chap 15

Transcription Initiation

• The sigma factor plays a crucial role in prokaryotic transcription initiation, helping to recognize and bind to specific DNA sequences, ensuring accurate transcription start sites.

Energy Source for mRNA Synthesis

• The energy required to create bonds between nucleotides during mRNA synthesis comes from the hydrolysis of high-energy phosphate bonds in nucleoside triphosphates (NTPs).

RNA (and/or DNA) Carbons

• The 2', 3', and 5' carbons of RNA (and/or DNA) are essential for phosphodiester bond formation, allowing nucleotides to link together, forming the backbone of these biomolecules.

tRNA and Charging

• tRNA (transfer RNA) plays a crucial role in protein synthesis, bringing amino acids to the ribosome during translation.
• tRNAs are 'charged' with specific amino acids, which are linked to the tRNA through an ester bond, forming an aminoacyl-tRNA complex.

Universal Genetic Code

• The genetic code is considered 'universal' because it is shared across all forms of life, from bacteria to humans, with the same sequence of nucleotides specifying the same amino acids.

DNA, Gene, Codon, Exon, and Chromosome

• DNA (deoxyribonucleic acid) is the genetic material containing the instructions for an organism's development and function.
• A gene is a segment of DNA that codes for a specific protein or functional RNA molecule.
• A codon is a sequence of three nucleotides that codes for a specific amino acid or stops protein synthesis.
• An exon is a segment of DNA that is included in the final mature RNA product after splicing.
• A chromosome is a thread-like structure composed of DNA and proteins that carry genetic information.

Nontemplate Strand (Coding Strand)

• The nontemplate strand, also known as the coding strand, is the DNA strand complementary to the template strand, which is used as a template for transcription.
• The coding strand is called as such because it contains the same sequence of nucleotides as the RNA transcript, with the exception of T being replaced by U in RNA.

Degeneracy of the Genetic Code

• The genetic code is considered 'degenerate' because more than one codon can code for the same amino acid, allowing for some flexibility in the genetic code.

Transcription, mRNA Processing, and Translation in Eukaryotes

• In eukaryotic cells, transcription occurs in the nucleus, mRNA processing (including splicing and capping) occurs in the nucleus, and translation occurs in the cytoplasm.
• This differs from prokaryotes, where all three processes occur in the cytoplasm.

Import of Sigma Factor and Initiation of Transcription

• The sigma factor is crucial for the initiation of transcription in prokaryotes, as it helps to recognize and bind to specific DNA sequences, ensuring accurate transcription start sites.
• The initiation of transcription is tightly controlled to ensure that transcription begins at specific locations, allowing for precise gene regulation.

Genetic Code and Amino Acids

• The genetic code cannot consist of only two nucleotides per codon because there are 20 amino acids, requiring a minimum of three nucleotides to code for each amino acid.
• Using four nucleotides per codon would be overkill, as it would allow for 256 possible codons, far more than the 20 amino acids that need to be encoded.