfer RNA, is a type of RNA molecule that plays a crucial role in translating genetic information from messenger RNA (mRNA) into proteins. Each tRNA carries a specific amino acid to the ribosome during protein synthesis, ensuring that the correct building blocks are assembled in the right order as dictated by the mRNA sequence. This process connects the flow of genetic information from nucleic acids to proteins, highlighting the relationship between nucleic acids and proteins in cellular functions.
Gene expression: Gene expression is the process by which information from a gene is used to synthesize functional gene products, typically proteins, that perform various functions in a cell. This process involves two main stages: transcription, where the DNA sequence of a gene is transcribed to produce messenger RNA (mRNA), and translation, where the mRNA is decoded to build a polypeptide chain. The regulation of gene expression allows cells to respond to changes in their environment and maintain homeostasis.
Pre-mRNA: pre-mRNA, or precursor messenger RNA, is an initial form of mRNA that is synthesized from a DNA template during transcription. It undergoes several processing steps, including capping, polyadenylation, and splicing, to become mature mRNA that can be translated into proteins. This process is essential for the regulation of gene expression and the proper functioning of cells.
3' poly-A tail: The 3' poly-A tail is a string of adenine nucleotides added to the 3' end of a pre-mRNA molecule during RNA processing. This modification plays a crucial role in enhancing the stability of mRNA, facilitating its export from the nucleus to the cytoplasm, and aiding in the translation process. The presence of the poly-A tail is essential for the proper functioning of eukaryotic genes, as it influences mRNA lifespan and translation efficiency.
Spliceosome: A spliceosome is a complex molecular machine composed of RNA and protein that is responsible for the splicing of pre-mRNA, which is a crucial step in the process of gene expression. By removing non-coding sequences called introns from pre-mRNA and joining together the coding sequences known as exons, spliceosomes ensure that the final mRNA product is correctly processed for translation into proteins.
Anticodon: An anticodon is a sequence of three nucleotides located on transfer RNA (tRNA) that is complementary to a corresponding codon on messenger RNA (mRNA). This complementary pairing is crucial during the process of translation, as it ensures the correct amino acid is brought to the growing polypeptide chain. Anticodons play a vital role in the translation of genetic information into proteins, linking the nucleotide sequence of mRNA with the amino acid sequence of proteins.
Codon: A codon is a sequence of three nucleotides in mRNA that specifies a single amino acid or a termination signal during protein synthesis. Codons are fundamental in the translation process, as they determine the sequence of amino acids that will form a protein, ultimately influencing the protein's structure and function. Each codon corresponds to specific amino acids or signals, enabling the accurate translation of genetic information from RNA to protein.
5' cap: The 5' cap is a modified guanine nucleotide added to the 5' end of eukaryotic mRNA transcripts during RNA processing. This structure plays a crucial role in mRNA stability, nuclear export, and initiation of translation, ensuring that the mRNA is properly recognized and utilized by the cellular machinery.
Exons: Exons are the coding sequences of DNA that are transcribed into RNA and retained in the final mRNA molecule after splicing. They play a crucial role in the process of gene expression, as they provide the necessary information to produce proteins. In contrast to introns, which are non-coding sequences removed during RNA processing, exons are essential for translating genetic information into functional proteins.
Aminoacyl-trna synthetases: Aminoacyl-tRNA synthetases are a group of enzymes responsible for attaching the correct amino acid to its corresponding transfer RNA (tRNA) molecule during protein synthesis. This process, known as 'charging' tRNA, is crucial because it ensures that proteins are built accurately according to the genetic code, which is transcribed from DNA into messenger RNA (mRNA) and eventually translated into protein.
Splicing: Splicing is the process of modifying the primary RNA transcript by removing non-coding regions called introns and joining the coding sequences known as exons together. This process is crucial for generating a mature mRNA molecule that can be translated into a functional protein. Splicing allows for the possibility of alternative splicing, which can create different protein variants from a single gene, thus increasing the diversity of proteins that can be produced in a cell.
Silencer: A silencer is a DNA sequence that can bind proteins to inhibit the transcription of a gene, thereby playing a crucial role in gene regulation. Silencers function by preventing the assembly of transcription machinery at the promoter region of a gene, ensuring that specific genes are expressed only when needed. This regulatory mechanism is essential for maintaining proper cellular function and responding to environmental changes.
Introns: Introns are non-coding sequences of DNA that are found within genes. Unlike exons, which are the coding parts that get expressed in proteins, introns are spliced out during the RNA processing stage before the final messenger RNA (mRNA) is translated into a protein. This splicing process is crucial for creating mature mRNA that accurately represents the genetic information necessary for protein synthesis.
Repressor: A repressor is a type of protein that inhibits gene expression by binding to specific DNA sequences, preventing the transcription of genes into RNA. By doing so, repressors play a crucial role in regulating cellular processes, ensuring that genes are expressed only when needed and helping maintain cellular homeostasis.
Activator: An activator is a molecule or protein that increases the rate of transcription of a specific gene by binding to an enhancer or promoter region in the DNA. Activators play a crucial role in gene expression, helping to initiate the process of converting DNA into RNA, which is essential for producing proteins and regulating cellular functions.
Enhancer: An enhancer is a regulatory DNA sequence that increases the likelihood of transcription of a particular gene. Enhancers can be located far from the genes they regulate and function by providing binding sites for transcription factors, which help to assemble the transcription machinery at the promoter region of the gene. This interaction can significantly boost gene expression, playing a critical role in the regulation of genes in both prokaryotes and eukaryotes.
Promoter: A promoter is a specific DNA sequence located near the beginning of a gene that serves as the binding site for RNA polymerase and other transcription factors, initiating the process of transcription. It plays a crucial role in determining when and where genes are expressed, thereby influencing gene regulation and cellular function.
Rna polymerase: RNA polymerase is an essential enzyme responsible for synthesizing RNA from a DNA template during the process of transcription. This enzyme binds to specific promoter regions on the DNA and unwinds the double helix, allowing it to read the nucleotide sequence and create a complementary RNA strand. Its function is crucial not only for the production of messenger RNA (mRNA), but also for other types of RNA like transfer RNA (tRNA) and ribosomal RNA (rRNA), linking it to key processes such as gene expression and regulation.
Transcription factors: Transcription factors are proteins that help regulate the transcription of specific genes by binding to nearby DNA. They play a crucial role in controlling gene expression, enabling cells to respond to internal and external signals. By interacting with RNA polymerase and other components of the transcription machinery, these factors can enhance or inhibit the transcription process, ensuring that genes are expressed at the right time and place.
Transcription: Transcription is the biological process by which genetic information encoded in DNA is copied into messenger RNA (mRNA). This step is crucial for gene expression, as it allows the information stored in DNA to be translated into proteins, which perform various functions in the cell. Transcription occurs in the nucleus of eukaryotic cells and involves several key enzymes and regulatory elements that ensure accurate copying and processing of the genetic code.