RNA Structure & Synthesis Overview

Ribosomal RNA (rRNA) is a type of RNA that forms part of the ribosome, the cellular machinery responsible for protein synthesis.

Transfer RNA (tRNA) is a type of RNA that acts as an adaptor molecule, carrying specific amino acids to the ribosome during protein synthesis.

Messenger RNA (mRNA) carries genetic information from DNA in the nucleus to the ribosome in the cytoplasm, where it directs protein synthesis.

Prokaryotic rRNA consists of three main types: 23S, 16S, and 5S rRNA. These rRNA molecules are involved in the formation of the prokaryotic ribosome.

Eukaryotic rRNA consists of four main types: 28S, 18S, 5.8S, and 5S rRNA. These rRNA molecules are involved in the formation of the eukaryotic ribosome.

Svedberg units (S) represent a measure of sedimentation rate, which is related to a molecule's size and shape. It's used to classify rRNA molecules.

tRNA molecules are the smallest RNA molecules, approximately 4S in size. They are structurally unique and have a specific 3' end where an amino acid attaches.

Each of the 20 common amino acids found in proteins has at least one specific tRNA molecule that can carry it to the ribosome. This ensures the correct amino acid is added to the growing polypeptide chain.

tRNA molecules are essential for accurate translation of the genetic code. They act as 'adaptors' that decode the mRNA sequence and bring the correct amino acid to the ribosome.

mRNA is the most diverse type of RNA in terms of its size and sequence, ranging from 500 to 6000 nucleotides. This variability reflects the diverse protein coding requirements of the cell.

mRNA carries genetic information from the DNA in the nucleus to the cytoplasm, where ribosomes can translate the instructions into proteins. This process is essential for gene expression.

Polycistronic mRNA carries information from more than one gene within a single RNA molecule. This type of mRNA is characteristic of prokaryotes.

Monocistronic mRNA carries information from only one gene within a single RNA molecule. This type of mRNA is characteristic of eukaryotes.

Eukaryotic mRNA has a poly-A tail, a long sequence of adenine nucleotides at the 3' end, and a 5' cap, a modified guanine nucleotide at the 5' end. These structures protect the mRNA and aid in ribosome binding.

The poly-A tail prevents degradation of the coding sequence by exonucleases, enzymes that degrade RNA from the ends. This ensures the mRNA remains intact and functional for a longer period.

The 5' cap helps the small ribosomal subunit find the start codon, the initiation site for protein synthesis. This ensures translation begins at the correct location.

RNA polymerase is a multi-subunit enzyme responsible for synthesizing RNA from a DNA template in prokaryotes.

Prokaryotic RNA polymerase recognizes specific nucleotide sequences called promoter regions at the beginning of genes to initiate transcription.

The promoter region on DNA contains consensus sequences that RNA polymerase recognizes. These sequences are typically centered around -35 and -10 bases upstream of the transcription start site.

The -35 sequence is a consensus sequence (5'-TTGACA-3') located about 35 bases upstream of the transcription start site. It's the initial point of contact for RNA polymerase.

The Pribnow box (also known as the -10 sequence) is a consensus sequence (5'-TATAAT-3') located about 10 bases upstream of the transcription start site. It's the site where DNA unwinding begins.

Mutations within the -10 or -35 sequences can affect the ability of RNA polymerase to bind and initiate transcription, potentially altering gene expression.

RNA polymerase synthesizes RNA from its 5' to 3' end, adding nucleotides one at a time to the growing RNA chain.

Prokaryotic RNA polymerase has a core enzyme and several auxiliary proteins that are essential for its activity and regulation.

The core enzyme of prokaryotic RNA polymerase consists of two α subunits, one β' subunit, one β subunit, and sometimes a fifth subunit called Ω. The core enzyme is responsible for the enzymatic activity of RNA polymerase.

The σ subunit (sigma factor) is an auxiliary protein that enables RNA polymerase to recognize and bind to promoter regions on DNA, thereby facilitating the initiation of transcription.

The holoenzyme is formed by the combination of the σ subunit with the core enzyme. This complex is fully functional and capable of initiating and elongating RNA transcripts.

Termination factors are proteins that recognize specific termination signals on DNA, signaling the end of transcription. These factors help release the newly synthesized RNA molecule from RNA polymerase.

Rho (ρ) factor is a termination factor in prokaryotic transcription. It's an ATP-dependent helicase protein that interacts with the RNA transcript and disrupts the RNA polymerase-DNA complex, causing the termination of transcription.

Transcription from bacterial operons involves the coordinated expression of multiple genes that are clustered together on the chromosome and transcribed as a single mRNA molecule. This allows for the simultaneous production of proteins involved in a specific metabolic pathway.

The lactose operon in E. coli is a classic example of an operon. It contains genes encoding enzymes involved in lactose metabolism, which are regulated in response to the availability of lactose and glucose.

The lac operon is regulated by a repressor protein that binds to the operator site, blocking transcription when lactose is absent. However, when lactose is present, it binds to the repressor, causing it to detach from the operator and allowing transcription to proceed.

The CAP binding site, promoter, and operator are regulatory elements within the lac operon. These sites control the binding of the CAP-cAMP complex and RNA polymerase, respectively, affecting the transcription of the lac operon genes.

When glucose is present, adenylyl cyclase is inactive, and the cAMP-CAP complex is not formed, preventing effective transcription of the lac operon genes. This ensures that the cell prioritizes the use of glucose as an energy source.